> Can anyone help me with the construction of a lewallen phasing
> network for 2
> 1/8 wave spaced 80m verticals or sort out my christman feed. I
> have used 2 x
> mfj1792 top loaded verticals. I have 40 radials 1/8 wave long on each
> antenna. I have tried the christman feed to these antennas but i
> suspect the
> phasing is not quite right as front to back is only about 15db. I used the
>
You see a lot of books talking about characterizing the
antennas and then designing a network for them. I have
found that is is much easier to just use an adjustable
phasing network and tune it until the currents are correct.
The adjustable phasing network is simply an adjustable
inductor and adjustable capacitor in an "L" network.
If you don't have these components, you can use an
ensemble of known fixed inductors and capacitors by cut and try.
First, check the feed coax to make sure it is exactly an
electrical quarter wave. In that case, you don't have to
measure the antenna current, you only have to measure the
voltage at the other end of the coax, as it will be proportional
to current.
You want the feed voltages equal in magnitude and 135 degrees
out of phase. You can measure each voltage vs ground with
an RF voltmeter and adjust them to be equal magnitude. Then connect
the RF voltmeter *across* the feed voltages and adjust for
a voltage 1.85 times as much as the individual voltages.
Iterate back and forth between magnitude and phase until
both are correct. This "3 voltmeter method" is described
in more detail at http://www.n6rk.com/driving_the_7_hex.pdf
pages 4 and 5. I made my own RF voltmeter with a
Schottky diode, bypass capacitor and pocket voltmeter.
I put about 100 mW into the antenna for these tests.
Rick N6RK
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