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 _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK