Message-ID: <3187.200102261523@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Mon, 26 Feb 2001 15:27:32 +0000
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Subject: LF: Antenna measurements/losses/insulators
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Dear LF group,

Re- antenna loss resistance: A basic difficulty with direct 
impedance measurement of an LF antenna is that the loss 
resistance is much smaller than the magnitude of the reactance. 
For example, an antenna with R(loss) = 50ohm, C = 500pF, has a 
capacitive reactance of 2.3405kohm. The magnitude of the 
combined impedance is 2.341kohm, and the phase angle 
88.8degrees, so quite close to that of the pure capacitance. A 
bridge will produce an accurate measurement of the capacitance 
component, but much larger errors will occur in measuring the 
resistive component, which is only a few percent of the total 
impedance. Even a very good bridge will struggle to get anything 
more than a rough figure for the resistance.

A better way to measure loss resistance is to cancel out the 
capacitive reactance of the antenna with a series loading coil (ie. 
tune it to resonance), and then measure the remaining resistance, 
as done by PA0SE.

A seperate measurement is then required to determine the loss 
resistance of the loading coil. This can be done by resonating the 
loading coil with a low-loss capacitor in place of the antenna, and 
measuring the resistance again.

Re: antenna losses - I use my "Scopematch" tuning aid (see LF 
Handbook and errata) to continuously monitor the antenna 
resistance while transmitting.  I also see the resistance go up (and 
the capacitance, to a smaller extent) when it is raining. The change 
occurs practically instantly when the rain starts, and is usually 
about 10 - 20%. I don't think wet insulators can be responsible - 
estimating the additional power loss at a couple of hundred watts in 
my case, they should dry themselves out quite fast!

I don't know why rain has so much effect, but my favorite theory for 
the major cause of loss resistance at the moment is that it is 
caused mainly by dielectric losses in the ground, where the electric 
field of the antenna penetrates to some depth at LF. This is 
contrary to the conventional view that the major losses are due to 
the resistance of the ground system. I don't think there is really a 
contradiction, just that amateur antennas have relatively high 
dielectric losses because they are smaller than conventional LF 
antennas. A bit of thought shows that a predominance of dielectric 
loss would explain lower loss resistance at higher frequency, and 
G3AQC's "footprint" effects, among other things.

Re: Insulators - Although better insulators, ie big glass or ceramic 
ones would help prevent the antenna catching fire or falling down, 
they would not actually stop corona discharge from taking place. 
This is a function of the field gradient around the antenna wires, 
and so needs attention to the conductors more than the insulators, 
hence the usefulness of corona rings. Prevention of corona is also 
a good idea from the QRM point of view.

Cheers, Jim Moritz
73 de M0BMU