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