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Antenna couplers galore!

The radio project I've built more of than any other is the antenna coupler (aka 'antenna tuner', 'transmatch', 'matchbox' etc).

Sometimes unfairly pilloried by the 'resonant antenna' brigade, I find them indispensible for home and portable operating, especially if you need to cover several bands from the one wire.

Correctly used they allow a 2:1 or 3:1 frequency range with a single end-fed wire or tuned feeder dipole. Even higher ratios are possible if you are willing to tolerate inefficiency at lower frequencies and split lobes at higher frequencies.

Limitations include having to make adjustments every time you change band and sometimes even frequency within a band. However that beats going outside changing jumper connections on link dipoles or putting up with the narrow bandwidth of traps (especially at lower frequencies).

While OK for a one-off 'let's give it a go' experiment, couplers also cannot compensate for inherently lossy systems like short verticals with poor ground systems or dipoles with long runs of coax that you're trying to match well away from the design frequency.

Most couplers described here are L-matches due to their simplicity and small size. Most are for low power (QRP) portable operating. But the principles scale up for higher power if larger inductors (with thicker wire) and variable capacitors (with wider plate spacing) are used.

Variations between them are to do with inductance and capacitance size (which allows matching of a wider range of antennas over a wider frequency range) and physical size (which needs to be minimised for backpack portable operating). I've also included a coupler specifically for balanced antennas.

Articles and videos on various antenna coupler projects

The versatile end-fed wire PDF ebook version

The above describes an end-fed wire and wide-range L-match coupler. It includes a resistive bridge to allow easy adjustment. It uses a larger variable capacitor, inductor and case than the others featured here so it is suitable for home station use with higher than QRP power levels. Many of the comments made relate to other L-matches described here so read this article first.

A compact L-match for portable use

Smaller than the previous one, this unit uses a medium-size air-spaced variable capacitor with a multi-position rotary switch. This gives many inductance settings and allows a wide range of antennas to be matched between 3.5 and 28 MHz. Once I didn't bring any wire and loaded up anything I could find on my local beach. The results were variable as you'll see in the video below.

A small L-match for end-feds

This was my first really small L-match. It allows a 20 - 22m end-fed wire to work on amateur bands from 7 to 28 MHz. There's only three inductance settings so not all antenna lengths will tune up on all bands. Way smaller and lighter than most commercial models this is the one to take when you have little space.

The above L-match in use on 12 and 10 metres

40/20/10m multiband no-tune end-fed

If you just want to cover two or three bands there are some antenna configurations where you can get this coverage with a pre-set matching unit. The one described here allows operation on 7, 14 and 28 MHz with a loaded element about 12 metres long. The coupling unit uses a pre-set compression trimmer that only needs to be adjusted once on set-up. Limitations include its narrow bandwidth and slightly inferior performance on 7 MHz compared to a full sized antenna.

Yet another L-match antenna coupler

A small 7 to 28 MHz L-match coupler with a wide inductance range. It does this by having a bank of inductors switched in and out. This gives more settings than couplers with just a rotary switch and allows a wide range of antennas to be matched. I really like this one!

Variable inductor L-match antenna coupler made from corflute/coroplast

More an experiment than anything else, this is an example of what can be built from scrap. It's light and performs well but I prefer other more robust enclosed couplers for portable operating.

A very small L-match antenna coupler

For a casual portable session at the beach or a national parks activation, this is the one I normally use. There's three inductance ranges so its matching range isn't as great as others featured. Most of the time though its small size and lightness make it a winner. 7-28 MHz.

A balanced antenna coupler and multiband tuned feeder dipole

Different from the rest, this versatile coupler contains two L-matches and a balun. This arrangement allows efficient matching of balanced tuned feeder dipoles. A switch also allows operation as a single L-match for unbalanced antennas like end-fed wires. A great performer with a very wide matching range it nevertheless spends most time at home due to its larger size than the others.

Parts you'll need

Most of the couplers described here use just two basic components; a variable capacitor and a variable inductor. Other parts needed include a switch the vary the inductor (unless you build the continuously variable one with the ferrite rod), a socket for the transceiver, connections for the antenna and some sort of enclosure. Many can be purchased on line. Links to examples are below.

Variable capacitor

Here you have a choice between a plastic dielectric variable capacitor and an air dielectric type. The plastic type is smaller, lighter and cheaper. However I wouldn't use it with more than about 5 to 7 watts. The air dielectric version is heavier but can take a higher transmit power and is easier to add a knob to. The value isn't that critical but aim for one with a maximum of around 200pF.

Variable inductor

Variable inductors aren't as readily available as variable capacitors. You either have to make your own or switch various taps on one or more fixed inductors. Inductors can be wound on a cardboard or plastic cylinder or iron powder toroid. The toroid approach is most common if you wish to save space. Enamelled copper wire, such as found in old power transformers, is good for the windings.

Switch

Depending on the coupler design you may need either toggle switches, toggle switches with a centre off position (harder to get but you can make do with two regular toggle switches) or rotary switches. Heavy duty switches are required if you wish to run high power.

Sockets to connect transceiver and antenna

Common antenna socket types include SO239 (to accept PL259 plugs) or BNC. Some homebrew QRP rigs even use RCA connectors to save money. Another option is you could have a flylead with plug to fit straight into your transceiver. This may make the coupler harder to pack but means that there's no risk of you leaving a coax patch lead behind. Another possibility is to use a BNC female as this can plug straight on to BNC equipped transceivers.

Case

Either scrounge a lunch box from the kitchen, make one out of circuit board material or consider one of these.

Antenna wire and feedline

Thin stranded insulated wire works well and will last for many portable outings. Most of the time I run this straight to the coupler. But if you're using the balanced coupler described above then you'll need some 300 to 450 ohm slotted ribbon for the feedline.

Other antenna and mast accessories

Couplers are great but you need a wire to load up. And it needs to be a good height. The simplest and lightest option is a wire thrown over a tree with some fishing line and a sinker. If your throwing arm isn't so hot or you're on a treeless beach or desert then it pays to bring an extendable mast. Consider also the best way to support it, like a sand spike or velcro strap to fasten it to a fence, gate or railing.

Further information

A selection of favourably reviewed antenna books to inform and inspire.

 

Disclosure: I receive a small commission from items purchased through links on this site.
Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.

 

Books by VK3YE

 

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(c) Peter Parker VK3YE 1997 - 2017.

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