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LF: <Tech> More on Antenna Loss

To: rsgb_lf_group@blacksheep.org
Subject: LF: <Tech> More on Antenna Loss
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Date: Tue, 30 Oct 2001 16:34:21 +0000
Organization: University of Hertfordshire
Reply-to: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>
Dear LF Group,

Thanks to those to commented on the antenna loss measurements. I added the data from ZL2CA, and also some provided by G3NYK to the graph of results from my own antenna (see attachment). The empirical formulas for the loss resistance of the 3 antennas work out to:
G3NYK (6m high, 30m long inv. L): Rloss = 4733/f^0.86 (for points 
below 600kHz only)
M0BMU(8m high, 40m long inv. L): Rloss = 1550/f^0.75

ZL2CA(Don't know, but must be quite big!): Rloss = 142/f^0.50

So as the antennas get bigger, the loss resistance gets lower and falls off more slowly with increasing frequency. This is reasonable compared with big commercial antennas, which have very low Rloss (a few ohms or less) which rises with increasing frequency.
The fact that loss resistance decreases as the antenna gets higher 
is consistent with the majority of loss occuring in the ground under 
the antenna - for a given antenna voltage, as the antenna gets 
higher, the E field at ground level will get lower, reducing the 
dielectric loss in the ground. This also explains why the (much 
higher) commercial antennas are not as significantly affected by 
this type of loss. It also ties up with G3AQC's "footprint" idea - 
increasing the area covered by top loading will also reduce the 
field at ground level, and result in lower losses.
It seems most people do not use a loading coil to resonate the 
antenna whilst measuring the loss resistance using bridge 
methods; the reasons I did were:
- The loading coil filters out most of the signal from the broadcast 
stations near my QTH - otherwise, these can put a watt or two into 
the unfortunate bridge detector, even though the antenna is not 
resonant.
-Having the antenna resonant means you are measuring just the 
resistance, rather than the resistance riding on a much larger 
capacitive reactance. This means that, for a given amount of 
resistance unbalance, the voltage at the bridge detector will be 
much higher, and so the SNR at the null will be better.
-Likewise, the effect of imperfect balance in the bridge transformer 
is less important when the antenna is resonant and purely 
resistive. This is especially so for errors in the phase between the 
voltages across the two bridge transformer windings, which will 
effectively transform part of the reactance into resistance.
Cheers, Jim Moritz
73 de M0BMU




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