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.

8 responses to “Coax Loop Revision

  1. Am intrigued about the bending of 25mm conduit into a 1.16m dia without snapping it – did you have to do anything crafty to get that happen? I’ve already had an expensive flop by trying plastic bendy garden edging – too heavy, too floppy!

    • Aha! Electricians use bending springs for that purpose, or a gas bottle burner to gently soften the pipe as you curve it. Takes a bit of practice! Paul

    • Hi Jonathan. There should be a picture of my loop number 2 on the website. I thought I was going to be able to bend the 25mm conduit but no longer have a suitable bending spring. It is possible to bend to this radius but conduit needs to be warm and you need to use a neat fitting bending spring with a pull-out wire attached. Probably easier to make it square. Bunnings have the cheapest conduit fittings. I resorted to making it square and used 25mm inspection elbows at the four corners. Bit tight getting the 3 turns of coax around the elbow but used a draw wire. I am in the process of making up a 1.8 m circular loop using 4 turns of copper wire. I have cut curved sections out of a sheet of 7mm ply to make a frame. 4 quarters to make the circle and another 4 quarters to overlap the joins making the ply 14 mm thick. the whole thing used glued and screwed. The rib of ply is about 70mm deep and my garden edging goes around that. The turns are then wound on to the garden edging. A little heavier than I thought but very sturdy. Will need a reasonable pole to support it outdoors. Will be mounted just above the house spouting line. There will be an article in due course on our website.

  2. Hi Bill,

    I recommend using polyethylene irrigation pipe rather than PVC pipe. PVC will probably lower the Q of the loop.

    • Hi Andy, Electrical PVC is fine. The rigid stuff plumbers use may be a different thing. I have RF-tested electrical conduit by putting a short length in a microwave (with a cup of water) on high for two minutes and it was barely warm. The rigid plumbing pipe/pressure pipe has metal particles to reinforce it so my guess is that it would fail the microwave test. Have used PVC on the lower section of aluminium ground mounted transmit verticals to avoid nasty shocks and the aerials have both performed amazingly well (was pushing 500W into them on occasion). Cheers, Paul

      • Check out

        The loss tangent (tan δ) of PVC is 2 orders of magnitude worse than nice-to-rf plastics like PTFE and PE.

        This is why quality RF connectors and coax use PTFE rather than cheaper PVC.

        This is why CAT6 cable has Polyolefin coating on the wires rather than PVC (and a PE spacer inside keeping each pair apart).

        (And why polystyrene capacitors are the bee’s knees for preamps!)

        Encasing your loop windings inside a PVC pipe is going to damage the Q as is using PVC coated “hookup” wire or mains cable.

        See also:

        • Hi again Andy
          Thank you for your valuable research and contribution. I have abandoned the idea of using wire inside the conduit frame. Although much more sensitive to radio signals than coax, it is much noisier (as expected) picking up all the electrical and other noises in a town environment. I have gone back to the original idea of using quality grade coax inside the conduit in order to reduce or eliminate the noise component. To this end the coax loop is very successful allowing me to hear signals not possible before from my townhouse location. I am reasonably confident that the performance of the 3 turn coax loop coiled in air has not been compromised by enclosing it in electrical conduit. Unfortunately I can not do a side by side comparison unless I make another loop. I can perhaps understand your concerns if I were building an HF loop, but this loop design is for the broadcast band i.e. 0.52 – 1.701 Mhz. There is always scope for improvement but this loop serves my purpose at this point in time. I contributed this article on the basis that others in my position might also find that this design would work for them also. Regards Bill

    • Hi Andy

      Thanks for that. Had already started construction as I had a few electrical conduit fittings left over from my electrical days. Another reason for using electrical as you can get inspection tees etc which make it handy for pulling conductor around loop pipe. I also want to go back to pulling in 7 or 8 turns of conduit wire before pulling in the coax. One more attempt at a conventional type loop. Because the conduit wire is enclosed in conduit it will be shielded from wind and should maintain it’s inductance in all weather conditions. This is just to compare sensitivity and electrical noise levels.

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