[Lowfer] SWR meters for LowFERs

[Lyle Koehler]

Bill Ashlock is correct in that there is no need to worry about SWR in the
usual sense, because in any good LowFER installation the transmitter is
located right at the antenna. But to obtain efficient power transfer from
the transmitter, the load should be resistive rather than reactive. The
impedance should be neither very high nor very low, to avoid excessive
voltages or currents in the final. Something in the 50 ohm vicinity is about
right, and an SWR bridge is perhaps the simplest homebrew instrument that an
experimenter can use to determine when the antenna is presenting a 50 ohm
resistive load.

As for a filter in the final output, it may seem unnecessary if you're only
interested in meeting the -20 dB Part 15 requirement on out-of-band
emissions, because the antenna itself is very narrow-band. But we also have
to remember that at higher frequencies, the antenna is much more efficient
than it is at LF. For example, a 15 meter loop of #10 wire has a predicted
gain (at 8 degrees elevation) of about -30 dBi at 185 kHz; -15 dBi at 500
kHz; -5 dBi at 1 MHz; and -1 dBi at 1.5 MHz. Similar numbers probably apply
to vertical LowFER antennas, although they come in many shapes and sizes. At
any rate, if the fundamental signal from my beacon can be heard at 1000
miles, I feel that the harmonics should be down a lot more than 20 dB in
order to avoid interference to some licensed radio service. So I'm willing
to pay the penalty of a fraction of a dB that might result from the
insertion loss of a properly designed filter working into a properly
designed antenna matching network.

Lyle, K0LR