If you want to deliver the maximum power to a load — say from a transmitter to an antenna — then both the source and the load need to have the same impedance. In much of the radio communication world, that impedance happens to be 50Ω. But in the real world, your antenna may not give you quite the match you hoped for. For that reason, many hams use antenna tuners. This is especially important for modern radios that tend to fold their power output back if the mismatch is too great to protect their circuitry from high voltage spikes. But a tuner has to be adjusted, and often, you have to put a signal out over the air to make the adjustments to match your antenna to your transmitter.
There are several common designs of antenna tuners, but they all rely on some set of adjustable capacitors and inductors. The operator keys the transmitter and adjusts the knobs looking for a dip in the SWR reading. Once you know the settings for a particular frequency, you can probably just dial it back in later, but if you change frequency by too much or your antenna changes, you may have to retune.
It is polite to turn down the power as much as possible, but to make the measurements, you have to send some signal out the antenna. Or do you?
Several methods have been used in the past to adjust antennas, ranging from grid dip meters to antenna analyzers. Of course, these instruments also send a signal to the antenna, but usually, they are tiny signals, unlike the main transmitter, which may have trouble going below a watt or even five watts.
New Gear
However, a recent piece of gear can make this task almost trivial: the vector network analyzer (VNA). Ok, so the VNA isn’t really that new, but until recently, they were quite expensive and unusual. Now, you can pick one up for nearly nothing in the form of the NanoVNA.
The VNA is, of course, a little transmitter that typically has a wide range coupled with a power detector. The transmitter can sweep a band, and the device can determine how much power goes forward and backward into the device under test. That allows it to calculate the SWR easily, among other parameters.
In Practice
This sounds good, but how does it work? Well, to find out, I took a long wire connected to an MFJ Versa Tuner II and fed the NanoVNA’s TX port to the tuner. With the tuner in bypass, the screen looked like the first image. It actually had a pretty low SWR near 14 MHz, but everywhere else was not going to work very well at all.
The wire across the HF spectrum
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The next step was to switch the tuner into the circuit. Ideally, you could infinitely vary the inductor and both capacitors, but making roller inductors is a cost, so many tuners — including this one — have switches that select taps on the inductor, meaning you can only change it in fixed steps. That isn’t usually a problem, though, because you can adjust the capacitors to make up for it.
Since you aren’t transmitting, there’s no rush, and you can easily switch things around and turn knobs until you can find a null. If you were using the actual transmitter, you’d want to avoid switching the inductor “hot” because the switch contacts won’t appreciate any high-power RF.
The tuner create a few dips, one on the 40 meter band
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I centered the frequency range around 7 MHz and found the lowest setting I could on the tuner. Then, I zoomed back out to the entire HF band. Not bad.
I went through and found null spots for all the ham bands. It was also possible to measure the SWR for bands I can’t transmit on (for example, 15 MHz, to listen to WWV).
Once I had jotted down all the settings, it was time to reconnect the transmitter. Well, technically, a transceiver — in this case, an Icom IC-7300. Even without transmitting, having the knobs adjusted correctly definitely helped with receiving, often strikingly so.
But Did It Really Work?
My first attempt was to use the frequency exactly where I had tuned before switching in the transmitter. As you’d expect, the transmitter saw a low SWR and had no issues, but changing frequencies was a little different.
The knobs on the tuner are not especially precise. Some high-end devices have multi-turn knobs with counters to help you get exactly back to some setting, but this tuner has no such thing. So when the dot on the knob is on, say, “2,” it is hard to know for sure if it is exactly where you had it last time it was in the same position.
A quick CQ on 15 meters with questionable propagation conditions
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However, you can get close. Changing frequencies and tuner settings would sometimes give me a great SWR, but sometimes it was a little high (never any more than, maybe, 1.5:1). A minor tweak of the two capacitors on the tuner would resolve it quite easily.
A quick CQ on 15 meters resulted in the map you can see from the reverse beacon network. The furthest away I was heard was a bit more than 1,800 miles away. Not bad for a fairly short wire hung over a tree. Subsequent testing on several bands resulted in many contacts across four continents in a few hours.
Takeaway
Do you need to use a VNA to tune? No, but it sure is handy. Sure, it generates a tiny signal, but nothing like your transmitter. I like tuning very quietly and precisely without risking the expensive final amplifiers in my station. A good tuner can load up almost anything, and while you won’t get the performance you would get out of a proper antenna, you can still get on the air and have a lot of fun.
Of course, the VNA can do other things too. It can characterize components and modules like filters. You can even use them as time domain reflectometers to troubleshoot cables. It is worth noting that while I took pictures of the VNA so you could see what it would look like, it is actually better to use one of several programs on your PC that can create graphs and data that would be easy to work with. For example, I often use this one.
Want more things to do with your VNA? You can even map antenna patterns with one.