[Lowfer] new vertical

[Lyle Koehler]

Bill is correct in that total system resistance is important for a given
size vertical (or loop), but Eric is correct in that size is even more
important, if your present antenna is smaller than the Part 15 limit.

One way to estimate the total system resistance is to measure the antenna
current, using the clamp-on current probe on my web site (for a vertical) or
the technique shown in Bill Ashlock's loop article. The two techniques are
equivalent, except that my probe will introduce more series impedance, which
will cause serious errors in a low-impedance circuit like a series-tuned
loop. If you know the current at resonance, and the power output of your
transmitter, it's easy to calculate the resistance. Let's assume the final
amplifier has a DC power input of 1 watt and an efficiency of 80 per cent,
and that the measured current is 150 mA rms. Since resistance =
power/(current squared), in this case it is about 36 ohms.

Another way to determine resistance of a vertical, if you have a scope, RF
voltmeter, or even a home-made RF probe for your multimeter, is to put a
non-inductive pot (250 or 500 ohm carbon or composition pots should be fine
for this application) in series with the antenna, and adjust the pot until
the voltage across the antenna is one-half of the voltage applied to the
series combination of the pot and antenna. Then you use the multimeter to
measure the DC resistance of the pot, which is equal to the antenna
resistance. This measurement must also be made with the antenna tuned to
resonance.

Lyle, K0LR