Sanity Checking an Antenna

It is no simple matter to raise a large antenna onto a guyed tower. I've done far more than most hams and I still have to carefully plan and execute every step. From small yagis to the truly gargantuan, I've lifted them onto free-standing and guyed towers. I am well aware of the dangers so that the risks to property and people are always at the forefront of my attention.

It is aggravating when I and my friends make the effort to raise a beautiful big yagi onto the tower and find that it does not work. What can you do? Some problems can be fixed in place while in many cases, especially the largest antennas, it has to come down for inspection and repair. It should be obvious that the risks of raising and lowering antennas multiple times are greater than doing it once. Not to mention the annoyance of the friends taking time out of their lives to help.

Since antennas need to be well off the ground to work properly, testing and repairing large yagis on the ground might seem unlikely. While true there are still many tests that can be done on the ground. These include mechanical and electrical inspections. In the latter category are continuity tests, coil and trap tests, matching network tests, among others. You do what you can and then up it goes, hoping for the best.

Commercial antennas are easier to deal with than those you design and build. If you build a commercial product according to the instructions the antenna ought to work. They've done the hard work for you ahead of time. For home brew antennas you must deal with both construction issues and electrical design issues. There are more trouble areas to contend with.

However, what I driving toward is that testing an antenna is different from adjusting an antenna. What I am about to propose is the former, not the latter. For the moment let's assume that the antenna is properly adjusted, whether by following the instructions that come with a commercial product or a home built antenna that has been previously adjusted but is now on the ground.

I have previously described the design and construction of this symmetrical and reversible 40 meter Moxon yagi. Although the antenna worked and its performance closely matched the NEC5 design, the reversing function did not work. I had tested the individual components before assembly but did not do a full antenna test before raising it. I was in a hurry at the time to get it out of the growing hay. Better to raise and use it rather than disassemble the antenna and try again after the harvest. I got a few months of use out of it so the effort was worthwhile.

I gathered a crew and lowered the antenna in mid-August before heading to Manitoba. When I returned I diagnosed several electrical and mechanical issues and made repairs. This time I took the time to do a full antenna test before gathering my friends for an antenna raising.

But how can you do a full system test with the antenna on the ground? This is where we need to clearly differentiate testing and tuning: the antenna is already tuned (adjusted) so we just need to test it. The antenna has to be high (relative to wavelength) to be tuned, but not a test as a basic sanity check.

As pictured and tested, the Moxon is about 50 cm off the ground, with capacitance hat tips even lower. Even so it can be tested -- sanity checked. Although its behaviour will be very different than what you will measure when it on the tower, it will still "work".

First, the resonant frequency will be lower, perhaps much lower, due to ground coupling. In a sense, near fields are slowed due to the VF (velocity factor) of the soil. The amount is unpredictable but not too important for this type of test. Hook up your antenna analyzer and sweep through a large spectrum below its design range.

Second, the impedance at resonance will not be 50 Ω. Expect a higher SWR. In most cases the SWR will be moderate so that its ground-proximity impedance curve will remain recognizable.

Third, don't expect a directional antenna to exhibit the same pattern as it does in the air should you attempt an on-air test. Efficiency will also suffer due to ground loss. For a sanity check these attributes are of no great interest except insofar as excessive loss will push the SWR curve higher and may not display properly on the analyzer. In my experience this is rarely a problem. 

Sanity tests can be done for many kinds of antennas. I've done the same with tri-band yagis a meter or two off the ground. When properly constructed there will be clearly identifiable SWR dips below the 3 typical band ranges. 

In another case a ham acquired an older model tri-band yagi. There were no identifying marks remaining on the traps and a visual inspection was inconclusive. I suggested some quick experimentation by random arrangement of traps, one element at a time, sitting on trestles, and do an SWR sweep. When the element shows the expected SWR dips, set it aside and move on to the next one. Testing goes faster as the pile of traps gets smaller. 

This is an SWR sweep of the Moxon on the ground in its normal (forward) direction. The impedance is pretty good despite the almost 20% lower frequency. 

Next, I hooked up a battery to the reversing terminal to test it in its reverse direction. This time it worked. The sanity check was successful.

Notice that the SWR curve is not quite the same as in the forward direction. It's ~25 kHz higher. This could be a real difference or due to different ground characteristics under each element or slight height differences. The antenna would have to be lifted at least 0.1λ to discern whether the difference is real.

I didn't do that last test. The sanity check was good enough that I called my friends over this past week to raise the antenna. This time the antenna worked as it should when it was mounted on the tower, in both directions. The small discrepancy between the forward and reverse directions was still there so this was no testing anomaly due to ground proximity. The 25 kHz shift is most likely due to the reflector coils in each switch box not being perfectly identical. It isn't enough of a difference to worry about.

The 40 meter reversible Moxon is a heavy and awkward antenna so I hope that it continues to work. I'd like to avoid lowering it again. On the first evening of use I learned quite a lot about how it performs and compares to the 3-element yagi at twice the height. Many nights will be needed since propagation elevation angles and low angle absorption change day to day and even hour to hour.

Once I have enough experience with it I'll write an article on the antenna's performance. There is enough innovation in the design that might inspire others to emulate the antenna whether for 40 meters or other bands. This has been and continues to be a very interesting project. It was also illustrative of the advantages of sanity checking a large HF yagi on the ground.