[Lf] Antenna measurements/losses/insulators]

[Andre' Kesteloot]

James Moritz wrote:

> 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