Hello Mal & group,
Some LF operators are indicating that they are getting large RF amperage
into their loading coil/antennas systems for very low power output to
the antenna ie a few 100 watts.
The current you can push through your antenna system depends on the loss
resistance you encounter. And this is more than just the ground loss. There
is also the loss in the loadingcoil and in many cases also a considerable
loss in the 'environement' ie. objects such as trees and buildings near the
antenna.
Up to a certain level one can reduce the losses, by using a high-Q
loadingcoil and a good groundsystem (radials and earthpoles).
The loss resistance (on 136kHz) can go from a few Ohm (on a metal vessel at
sea) over a few tens Ohm (good conducting ground) to a few hundred Ohm
(poor soil, many obstacles near the antenna).
So this means that for 200W output power the antenna current can go from
10A (ie. on a metal vessel at sea) down to 1A (poor soil, many obstacles).
From what I have read on this reflector the loss resistance for 'amateur
antenna systems' goes from 20 Ohm up to 150 Ohm.
I am trying to figure out what procedure they are using to measure the
rf amps and where all the rf is going because the signals heard on LF
are low to average around the UK and in EU.
Antenna current is only one thing that determines your ERP, the other thing
is the radiation resistance of the antenna. For a 'pure vertical' (no
toploading) the rad. res. on 136kHz (in milli-Ohm) = 0.08 x h^2 (h =
antenna height in m). Toploading can quadruple the rad. res. (in you ave
infinite toploading), in practical cases toploading will double ... triple
the rad. res.
My system is designed for 50 ohms throughout and matched accordingly at
the base of the antenna via a coil with max of 400 microH.
Since the resistance you measure is for 99% (or more) the loss resistance I
would recommend to design the antenna for least resistance and match it to
50 Ohm using a transformer of L-C circuit.
Maybe most are heating up the ground around the antenna, and the loading
coil ,whereas my couple of amps are being radiated by the antenna.
Sorry to have to dissapoint you, but I'm afraid that also you use over 99%
of your power to heat the coil and soil. Even with a really big antenna
(lets' say 40m high and well toploaded) the rad. res. will not exceed a few
hundred milli-Ohms will you loss resistance most likely will be in the
range of several tens of ohms. A wild gues would be a rad. res of 0.3 Ohm
and a loss of 50 Ohm, meaning that only 0.6% of the power is radiated (and
thus 99.4% is heated).
But with the antenna system described here and a 400W TX (= 2.8A
antennacurrent) the ERP would be 4.4W, well exceeding CEPT limits ... to
stick to 1W ERP the TX power should be limited to 90W.
Maybe someone knows the answer.
The above is only my limited knowledge
73, Rik
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