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Re: LF: Re: Series loading coil

from [Dick Rollema] [Permanent Link][Original]
To: [email protected]
Subject: Re: LF: Re: Series loading coil
From: "Dick Rollema" <[email protected]>
Date: Sun, 01 Dec 2002 13:52:33 +0100
Reply-to: [email protected]
Sender: <[email protected]>
Sri I made a mistake in the equation for L1;

It should be L1 = L2 + L3 + 2k * square root L2 * L3

To M0WYE and All from PA0SE

Hugh, M0WYE wrote:

I've wound myself a coil on a bucket, the inductance is a bit low at the moment, the whole thing resonates no lower than 250kHz at present - so I've some work still to do before I transmit. I put lots of taps in, as suggested.
 
But I am puzzled. In the LF Experimenters Handbook, at the end of the description of your loading coil, it says
"by short-circuiting turns at the top and bottom end, inductance can be decreased in steps of one turn." Why short-circuit the unused turns? Surely that is like having short-circuited turns on a transformer and will make the coil very lossy ?
Why not just connect the vertical part of the antenna to the appropriate tap, and leave the others open ?

This subject has been discussed on the reflector several times before.
For coils to be used on the HF bands, for instance in the pi-network of a valved power amplifier or an aerial matching unit, it is usual to short circuit the unused part of the coil. If it is was left open the unused part, together with its distributed capacitance, could be come resonant  on the working frequency and absorb a large part of the power.
The short circuited part is only loosely coupled to the used part so there is only a small current flowing in it and this results in negligible power to be lost in the resistance of the short circuited turns.

That the coupling is loose can be easily shown on paper.
Using one of the formulas for the inductance of a single layer coil calculate:

a. Inductance L1 of the total coil
b. Inductance L2 of the used part of the coil
c. Inductance L3 of the part to be short circuited.

Now L1 = L2 + L3 + 2k * square root(L2 * L3).
k is the coupling factor; k is between 0 and 1.
You will find that k << 1. 

It is a different story for coils wound on a toroidal core of ferrite or powdered iron. k can then be almost one and the unused turns should NOT be short circuited, though this is often seen in designs for aerial tuners.

For a loading/matching coil for the 136kHz band the chance of self-resonance in the unused part is remote.
But when the unused turns are left open the voltage at the top end of the coil is higher that when they are short circuited.
That is why I short circuit them.
I found the aerial current being the same with the unused part open or short circuited. So the loss is negligible.

73, Dick, PA0SE

]
----- Original Message -----
From: Dick Rollema
To: LF-Group
Sent: Tuesday, November 05, 2002 10:18 AM
Subject: LF: Series loading coil

To All from PA0SE

I also use the series coil system for tuning and matching the aerial to the 136kHz transmitter.
The coil was made by bolting four 1m long and 12.5cm diameter PVC pipes together and putting on 200 turns of 1.4mm diameter copper wire with black PVC insulation as used for house wiring.
Inductance is 4.4mH and unloaded Q  about 350.
Starting from the top I made a tap at every 10 turns by putting a twist in the wire. Heating with a soldering iron melts the insulation that then easily comes off to  bare the wire.
The bottom 10 turns have a tap at each turn. By short circuiting turns from the top and the bottom the coil can be adjusted in steps of one turn. For fine tuning a variable capacitor is in parallel with the coil. As this increases the current in the coil and thus the loss in it (proportional to current squared!) I adjust the coil so that only minimal capacitance is needed.
The capacitor shown started to arc over at 60W of power. It was replaced by a vacuum capacitor rated at 5000V maximum.  When transmitter power was increased  from 120W to 320W I connected the capacitor over half the coil in order not to exceed  the maximum voltage.

The transmitter is connected to one of the turns at the bottom end. I don't worry about SWR and simply select the tap giving maximum aerial current (3A in dry weather).

A better picture of the coil above can be found on page 62 of G3LDO's The low frequency experimenter's handbook. The coil with the vacuum capacitor is shown on the cover of the book.

73, Dick, PA0SE
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