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September 26th 04, 08:09 PM
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Ed's one meter vertical over a large conductive ground plane has an
effective height of close to 0.5 meters up to something like 20 MHz. With an amplifier at the base of the vertical that has high input impedance and some tailored feedback, one can have a system that can be used to measure field strength with very little frequency dependence. I use a miniaturized version of such an antenna as a probe in a TEM cell. I have seen the use of resistively loaded (short) dipoles connected to resistively loaded transmission lines used by the NBS (as it was then called) to measure field strength with minimum disturbance to the field. These are all receiving antennas with essentially uniform performance over very large frequency spans. My feeling is that to have a small variation in transmitting gain over more than something like five to one will require an adaptive antenna system. (I am assuming antennas with an "average" gain that is close to one - no resistive loading.) 73 Mac N8TT -- J. Mc Laughlin; Michigan U.S.A. Home: "Fractenna" wrote in message ... I regularly use an active 41" monopole to accurately measure electric field strength over the range of 10 kHz to 30 MHz. That's a ratio of 3000:1, and that is 25-year old technology. -- Ed WB6WSN And your variation of gain, excluding mismatch, is...? 73, Chip N1IR The array is RX only, and G/T is important for each individual element, over a broad range. You need a truly compact element that is wideband. Wideband means about the same gains and impedances across a very wide range. Mediocre broadband RX antennas have been around since WWII. The discussion on TX, albeit interesting, does not apply. Also, mismatch produces dramatic signal drops without an ATU in conventional designs, such as inverted V's. The assumption has been 'no ATU' in this project, because the costs are prohibitive. The one mitigating issue is that the sky temperature --and this system is designed to synthesize a measurement sky tempertaure with high angular resolution--increases with lower frequency (below about 1GHz) because the emission is non-thermal. Thus the signal to noise gets better in VHF. 73, Chip N1IR |
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