These Capacitors are a Cheap Gimmick

If you search through an electrical engineering textbook, you probably aren’t going to find the phrase “gimmick capacitor” but every old ham radio operator knows about them. They come in handy when you need a very small capacitor of unknown value. For example, if you are trying to balance the stray capacitance in a circuit, you might not know exactly what value you need, but you know it won’t be very much. That’s when you want a gimmick capacitor.

A gimmick capacitor is made by taking two strands of insulated wire and twisting them together; the length and the tightness of the twist determine the capacitance. Tightening or loosening the twist, or trimming some of the wire off, makes it tunable.

These are most commonly found in RF equipment or high-speed logic because of the small capacitance involved — usually about 1 to 2 pF per inch of twist or so. The thicker the insulation, the less capacitance you’ll get, so it is common to use magnet wire or something else with a thin insulating layer. You can take this one step further and decrease the spacing by stripping down one wire as long as it isn’t going to touch anything else.

Obviously, the insulation needs to be good enough for the voltage on them, an important consideration in tube circuits, for instance. But other than that, a gimmick capacitor is a straightforward tool to have in your box of design tricks. Can we take this further?

PC Board Gimmicks

You might wonder if the technique can be applied to PC boards. The answer is yes — sort of. Unless you use very thin boards, or thin layers in multilayer boards, it takes a lot of board real estate to get even a small capacitance. Also, typical PCB material can change over time with moisture or other effects. Practically, unless you use special board material and thicknesses, it isn’t very useful. There has been work on laying out linear capacitors on IC substrates using fractals, but we aren’t sure how that would translate into a PCB layout. We’ve seen lots of other PC trace components like antennas, shunt resistors, inductors, and transmission lines.

You can see I made a gimmick just bigger than two inches. I then went looking for something around the lab that had the ability to measure such a small capacitor. The component tester couldn’t. I have a nice digital multimeter that has a special plug-in for measuring capacitors and thermocouples, but it wouldn’t reliably read anything under 25 pF. I was thinking about building up a circuit to test when I realized I should search Hackaday first.

Hackaday Saves the Day

[Jonathan’s] capacitance meter is just what I needed and I even threw it out to an Arduino that was already hooked up using the Arduino Create web interface, so that was easy. I actually used the newer “Mark II” code but it works the same for the low values I was measuring. I calibrated with a garden variety 10 pF ceramic. It probably isn’t that accurate, but I really only wanted to see the change more than the actual value, so I thought this was sufficient.

The two inch (call it 6 cm) gimmick reads about 5.5 pF. That might not be totally accurate, but I was expecting about 4.5 pF and the magnet wire insulation is quite thin, so it’s in the right ballpark. Let’s take it as a baseline to measure change. I then cut about 1.5 cm of the capacitor away — about 25% — and the reading became 3.7 pF. Another centimeter brought it down to 2.6 pF.

Of course, hand-wound pitch isn’t very accurate, nor were my cuts or measurements, but that works out to just around 1 pF per centimeter. Obviously, your results are going to depend on your winding and the kind of wire you use. [Harry Lythall] suggests folding a single piece of wire, holding it with pliers, and twisting. Then you cut the loop when you are done.

That’s a Wrap

It is easy to forget that any two conductors near each other will have capacitance. Another common makeshift capacitor is a length of coax with connections at one end and open at the other. RG-8, for example, is about 30 pF per foot of cable. There’s even an online calculator that will tell you how much coax you need for any given value. This varies by coax type, of course, so remember to cut a little long and trim!

The next time you need a small adjustable capacitor — especially in a lab setting — don’t forget about the gimmick. Be sure to experiment with different kinds of wire if you are trying for larger values. We’ve seen this trick used in RF filters. In the case of the gimmick, you may be thinking small, but when you are really looking for high voltage capacitors, you can make those, too.

Free E-Book: Software Defined Radio for Engineers

We really like when a vendor finds a great book on a topic — probably one they care about — and makes it available for free. Analog Devices does this regularly and one you should probably have a look at is Software Defined Radio for Engineers. The book goes for $100 or so on Amazon, and while a digital copy has pluses and minuses, it is hard to beat the $0 price.

The book by [Travis F. Collins], [Robin Getz], [Di Pu], and [Alexander M. Wyglinski] covers a range of topics in 11 chapters. There’s also a website with more information including video lectures and projects forthcoming that appear to use the Pluto SDR. We have a Pluto and have been meaning to write more about it including the hack to make it think it has a better RF chip inside. The hack may not result in meeting all the device specs, but it does work to increase the frequency range and bandwidth. However, the book isn’t tied to a specific piece of hardware.

Make no mistake, the book is a college-level textbook for engineers, so it isn’t going to go easy on the math. So if the equation below bugs you, this might not be the book you start with:

[Di Pu] and [Alexander Wyglinksi] have an older similar book, and it looks like the lecture videos are based on that book (see video below). The projects section on the website doesn’t appear to have any actual projects in it yet, although there are a couple of placeholders.

We have enjoyed Analog’s book selections in the past including The Scientist and Engineer’s Guide to Digital Signal Processing which is a classic. If you visit their library you’ll find lots of books along with classes and videos, too.

If you want something a bit less academic, there’s always [Ossmann’s] videos. Or if you’d rather just use an SDR, there are plenty of inexpensive options to choose from.

A cleverly concealed magnetic loop

We’re sure all radio amateurs must have encountered the problem faced by [Alexandre Grimberg PY1AHD] frequently enough that they nod their heads sagely. There you are, relaxing in the sun on the lounger next to the crystal-blue pool, and you fancy working a bit of DX. But the sheer horror of it all, a tower, rotator, and HF Yagi would ruin the aesthetic, so what can be done?

[Alexandre]’s solution is simple and elegant: conceal a circular magnetic loop antenna beneath the rim of a circular plastic poolside table. Construction is the usual copper pipe with a co-axial coupling loop and a large air-gapped variable capacitor, and tuning comes via a long plastic rod that emerges as a discreet knob on the opposite side of the table. It has a 10 MHz to 30 MHz bandwidth, and should provide a decent antenna for such a small space. We can’t help some concern about how easy to access that capacitor is, on these antennas there is induced a surprisingly large RF voltage across its vanes, and anyone unwary enough to sit at the table to enjoy a poolside drink might suffer a nasty RF burn to the knee. Perhaps we’d go for a remotely tuned model instead, for this reason.

[Alexandre] has many unusual loop projects under his belt, as well as producing commercial loops. Most interesting to us on his YouTube feed is this one with a capacitor formed from co-axial soft drink cans.

Thanks [Geekabit] for the tip.

Homebrew SDR Ham Radio in 9 parts

It used to be homebrew ham gear meant something simple. A couple of active devices that could send CW. Maybe a receiver with a VFO. But only the most advanced builders could tackle a wide range SSB transceiver. Today, that goal is still not trivial, but it is way easier due to specialty ICs, ready access to high-speed digital signal processing, and advances in software-defined radio techniques. [Charlie Morris] decided to build an SSB rig that incorporated these technologies and he shared the whole process from design to operation in a series of nine videos. You can see the first one below.

The NE612 is a child of the popular NE602 chip, which contains a Gilbert-cell mixer, and an oscillator that makes building a receiver much easier than it has been in the past. The chips are set up as direct conversion receivers and feed a Teensy which does the digital signal processing on the recovered audio.

One nice thing about the Teensy is that it has an accessory audio board that makes it easy to connect audio inputs and outputs to the device. The DSP does work on the received audio and the transmit audio. There’s also a few other stock parts like an LCD, an encoder, a speaker, a microphone, and things like that. There’s also a digital clock generator (an Si5351), but again all that is common off-the-shelf stuff these days.

The first video is a bit introductory, but by video number two he jumps right into the wiring and why all the circuits work. By the third video, the receiver is actually working and it sounds pretty good. Because the receiver needs I and Q outputs, there are actually two NE612s operating out of phase with each other.

Posted

Foto’s velddag 2018 – Daîromont Vielsalm

Het was een mooie velddag met mooie foto’s:

Velddag 2018: Dairomont (nabij Malmédy)

Na de succesvolle velddagen in Bellevaux is de VRZA Zuid-Limburg in 2018 verhuisd naar Dairomont, vlakbij Malmédy. Een uurtje rijden vanuit Maastricht ligt dit vakantiehuis bovenop een plateau met een prachtig uitzicht over de omgeving, zoals je op de foto kunt zien.

De velddag vindt plaats vanaf donderdag 10 mei (Hemelvaartsdag) tot en met zondag 13 mei. Tijdens deze dagen zullen 8 leden van de VRZA hun kamp opslaan en met diverse antennes en transceivers proberen zoveel mogelijk verbindingen te maken en experimenten te doen.

 

De velddag is natuurlijk ook te bezoeken. Hou er wel rekening mee dat er voor de maaltijden niet is gerekend op mee-eters dus zorg zelf voor eigen eten, indien nodig. Voor de consumpties wordt een bijdrage gevraagd. Wil je langs komen? Van harte welkom! Meld je even in via de repeater van Botrange: 439.0125 MHz (shift -7.6MHz, Tone – 131,8Hz), die staat continu aan tijdens de velddag.

Parkeren in de buurt zal geen probleem zijn maar hou er rekening mee dat dit huisje in een gewoon dorp ligt dus hou rekening met omwonenden. De locatie is gewoon met de auto of motor te bereiken, je hoeft nu eens niet door het bos!

Weersverwachting

Weersverwachting
Vielsalm

Weeronline.nl – Meer weer in Vielsalmweeronline.nl Altijd jouw weer

 

Het adres: Dairomont 27, 6698 Vielsalm, België: 

Route
U volgt de A2 richting Maastricht – Luik
Net voor Luik neemt u de autosnelweg
Antwerpen – Luik – Aaken
(hier opletten, kort na elkaar juiste afslag kiezen! (Aaken-Vervier)
Na ± 10 km volgt u de autosnelweg richting Verviers – Spa – St. Vith.
Bij afslag Malmedy gaat u rechts richting Stavelot en daarna Trois-Ponts
(na 2 tunnels linksaf, daarna weg volgen)
In Trois-Ponts volgt u richting Hamoir – Huy.
(Neem bij voorkeur deze route. Uw navigatiesysteem geeft mogelijk
de kortste route (langs de kerk) aan! Deze is echter alleen toegestaan
voor lokaal verkeer en erg steil en smal!)

