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WA8LMF,?Thanks for the great explanation!!?

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66pacific.com has a mag loop calculator. Looks like a 40M mag loop requires twice the capacitance (121 pF) of a 30M mag loop (59 pF), given the same 20 foot circumference conductor. So, would substituting a 9.06 foot center conductor + dielectric of RG-8 coax make your design resonant on 40M - can you change bands by using a longer center conductor + dielectric of RG-8 coax? Or are other modifications to the antenna necessary?

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Yes...? You can tune the loop to any frequency by varying the capacitor value.??? Just note that as you go down in frequency, the capacitance value,?RF voltage and RF current will increase with the inverse square of the frequency if the loop size remains the same.? At the same time, the efficiency of the loop will decrease as the loop becomes a smaller and smaller percent of the natural wavelength.?

Your calculation sounds about right. My values were determined with the "KI6GD Loop Calculator" which used to have it's own website - "magneticloop.com". The site is long gone, but the calculator has been archived on many other sites including here:
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I have also included the tool?? KI6GD-MagLoopCalc-1.6.exe? in my "Ham Software MegaPak" collection that I sell on 32GB flashdrives at ham fests and radio club meetings.??

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On Wed, Jan 1, 2025 at 06:12 PM, WA8LMF wrote:

WA8LMF, I am going to try building your 30M mag loop antenna. On your website the design is a bit different; the RG-8 center conductor + dielectric is soldered to the copper tubing. It appears the design posted here, the RG-8 center conductor + dielectric is NOT soldered to the copper tubing. I don't understand antenna design well enough to determine how this tuning capacitor is working in each design, would you please explain this aspect of each design? Just want to be sure that I build it correctly. How much capacitance is needed for the design posted here? Thanks!

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A mag loop is a resonant tuned circuit -- the tubing forms a one-turn coil resonated by a capacitor connected across the open gap.? ?? Normally the capacitor is an external one, often a variable one remotely controlled by a motor. A smaller loop, usually about? 1/5th the diameter of the main loop,? forms the primary winding of an RF transformer to couple RF from the transmitter into the main loop. The main loop is the secondary of the transformer.? The construction of a mag loop is complicated by the immense RF voltages (multiple kilovolts) across the capacitor, and the huge RF currents (10s of RF amps) circulating in the loop at anything above QRP power levels.

In my effort to create the absolutely simplest mag loop for a single frequency, I did away with the external capacitor completely.? The coax+dielectric inserted into the tubing forms one plate of a capacitor. ? The inside surface of the tubing itself forms the other plate of the capacitor.? It just so happened that the inside diameter of 3/8th refrigeration tubing was a perfect fit for the center conductor + dielectric of RG-8 coax.? [Don't use foam dielectric coax like LMR400 or RG-214 - the dielectric which is part polyethylene and part air has a different dielectric constant that causes the capacitance per foot to be different.]

Initially, I inserted enough coax+dielectric into only one side of the loop and soldered the end of the coax conductor to the tubing on the opposite side of the gap.? It worked perfectly to resonate the loop on the first try --BUT-- the immense RF voltage that appears across the capacitor? (3-4 KV at 100 watts) exceeded the breakdown voltage of the dielectric.? The dielectric arced over, melted and scorched with a full 100 watts TX power for more than about 20-30 seconds!? ? (The loop worked perfectly for a QRP FT-817 at 5 watts out, but was useless for a full-power 100-watt HF rig.)

My second pass involved inserting coax into BOTH sides of the tubing, essentially creating two capacitors in series (one on each side of the opening), doubling the breakdown voltage. But two caps in series have half the value of a single cap of the same value.? I worked around this by making each cap twice as large by making the coax section on each side twice as long.??

This modified design (documented in the PDF)? worked successfully and has been on-the-air on 30-meters at 100 watts TX for nearly a decade now. ?

