Topband: Measuring field strength

[Telegrapher9 at aol.com]

Tom,

thanks for the excellent data.We have two things were are discussing; whether 
a decrease in input impedance follows the field strength trend and how well 
NEC-2 can model all of this. 

I ran a simulation of your ground mounted 7 MHz antenna. To make the 32 
radial impedance match between the sim and actual I added 19.5 ohms of base 
resistance. The antennas were driven with the same power. Here is actual vs. 
simulated using NEC-2 (with radials 3" above 'average' Sommerfield GND). 

CONDX       actual R      sim R       actual FS     sim Fs

4 radials        99.0        56.8        -4.75 dB       -0.15 dB

16 radials      58.4        54.2        -1.50 dB            0 dB

32 radials      53.7        53.7              0 dB            0 dB

NEC-2 shows a trend having the same polarity but the magnitude is way off. 
Not good correlation. 

As for using the measured input impedance (of a ground mounted vertical) to 
tell how well an increase in the number of radials works, let's see. Assuming a 
radiation resistance of 36 ohms we would expect a change of 2.3 dB going from 
4 to 16 radials and 0.4 dB going from 16 to 32 radials based on the base 
impedance measurements. Your FS measurements show 3.3 dB and 1.5 dB respectively. 
The correlation between measured relative FS and calculated relative FS based 
on input impedance measurements, for the 4 to 16 radial change, is 4.75 vs. 
3.3 dB. For the 16 to 32 radial change it is 1.5 vs 0.4 dB. The base impedance 
method appears to underestimate the FS change by a bit more than 1 dB. We could 
delve into the uncertainty of your measurements but for now let's say that a 
ham used the equipment that you used. The question is "ould he make a 
different decision based on base impedance than on FS?" I think he might. He might 
read the base impedance, make the calculations, and conclude that adding 16 more 
radials to the 16 radial case would give only 0.4 dB and might decide to skip 
the extra 16 radials. But using the FS measurement he would see a 1.5 dB 
increase and decide to add the extra 16 radials. 

    Dave WX7G