Hackaday – Pagina 8

How Is Voyager Still Talking After All These Years?

The tech news channels were recently abuzz with stories about strange signals coming back from Voyager 1. While the usual suspects jumped to the usual conclusions — aliens!! — in the absence of a firm explanation for the anomaly, some of us looked at this event as an opportunity to marvel at the fact that the two Voyager spacecraft, now in excess of 40 years old, are still in constant contact with those of us back on Earth, and this despite having covered around 20 billion kilometers in one of the most hostile environments imaginable.

Like many NASA programs, Voyager has far exceeded its original design goals, and is still reporting back useful science data to this day. But how is that even possible? What 1970s-era radio technology made it onto the twin space probes that allowed it to not only fulfill their primary mission of exploring the outer planets, but also let them go into an extended mission to interstellar space, and still remain in two-way contact? As it turns out, there’s nothing magical about Voyager’s radio — just solid engineering seasoned with a healthy dash of redundancy, and a fair bit of good luck over the years.

The Big Dish

For a program that in many ways defined the post-Apollo age of planetary exploration, Voyager was conceived surprisingly early. The complex mission profile had its origins in the “Planetary Grand Tour” concept of the mid-1960s, which was planned to take advantage of an alignment of the outer planets that would occur in the late 1970s. If launched at just the right time, a probe would be able to reach Jupiter, Saturn, Uranus, and Neptune using only gravitational assists after its initial powered boost, before being flung out on a course that would eventually take it out into interstellar space.

The idea of visiting all the outer planets was too enticing to pass up, and with the success of the Pioneer missions to Jupiter serving as dress rehearsals, the Voyager program was designed. Like all NASA programs, Voyager had certain primary mission goals, a minimum set of planetary science experiments that project managers were reasonably sure they could accomplish. The Voyager spacecraft were designed to meet these core mission goals, but planners also hoped that the vehicles would survive past their final planetary encounters and provide valuable data as they crossed the void. And so the hardware, both in the spacecraft and on the ground, reflects that hope.

Voyager primary reflector being manufactured, circa 1975. The body of the dish is made from honeycomb aluminum and is covered with graphite-impregnated epoxy laminate skins. The surface precision of the finished dish is 250 μm. Source: NASA/JPL

The most prominent physical feature of both the ground stations of the Deep Space Network (DSN), which we’ve covered in-depth already, and the Voyager spacecraft themselves are their parabolic dish antennas. While the scale may differ — the DSN sports telescopes up to 70 meters across — the Voyager twins were each launched with the largest dish that could fit into the fairing of the Titan IIIE launch vehicle.

Voyager High-Gain Antenna (HGA) schematic. Note the Cassegrain optics, as well as the frequency-selective subreflector that’s transparent to S-band (2.3-GHz) but reflects X-band (8.4-GHz). Click to enlarge. Source: NASA/JPL

The primary reflector of the High Gain Antenna (HGA) on each Voyager spacecraft is a parabolic dish 3.7 meters in diameter. The dish is made from honeycomb aluminum that’s covered with a graphite-impregnated epoxy laminate skin. The surface of the reflector is finished to a high degree of smoothness, with a surface precision of 250 μm, which is needed for use in both the S-band (2.3 GHz), used for uplink and downlink, and X-band (8.4 GHz), which is downlink only.

Like their Earth-bound counterparts in the DSN, the Voyager antennas are a Cassegrain reflector design, which uses a Frequency Selective Subreflector (FSS) at the focus of the primary reflector. The subreflector focuses and corrects incoming X-band waves back down toward the center of the primary dish, where the X-band feed horn is located. This arrangement provides about 48 dBi of gain and a beamwidth of 0.5° on the X-band. The S-band arrangement is a little different, with the feed horn located inside the subreflector. The frequency-selective nature of the subreflector material allows S-band signals to pass right through it and illuminate the primary reflector directly. This gives about 36 dBi of gain in the S-band, with a beamwidth of 2.3°. There’s also a low-gain S-band antenna with a more-or-less cardioid radiation pattern located on the Earth-facing side of the subreflector assembly, but that was only used for the first 80 days of the mission.

Two Is One

Three of the ten bays on each Voyager’s bus are dedicated to the transmitters, receivers, amplifiers, and modulators of the Radio Frequency Subsystem, or RFS. As with all high-risk space missions, redundancy is the name of the game — almost every potential single point of failure in the RFS has some sort of backup, an engineering design decision that has proven mission-saving in more than one instance on both spacecraft over the last 40 years.

On the uplink side, each Voyager has two S-band double-conversion superhet receivers. In April of 1978, barely a year before its scheduled encounter with Jupiter, the primary S-band receiver on Voyager 2 was shut down by fault-protection algorithms on the spacecraft that failed to pick up any commands from Earth for an extended period. The backup receiver was switched on, but that was found to have a bad capacitor in the phase-locked loop circuit intended to adjust for Doppler-shift changes in frequency due primarily to the movement of the Earth. Mission controllers commanded the spacecraft to switch back to the primary receiver, but that failed again, leaving Voyager 2 without any way to be commanded from the ground.

Luckily, the fault-protection routines switched the backup receiver back on after a week of no communication, but this left controllers in a jam. To continue the mission, they needed to find a way to use the wonky backup receiver to command the spacecraft. They came up with a complex scheme where DSN controllers take a guess at what the uplink frequency will be based on the predicted Doppler shift. The trouble is, thanks to the bad capacitor, the signal needs to be within 100 Hz of the lock frequency of the receiver, and that frequency changes with the temperature of the receiver, by about 400 Hz per degree. This means controllers need to perform tests twice a week to determine the current lock frequency, and also let the spacecraft stabilize thermally for three days after uplinking any commands that might change the temperature on the spacecraft.

Double Downlinks

An Apollo-era TWTA, similar to the S-band and X-band power amps used on Voyager. Source: Ken Shirriff

On the transmit side, both the X-band and S-band transmitters use separate exciters and amplifiers, and again, multiple of each for redundancy. Although downlink is primarily via the X-band transmitter, either of the two S-band exciters can be fed into either of two different power amplifiers. A Solid State Amplifier (SSA) provides a selectable power output of either 6 W or 15 W to the feedhorn, while a separate traveling-wave tube amplifier (TWTA) provides either 6.5 W or 19 W. The dual X-band exciters, which use the S-band exciters as their frequency reference, use one of two dedicated TWTAs, each of which can send either 12 W or 18W to the high-gain antenna.

The redundancy built into the downlink side of the radio system would play a role in saving the primary mission on both spacecraft. In October of 1987, Voyager 1 suffered a failure in one of the X-band TWTAs. A little more than a year later, Voyager 2 experienced the same issue. Both spacecraft were able to switch to the other TWTA, allowing Voyager 1 to send back the famous “Family Portrait” of the Solar system including the Pale Blue Dot picture of Earth, and for Voyager 2 to send data back from its flyby of Neptune in 1989.

Slower and Slower

The radio systems on the Voyager systems were primarily designed to support the planetary flybys, and so were optimized to stream as much science data as possible back to the DSN. The close approaches to each of the outer planets meant each spacecraft accelerated dramatically during the flybys, right at the moment of maximum data production from the ten science instruments onboard. To avoid bottlenecks, each Voyager included a Digital Tape Recorder (DTR), which was essentially a fancy 8-track tape deck, to buffer science data for later downlink.

Also, the increasing distance to each Voyager has drastically decreased the bandwidth available to downlink science data. When the spacecraft made their first flybys of Jupiter, data streamed at a relatively peppy 115,200 bits per second. Now, with the spacecraft each approaching a full light-day away, data drips in at only 160 bps. Uplinked commands are even slower, a mere 16 bps, and are blasted across space from the DSN’s 70-meter dish antennas using 18 kW of power. The uplink path loss over the current 23 billion kilometer distance to Voyager 1 exceeds 200 dB; on the downlink side, the DSN telescopes have to dig a signal that has faded to the attowatt (10^-18 W) range.

That the radio systems of Voyager 1 and Voyager 2 worked at all while they were still in the main part of their planetary mission is a technical achievement worth celebrating. The fact that both spacecraft are still communicating, despite the challenges of four decades in space and multiple system failures, is nearly a miracle.

Wind-Up Tape Measure Transformed Into Portable Ham Antenna

If there’s one thing that amateur radio operators are good at, it’s turning just about anything into an antenna. And hams have a long history of portable operations, too, where they drag a (sometimes) minimalist setup of gear into the woods and set up shop to bag some contacts. Getting the two together, as with this field-portable antenna made from a tape measure, is a double win in any ham’s book.

For [Paul (OM0ET)], this build seems motivated mainly by the portability aspect, and less by the “will it antenna?” challenge. In keeping with that, he chose a 50-meter steel tape measure as the basis of the build. This isn’t one of those retractable tape measures, mind you — just a long strip of flexible metal on a wind-up spool in a plastic case. His idea was to use the tape as the radiator for an end-fed halfwave, or EFHW, antenna, a multiband design that’s a popular option for hams operating from the 80-m band down to the 10-m band. EFHW antennas require an impedance-matching transformer, a miniature version of which [Paul] built and tucked within the tape measure case, along with a BNC connector to connect to the radio and a flying lead to connect to the tape.

Since a half-wave antenna is half the length of the target wavelength, [Paul] cut off the last ten meters of the tape to save a little weight. He also scratched off the coating on the tape at about the 40-meter mark, to make good contact with the alligator clip on the flying lead. The first video below details the build, while the second video shows the antenna under test in the field, where it met all of the initial criteria of portability and ease of deployment.

Commodore Promotional Film From 1984 Enhanced

Over on Retro Recipe’s YouTube channel, [Perifractic] has been busy restoring an old promotional video of how Commodore computers were made back in 1984 (video below the break). He cleaned up the old VHS-quality version that’s been around for years, translated the German to English, and trimmed some bits here and there. The result is a fascinating look into the MOS factory, Commodore’s German factory, and a few other facilities around the globe. The film shows the chip design engineers in action, wafer manufacturing, chip dicing, and some serious micro-probing of bare die. We also see PCB production, and final assembly, test and burn-in of Commodore PET and C64s in Germany.

