Coax Loop Revision

Bill Marsh has been experimenting further with his coax loop for MW and the X-band. Here are the results:


The construction of Loop Number “1” (see below) was successful but left a number of unanswered questions regarding design criteria to be resolved. My 8.8 metre length of coax was resonating above 1.8 MHz and it became apparent with more research that it should resonate close to the highest frequency of interest, in my case 1.7 MHz for optimum performance. This meant a longer length of coax. It must be remembered that this type of loop can only tune down in frequency by adding capacitance to lower the resonant frequency. It is not possible to tune to a higher frequency. I was guided by the formulae used by Kelvin Brayshaw i.e  300/freq in Mhz x 0.084% = metres of coax. The ARRL quote a multiplier of 0.085%. It became obvious that this formulae possibly only works with a single turn loop. In my case I was making a 3 turn loop and at a lower frequency. Other factors were having an effect on the resonant frequency of the loop, were the velocity factor of the coax and the additional inter-turn capacitance. With a 3 turn loop as I lowered the resonant frequency this multiplication factor also reduced to close to 0.062% when I finally reached the ideal length of coax.

The following outlines my steps toward the end result.

I had a piece of 18 mm Heliax which was 13 metres in length. When I coiled this into 3 turns it resonated at 1.7 Mhz. This has become my radio workshop antenna and resulted in the dismantling of my wire antenna strung around the section. My valve radios are now hearing signals not possible on the external wire antenna due to electrical noise pickup. The loop is near my workbench (outside and hidden between garage wall and fence) Very short wires come through the wall to a 3 gang tuning capacitor and a rotary switch to select 1, 2 or all 3 sections of the gang.

I had bought a 15 metre length of freeview type cable to experiment with. When I coiled this into 3 turns it resonated at 1.49 Mhz. I next coiled this 15 metre length of coax into a 5 turn loop and it resonated at 1.12 Mhz. I then decided to space the 5 turns by 1 wire diameter. As the overall diameter of this coax cable is 8 mm I decided to use a length of 9 mm nylon rope as a spacer which is slightly spongy and as a result gives a good uniform spacing between the turns. This moved the resonant frequency up to 1.35 Mhz.

At this stage I decided to abandon the idea of a 5 turn loop in favour of a 3 turn loop. I also concluded it was only necessary to check the resonant frequency of the loop as any loop can be tuned down in frequency by adding capacitance.

My starting point was to go back to my 15 metre length of freeview type cable and progressively reduce the length by 1 metre at a time. In order to keep the braid break central to the loop this meant cutting half a metre from each end of the coax. As the coax was shortened my garden edge support had to be reduced in circumference. With 15 metres the circumference of each turn is 5 metres.

The results of each change are as follows:

15 metres close spaced gave a resonant frequency of 1490 khz,

15 metres 1 x diameter spaced gave a resonant frequency of 1660 khz,

14 metres close spaced gave a resonant frequency of 1530 khz,

14 metres 1 x diameter spaced gave a resonant frequency of 1730 khz,

13 metres close spaced gave a resonant frequency of 1590 khz,

13 metres 1 x diameter spaced gave a resonant frequency of 1800 khz,

12 metres close spaced gave a resonant frequency of 1630 khz,

12 metres 1 x diameter spaced gave a resonant frequency of 1910 khz,

11 metres close spaced gave a resonant frequency of 1750 khz,

11 metres 1 x diameter spaced gave a resonant frequency of 2200 khz,

After 3 changes in length it was apparent that results were following a straight line graph and that frequency could be easily determined.

For my purposes I have decided to make my loop 3 turns using 11 metres of coax close spaced. The reason for choosing close spacing is that I intend to enclose the loop inside 25 mm PVC electrical conduit. 25 mm being chosen for ease of threading the coax into the loop and also for the strength available with this diameter conduit. This loop is approximately 1.16 metres diameter. This loop also appears to be much more sensitive than my loop number “1” probably due to the larger diameter and greater capture area.

Anyone contemplating making a loop along these lines should be aware that not all 3 gang tuning capacitors (ex old valve radios) are equal. The minimum capacitance is the critical factor for these loops and any capacitor chosen must have the lowest capacitance possible when the plates are fully unmeshed. A number of the older variable capacitors had quite high minimum capacitance due to the way they were manufactured. Some of the last variable capacitors manufactured had very low minimum capacitance and also quite high capacitance when fully messed. Those which have the fixed plates supported on ceramic mounts generally have low minimum capacitance, typically 8 pico farads. It is this minimum capacitance that sets the upper frequency limit of the loop. One way around this is to aim for a slightly higher resonant frequency when choosing a length of coax. You should also consider that different types of 75 ohm coax may give different results to those above due to differing capacitance values per lineal metre. It is therefore suggested that you start with a slightly longer piece of coax than needed, and progressively shorten each end equally until you get the desired result. Obviously any permanent supporting frame comes last when the final circumference of the loop is known.

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