Even buiten de bebouwde kom van Trois-Ponts gaat u de eerste weg links
(bord richting Fosse sur Salm).
U volgt deze weg via Mont de Fosse en Bergeval en u nadert dan Mont Saint Jacques.
Bij het eerste huis links van de weg ± 50m voor de kerk gaat u bij de kruising (richting Vielsalm) linksaf.
Het eerstvolgende huis links van de weg is uw vakantiebestemming!
(Nabij het kerkje van Mont Saint Jacques, dus wat afwijkend van punt A op het kaartje hierboven op deze pagina!)

Temperature Sensor and Simple Oscillator Make a Value-Added HF Beacon

Sometimes the best projects are the simple, quick hits. Easily designed, fast to build, and bonus points for working right the first time. Such projects very often lead to bigger and better things, which appears to be where this low-power temperature beacon is heading.

In the world of ham radio, beacon stations are transmitters that generally operate unattended from a known location, usually at limited power (QRP). Intended for use by other hams to determine propagation conditions, most beacons just transmit the operator’s call sign, sometimes at varying power levels. Any ham that can receive the signal will know there’s a propagation path between the beacon and the receiver, which helps in making contacts. The beacon that [Dave Richards (AA7EE)] built is not a ham beacon, at least not yet; operating at 13.56 MHz, it takes advantage of FCC Part 15 regulations regarding low-power transmissions rather than the Part 97 rules for amateur radio. The circuit is very simple — a one-transistor Colpitts oscillator with no power amplifier, and thus very limited range. But as an added twist, the oscillator is keyed by an ATtiny13 hooked to an LM335 temperature sensor, sending out the Celsius and Fahrenheit temperature in Morse every 30 seconds or so. The circuit is executed in Manhattan style, which looks great and leaves plenty of room for expansion. [Dave] mentions adding a power amp and a low-pass filter to get rid of harmonics and make it legal in the ham bands.

Beacons are just one of the ways for hams to get on the air without talking. Another fun way to analyze propagation is WSPR, which is little like an IoT beacon.

 

SDR IF Experiments

The R820T tuner IC is used in the popular Airspy software defined radio (SDR) as well as many of the inexpensive RTL SDR dongles. [TLeconte] did some experiments on intermediate frequency (IF) configuration of the chip, and you’ll find his results interesting.

Using 5 million samples per second and the device’s real mode, the tests look at a what comes out when the IC reads a noise source. There are two registers that set the IF parameters, but the tests show the effects these registers have in precise terms.

According to the post, there are three things you can set:

  • Coarse IF filter bandwidth : narrow/mid/large
  • Manual fine tuning IF filter bandwidth from 0 (large) to 15 (narrow)
  • High pass filter frequency from 0 (high) to 15 (low)

Some of the settings don’t make sense — at least at the 5 MHz sample rate — because of aliasing. However, it is instructive to see what each setting does. [TLeconte] uses Octave to visualize the data.

If you want to know more about SDR in general, we have something to get you started. If you want to do your own testing, consider using GNU Radio.

An SSB Transceiver Using Only One Device? Surely not!

There are a multiplicity of transmission modes both new and old at the disposal of a radio amateur, but the leader of the pack is still single-sideband or SSB. An SSB transmitter emits the barest minimum of RF spectrum required to reconstitute an audio signal, only half of the mixer product between the audio and the RF carrier, and with the carrier removed. This makes SSB the most efficient of the analog voice modes, but at the expense of a complex piece of circuitry to generate it by analog means. Nevertheless, radio amateurs have produced some elegant designs for SSB transmitters, and this one for the 80m band from [VK3AJG] is a rather nice example even if it isn’t up-to-the-minute. What makes it rather special is that it relies on only one type of device, every one of its transistors is a BC547.

In design terms, it follows the lead set by other simple amateur transmitters, in that it has a 6 MHz crystal filter with a mixer at either end of it that switch roles on transmit or receive. It doesn’t use the bidirectional amplifiers popularised by VU2ESE’s Bitx design, instead, it selects transmit or receive using a set of diode switches. The power amplifier stretches the single-device ethos to the limit, by having multiple BC547s in parallel to deliver about half a watt.

While this transmitter specifies BC547s, it’s fair to say that many other devices could be substituted for this rather aged one. Radio amateurs have a tendency to stick with what they know and cling to obsolete devices, but within the appropriate specs a given bipolar transistor is very similar to any other bipolar transistor. Whatever device you use though, this design is simple enough that you don’t need to be a genius to build one.

Via [G4USP]. Thanks [2ftg] for the tip.