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Re: LF: LF Loading coil designs

To: [email protected]
Subject: Re: LF: LF Loading coil designs
From: "James Moritz" <[email protected]>
Date: Mon, 22 Oct 2001 11:57:26 +0000
In-reply-to: <001101c15997$35e4b7c0$c657073e@computer>
Organization: University of Hertfordshire
Reply-to: [email protected]
Sender: <[email protected]>
Dear Martin, LF Group,

For relatively low powers like 100W, a loading coil wound on about 100mm dia PVC tube using about 0.8 or 1mm diameter enamelled wire is fairly satisfactory. It is not the most efficient loading coil possible, but the difference between it and a more elaborate coil with litz wire, etc is probably not going to be more than 1dB or so with this type of antenna, and heating will not be a problem with 100W. The brown PVC main drain pipe sold by DIY shops is OK; I have yet to find any type of plastic pipe with significant loss at LF.

As a very rough guess, you will need about 4 - 500 turns for the G5RV. Put in taps every 10 - 20 turns for one half of the coil, for coarse tuning. A variometer is ideal for fine tuning, but if you want to avoid the work involved, a simple fine tuning method that works quite well is to use a large ferrite core (one from an old SMPSU with the windings stripped off is OK) which can be slid in and out of one end of the loading coil. You can find the resonance point roughly by peaking up a received signal on the band ( or DCF39 on 138.83kHz).

You may well get a reasonable impedance match simply by connecting the loading coil in series with the antenna and TX output, and tuning for resonance. The resistance of the resonant antenna will probably be in the range about 20 to 100 or so ohms, depending on all sorts of factors. To improve the matching, either a seperate transformer wound on a ferrite core can be used (probably better on the whole), or the loading coil can be used as an auto-transformer by grounding the "cold" end and having several taps every few turns from the ground end. The tap can be selected for best matching, although there will be some interaction between tuning and loading adjustments.

The best way to check performance is with an RF ammeter in the antenna down lead - either thermocouple or one of the current transformer/rectifier designs around. I guess about 1-2 A will be quite good going. However, although max antenna current means max radiated signal, it does not neccesarily mean good matching for the TX, so one of the LF SWR bridge designs will help a lot for setting up.

If possible, have the loading coil directly at the bottom of the antenna feeder. Lifting the coil a few feet off the ground will reduce losses. Some kind of shelter to keep the water off is advisable.

Hope this info is helpful - hope to see you soon on 136k!

Cheers, Jim Moritz
73 de M0BMU




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