Topband: Measuring field strength

[Telegrapher9 at aol.com]

Dave,

let's see if we can obtain an accurate change in field strength from added 
base resistance in NEC-2. Here's a 7 MHz vertical mounted 1' above average 
Sommerfield GND. The vertical has 16 resonant radials. 

36.9 ohms
Added 0 ohms to base
1 amp source
19.7 uA at RX antenna (relative field strength)
 
Add 5 ohms to base
41.9 ohms
0.9384 amp source (to maintain the same 36.9 watts input)
18.5 uA at RX antenna (relative field strength)
 
Add 10 ohms to base
46.9 ohms
.887 amp source (to maintain the same 36.9 watts input)
17.5 uA at RX antenna (relative field strength)

The relative field strength is exactly as one would calculate given the 
source current into 36.9 ohms (in this case we can 'assume' that the radiation 
resistance is 36.9 ohms). 

I am not claiming that NEC-2 accurately calculates feedpoint impedance over 
other than perfect GND. When limited to the case of relative comparisons 
between similar antennas it can give useful results. In the case of the added 
radials mentioned in the email that began this string, we are comparing antenna A + 
some radials to antenna A + more radials. The original proposed measurement 
method was to use field strength as a direct indication of radial effectiveness. 
My suggestion uses an indirect method as an indication of radial 
effectiveness. In the first case distance measurements are needed. In my method the data 
already exists in the form of base impedance measurements. It's just another 
(and possibly easier) way to arrive at the same end result. 

Relative measurements can be SO much easier than pegging a number to NIST.

    Dave WX7G