Antenna Test Gear

2018-10-29 How do you tell whether an antenna will work? Perhaps the best way is to use it! Second best is to read other's experience with a particular antenna or antenna type. This is probably more subjective than your own on-air use. Not only are your reading (or talking) about the actual antenna performance, but that is all filtered through the other person's situation and opinions which will be different than your own. If I've learned anything about antennas it is that my own prejudices on the topic, coupled with propagation conditions which can be dramatically different every time you hit the key or press the push-to-talk button, both factors severely clouding the actual "objective" performance of a given design or antenna. Few of us have contexts and equipment that allow us to make truly objective and accurate measurements that "prove" the performance of any given antenna. Ultimately the true measure is whether the guy I want to hear and be heard by can usefully hear me and I him. If so, the combination of power, transmitting/receiving skills, propagation and antenna work!

But there are some tools that help us get to that "working" place. SWR measuring tools are close to indispensable. Thankfully most rigs today include some form of SWR measurement. The primary and minimal point of SWR is that it must be low enough at the transmitting terminals that the transmitter can push  signal into the feedline, hopefully all it is capable of. With today's rigs that is generally somewhere around 2.5:1 or below. If the transmitter simply won't transmit or reduces the power so much that it is as if it weren't transmitting, all else is irrelevant.

Everyone pretty well agrees that the "best" antenna will look to the feedline and then the transmitter to be 50 ohms resistive, which is approximately the case with a resonant half wavelength dipole. That supposedly minimizes losses in the feedline and delivers the most of the power sent into the feedline by the transmitter to the antenna. Of course a dummy load, by that measure is a perfect antenna since it is a 50 own resistor across a 50 own feedline which attaches to the transmitter which is designed to see a 50 ohm resistor. That also means that the feedline length, generally coax, is not important beyond the fact that the longer it is the more power is lost in the wire of the feedline. But, the dummy load is actually a terrible antenna since almost all of the energy fed to it is dissipated as heat rather than RF into the outside world.

That brings us to the second most important aspect of an antenna. Assuming it looks good to the transmitter and feedline, how well does it radiate the power it is receiving? The standard for that is a resonant half wavelength dipole. It looks very much like a 50 ohm resistor to the feedline and transmitter and according to people who have researched such thing for many years delivers most of the power it receives to the outside world.

That brings me to the next issue. Given that the antenna itself is both receiving and delivering most of the power that the transmitters sends to it, how much of that power is going where I want it to go? Answering that question requires mostly magic! I'm joking but it's not far off as things like surrounding objects; the ground below, trees, buildings, countryside, ionosphere, sunspots, solar storms, and more now come into the equation. And when it comes to communications all those factors ALSO impinge on "the other end" as well. A perfect signal going into a perfect antenna over perfect ground, etc. will appear terrible at the other end if the other guy is using a dummy load as his antenna!

SWR Bridge

My conclusion is that though the lowly SWR is both blessed and cursed by many in terms of its ultimate usefulness, it is just about the ONLY thing most of us will ever be able to use somewhat helpfully as it deals with an essential thing that we have some influence over through decisions we make; i.e. dipole, vs, vertical, inverted vee, traps, links, etc. All of those are irrelevant if our transmitter won't transmit! So for me, the most essential piece of antenna related test equipment is an SWR bridge of some sort at the antenna terminal of our transmitter.

Over the years I've had many of them. Only a couple were stand-alone type like this MFJ-860 model. Some were integrated into a tuner like this MFJ-971 tuner. And I believe every purchased rig I own now has one incorporated into it so usually no outboard SWR bridge is needed. But I wouldn't be caught without one anymore. Almost every time I tap a CW key or press a push-to-talk button I glance at the SWR.

Antenna Analyzer

2018-11-07 An antenna analyzer is a very useful, almost essential piece of test gear, especially if you want to build your own antennas. I borrowed an MFJ-259b (now superceded by the MFJ-259c) from a neighbor when I built the 40/20 meter fan and trap inverted vee antennas for Field Day. It was so helpful. I don't think I could have made them without it.

Then when I got my Xiegu X5105 QRP rig back in August I began playing with its SWR sweep feature. It is not as good as a true stand-alone analyzer but it is pretty good, far better than anything I've ever had before. I've used it exclusively while building and tuning my 18, 20, 30 & 40 meter link inverted vee and my trap version of the same antenna. It works so well that I cancelled an order for a MFJ analyzer. I wouldn't recommend the X5105 just to get that function but it is a very effective tool.

Antenna Bridge

Another useful piece of antenna test gear that is much less expensive than a true analyzer is the MFJ Antenna Bridge. I've had the 204B model for some time which has been superseded by the MFJ-207. The one I have has one major drawback, the frequency dial is not all that accurate. I actually added a CW audio output frequency counter to mine which helps but the signal level is too low for it to read anything above 20 meters. Never-the-less it is a major enhancement. The Antenna Bridge will tell you the resonant frequency and the approximate "resistance" at resonance. It is pretty close and is far better than nothing. I've used it a lot over the years when I had no other practical option and got my antennas pretty close to what I wanted.

Field Strength Meter

Another piece of gear I have "always wanted" and wondered what it would tell me is a field strength meter. it just makes sense in concept. And so a YouTube video on building a cheap field strength meter caught my attention. And since I had the very digital meter used in the video and all the parts lying around, I decided to build one. I have yet to use it but it looks impressive!