More About Antenna Tuners

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More About Antenna Tuners

18 March 2021
by Steve Ford, WB8IMY, ARRL Contributing Editor

In the January/February 2021 issue of On the Air, we spent several pages talking about antenna tuners, in the articles “Two Types of Antenna Tuner,” “Untangling SWR,” and “Antenna Tuners: Making a Match.” Here’s a closer look at how they work, how to buy one, and other considerations.

Why Use an Antenna Tuner

When you reduce the function of an antenna tuner to the bare minimum, it’s as simple as this: It matches the impedance of the antenna system — measured in ohms — to the impedance the transceiver needs to see, which is 50 ohms.

What’s the point of doing that? Well, modern transceivers are only designed to handle a narrow range of antenna impedances. They certainly could be more flexible, but that would add substantial cost — and physical size — to the radio. So instead, manufacturers design radios to deal with 50-ohm impedances at their outputs.

If the antenna system impedance deviates too far from 50 ohms, the result is an elevated standing wave ratio, or SWR, at the radio. We explain SWR in the January/February 2021 issue (see “Untangling SWR”), so I won’t get into it here. Suffice to say that an elevated SWR can be problematic for your radio.

An elevated SWR can cause high voltages to appear in your transceiver’s output circuitry, and this is never a good thing. Transceiver manufacturers try to protect your radio from damage by including something called a foldback circuit. It sounds complicated, but all the circuit does is reduce the radio’s output whenever it detects that the SWR has exceeded about 1.5:1.

So, if you want to load all your available power into the antenna system, you need to get the SWR at the transceiver below 1.5:1. That’s where the antenna tuner comes to the rescue. By using different amounts of inductance and capacitance, the antenna tuner can convert the antenna system impedance to 50 ohms for the radio. The result is a 1:1 SWR, and your transceiver is happy to dump its power into the circuit.

Seems simple, doesn’t it? But as you can imagine, there is a catch — and it’s a significant catch. Many hams don't realize this, but when an antenna tuner achieves a low SWR for your transceiver, the SWR between the antenna tuner and the antenna is unchanged.

For example, if you have a 2.5 to 1 SWR between the antenna tuner and the antenna, the SWR in that portion of the feed line — let’s say it is your coaxial cable — will remain at 2.5 to 1. It’s from the antenna tuner to the radio where the SWR will be 1 to 1, or something close to it.

Having an elevated SWR between the antenna and the antenna tuner can result in some portion of your power being lost as heat. How much is lost due to SWR depends on the type of feed line you are using and the frequency at which you’re operating.

If you’re using coaxial cable and operating on one of the higher HF bands, such as 10 meters, this loss can become serious if the SWR is high. On the other hand, if your feed line is something like open-wire line, the loss will be minimal regardless of the SWR. Now you know why many hams prefer open-wire feed lines, but that’s a topic for another time.

The key point to keep in mind is that the SWR reduction only occurs between the antenna tuner and the radio. By adding an antenna tuner to the equation, what you’ve done is make it possible for the radio to put as much power into the feed line as it can. Power is still being lost between the tuner and the antenna, but at least you have more power in the whole system to compensate.

I don’t want to underplay the importance of antenna tuners. They can be critical components in some ham stations. Just keep their limitations in mind and don’t expect miracles.

Types of Antenna Tuners

Increasingly, manufacturers have been including tuners inside some of their more expensive transceivers. Most of these tuners are limited, though. They can usually only handle SWRs up to about 3:1. If you’re dealing with an SWR higher than 3 to 1, chances are the antenna tuner in your radio will not be able provide the one-to-one match your radio wants to see.

If this is the case, you may need an outboard tuner. This can be a manual or an automatic model. It’s worth noting that all built-in tuners are of the automatic variety.

With that in mind, let’s talk about automatic tuners first. The typical auto tuner is box that contains coils, capacitors, switching relays, and some sort of microprocessor control circuitry.

