John, do your NEC4 numbers take in to account the signal that actually
arrives at the element insulators and the amplifiers with their inherent
input capacitances as a load on the element source capacitances. Or are you
using NEC gain numbers to get the 0.5 dB per foot?
Lee K7TJR
>>>A while back, while designing my own "active" receiving array of short
verticals, I did a NEC-4 analysis of how the behavior of a very short
nonresonant vertical varied with height. The vertical I modeled used a
4-foot ground rod inserted in the ground, which is typical of "active"
vertical systems. NEC-4 is necessary to model anything buried in the
ground. NEC-2 won't do it.
The gain of a very short vertical with just a ground rod is obviously very
low, hence the need for amplification. The principle effect of increasing
height is to increase signal output. The NEC-4 analysis showed a scaling of
roughly 0.5 dB of increased signal output on 160m for each 1 foot increase
in length over the range of 10 to 25 feet. Keep in mind that this is
strictly a modeled result with the usual caveats, and I don't have any
measured data to confirm it. However, I do believe the trend shown by NEC-4
is correct, if not the exact numbers.
On 160m, you want to be sure that the system noise floor is set by ambient
atmospheric noise and not the internal noise of the amplifier that provides
receiving gain. This is where the signal level delivered by the vertical
matters, because the signal includes the atmospheric noise (as well as local
man-made noise) picked up by the vertical. You want the external noise to
overwhelm the internal noise of the amplifier. The point at which this
happens depends on how noisy your location is. Ironically the quieter your
environment, the more "signal" you need from the vertical to be sure this
happens for a given amount of amplifier noise, and therefore the longer the
vertical required.
Once you reach the point where the atmospheric noise dominates the amplifier
noise, there is really no receiving benefit to a longer vertical in
performance terms.
73, John W1FV
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