[Lowfer] SWR meters for LowFERs

[Lyle Koehler]

Bill, your measurements indicating low harmonic content are probably
correct, although I'm not sure I agree with your reasons. First of all, both
loops and verticals exhibit resonances on all harmonics of the fundamental
frequency. The resonances for a loop system are easier to predict, since the
series capacitor probably doesn't start to exhibit weird resonances until
somewhere above the HF range. However, the series capacitor looks almost
like a short circuit at HF, so it does nothing to keep high-frequency
currents from flowing in the loop. Modeling results show that a 15-meter
square LowFER loop would make an excellent NVIS antenna if we ever get the
proposed 60-meter ham allocation (5.25 to 5.4 MHz), with an SWR (50 ohm
system) under 2:1 over most of the band, and a gain of about 7 dBi at 90
degrees elevation. Hmmm; maybe I *should* put up a loop!

The loop matching transformer doesn't inherently attenuate high frequencies.
Type 43 and 77 material can be used in broadband transformers and baluns
through the HF range. However, the transformer steps down the voltage from
the transmitter, and the loop exhibits a fairly high reactance until it
reaches the first resonance at 5 MHz, so you are correct that not much HF
current flows in the loop. By the time you get to 5 MHz, the harmonic
content from the transmitter is very low anyway. I expect that the same
thing is true for vertical antennas, although the analysis is more
complicated because of possible stray resonances in the loading coil. But I
like to be able to go to the end of my driveway and find no trace of LEK
harmonics on the AM broadcast band, so I'll keep the filter in the circuit.

It would be interesting to try a loop at my location, but there are three
reasons why I haven't done so:

1. No convenient 50-foot high trees that could support a heavy loop.

2. If I apply the 15-meter cylinder rule to a good vertical installation (15
meter overall height with 15 meter top hat diameter), I would expect
approximately 3 dB more signal in *all* directions than I could get in the
most favored directions from a 15 meter square loop of 1 inch copper pipe.
Agreed, it would take a lot of work to install a ground system like those
used by TEXAS and OK and achieve that kind of efficiency in a vertical
system, but erecting 1 inch copper pipe would be no picnic, either.

3. Last but far from least, I've already got a LowFER antenna that ain't
busted!

Lyle, K0LR