On 8/4/2012 10:47 AM, Tom W8JI wrote:
>
> The reason I say this is because I know for an absolute fact.....base
> impedance can vary all over the place with unrelated or unexpected changes
> in efficiency. A radial system here that made base impedance of a 1/4 wave
> vertical 50 or 60 ohms delivered the SAME field strength as another system
> that made base impedance 35 ohms or so. This was for the exact same height
> antenna above the radial field height.
Confirming Tom's test. When I first built my 230 foot diameter ground
screen, consisting of #16 wires on a 3x3 foot grid with crossovers
soldered, I put up a 40 meter vertical in the center to test
the effectiveness of the grid. I placed an HP 8640 signal generator
running on a battery at the center of the grid and grounded it to the
nearest wires. I measured the driving impedance with a very
accurate RF bridge consisting of a transformer, variable resistor
and a null detector. Tune sig gen to resonance, null, then
measure resistor with ohmmeter.
I was shocked to discover a driving impedance of 70 ohms. I set up a
receive antenna 1000 feet away monitored by an HP
spectrum analyzer. I started adding radials to the vertical, the
ends of which were soldered to the grid. I monitored the field
strength while doing this and found that it decreased with additional
radials until it reached 36 ohms at 60 radials. I continued to
add radials until I had 120. During all these changes, there was
no measurable change in field strength. The spectrum analyzer
could resolve 0.1 dB.
So field strength is the gold standard, and impedance is the
lead standard.
Rick N6RK
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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