Check out the video description, where [Perifractic] goes over the processes he used to clean up video and audio using machine learning. If restoration interests you, check out the piece we wrote about these techniques to restore old photographs last year. Are there any similar factory tour films, restored or not, lurking around the web? Let us know in the comments below.

A Simple EMF Detector And Electroscope You Can Make From Junk Box Parts

2N2222 devices used, but practically any junkbox NPN will do

Electromagnetic fields are everywhere, all around us. Some are generated naturally, but in vast majority of cases, it’s we humans that are generating them with artificial, electronic means. Everything from your mobile phone to the toaster will emit some sort of signal, be it intentional or not. So we think it only befits the general electronics-orientated hacker to have some way of sniffing around for these signals, so here is [Mirko Pavleski] with his take on a very simple pair of instruments to detect both static and dynamic electromagnetic fields.

CMOS clock input connected directly to the antenna. Warning! ESD damage risk!

The first unit (a simple electroscope) uses a cascade of 2N2222 NPN bipolar transistors configured to give a high current gain, so any charge near the antenna will result in increasing currents in subsequent stages, finally illuminating the LED. Simple stuff.

The second unit relies on the extremely high input impedance of the old-school CMOS 4017 decade counter, which is likely of the order of 100 MΩ or even more. Normally you would not leave such a CMOS input floating, or even connect it with too long a PCB trace — lest it pick up a stray signal —but for detecting alternating EM fields, this appears to work just fine. Configured as a simple divide-by-ten, when presenting 50 Hz AC, the LED can be seen to flash at 5 Hz.

Simple stuff, and this scribe has all those exact parts in the junk box, so will be constructing these shortly!

We’ve covered electroscopes for years, here’s a modern twist on a famous classic experiment, and some hair-raising experiments to get you started.

Ham Antenna Fits Almost Anywhere

[G3OJV] knows the pain of trying to operate a ham radio transmitter on a small lot. His recent video shows how to put up a workable basic HF antenna in a small backyard. The center of the system is a 49:1 unun. An unun is like a balun, but while a balun goes from balanced line to an unbalanced antenna, the unun has both sides unbalanced. You can see his explanation in the video below.

The tiny hand-size box costs well under $40 or $50 and covers the whole HF band at up to 200 W. The video shows the inside of the box which, as you’d expect, is a toroid with a few turns of wire.

The proposed antenna is an end-fed dipole fed with the unun. These are somewhat controversial with some people swearing they can’t work and others saying they are amazing. We are guessing they may not outperform a perfect antenna system, but we also know that you can have a lot of fun with almost any kind of radiator.

The element is about 33 ft long, but to make it fit, you can bend the antenna to fit your lot. Again, it is probably not optimal, but better than nothing. Erecting a wire antenna like this is easy and just requires some insulators and supporting rope or string. Using thin wire and low-profile rope, you can hide it nearly anywhere.

Does it work? Seems to in the video, at least judging by the SWR. As [G3OJV] says, why not try it before dismissing it?

We’ve seen other options, of course. We’ve also seen these end-fed antennas made with tiny band traps.

Arduino + Ham Radio = Texting

Over on the Spectrum web site, [Dale] — a relatively new ham radio operator — talks about his system for sending text messaging over VHF radios called HamMessenger. Of course, hams send messages all the time using a variety of protocols, but [Dale] wanted a self-contained and portable unit with a keyboard, screen, and a GPS receiver. So he built one. You can find his work on GitHub.

At the heart of the project is MicroAPRS, an Arduino firmware for packet radio. Instead of using a bigger computer, he decided to dedicate another Arduino to do everything but the modem function.

You can probably figure out the rest. A $10 GPS, a battery pack, a charge controller, and a few user interface parts like an OLED screen and a keyboard. In addition, there’s an SD card to store messages.

Of course, we couldn’t help but notice that our cell phone has a keyboard, screen, GPS, and storage. We might have been tempted to work out a way to connect the radio to it by Bluetooth. But we have to admit the little HamMessenger setup is cool-looking and probably lasts longer on a charge than our phone, too.

Heathkit IM-13 VTVM Repair

If you are under a certain age, you might not know the initialism VTVM. It stands for vacuum tube voltmeter. At first glance, you might just think that was shorthand for “old voltmeter” but, in fact, a VTVM filled a vital role in the old days of measuring instruments. [The Radio Mechanic] takes us inside a Heathkit IM-13 that needed some loving, and for its day it was an impressive little instrument.

Today, our meters almost always have a FET front end and probably uses a MOSFET. That means the voltage measurement probes don’t really connect to the meter at all. In a properly working MOSFET, the DC resistance between the gate and the rest of the circuit is practically infinite. It is more likely that a very large resistor (like 10 megaohms) is setting the input impedance because the gate by itself could pick up electrostatic voltage that might destroy the device. A high resistance like that is great when you make measurements because it is very unlikely to disturb the circuit you are trying to measure and it leads to more accurate measurements.