A side benefit is that the 2nd-gen loop is more efficient. ? In a small loop like this, the radiation resistance is a tiny fraction of an ohm, yielding an RF current flow of many 10s of RF amps at the 100-watt power level.?? It is a struggle to get the resistance of any joints in the RF conductor path down to the tiny fraction of an ohm required. ?? It's like high-current 12 VDC power wiring - even a tiny fraction of an ohm of resistance in ring terminal crimps, hose clamps, soldered pipe elbows, etc can lose 1/2 or 2/3rds of the transmitted power.? You wind up heating the joints instead of radiating RF!? I cringe when I see the commercial mag loops made from LMR cable with PL-259 plugs crimped to each end. ?

In my design, there ARE NO joints!? Just a single length of copper conductor with the capacitor inside it. ? Note that to prevent RF arc-over the gap between the two ends of the loop needs to be around a half-inch or greater. ?

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East coast folks are a different breed of people as we say on the west coast.?

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I have fully migrated my 60-meters APRS receive-igate-mapping webserver to 40 meters for the indefinite future.?? As you can see on the live UI-webserver live map, I am seeing VERY few stations on 40M except in the later part of the evening.? We seem to be currently having another? relatively-minor geomagnetic storm at the moment.??

I will continue to watch and monitor 40 meters in anticipation of the 30-vs-40 shoot-out on my trip up north on the 8th of Feb.???

I may have a chance sooner for a road trip (around the end of this month) as I drive my sister who lives in Kalamazoo, Mich to the Grand Rapids, MI? air port for an overseas trip. This trip will not take me as far afield as the February one, but it will be a total of about 200 miles in a triangular route -- Lansing-Kalamazoo-Grand Rapids-Lansing.?? All under daylight conditions.?

The 40-meters APRS igate/webserver is at:

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Since this was a dual-mode (AX.25 packet -and- VARA) receive system on 60 meters, it still is on 40.? If you want, you can take a shot at classic AX.25 packet APRS.? Just leave the radio on the same frequency (7083.5 USB) and use a soft TNC like the UZ7HO Soundmodem or DireWolf instead of VARA.? The results from any receive on 40M classic packet APRS will be here on my web server:

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FYI.? im KD2UWR and will be mobile from Fri 1/3 until Feb 2/2 as KD2UWR-7.? I will leave this VARA RX station in NY running.? I typically have great prop into EU on 40M but maybe not many in EU use 40M APRS VARA

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?At least, not on this frequency -- a widely-used European voice channel overlaps it ......

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WA8LMF, oh really? I'm curious about the antenna thing you're talking about. Got any more details? I have another station I could add to 30m for a while.?

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Here are the construction details for my homebrew 30M mag loop that will handle 100 watts RF.?? Small magloops can have 100s of RF amps circulating in the main loop, and thousands of RF volts across the resonating capacitor.??? The most complex problem is making or acquiring a VERY high voltage variable capacitor that can be tuned remotely.? Once you operate on only a single frequency, such as on APRS, this issue goes away, and a loop can be built very easily.???

File??? 30m-20-foot-circ-Magloop.pdf??? attached

(This is of the hundreds of documents in the \Documents folder of the "Ham Software MegaPak" collection that I sell on 32GB flash drives at ham fests and radio club meetings.)

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All this is very impressive. When I first saw these HF aprs threads I thought I might have to replicate one of those Harris NVIS magnetic loops but a Shark stick is so simple and cheap. And in either case a high Q antenna is no issue when you are sitting on a single frequency. I’ll likely order a stick and try to get in on the fun. I’m in E. WA.

Thanks to all of you for the inspiration and Happy New Year.

David, KE7RGP

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The mag loop does have one major advantage.? It's so enormously hi-Q and narrow-banded that it acts as a pre-selector for the transceiver's receiver.? If you want to dedicate one radio to 30 or 40 meters APRS exclusively, and use another HF rig for voice or other digimodes on other HF bands you can do it.??

My experience here with a home-brew 30-meter mag loop (effective bandwidth about 10 KHz) connected to a Yaesu FT-891 is that I can run my other HF radio on any HF band other than 30 meters without the slightest effect on 30M receive.?