Hello Alan, Stefan,
in my mail I just wanted to point out that maximum current at the TX output (one end of the coax cable) not coincidences with maximum current at the antenna (other end of the coax cable).
So tuning (adjusting the loading coil) for maximum current at the TX will not result in maximum ERP.
I am aware that a transmission line can transform the antenna impedance, but at first I was suprised that it happend so significant with a short (compared to the wavelength) cable.
This evening I did some simulations using SIMetrix (http://www.simetrix.co.uk/) and this confirmed more or less my measurements.
For clearity: at this moment the Rloss (+ Rrad) is about 60 Ohm and I did not care to match it to 50 Ohm. The only tuning was done with the loading coil (cancel out any reactive part of the antenna impedance).
73, Rik ON7YD - OR7T
Rik, Stefan, I follow your points but there would seem to be something strange in Rik's antenna or his appreciation of what/how its working.
Contrary to an earlier assertion, the antenna current flowing through Rrad generates the radiated power, so lower current, if Rrad doesnt change, MUST mean lower tranmitted power. It matters not that the SWR is unity. A unity
SWR indication at the transmitter says that the impedance at the end of the coax is 50ohms, but it does not necessarily indicate the the the antenna is resonant i.e the inherent capacitance of a short antenna totally compensated by the loading inductance.
If we assume that Rrad+Rloss does not change across the band the matching transformer should transform the value of Rrad+Rloss to 50 ohms across all the band. The coax will now be matched and will present 50ohms at the shack end. If this does not happen I
would be inclined to suspect currents induced in the coax braid. SWR indicators can easily be confused by induced braid currents......could this be the cause of Riks strange effects? This might be a function of having a "better" ground in the shack than at
the base of the antenna.
Alan
G3NYK
----- Original Message -----
Sent: Friday, November 08, 2013 6:53 PM
Subject: Re: LF: remote antenna tuning
Oh yes, or, a phase meter (M0BMU) and a battery operated variometer motor. Without an additional cable to the shack. Then the SWR meter in the shack confirms that everything is fine.
73, Stefan
Am 08.11.2013 19:40, schrieb pat:
Hi All,
A suggestion: battery operated tuning device at antenna end and "wireless" link back to shack. Wireless can operate at a non-interfering frequncy (HF/VHF/UHF/Optical).
73
On 08/11/13 16:09, Stefan Schäfer wrote:
Hi Rik,
Am 08.11.2013 16:06, schrieb Rik Strobbe:
Hi Stefan,
remote tuning of the loading should be done by optimizing SWR (at the TX), not for maximum RF current.
If one keeps that in mind there is no problem.
...which is in agreement with what i said.
But once you have matched your antenna to 50 Ohm on resonance and you can only vary the reactive part of the antennas impedance (after doing QSY), then you will get the maximum antenna current at best SWR (assuming that Rrad+Rloss is constant which is not to
far from reality).
I am not sure if I will keep the remote variometer tuning. It needs some extra wires to the loading coil and I am not sure how long the small DC motor will function properly with all the rain and frost coming up.
Here it holds since more than 2 years for LF now..
73, Stefan
--
73 de pat g4gvw
es gd dx
qth nr Felixstowe
East Coast UK
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