We take that for granted today, but a typical voltmeter in the old days was just a meter with some resistors in front of it. While a good meter would have relatively high resistance, it wasn’t as high as a FET. However, with a tube amplifier, a VTVM could also show a very high resistance and still make good measurements. The Heathkit meter used a dual tube as an amplifier along with some input resistor dividers to provide an 11 megaohm input. There was also a rectifier tube switched in to make AC measurements. In the end, the amplifier drove a conventional analog meter, but that load was isolated from the device under test so its relatively low resistance wasn’t important.

The repair seemed pretty simple, but it was fun to see the inside of one of these. Compare it to a digital meter today and it seems very strange, doesn’t it? If you want to read more about how VTVMs were used, there’s a copy of a 1951 Sylvania book about them online. Some people still prefer meters that move and, we admit, for certain tasks they beat even a digital bargraph.

The Low-Down On Long-Wave: Unlicensed Experimental Radio

In the 125 years since Marconi made his first radio transmissions, the spectrum has been divvied up into ranges and bands, most of which are reserved for governments and large telecom companies. Amidst all of the corporate greed, the “little guys” managed to carve out their own small corner of the spectrum, with the help of organizations like the American Radio Relay League (ARRL). Since 1914, the ARRL has represented the interests of us amateur radio enthusiasts and helped to protect the bands set aside for amateur use. To actually take advantage of the wonderful opportunity to transmit on these bands, you need a license, issued by the FCC. The licenses really aren’t hard to get, and you should get one, but what if you don’t feel like taking a test? Or if you’re just too impatient?

Well, fear not because there’s some space on the radio spectrum for you, too.

Welcome to the wonderful world of (legal!) unlicensed radio experimentation, where anything goes. Okay, not anything but the possibilities are wide open. There are a few experimental radio bands, known as LowFER, MedFER, and HiFER where anyone is welcome to play around. And of the three, LowFER seems the most promising.

Gettin’ The Band Back Together

Before we dive into what the LowFER, MedFER, and HiFER bands actually are, it’s worth noting that these rules apply in the US only. That’s not to say that these bands are illegal elsewhere, but be sure to check your local frequency allocations before firing up a transmitter.

Ground wave radio propagation along the surface of the Earth. Courtesy of Electronics-Notes.com

LowFER, as the name would suggest, contains the lowest frequency range of the three, falling between 160 kHz and 190 kHz, with a whopping wavelength of around one mile. Also known as the 1750-meter band, this frequency range is well-suited for long transmission paths through ground wave propagation, a mode in which the radio signals move across the surface of the earth. This can easily carry even low-power signals hundreds of miles, and occasionally through some atmospheric black magic, signals have been known to travel thousands of miles. These ground wave signals also travel well across bodies of water, especially salt water.

MedFER is the medium frequency experimental band, specifically running from 510 kHz to 1,705 kHz. Now that range may sound similar, and it should because it’s also known as the AM Broadcast band! That’s right, you can listen in on this one with your old AM radio. There’s a catch though — amateur experimenters are limited to 0.1 W of transmit power, and can only use a three-meter long antenna. While that’s fine for playing around, there’s little chance of being heard very far away over the 500 W professional stations with massive antennas that dominate the band.

And then there’s HiFER, the high-frequency experimental band. Much narrower than the others at only 14 kHz wide, it sits centered on 13.56 MHz. This band is commonly used for many RFID applications, including keycards, public transportation payments, and Nintendo Amiibo. Experimentation on this band is limited to extremely low power levels, and at such power levels signals only travel a few inches, which is perfect for RFID.

While there’s a lot that can be done on any of these bands, LowFER seems to be the one that yields itself to some serious fun. MidFER and HiFER both restrict power used so low that you’re not reaching outside of your house, or even arm’s length, respectively.

Low Frequencies, High Expectations

Like the other bands, LowFER does have some restrictions — but they’re much less limiting. First and foremost, the power into the last change of the transmitter can’t exceed 1 W. That’s still fairly low power, but there are some digital modes, such as WSPR, that are known to propagate around the world at 1 W on some frequencies. Antenna lengths are also limited to 15 meters– which seems awfully short compared to the nearly-two-kilometer wavelength. Generally, the length of such a wire antenna should be tuned to a fraction of a wavelength — 1, ½, ¼, etc. for maximum efficiency. In this case, “antenna length” also includes the transmission line between the radio and the antenna. For this reason, it’s common to connect antennas directly to LowFER radios to maximize the radiating length of the antenna.

As you may imagine, because the frequencies we’re dealing with here are so low, there are few commercially available solutions that let you get on-the-air with LowFER– but when has that ever stopped the hacker and amateur radio communities? Even with these limitations, we’ve seen some wonderful kHz-range projects, like this Altoids Tin Beacon and this Arduino-based transmitter. If you want to start out by listening in, there are a number of beacons on the air right now.

