Ron:
I will give you my thoughts. Since the price for them is zero they may have
exactly that value.
If we assume your readings are correct (and they sound quite reasonable to
me based upon my measurements for a similar system made with a General Radio
1606A RF bridge), then the input impedance is on the order of 20-j50 say.
An L network consisting of 2.2 uH in series followed by 3.5 uH across the
input (after the series coil) will give you 1:1 at 1850 kHz. If the Q of the
coils is 300 you will have 12 watts of loss for 1500 watts input. If the Q
is 100 you will have 36 watts of loss for 1500 watts input. Even with a Q of
50 your losses will only be 0.4 dB. At 1500 watts input the current in the
series coil will be less than 9 amps; the shunt arm less than 7 amps.
If your input impedance is 15 -j50 then your network will be 2.3 uH series
and 2.8 uH shunt and with coil Q's of 100 the loss will be 0.05 dB.
If the input impedance was 15 -j150 then the network would be 11 uH series
and 2.8 uH shunt and with coil Q's of 100 the loss would be 0.4 dB.
As you can see, matching using an L network at the input can produce good
results with fairly simple to create coils. You can tune the base network to
the center of the band segment of interest and it should be broad enough to
reduce the SWR over a reasonable range of frequencies. If you are using
RG-213 to feed the antenna, the losses at 1.8 MHz for reasonable lengths and
VSWR's up to say 5:1 will be trivial. That should allow you to use your
tuner at the shack to match the transmitter as required. Since the SWR of 15
-j150 is about 33:1 and 15 -j50 about 7:1 the tuner losses should be much
less when you "prematch" at the input and fine match at the station.
I hope that is useful information.
Tod, KØTO
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