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 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.
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.
QRP L-matches with small variable capacitors
About getting a wider matching range if your L-match has only a small variable capacitor.
Solderless breadboard antenna coupler
A very simple QRP L-match that requires no coil winding, no variable capacitors and no soldering. Based on a solderless breadboard you substitute various series and parallel fixed inductors until you get a
good match. The inductors are pre-wound RF chokes in the few microhenry range. The capacitors are disc ceramic though other types would give less loss. In the video I demonstrate the unit's construction then use it to
make some long distance contacts. While fiddlier to operate than if you had a variable capacitor it is still an interesting project to try. Once you've established the right values of parts you could even make it very
small (eg fit inside a BNC or PL259 connector) if using it with a monoband QRP transceiver that you only use one type of antenna (eg an end-fed half wavelength wire) with.
A 160 - 10m antenna coupler
I built this nearly 30 years ago. It hadn't been used for a long time. I took it off the shelf to try portable with an
end-fed 20 metre long wire. It works! Even down to 160 metres! Watch the video to find out more about this very simple one-capacitor antenna coupler.
An L-match antenna coupler that needs no variable capacitor
Variable capacitors can sometimes be hard to find. Described is a 1.8 to 18 MHz L-match antenna coupler that
uses double sided fibreglass printed circuit board as a substitute. You don't get a continuous tuning range but if there
are enough taps on your inductor you should still be able to have fine enough adjustment to obtain a good match on most bands.
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.
Will an unbalanced antenna coupler work with balanced antennas?
Short answer is yes! More in the video.
Plug-in coupler for 80m
A solution when space is tight and you're only using an end-fed half wavelength wire.
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 is available via VK3YE Books.
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.