Depending on the design of the tuner, it goes into action whenever you apply RF power or press a button. It uses sets of relays to rapidly select combinations of inductors and capacitors, switching from one to the other until it finds a combination that provides a low SWR for the radio.

All this mixing and matching is guided by the microprocessor. Once it finds a combination that works, it records the settings in memory for future use.

Among the automatic tuners on the market, you will even find a few that are designed for remote installations. These auto tuners are particularly interesting because they do the matching at the antenna — or close to it. Assuming the length of feedline between the remote tuner and the antenna itself is short, that means in the rest of the feed line going way back to your station the SWR is maintained at 1:1. As a result, the loss in your feedline will always be as low as possible, which is a clever trick when you think about it.

It’s common to find hams using remote tuners at the bases of antennas that consist of nothing more than a single tube or wire. Thanks to the remote tuner, they can use the wire on several different bands because the tuner will provide the low SWR for the coax back to the radio.

The other type of antenna tuner that you’ll find are manual tuners. And as the name implies, you have to adjust these yourself. Your typical manual antenna tuner will have three primary controls. There will be two controls attached to variable capacitors so you can change their settings by tuning one knob or the other. There will also be a control for the inductor.

This could be a knob that continuously changes the inductance — what is known as a variable inductor — or simply a switch that lets you select different fixed amounts of inductance. Many manual antenna tuners will also have meters on the front, so you can see the SWR.

Using a Tuner

Manipulating a manual tuner takes a little practice. The first step is to just turn up the volume on your transceiver and adjust the tuner controls until you hear as much noise as possible. Now, transmit a small about of power, say 10 watts or so, and adjust the controls as you watch the SWR meter. The idea is to bring the SWR as close to 1:1 as you can.

Once you've found the lowest SWR, it’s a good idea to write down the settings of the controls you can go back to them easily. Keep in mind that you must adjust antenna tuners at low power. This goes for automatic antenna tuners as well. You don't want a lot of RF energy present while tuning because that may result in high voltages inside the tuner — high enough to cause considerable damage.

Tuner Shopping Tips

If you’re in the market for a manual antenna tuner, make sure you get one that is rated for power you think you’ll be using at your station. If you’re using a 100 W transceiver, make sure your tuner is rated for a minimum of 100 W continuous power not peak power, not power for a short period of time, but continuous power. This is important. You’ll need that continuous power rating if you’re operating digital modes like FT8 because they require your radio to constantly generate output.

Unlike SSB or CW, the power remains essentially at one level when you are using most digital modes, possibly for an extended period. Your antenna tuner needs to be robust enough to handle this kind of operating.

You may see some tuners that are advertised as being rated for, let’s say, 200 W. But look closer at those specifications and sometimes you'll see something like 200 W PEP — peak envelope power. That’s momentary power, not continuous power. The continuous power rating is usually much lower. Of course, if you don’t think you’ll ever want to operate at high continuous power levels, save your money and pick a tuner based on the PEP or SSB power rating.

Also, when you’re shopping for a manual tuner you will find some that do not have meters. These are less expensive because they rely on you using the SWR meter built into your transceiver. Frankly, I would recommend investing more and getting an antenna tuner that has the metering already built in so it’s right there when you need it. In manual tuners, you’ll also find some additional features like switches that allow you to select different inputs and outputs. Some antenna tuners also have built-in dummy loads for antenna testing and troubleshooting.

When considering an automatic tuner, pay attention to how it is powered. All auto tuners require dc power to operate. Do you have to run a separate wire to the tuner to supply the DC voltage? Or can you send the DC voltage in the coaxial cable through what is called a bias-T?

Also, will the automatic tuner operate whenever it senses that your transmitter is generating RF power? Some do, but some don’t. Some automatic antenna tuners require you to push a button to start the tuning. If the antenna tuner is sitting right beside you on your operating desk this isn’t a big deal. But, of course, if you have the antenna tuner in another location this could be a hassle. For an antenna tuner that won’t be nearby, look for a model that offers RF-sensed tuning, or what is sometimes called RF activated tuning.

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