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102" whip
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#2
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102" whip
Your tests really do not measure gain perse of the antenna but more the efficiency of the antennae tested. Now that can be calculated into a dB ratio and presented as gain. In reality it is not actually the gain of the antenna. The db expressed was not intended to be the gain (dbd, or dbi) of the antenna. That would reconcile the large variations of readings between the extremes. Radiated power(Pr) from a 1/4 antenna is 18.27* Im^2 = Pr where Im is the magnitude of the antenna current. Gain calulations of an antenna factor in power loss to heat and other factors(Pl). Thus the total power into an antenna is the sum of power radiatied as a TEM wave and those losses which are incurred. While at very small power levels the heat loss in an antenna does apporach zero. There are other losses besides heat. The largest of all is ground loss. Mobile installations can be as poor as 5% efficient. That results in only 5% of the total power input actually being radiated. A very good mobile installation can be as good as 65% efficient. In dBs that would be about 11dB. Lastly,I beleive that you mentioned that the distance separating the two antennea were about 200 feet. A better distance would be 350 feet minimum. This will get you beyond 10 wavelengths separation. In that distance, near field power is a nonfactor and the TEM wave off the antenna is fully formed. The near fields from an antenna are measurable out to about 3 wavelengths from the antenna. It is there that the TEM wave is nearly formed and by ten wavelengths from the antenna is its strongest. Trust me antenna efficiency is far more important than antenna gain in a mobile installation. HIgh ground losses, can make the best mobile antenna look horrible. james OK, so in the real world we are never effected by "antenna efficiency", "ground loss" ect.ect.? The test is as stated. It was never intended to be a lab test. It was intended to show what would happen if this particular test scenario was followed. |
#3
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102" whip
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