Bandwidth is obviously an issue down low, so LowFER applications probably want a microcontroller- or computer-based solution driving them, so there’s nothing to stop you from keeping the link running 24/7. The long antennas required also favor fixed operation. Intra-Hackerspace low-bitrate data networks?

How Low Can You Go?

So, now it’s your turn. What will you do with LowFER? Build a tiny transmitter and try to talk to a far-away friend? Send some waterfall art out into the æther, hoping some distant hacker sees it? Maybe even just engage in some good-old fashion CW. Although LowFER has been around for a while, we feel that there’s still a ton of untapped potential here for some crazy hacker fun. Just make sure to check (and obey!) your local laws, and tell us about anything awesome you do!

Sign Detects RF To Show You Are On The Air

Like a lot of hams, [Stuart] wanted an “on the air” sign. These signs often connect to a PTT switch or maybe an output from the transmitter that also does things like switches antennas or switches in an amplifier. [Stuart’s] version, though, simply senses the radio frequency emissions from the transmitter and lights up that way. You can see two videos about the sign, below.

Honestly, we are a little worried that he might have too much RF at his operating position. Presumably, the device is pretty sensitive, especially if there’s any actual antenna on the sign. A comparator and a pot let you set the sensitivity so it doesn’t light up when your garage door opens.

The sign itself looks great thanks to a laser cutter. LEDs light up the entire sign in either red for on the air or another color for receiving operations. The acrylic sign is edge-lit which gives it a very nice effect.

We always enjoy edge-lit projects. Reasonably simple to do, and the effect looks great. If you have a laser cutter, you can get some very professional-looking results. If you are interested in the FT8 communication mode, we’ve talked about it before.

Tuning Electrically Short Antennas for Field Operation

Even as the current sun spot cycles do not favor radio operation in the HF band (defined here as 1.5 to 30 MHz), there are military and amateur radio applications for 20 W battery-operated radios with whip antennas. In general, the whip antenna which makes the radio portable is not optimized for signal propagation: a whip antenna has no ground return or proper counterpoise. While some users drag a wire up to 8 m behind, this is not an ideal solution.^1-2

This article explores optimizing antenna performance for HF and VHF (defined here as 30 to 108 MHz) manpacks using an antenna tuning unit (ATU). Two Rohde & Schwarz battery-operated manpacks with internal ATUs were used for testing, comparing their internal ATUs with the performance obtained using external tuners. The two R&S units are multiband, multirole and multimode software-defined radios (SDR), covering HF and VHF (R&S®MR3000H) and 25 to 512 MHz (R&S®MR3000U, shown in Figure 1). They are similar to other HF, VHF and UHF transceivers popular with radio amateurs.

Figure 1 R&S®MR3000U manpack radio, covering 25 to 512 MHz.

Figure 2 Original dipole made by Heinrich Hertz in 1887 used balls at each end to form a capacitive load (Source: Deutsches Museum in Munich).

SHORT ANTENNAS

Figure 3 Measured impedance of a 2 x 5 m non-resonant dipole antenna from 2 to 30 MHz.

The first resonant dipole antenna, developed by Heinrich Hertz in 1887, was driven from a “noisy” spark gap transmitter (see Figure 2). Both sides were equally long and used end-loading metal balls, acting as a capacitive device to reduce the length of the two λ/4 resonant segments.³

Now consider a symmetrical, non-resonant dipole, each side 5 m long, with the center point connected to a battery-operated network analyzer approximately the same size and surface area as a manpack. The analyzer is connected to the symmetrical dipole with a mechanically small, 1:1 symmetrical-to-asymmetrical ferrite transformer covering 1 to 60 MHz. Figure 3 shows the measured impedance of the antenna from 2 to 30 MHz, which covers both tactical military and some amateur radio bands. The typical mobile/portable application using a vertical antenna reflects the evolution of the dipole to a monopole: a symmetrical two-wire antenna made asymmetrical with a transformer and best performing with a set of resonant radials and a counterpoise or some kind of grounding.

The magnetic field of the antenna is generated by RF current in the antenna wire or rod, perpendicular to the antenna. The electric field of the antenna is needed for resonance. Many antennas are bent at the end to make them mechanically smaller. An extreme example is the capacitive hat; another is a “loading” coil located about two-thirds of the length – although this reduces the usable bandwidth.

The electrical equivalent of an electrically short antenna is given by

where C_a is the equivalent capacitance of the antenna in pF, D is the diameter of the wire, R_F the input-output impedance of the λ/4 antenna, R_S the radiation resistance, Z_a the characteristic impedance of the wire and l the length of the element.

Figure 4 Vertical antenna showing the current loop through the ground.

