Back in the loop

My main project for at least the last 12 months has been building a solid magnetic loop antenna and its companion automatic loop controller. I’ve been roughly tracking its progress at my magnetic loop antenna project page on this blog.

As usual, life has got in the way, but I want to get back on track and complete the project. To start pumping some RF current through it again, over the weekend I spent a short time playing with the loop on WSPR on 40, 30 & 20m. The tests were too brief but they certainly confirm that the loop is capable of transmitting a signal in spite of the fact the loop is only half a metre above ground and surrounded by metal garden furniture, a steel framed awning and gutters.

I used the WSPR Beacon android app to control my transmitter. There was some discrepancy (tens of Hz) between the actual output frequencies on the app and those shown on WSPRnet. I also found that tuning the loop to each WSPR frequency using the iP30 antenna analyzer was easy and the KX2 gave lower SWR figures.

The brief test became an exercise in understanding theWSPRnet results taking into account propagation and loop orientation which was aligned north-south.

This map view combines all 20 spots of the 1W VK2RH transmissions from grid square QF56oc. The first test was logged at 2017-05-07 01:36 UTC. (I’ve trimmed repeated info from the chart below to improve its fit on the page.)

 05:24 14.097001 -15 1 VK4ALR QG56fk 1151 356
 05:24 14.097016 -26 0 VK4TDI QG62lm 733 14
 04:48 10.140109 -22 0 VK4TDI QG62lm 733 14
 04:48 10.140094 -23 0 VK7TW QE37pc 1057 198
 04:48 10.140091 -17 0 VK6XT OF86td 3086 261
 04:40 10.140095 -27 0 VK7TW QE37pc 1057 198
 04:40 10.140090 -4 0 VK3WE QF32se 547 216
 04:40 10.140090 -22 0 ZL1RS RF64vs 2069 101
 04:40 10.140092 -15 0 VK6XT OF86td 3086 261
 04:40 10.140091 -16 0 ZL3GA RE66ho 2130 126
 03:18 7.040121 -24 0 VK3BAL QF22mc 711 230
 03:18 7.040134 -7 0 VK3AXF QF33fn 516 235
 03:18 7.040135 -18 0 VK4MOB QG62ol 734 16
 03:18 7.040130 -18 0 VK3DXE QF21nv 720 228
 03:18 7.040128 -12 0 VK2TPM QF56of 14 0
 03:18 7.040129 -14 0 VK7DIK QE38cu 918 207
 01:36 7.040183 -16 -1 VK3AXF QF33fn 516 235
 01:36 7.040177 -16 -1 VK2TPM QF56of 14 0
 01:36 7.040184 -24 -1 VK4MOB QG62ol 734 16
 01:36 7.040179 -21 0 VK3DXE QF21nv 720 228

40 metres favoured north-south, while 30 metres was literally an all-rounder and 20 metres was too brief. These results probably say more about propagation than the loop, not to mention the heavy lifting done by all the reporter stations extracting my down to -26 or -27 dB signals from the noise! Impressive all round!

I wonder how many people are using the Sotabeams WSPRlite antenna tester device. Certainly looks tempting, especially for longer term antenna evaluation.

In any case, the main purpose of today’s exercise was to re-start the loop project. The To Do list includes

  • building & installing the SWR bridge into the loop controller,
  • deciding on the best way to couple the stepper motor shaft to the tuning capacitor shaft,
  • and wiring it all together with appropriate coax and control cables.

3 Responses to “Back in the loop”

  • Lowell KM4QEG:

    Re item two on the “To Do.”

    There are dozens of types of nylon and plastic tubing that will do a great job of this. They allow and compensate for shaft mis-alignment, are dilectric by nature, and the stiffer varieties will have minimal backlash.

    They can be softened with heat to allow fitting to different shaft sizes and are easily anchored with small compression rings typically used for gas/vacuum tubing on small engines.

    Lowell Haney
    Rome GA USA

  • Charles Kollett:

    Nice article and very professional construction. I am an aerospace design engineer and this is the kind of quality not seen that often. I am also very interested in loop antennas and have been wanting to build one which will tune 80-10 meters. Curious, did you use a mandrel to bend the aluminum tubing?

    Very best
    Charles Kollett
    Queen Creek, AZ. USA

  • richard g3cwi:

    I use my WSPRlites a lot. But that’s not surprising really! There is a good Facebook group where you can ask questions:

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