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Message-ID: <8DD8B718050F41638ABA7A5AB1C37101@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <AANLkTi=LvH+=FbTmozrN6aK4scH2_p3yoHNB9BF3FMhb@mail.gmail.com>
In-Reply-To: <AANLkTi=LvH+=FbTmozrN6aK4scH2_p3yoHNB9BF3FMhb@mail.gmail.com>
Date: Mon, 17 Jan 2011 21:45:22 -0000
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Subject: LF: Re: LF Antena impedance and matching
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Dear Andy, LF Group,

The idea that any short feeder of only a few electrical degrees has 
virtually no effect on impedance seems to be popular, but it just isn't true 
as you show here. A more realistic statement would be: If the load impedance 
is of a similar magnitude to the feeder Zo, the short feeder will have 
little effect on the impedance. If the load impedance is much higher than 
Zo, the feeder will act as a shunt capacitor (100pF/m for Zo 50ohm), and if 
the load is much lower impedance, it will behave as a series inductance 
(0.25uH/m for Zo = 50ohm).

Since the impedance at the point the feeder connects to the antenna tuner is 
at a fairly high impedance in this case, the 12m of 50ohm feeder behaves 
roughly as a 1200pF shunt capacitor, giving the parallel resonance peak. 
Moving the variometer to the antenna end of the feeder gets rid of a third, 
higher, resonant frequency, approximately where the variometer inductance is 
series resonant with the cable capacitance. But it does nothing to get rid 
of the parallel resonance in your plot, just shifts it a bit in frequency - 
all you are doing is adding to the loading coil inductance by a small, 
variable amount. I think with a high-Q antenna, these multiple resonances 
are unavoidable.

The variometer-in-shack scheme becomes more manageable if the feeder has 
less capacitance. This can be done by having a high Zo feeder - probably the 
best feeder is just 2 wires, separated by a metre or more. This has a high 
Zo, more similar to the off-resonance impedance of the antenna, and a 
capacitance of probably < 10pF/m - the parallel resonance would still be 
there, but would be much further from the wanted series resonant frequency. 
I have used such an arrangement a few times, and it worked rather well - the 
wire between the variometer and loading coil needs to be reasonably well 
insulated and spaced from conducting objects. The wire from TX earth to 
antenna ground can just be laid on the ground.

Cheers, Jim Moritz
73 de M0BMU