Figure 5 A poor ground near the antenna’s base results in losses from the return current (a), while a ground network or counterpoise reduces the losses (b).⁴

Figure 6 Current distribution in a short antenna, including the ground current.³

Grounding is necessary to close the loop for the currents. Figure 4 illustrates the behavior of the electric field lines for a vertical antenna over ground. The field lines penetrate the surface of the Earth and produce a current that flows back to the ground point, incurring heat losses. The antenna’s efficiency, η, is defined as

where R_S is the radiation resistance and Rv the total effective loss resistance. For electrically short antennas with radiation resistance values of only a few ohms, the resulting antenna efficiency can be very low, especially with long-wave and extremely long-wave communication systems. In such cases, Rv can be reduced with a ground network or a wire network extending over the ground as a counterpoise, especially with unfavorable ground conditions (see Figure 5). All symmetrical antennas not excited with respect to ground, such as dipoles, benefit from the antenna’s independence from the ground resistance – as long as the entire antenna is elevated above the ground.

Electrically short antennas, typically λ/10 or shorter, look like a capacitor with a typical capacitance of 25 pF/m of length, e.g., 75 pF for a 3 m rod. At 2 MHz, where the wavelength is 150 m, an inductor of 84 μH is required for resonance. The radiation comes from the current in the antenna, not from the voltage; the voltage is maximum at the end. To better understand the radiation, consider the case where the whip antenna’s length, l, is λ/4. The vertical radiation pattern of the antenna over ground is1,3

The radiated power, RF current and radiation resistance of the λ/4 antenna over ground is determined from the power radiated in the half sphere. Solving for R_S:

where C is Euler’s constant (0.5772) and C_i(x) is the cosine integral. The radiation resistance of the electrically short antenna based on the electric field can be calculated from:

The radiation resistance of the short antenna is obviously very low.

EFFECTIVE HEIGHT

To calculate the effective height of an electrically short antenna, consider that the open circuit voltage, V0, of the antenna is proportional to the antenna field strength, E, where the antenna is located:

The proportionality factor heff has the dimension of length and is known as the effective height. If the current in the antenna is independent of location (i.e., a Hertzian dipole), then heff corresponds to the antenna’s geometrical length, ^l. Otherwise, the effective height will be less because of the non-uniform current distribution. In the general case, heff is determined by converting the current area into a rectangle with the same area and the maximum current, I₀, at its base (see Figure 6). Its height is then equal to h_eff. Computationally,

The effective height is related to the effective area, A, and characteristic impedance Z03-4 as follows:

Figure 7 Theoretical input impedance of three wire antennas with different diameters.⁴

Figure 8 Measured impedance of a 35 ft. whip antenna used on a ship.⁵

Figure 9 Center loading a whip antenna creates a larger integrated surface for the current than base loading, which improves radiation.6 The two figures are not to scale.

Figure 7 shows the differences in theoretical impedance of wire antennas with different wire diameters. Figure 8 plots the measured impedance versus frequency of a 35 ft. long whip antenna, showing the maximum real impedance is approximately 600 Ω at 10 MHz (λ/2), and the real loss resistor is never below 10 Ω. The imaginary part of the impedance is −400 Ω at 2 MHz and approximately 200 Ω at 9 MHz.

ANTENNA LOADING AND TUNING

Where a whip antenna is loaded affects the antenna’s performance. Figure 9 compares base and center loading. With center loading, both the radiation resistance and integrated surface are larger, which are better for radiation.

The typical lowpass configuration for antenna tuning is a series L, shunt C network, also called a Collins filter (see Figure 10). For the 35 ft. whip antenna operating at 2 MHz (Figure 8), the network needs an inductor of 100 μH and a shunt capacitor of 262 pF. Assuming a 4:1 input transformer and the sum of the radiation and ground loss resistance to be larger than 12.5 Ω (50/4), the tuner only needs two variable elements, and either the left or right capacitor bank in the figure can be eliminated. The right capacitor bank is used when the load resistance is greater than 50 Ω (12.5 Ω using the 4:1 transformer), and the left capacitor bank is used when the load resistance is less than 50 Ω (12.5 Ω using the 4:1 transformer). In this case, at 2 MHz, R = 12 and Xc = −400 Ω, the required series inductor will be 33.42 μH and the left capacitor bank is used to set a value of 3.18 nF. The most inductive impedance is at 9 MHz, approximately 600 +j200 Ω, such that 25 μH and 370 pF are needed for tuning.

The inductances for tuning are found in commercial input/output switchable antenna tuners operating from 1.5 to 30 MHz. The tuning network looks like an “L,” where the switched shunt capacitors can be connected to either side of the inductors.⁷ Figure 11a shows the matching ranges of the tuner with the capacitors on each side of the inductors, and Figure 11b shows a simplified schematic of the main capacitor bank that is switched to either the input or output side of the inductors. The inductors are wound with 0.048 in. diameter, enameled copper wire and have approximately 0.04 μH step size. The capacitor step size is 9.375 pF. This tuner has nine switchable inductors, 10 switchable capacitors and two positions for one of the capacitor banks (i.e., on either side of the inductors), yielding 1,048,576 tuning combinations. The 6 pF capacitor at the output represents the ceramic antenna mount or connector. For best performance, antenna tuners use air core coils or very low permeability powdered iron cores. As such, some of the measured intermodulation distortion (IMD) products result from ferrite saturation, not from the power amplifier driving the antenna.^8-9

Figure 10 Schematic of a Collins antenna tuner with an input transformer.

