Lee,
The NEC-4 analysis is based strictly on gain and does not take into account
the loading of any amplifier input capacitance, which can modify the scaling
of signal level obtained at the output of the amplifier with increasing
vertical height. A very short vertical, in the range of 10 to 25 feet,
exhibits a feedpoint impedance that is almost entirely capacitive reactance.
NEC-4 shows that the capacitance is a few ten's of pF, with the capacitance
increasing with increasing length. On my own 15 foot verticals, which uses
4 foot ground rods, I measured the vertical's capacitance as about 50 pF on
160m using an accurate impedance analyzer.
This means that the voltage at the output of any feedpoint amplifier will
depend on the voltage divider relationship that occurs between the
capacitance of the vertical and the input capacitance of the amplifier.
This reduces the maximum voltage output that is available from the vertical
at the amplifier's output. The maximum output is obtained (theoretically)
when the input impedance of the amplifier presents a conjugate match to the
antenna impedance, which, in this case, would be inductive (a very large
inductance, in fact). But this is the same as saying the vertical is now
resonated to the frequency of interest by the amplifier's input inductance.
As a practical matter, the amplifier gain can easily make up any
inefficiency in coupling signal out of a short vertical, subject to the
considerations of noise added by the amplifier, as I discussed in my earlier
post.
73, John W1FV
-----Original Message-----
From: Lee STRAHAN [mailto:k7tjr@msn.com]
Sent: Friday, August 26, 2016 9:14 PM
To: jkaufmann@alum.mit.edu; topband@contesting.com
Cc: Gary@ka1j.com
Subject: active antenna height
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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