A Beginner’s Guide to Antenna Tuners
last updated 30 November 2024.
what is an antenna tuner?
Antenna tuners may go by different names: transmatch, matchbox, ATU (“aerial tuning unit”), ASTU (“antenna system tuning unit”), antenna coupler, “tuna”, etc. Some hams will insist that a specific name is better than others, and others may argue that there are differences among them. Don’t worry about it. “Antenna Tuner”, or just “tuner”, is the most common term these days, including for commercial products, so that is what I’m using here. You may use whatever term you prefer.
Often it is a box that sits on the operating desk near the radio, with knobs to twiddle so the transmitter sees a low SWR, although there are some variations. For example, it may be built into some radios, or it may adjust the match automatically, so you don’t have knobs to play with.
why would I need one?
Most modern transmitters are designed for optimum performance into a 50 ohm impedance at the antenna jack on the back of the radio. If you use a 50 ohm antenna, and connect it to the radio using 50 ohm coax cable, then the radio should see 50 ohms. In that case, the SWR would be 1 : 1. That’s an ideal situation.
But sometimes your antenna isn’t exactly 50 ohms, especially across some of the wider bands like 80m or 10m. The SWR may be low at, say, 28.5 MHz, but will increase on either side of that. At 29.5 MHz, it may be too high for optimum performance of the transmitter.
There are some types of antennas that work well, but have an impedance other than 50 ohms. And especially for multi-band operation, an antenna that is a good match on one band may have a high SWR on other bands. And you might use a feedline with an impedance other than 50 ohms.
so what does an antenna tuner actually do?
An antenna tuner is, basically, an adjustable impedance transformer. By adjusting the controls, a high SWR (that is, an impedance that is far from 50 ohms) at the end of the coax in the shack can be transformed to 50 ohms, or close enough that the transmitter will operate properly and deliver full output power into the feedline.
do I need one?
Sometimes you might. Other times you don’t.
If you are using an antenna that is close to 50 ohms (has an SWR of less than about 2 : 1) and you are feeding it with 50 ohm coax, then you probably don’t need one. This is often the case on VHF/UHF, or when using an antenna like a dipole or beam on the bands that it is designed for.
But sometimes we want to use one antenna on different bands, or press a random length of wire into service as an antenna, or otherwise do something different. In that case, if the SWR is too high for the transmitter to deliver full output power, then an antenna tuner will improve the match and allow the transmitter to deliver full power.
how do I use it?
Basically, you put it between the transmitter and the antenna when the SWR is too high for the transmitter to operate normally. Then you twist the knobs to lower the SWR, as indicated on an SWR meter. (Or, for an automatic tuner, apply some power and let the tuner adjust itself for low SWR.)
Typically, the antenna tuner is connected into the feedline between the antenna and the transmitter like this:
The signal from the transmitter passes through the SWR meter (which may be built into the radio or into the tuner, for convenience), through the tuner, and to the antenna. Some antenna tuners also include an SWR/power meter, antenna switch and/or a dummy load – combining several useful shack accessories into one box. But the basic function is the same.
Some tuners have a “bypass” option, that lets you remove the tuner circuitry from the signal path without physically disconnecting the cables to the box. This is convenient if you only need to use the tuner on some bands.
are there different types of antenna tuners?
There are different types in several different categories.
For example, we already mentioned two types of antenna tuners: “manual” (where the controls are adjusted by the operator), or “automatic” (where a processor in the box makes the adjustments for you when RF is applied). Actually there is a third type, “switched”, that is less common in ham use, but once was more common in marine HF use. A “switched” tuner is useful when the same antenna is used on a number of fixed frequencies or bands. It consists of a number of switched circuits that initially are adjusted manually for each frequency or band, then the required circuit is switched into the circuit as needed for each band. These can be switched manually, or controlled by the bandswitch in the radio.
Another distinction is between “desktop” and “remote” tuners. As the name implies, a “desktop” tuner sits at the operating position (which is convenient when the tuner requires manual tuning), while a “remote” tuner can be positioned out at the antenna feedpoint, which may be more efficient.
A third category of antenna tuners is “balanced” or “unbalanced”. A “balanced” tuner is intended to match a balanced feedline, like twinlead or ladder line. An “unbalanced” tuner is intended to match an unbalanced load, like coax cable, or an end-fed wire fed against ground. An unbalanced tuner can be used as a balanced tuner with the addition of an effective balun between the tuner output and the balanced feedline: some tuners provide an internal balun that can be used for this purpose.
how do I adjust a manual tuner?
This section got too long, so I’ll put it in a separate page.
so with an antenna tuner I can use any antenna?
Well, not really. All antenna tuners have limitations. And, even if you can adjust the antenna tuner for a low SWR, that doesn’t mean that the antenna is necessarily going to work well. If the antenna isn’t going to radiate effectively at the frequency being used, then it isn’t going to work very well, regardless of how low the SWR is. Yes, I have matched a 50cm (18″) clip lead on 80m. Was it an effective radiator? Of course not. (I suspect that the ground wire radiated more power than the “antenna”.)
Every tuner has limitations on the range of impedances that it can match to a low SWR. And, often, a smaller range of impedances that it can match efficiently.
Let me point out here that, while we use SWR as an indicator of proper tuning, an antenna tuner actually matches impedances, not SWR. Here’s why that makes a difference. A resistive load of 5 ohms and one of 500 ohms both cause an SWR of 10 : 1 in a 50 ohm coax. But the tuner settings to match these will be very different, and some tuners may match one but not the other. When you change the coax length, the impedance will change, even if the SWR stays the same, so it will require different tuner settings, and, in some cases, the tuner might not be able to match the antenna with some lengths of feedline.
How much loss is there? That depends on the load impedance, the tuner design, and how it is adjusted. It can be as high as half of your power – in which case the tuner components may overheat. I’ve seen coils (usually the part with the most loss) get hot enough to melt the plastic support rods, or even unsolder itself, when matching extreme loads. Modes with a high duty cycle, such as AM, FM, or some digital modes, are often more of an issue because the heat builds up faster.
In more normal cases, losses may be around 5 to 10%. That doesn’t make a big difference in your signal strength at the other end, but it’s a good reminder not to use the tuner when you don’t need it. On the other hand, if an antenna tuner allows you to operate when otherwise your rig would reduce power because the SWR is too high, then it serves to increase your radiated power.
The other issue is loss in the coax. When the coax between the antenna and the antenna tuner is operated at a high SWR, the losses increase. Longer and/or smaller diameter cables have higher losses. Using a remote tuner at the antenna feedpoint reduces the cable losses compared to using a tuner in the shack. How much loss? That depends on a lot of factors. But if the SWR measured in the shack without the tuner is higher than about 5 : 1, it may be worthwhile using one of the available calculators to determine the actual loss in your situation, to see if that is acceptable for your operating.
does an antenna tuner really tune the antenna?
Some people make a big deal out of this, but it really is just a useless exercise in semantics. If that happens to you, just smile and repeat what an antenna tuner really does (regardless of the name you use for it): “It provides an impedance transformation to enable the transmitter to deliver full output power”.
does the antenna tuner “fool the transmitter”?
This is another common myth, or misunderstanding. No “fooling” is involved: the transmitter really sees it’s design load impedance, so it can deliver full output power. That’s the objective.
There are other similar myths and misstatements about how antenna tuners work, and I can’t list them all here. Don’t worry. Just repeat what the tuner does, and refuse to play useless semantic games.
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