Magnetic loops for HF pedestrian mobile - Part 1

A look at two 1 metre diameter magnetic loop antennas for 40 metres pedestrian mobile.




FIRST LOOP

The first loop uses RG213 braid for both main and transmitter loops (encased in flexible ribbed tubing) and a large variable capacitor. Important values: main loop - 3 metres RG213, small loop 60 cm RG213, tuning capacitor up to 150 or 200pF. With these values the loop should cover at least 7 to 18 MHz, with 21 and 24 MHz available if you're lucky.

The loop tunes sharply and works well but was found too heavy for serious pedestrian mobile operating. It's also bulky to carry - it can't be folded down. Not as good as a loop made from solid copper but it's a good compromise and contacts have been made with it indoors.

SECOND LOOP

The second loop is made from 3 core extension cord (wires soldered together) with similar dimensions to above loop. It's also mounted on a broom stick but has bamboo rods to steady the lighter cord. A 5 - 50pF beehive trimmer capacitor is used to tune it to frequency. 100pF is wired in parallel for 40 metres - leave it off if you want 20 metres. Usable bandwidth approx 50-80 kHz - wide for a magnetic loop due to its low Q and efficiency.




The second loop (in particular) breaks all the rules of sound magnetic loop design. However it meets its design objective of being a lightweight pedestrian mobile antenna superior to the small whip used previously. This exercise shows that even a greatly compromised magnetic loop will beat a small whip for HF pedestrian mobile work.

On-air tests show that the second loop is sufficient to produce interstate contacts on 40m. A future version will be electrically the same as this but foldable.

MEASURED TESTS (loops used indoors compared to full-sized outdoor antenna - WSPR to VK7DIK)

First loop: -17 to 20dB
Second loop: -26 to 28dB

Part 2 follows.

Category:

Science & Technology

Tags:

• amateur
• ham
• radio
• QRP
• FT-817
• magnetic
• loop
• antenna
• portable
• pedestrian
• mobile

[oimmuk]

I have been looking at small loops the one thing that I am fuzzy on is the feed loop, I cannot find a good example to show how to make it.

[vk3ye]

@oimmuk Have another look - particularly the bit from 2:20. The feed loop is just that - 1/5 the size of the main loop wired straight to the feedline.

[TheHamNut]

where can we get more detailed plans for this antenna? And also is there a formula so one could build one for 20 meters? Keep the videos coming you do a great job. Hope to get some videos up soon on my channel. Good Luck, Good DX & 73'

[vk3ye]

@TheHamNut More detailed info has been added to the description including values for 20 metres. Glad you enjoyed the videos and look forward to seeing yours.

[9w2pju]

hi, can i have the diagram ?

[VictoreCelia]

nice job

[alexgrim2000]

Great work,

Continue your expiriences.

Small magnetic loop antennas are underestimated and you will be amazed how far it can send your QRP signals.

I am working on SML antenas for more then 10 years, and have designed circa 40 diferent models.

73s.

PY1AHD ALEX