Removing RF from the Feeder of the G0CWT loop Click Here

A 160 Metre Transmitting and Receiving Thin Wire Magnetic Loop  Capable of  DX

Based on the patent GB 2285712 A

Designed and Patented by Ben Edginton G0CWT.

The thin wire 160 metre G0CWT  Loop that I use is shown in the Sketch.

The dimensions starting at the bottom left of the wire are 30 foot up to the chimney, then 58 foot across to the mast, 12 foot down to the nylon tie and 28 foot back down to the shack roof. a total of 128 foot of wire.The Loop is pulled into shape by the nylon cord from both the chimney and the mast.

The box shown on the roof of the shack contains a variable capacitor and the ferrite transformer which uses three turns for the primary winding to connect to the 50ohm coax feeder, The secondary is two turns and connects between the loop and the capacitor. A description of the ferrite core can be found at http://members.aol.com/benprom If more information is required E-Mail me at -- benprom@aol.com --

The shape of the Loop is not important but will change the direction of the main radiation which is normally away from the capacitor and through the loop center point.

When 128 foot of wire is used in this loop it can be resonated on the 160 meter band and is capable of DX on this band.

On forty metres this 128 foot loop is a full wave loop and by switching out the capacitor and switching in extra turns on the secondary making a total of six turns it becomes an excellent antenna on forty metres and will also work on all the other bands through to ten metres (exept but 80mtrs )although an ATU may be needed on some of the bands to get a one to one SWR

The only exception is 80 metres this is because it is a one half wave length loop and the feed point impedance becomes very high. This makes it necessary to use an additional  matching transformer to bring it down to 50 ohms and this is not very easy to acheive. for this reason I still use the 64 foot loop for 80mtrs .

I have used the 128 foot thin wire loop on all the other bands and find that I can get through a pile up with less than 100 watts and can have a TV switched on in the same room while transmitting without any sign of interference.

I still favour the 64 foot G0CWT Loop for 80 metres it uses a thick wire element made from multi core cable and can be tuned to resonance at 80 metres giving it a better radiation pattern towards the west details can also be found at http://members.aol.com/benprom-it is excellent for DX as well as inter G work.

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Problems caused by  the capcitance between the transformer windings. 

When using the G0CWT Loops on 400watts it was found that some RF feedback along the coaxial cable was experienced .

This was finally traced to the capacitance that was present between the primary and secondary of the coupling transformer.

This was mostly because three transformers had been connected in parallel in order to carry the extra currant.

At first the problem was tackled by using wire that had relatively thick insulation to wind the transformers and although there was some improvement there was still some RF feedback.

After some thought it was decided that it would be better to tackle the problem from a different angle by isolating the coaxial feeder from the transformer by using a broad band Balun as described by W1JR in Ham Radio sept 1978.

The results where more than satisfactory because when the Balun was in place not only did it remove the RF from the coaxial feeder but also improved  the SWR changing because of the transformer capacitive coupling.

The balun was wound onto a large ferrite ring of unknown specifications (black in colour}with  RG58  coax by first taking the coax over the top of the ring and winding six turns through the ring and then taking the coax across the under side of the ring to the point opposite to the start and winding six more turns.

All that was then necessary was to connect one end of the balun to the feeder and the other end to the input of the transformer.

This cured the problems and now I intend to use the balun on all of  my loops.

benprom@aol.com

{c} 12-2-2,000 Ben Edginton G0CWT

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by Ben .Edginton G0CWT  [c]