[Lowfer] MP 137.780 ON

[Lyle Koehler]

I've gone through the same exercises as John Andrews to calculate the
frequency dependence of loop and vertical antennas. Like John, I seem to
have discarded the scraps of paper on which I made the calculations. Rather
than starting over from scratch, I got lazy and used the antenna modeling
software. Here are the results comparing gains at 8 degree elevation angles.

1) LEK vertical with coil Q of 500 and with ground losses of 20 ohms;
loading coil inductance adjusted for resonance at each frequency.

Gain at 185 kHz = -23.4 dBi

Gain at 137 kHz = -26.1 dBi

Difference = 2.7 dB

2) 15 meter square loop of 0.5" copper with additional 0.22 ohm fixed series
resistance. The fixed resistance is added to account for differences between
Bill Ashlock's measured antenna current at 1 watt, and the predicted current
based on copper losses in the loop.

Gain at 185 kHz = -26.4 dBi

Gain at 137 kHz = -31.1 dBi

Difference = 4.7 dB

You can plug in different assumptions for loss resistances and get
considerably different results, but I think these are in the right ballpark
for practical antennas. The net result is that both kinds of antenna will
have lower efficiencies (for a given size) at 137 kHz than at 185 kHz.

As for why Mitch isn't burning up receivers with 75 watts when 1-watt
signals can be copied at such great distances, welcome to the world of RF. A
power ratio of 75 (18.75 dB) sounds like a huge amount. However, noise
levels and propagation path losses can vary by much more than that between
winter and summer, or even from day to day. At a distance of a few hundred
miles, 100 watt signals will be readable more often than 1 watt signals, but
not all the time!

Lyle, K0LR