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Re: LF: Antenna Current

To: [email protected]
Subject: Re: LF: Antenna Current
From: "Vernall" <[email protected]>
Date: Fri, 11 May 2001 10:23:25 +1200
References: <[email protected]><[email protected]> <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
Hi all following this theme,

I have thought of a further test to explore phase differences of current
into the cold end of the loading coil and current making it out the hot end.

With two RF ammeters, one bottom and one top, observe if current peaking
tracks near resonance or whether peaks are a little apart, as the antenna is
tuned across resonance (either by variometer or frequency change of the

If the bottom current is comprised of I and Q components, then the matching
will not be purely resistive SWR when the current out the top is peaked.

I currently have a lack of facilities to do tests with two ammeters on my LF
antenna, so I apologise for putting up suggestions for possible tests but
not substantiating with practical measurements of my own.

With the monitoring I do have, which is a current transformer partly up the
loading coil (I have four series inductors to make up my loading), an L
match at the "cold" end to give close to 50 ohms, and a good SWR meter in
the 50 ohm line from the transmitter, I can say that I never get maximum RF
current concurring with minimum SWR, but they are offset a little.  This
observation supports the supposition that when the radiating current
(current out the top of the loading coil) is peaked, there is also a
reactive current going in to the bottom of the coil to "feed" stray
capacitance to ground, so the impedance "seen" at the cold end can not be
purely resistive.  If the variometer is adjusted for minimum SWR, what that
is doing is providing a small amount of inductive reactance to make the
input appear resistive (and 50 ohms because of my L match) but doing this
actually reduces the radiating current by a small amount.  So these
observations support the supposition that a current I+Q goes in the cold end
of the loading coil, and I comes out the hot end.

Another point is where to put an RF ammeter to use for tuning for maximum
radiation:  it would seem to be far better in the "up wire" at the "hot" end
of the loading coil, as radiating current is what facilitates QSOs.

73, Bob

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