Figure 11 Tuner matching options: the right capacitor bank is used when the load resistance is > 50 Ω, and the left capacitor bank is used when the load resistance is < 50 Ω (a). Tuning network similar to that of the ICOM AT130 (b).

Figure 12 T-configuration ATU.

Figure 13 Measured S₁₁ of a 5 m wire antenna with 8 m ground wire, 2 to 55 MHz.

Occasionally, a tuner design uses a highpass, T-configuration rather than a π (see Figure 12). The LC1 forms one resonant circuit, LC2 forms the other circuit and the two are tuned to the resonant frequency. The left loop and the right loop are dependent. Generally, when the value of C2 is too small, input resonance occurs yet there is no loading and no output power obtained. This is the flaw of the design: the highpass filter is mathematically over-determined. The left loop can be in resonance, and no output power (voltage) is available in the right loop. Other drawbacks of this frequently-used design are higher loss, the lack of harmonic suppression and LC combinations where the tuner absorbs most, if not all, the power without tuning the antenna perfectly.⁶ For electrically short antennas, this can be overcome with a voltage probe at the output, such as a pre-ignited neon bulb. The neon discharge tube glows when there is maximum voltage at the output of capacitor C2. If the frequency is below 30 MHz, a blue color is emitted; if the frequency is above 30 MHz, a pinkish color is emitted.

2 MHz TUNING

For a 5 m wire antenna with an 8 m ground wire and π tuner, Figure 13 shows the measured antenna impedance, and Table 1 summarizes the antenna impedance and the L and C values needed for matching, assuming a 1:1 (50 Ω) input transformer.10-11 The LC combination minus the capacitance value of the antenna rod or wire must resonate at the test frequency. Figure 14 shows the antenna resonance tuning at 2 MHz, and Figure 15 shows the simulated Z11 vs. frequency, with Re{Z11} plotted in Figure 15a and Im{Z11} in Figure 15b. The lower curve in Figure 15a closely matches the desired 20 Ω at 2 MHz; however, it assumes no losses in the tuner. The upper curve, assuming a realistic Q of 200, shows 22 Ω at 2 MHz. As power is I2R, a 10 percent increase in Re{Z11} causes a power loss of approximately 10 percent, from 20 to 18 W available for radiation. These losses are frequently overlooked. While the two-element tuning network produces the desired real and imaginary values, the 8 m ground wire is too short at this frequency, so there is no useful grounding.

Figure 14 Resonance tuning for the 5 m wire antenna with 8 m ground wire at 2 MHz.

Figure 15 Simulated Re{Z₁₁} (a) and Im{Z₁₁} (b) at 2 MHz.

GROUNDING

Figure 16 ICOM antenna tuner with 90 degree offset coils.

Figure 17 Tuning range of ICOM AH4 antenna tuner.¹³

An antenna tuner can improve the matching but not the radiation. At a single frequency, the asymmetric dipole shows a resonance close to 12 MHz, the antenna to case impedance is 37 Ω – almost purely resistive – and the system works. A symmetrical, non-resonant antenna like an inverted V with an elevated tuner and an asymmetrical antenna cable isolating the radio and RF, by actually grounding it, will give superior results. If this is not possible, to obtain a symmetrical antenna system a ground connection to some electrical wire, fence or similar structure is recommended. If the built-in antenna tuner can tune end-fed, high impedance, low current, long wires, this may be a good solution, although the radio is no longer a manpack, more of a portable solution for a stationary setup. Some counterpoise is needed for good results.

Table 2 shows field strength measurements with a 2 x 5 m non-resonant, wire dipole in a V configuration, tuned using an ATU; the measurement is made at 10 m distance using a test receiver with antenna. For comparison, the table includes the frequency dependent field strength for the 3 m rod antenna.

Radio amateurs often use ICOM tuners. Generally, they are reliable in all weather conditions and, with a huge tuning range, find a good match and fit all the 100 W radios, even non-ICOM radios with a simple adaptor. The tuner layout shifts the coils by 90 degrees to minimize magnetic coupling (see Figure 16). Any number of inductors will not mutually couple if they are placed with their axes forming a 54.74 degree angle to a common plane,¹² although such placement is not always physically ideal. The ICOM AH-4 tuner has nine inductors, 10 capacitors and two positions, equating to 1,048,576 tuning combinations. The tuning range is shown in Figure 17. Part of the magic of the tuner’s effectiveness is the search algorithm.

Figure 18 shows the measured S₁₁ from 2 to 30 MHz of a 2.5 m whip antenna with two ground configurations, floating and next to the body. Floating is the worst case. For a VHF/UHF manpack, Figure 19 shows the antenna S₁₁ measured from 20 to 200 MHz.

