Wide Band Tuned Loop

Wide band tuned loop? That sounds like a contradiction of terms and for many years the experts have told us that magnetic or tuned loops have an extremely low radiation resistance of around 0.01 ohms. Next they say the current is uniform around the loop and so we must use large diameter tubing to stop most of the RF being lost in resistance, heat and skin effects. And finally that the loop is very inefficient.

Well I don’t agree with them, and I will now tell you why. Ever since I first used a ferrite transformer to successfully couple a ¼ wave wire loop to a transceiver I have known that the input impedance of a quarter wave loop must be about 22.2 ohms when fed next to the tuning capacitor.
It had to be because the only transformer ratio that would give an SWR of 1 to 1 used a 1.5 to 1 turns ratio and that means 50 ohms down to 22.2 ohms. This can easily be confirmed.

This means that because of the relatively high impedance at this feed point losses from dc resistance in the loop are at a minimum and thick tubing is not necessary.

In order to prove this I have made a 32 foot ¼ wave loop for 40 meters and deliberately used a wire that is normally used for soil warming and has a total DC resistance of 7.5 ohms.

This 32 foot of resistance wire was pulled up into position as a loop (see picture number 1) and I found that it would tune on 7.0 MHz but that the SWR was very high this turned out to be because the 1.5 to 1 turns ratio of the transformer was now looking at a total resistance of 22.2 ohms plus 7.5 ohms = 29.7 ohms.
At first I settled for a 1 to 1 transformer and found that it gave me a 1.4 to 1 SWR this allowed the transceiver to put out 50 watts without the ALC shutting it down.
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The receive was very good with no noticeable loss, so now I needed to test the transmit capability.
To do this I used my computer to connect me to a DX Tuner on the Internet.
This allowed me to monitor my signal from a distance (I did not want to involve other Amateurs at this stage of the game)

The Atlantic DX receiver at Ilfracombe was not being used so I switched on the transceiver and put out a test signal.
I was delighted to hear the tone being received in Ilfracombe and returned to me via the Internet.
This proved to me that my lossy loop was actualy radiating a great deal of the 50 watts that I was putting into it.

Now I needed to check it against a loop that was made with copper wire of low resistance.
I set up this second 32 foot wire loop as far away from the resistance wire loop as I could by hanging it inside the 64 foot 80 meter loop. see picture number 2. I then tuned both 32 foot loops to 7.050 MHz.

Now I could alternate my transmissions between the copper wire loop and the resistance wire loop and listen to my own transmissions on DX tuners when they were not being used by short wave listeners.

Surprisingly there was almost no discernable difference in the signal strength that the remote tuners were receiving.

A few more experiments and a few days later I thought it was time to see how it performed in a live contact test. I tuned both loops to 7.05 MHz and listened around finally a station ON3CYN came up just below and I tuned up and gave him a call on the resistance wire loop.
He came right back and gave me a report of 5 by 8 to 9 and told me he was activating a Windmill number 2925 in Belgium. We talked for some minutes and I left him to call in a few of the others that were calling him. Things had gone so well I completely forgot to try him with the good copper wire loop.

My next opportunity to call a notable station was IO2MET Antarctica special event at 5-15 on the 21-2-2006 on 7.045 MHz I was still using the 7.5 ohm resistance loop and about 75 watts, he gave me the traditional 5-9 and said I was in the log and that his QSL manager was IK2IWU he then went back to others who were calling him.