PRACTICAL DETAILS OF THE
The G0CWT Loop differs from other magnetic loops in the way that it is fed with RF power, it uses a ferromagnetic transformer instead of the usual faraday shielded coupling loop or gamma matching device.
It consists of 64ft or 19.5 metres of wire and a cane or fibre glass spreader which is pulled up to the top of a mast or other support to form a loop,
The two sides are tied with nylon ropes to hold them apart, or to another spreader, whichever is the most convenient.
The ends of this loop are now connected to the tuning and matching assembly which consists of a wide spaced capacitor of about 300 pf in full mesh and the all important ferrite matching and loading transformer.
This transformer is wound through two ferrite tubes with nine turns for the primary winding tapped at three turns, and two turns for the secondary winding the ends of which are coupled one to the loop and the other to the capacitor the other end of the loop is taken to the capacitor
There is a choice of tuning arrangements and if it is possible to bring the ends of the loop to the shack wall or window then fibre glass rods can be used to extend the controls and allow for switching and tuning inside the shack.
If care is taken it is also possible to bring the ends of the wire loop just inside the shack.
The better method is to use a geared motor to turn the capacitor and a relay to switch from the 80 to 160 mtrs band.
Another possibility is that you may be happy to use this antenna dedicated to just a part of one band and then it can be tuned up and left in that position.
As it is not a high Q loop the bandwidth is in the region of 40 Kcs on 80 mtrs and 14 kcs on 160mtrs .
The erection of the wire loop itself is mainly common sense and its shape will be dictated by your QTH and whether you use poles, trees or walls and buildings as the supports for your aerials.
Keep the aperture of the loop as large as possible although it will work even if you are limited to a rectangle.
The bottom of the loop can be about two to four metres above the ground and the top up to ten mtrs above the ground the sides will be around five metres long.
In my case a lot of the wire loop runs through a tree and the loop is not at all square going to a point at the top and about six meters across the base at three mtrs above the ground.
The tuning and matching unit is placed at one corner on the east side and although the antenna has very good N.V.I.S. properties for inter G work the position of the Capacitor and Transformer help the loop to radiate best in a westward direction for DX purposes.
TRANSFORMER, SPECIFICATIONS For The G0CWT 64
The transformer used with the prototype is wound on two ferrite tubes placed side by side as shown .
The ferrite tube dimensions are 28mm long, 17 mm outside diameter, and nine mm inside diameter.
The primary winding consists of nine turns tapped at three turns and the secondary is two turns
You may have asked yourself why go to all this trouble when a measured length of wire will will make a good antenna
That might well be the case on 10 Mtrs to 40 Mtrs where the size and height are not too difficult to achieve but on 80 and 160 most Amateurs have to compromise unless they live on a farm with 20 mtr high trees.
However, the wire magnetic Loop makes it possible to achieve top performance from a postage stamp garden and there are no radials or ground plains to worry about.
To get the power into a loop when using a wire instead of metal tubing a gamma matching device or Faraday coupling coil can not be used as there is no way to attached them to a wire loop satisfactory, I tried but with little success.
the ferrite transformer turned out to be the solution and worked better than any other method allowing the loop to compete with a full size dipole and remain stable in all conditions.
Using a transformer to load the Loop also makes it possible to place the transformer in the same container as the tuning capacitor to make a tuning and loading unit.
The fact that after testing this loop I decided to file a patent on it should tell you how well it works.
The wire that I use is old computer cable or coax both work well although I have also used thin flexible wire for portable use and had very good results.
The variable capacitor can be of any reasonably wide spaced type and due to the circumference of the loop the voltages across it are not as high as would be found on the high Q loops of tubular construction.
As the drawings show it can be constructed in many ways and it is very stable in use even in high and blustery winds.
Although when constructed with 19.5 mtrs of wire it is basically an 80 mtr and 160 mtr aerial, with the aid of an A.T.U. it can become quite useful on other bands.
There are two main ways to tune up the loop depending on whether you are using a remote control unit or have to tune manually away from the Transmitter.
If you have fitted the remote tuning it is only required to switch the transformer to the band that you are interested in and then tune the capacitor for the strongest signal.
Then transmit on low power and tune for the best standing wave ratio reading, an SWR of one to one can be expected with this aerial.
If you have not fitted remote tuning then put an SWR meter in line where it can be seen from the loop tuning controls.
Switch to the band that you require and then feed in a little R.F. from the transmitter and tune the capacitor for the best SWR.
A third way that tuning can be done at the loop is by fixing a neon bulb on a short length of wire close to the loop input, then as you tune it will increase in brightness as the loop comes into resonance.
I Hope this Article will encourage you and other Amateurs to construct and use the G0CWT Loop or as my grandson calls it,the Magic Loop. it certainly lives up to this name on occasions.
Some of the best contacts I have achieved so far have been to America, New Zealand and Aruba in the Caribbean sea on 80 mtrs and to North America on 160 Mtrs
Since first writing this I have received a letter from G0CFQ with a report of the stations he has worked using a 64 foot loop in his garden and a 32 foot loop in his loft and he has proved beyond any doubt that this antenna is capable of serious DX , by working stations in Africa ,Asia, N America, S America, Chatham island, New Zealand, Brazil, South, Shetlands, Venezuela, Argentina, Barbados, Jamaica, to mention only a few and he uses a maximum of 100 watts on 80 metres with a G0CWT 64 foot loop in his garden and a 32 foot 40metre version in his loft.
All the above antennas are based on the Patents No 9400764.8 and GB2285712 filed by me on the 17-01-1994.
B.Edginton G0CWT Email firstname.lastname@example.org [c] 2001
© Ben Edginton 2012 Last Updated:May 2012