HF PROPAGATION

Recently, HF communication has fallen out of grace and HF manpacks have become less interesting. The 11-year sunspot cycle causes poor propagation, making the band unattractive. Appliances such as long-life LEDs produce high conducted and radiated interference, and noise blanker implementations in SDRs are not very effective. The difference between summer and winter propagation is also a factor. In the summer months, beginning in May, the D layer of the ionosphere makes day propagation up to 15 MHz difficult; low frequency night propagation works better up to 10 MHz. While propagation forecasts are available on the internet, particularly for long distance connections, 10 to 60 mile connections are more complex: too far apart for VHF/UHF and too close for HF. Frequencies from 1.5 to 8 MHz would work well, but effective use of those frequencies requires better antennas than manpacks have, and local RF noise does not help. The D layer further complicates use of the band. During the day, 5 to 8 MHz provides 300 to 500 mile coverage, which increases to at least 2000 miles shortly before sunset. Two MHz is better for close communication. The old ship SOS frequency (2.182 MHz) has been replaced by satellite telephones.^6,12-16 The 20 m radio amateur band for voice operation covers 14.15 to 14.35 MHz. Even for this small difference, propagation may vary significantly, as the coherence bandwidth is small.¹⁷

However, the amateur radio communities hang on to low-power operation (QRP) and remain true believers. During hurricane season and other natural disasters, using these portable stations saves lives.

SUMMARY

For radio communications, the antenna is probably the most critical part of the link, so grounding and antenna tuner losses should be avoided as much as possible. The inductor is always the lossy part of the ATU, while the capacitors, typically mica, are infinitely better.

For best operation, antenna radials should be λ/4. One is sufficient for tuning; up to four will produce a symmetrical azimuth pattern. When λ/4 is not possible, radials several wavelengths long will do. Connecting the HF radio ground to a large metallic object is a good choice. For the tests supporting this article, a grounding spear of about 10 in., similar to a tent support, was used.

These requirements for optimum antenna performance make HF manpack radios somewhat complicated and unattractive. Nonetheless, the well matched and radiating antenna provides the most success, and some of these highly portable radios (see Figure 20) provide vital communications in disaster areas – recently in Puerto Rico and South Florida.

Figure 18 Measured S₁₁ of a 2.5 m whip antenna with floating ground (a) and ground next to the body (b).

Figure 19 Measured S₁₁ of an antenna for a VHF/UHF radio, from 20 to 200 MHz.

Figure 20 VHF/UHF (left) and HF/VHF (right) manpacks. The UHF/VHF manpack shown uses a thick vertical pole antenna, and the HF manpack uses a 5 m dipole (the yellow wire).

References

U. L. Rohde, “Die Anpassung von Kurzen Stabantennen fur KW-Sender,” Funkschau 1974, Heft 7.
U. L. Rohde, “Match Antenna Over 1.5 to 30 MHz Range with Only Two Adjustable Elements,” Electronic Design 19, September 13, 1975.
L. Bergmann and H. Lassen, “Ausstrahlung, Ausbreitung und Aufnahme Elektromagnetischer Wellen,” Berlin, Verlag Von Julius Springer, 1940.
A. Vlcek, H. Hartnagel and K. Mayer, “Zinke.Brunswig Hochfrequentztechnik 1 Hochfreqenzfilter, Leitungen, Antennen, 6th Edition,” Springer.
W. E Sabin and E. O. Schoenike, “Single Sideband Systems and Circuits, 2nd Edition,” McGraw Hill, pp. 582–605.
U. L. Rohde, M. Salazar-Palma and T. Sarkar, “Electrically Short Antennas,” www.synergymwave.com/articles/2016/Antenna_presentation.pdf.
Chapters 22, “Antennas,” and Chapter 18, “The ARRL Handbook for Radio Communications 2013,” pp. 18.6.
U. L. Rohde, “Theory of Intermodulation and Reciprocal Mixing: Practice, Definitions and Measurements in Devices and Systems, Part 1,” QEX, November/December 2002.
U. L. Rohde, “Theory of Intermodulation and Reciprocal Mixing: Practice, Definitions and Measurements in Devices and Systems, Part 2,” QEX, January/February 2003.
“Antenna (radio),” Wikipedia, en.wikipedia.org/wiki/Antenna.
“Antenna Tuner,” Wikipedia, en.wikipedia.org/wiki/Antenna_tuner.
U. L. Rohde, J. C. Whitaker and H. Zahnd, “Communications Receivers: Principles and Design, 4th Edition,” McGraw Hill, 2017.
K. Siwiak and Y. Bahreini, “Radiowave Propagation and Antennas for Personal Communications, 3rd Edition,” Artech House, 2007, pp. 20–22.
R. A. Sainati and D. E. Fessenden, “Performance of an Electrically Small Antenna Amplifier Circuit.”
J. Taylor, S. Franke and W. Somerville, “Work the World with WSJT-X, Part 1,” QST, October 2017, pp. 30–36.
J. Taylor, S. Franke and W. Somerville, “Work the World with WSJT-X, Part 2: Codes, Modes and Cooperative Software Development,” QST, November 2017, pp. 34–39.
U. L. Rohde, “Receiver Measurements, How to Evaluate Receivers,” QEX, July/August 2005

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