That was a really interesting idea, thanks. I can occasionally get a contact through the speakers, but usually not (it seems random, one time it will connect, and the next time not). Very occasionally the sound will cut off for part of a sequence and then continue again. I expect there is still some problem with the sound subsystem in the Ubuntu/WINE installation.

The tones sound identical from each computer. Is there anyone who has successfully gotten VARA FM to work using Ubuntu 24.04 and WINE? Thanks, Ron

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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 The site is long gone, but the calculator has been archived on many other sites including here: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. ?

I did misspeak, the waterfall is on VARA HF (which is working fine). The fact remains, I can hear my transmission and the other station's response on a separate HT when running Winlink and VARA FM on the Windows PC. When using the Ubuntu laptop I hear my transmission but the other station simply never responds. Ron

I can hear my transmission and the other station's response on a separate HT when running Winlink on the Windows PC. When using the Ubuntu laptop I hear my transmission but the other station simply never responds. Ron

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Nice, you could prob go down to 2 min intervals. I do this on my mobile with smart beaconing with varadroid by Mike na7q. Works awesome with the trusdx 4 watts 40m!?

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NO!!!?? You can't do that kind of rate on HF!????

1)?? The transmission time is much longer on VARA than on the usual 1200-baud 2M VHF.? A simple beacon, especially if it is not in a compressed format like Mic-E,? can take 20-30 seconds.?? At a two-minute repetition rate, you are occupying 1/5 th to 1/4 of the total capacity of the channel.?

Note that on a random-access channel with no coordinated timing of the users, that if the channel is occupied more than 18-20% of the time, the probability of data collisions between stations goes up sharply. (The so-called Aloha" limit.)

2)?? On two meters, you are occupying the channel over a city or county-sized area. Many users around even a single state can be using the channel at the same time.??

One HF, you are occupying the channel over ONE-THIRD of North America or more.

On HF, a reasonable beacon rate is around 15-20 minutes for a mobile, if the channel is being used with multiple users.

I am running Winlink Express and VARA FM using Wine 9.0 on Ubuntu 24.04. VARA FM shows signals on the waterfall, and triggers PTT on my Yaesu FTM-400 using either a SignalLink (using VOX), or a Digirig Mobile (using a COM port). Since the SignalLink performs PTT using VOX I know there is audio coming in/out to the radio (there is 40W coming out but I have no easy way of checking the FM modulation.)

The problem is that no Winlink station responds to my transmission. If I switch the Linux laptop for a Windows PC then a local station immediately responds using VARA FM, so the problem is on the Ubuntu laptop, not with the cables, Signalink, or Digirig. FYI, VARA HF works fine on the Linux laptop using the USB cable from an ICOM 7300.

Any direction on where to start is greatly appreciated. Ron/K2RAS

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