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:[email protected]] Sent: Friday, August 26, 2016 9:14 PM To: [email protected]; [email protected] Cc: [email protected] 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 _________________ Topband Reflector Archives - http://www.contesting.com/_topband
