Regards

Peter, G3LDO

Many years ago when I did 160m mobile I had a setup in which the mobile
antenna had a loading coil that used a copper disc approx. 150% of the
coil dia. placed with a centre boss that could be adjusted and locked
along the whip with a screw at a point close to the coil. I used to
puzzle about the difference between capacitive or "shorted-turn" effects
but in the end just accepted that it "worked" for whatever reasons. From
memory I think the coil was about 5 inches dia.
73

On Sat, 2011-01-29 at 19:15 -0500, John Andrews wrote:

Peter,

It used to be common MW broadcast practice to use "eddy current disks",
which were aluminum rings. In some cases, the ring was put inside the
coil, with the ability to rotate it in and out of the plane of the
turns. A simpler setup was to have it at one end, mounted on a threaded
rod, allowing it to be moved along the axis of the coil. I'm sure there
were consequences for "Q", but I don't recall any heating issues.

John, W1TAG

On 1/29/2011 6:43 PM, Alan Melia wrote:

Hi Peter I think this is analogous to putting an aluminium or brass slug
into a coil as with the VHF Toko range where there is an optional ferrite of
aluminium.core. I have never measured Qs I suspect it is a bit lossy maybe
that is why a single shorted turn is used for a loading coil.

Alan
G3NYK

----- Original Message -----
From: "Peter Dodd"<[email protected]>
To:<[email protected]>
Sent: Saturday, January 29, 2011 11:10 PM
Subject: LF: LF coil tuning

The traditional method of tuning an LF loading coil is to use a

variometer.

I have been investigating methods of tuning coils for mobile use and one
method that appeared briefly  in one of the ARRL Handbooks of the 50s
was the shorted turn method of tuning.

I tried a home made coil of 136uH on a 5cm diameter former and using a
shorted turn of 2mm insulated wire and obtained a tuning range of 32uH.
(136 - 102uH).

Has anyone out there tried this?

Regards

Peter G3LDO

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