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LF: Re: The case of the missing amps

To: [email protected]
Subject: LF: Re: The case of the missing amps
From: "Vernall" <[email protected]>
Date: Thu, 17 May 2001 09:58:07 +1200
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
To all,
Thanks to Dick PA0SE for doing further tests.  Replacing the antenna with a increased capacitance from a vacuum variable has lower losses than the "antenna circuit" but it is a reasonable approximation for gathering measured data.
My earlier bulletins assumed that the current going in to the cold end of the coil could be considered as I+Q components.  I now think this is an over-simplification, as it is not the case that a myriad of stray capacitance to ground can be replaced by a single shunt capacitor at the feedpoint (or anywhere else), and then assume a perfect inductor.  The real situation involves distributed capacitance, along the coil.  I'm no longer keen on an I+Q current model, as if all stray capacitance was a single capacitor at the feedpoint.
Test data appears to support that there is only one system resonance, and the input impedance is resistive at this resonant point.  This is reasonably consistent with the expected performance of a series resonant circuit (traditional theory of LF and MF vertical antennas).  The distributed capacitance lowers the resonant frequency compared to a perfect inductor.  
If any simple model is to be considered, it could be that the RF current divides between the capacitance of the antenna system proper (current going in to the antenna wire) and the net stray capacitance of the coil to ground.  PA0SE found this apportionment to be about 95/5.  Data from a ZL advised privately to me, for a loading coil in a metal box, shows a ratio of close to 90/10, and this is likely because the metal box definitely has higher shunt capacitance, but also likely to be low loss "flux catching".
A practical result is that it is generally safest to place an RF ammeter in series with the cold end of the loading coil, and maximum current (at loading coil resonance) does concur with maximum radiation.  The trap is to claim that this indicated current is the antenna current, but as recent bulletins have underscored, the current in the antenna wire (from the coil hot side) is less, by some 5 to 10%.
73, Bob ZL2CA 
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