VHF magnetic loop antenna

VK3YE is a real experimenter. This is a recent video describing the use of a tin can as a VHF loop antenna.

Several years ago - 2012?

 This was me at my old QTH on 10m CW in the garden with a loop on the FT817. I think it was 2012.

VLF RX loop

This was the RX VLF loop antenna I used for my earth-mode tests when I was fit. It was tuned to the RX frequency that was usually about 8-9kHz. The best orientation was with the loop  on the ground. The pre-amp in the blue box was tuned to the RX frequency.

As I recall, it was about 30 turns of 0.2mm PVC covered wire.

It is still in the back of the garage, I think. It is many years since I did any tests at VLF.

Loop TX antenna

Last year, I used this tiny antenna on many HF bands. 

It is only about 50cm across and sits on the shack windowsill, but worked remarkably well on FT8. 

I must try it again on bands like 17m.

10m FT8 RX

There is widespread Es on 10m this morning. A few moments ago (it is now 0917z) I turned on 10m FT8 RX with the FT817ND and the tiny indoor loop antenna. Already, 49 stations have been spotted.

UPDATE 0948z:  85 stations spotted on RX.

UPDATE 1214z:  198 stations spotted on RX. The furthermost is VO1CH (3737km).

UPDATE 1604z:  325 stations spotted on RX.

UPDATE 1904z:  467 stations spotted on RX with the furthermost PY5BH (9903km).

UPDATE 1954z:  514 stations spotted on RX today.  See map.

UPDATE 2130z:  547 stations spotted today. Soon QRT.

How far down is my loop?

As you may know, I have no external antenna for 10m at present. Instead I use the 50cm loop on the shack windowsill. 

It is extremely hard to measure the difference in performance of, say, a dipole and my loop. Even over quite small distances signal levels can vary greatly. There are many factors: location, time, directivity, polarisation, radiation angle etc. 

The best I could do is compare signal levels at similar times in the best and worst directions of the loop with that received by others locally. The best I can work out is that the loop is up to 6dB down. There are quite wide errors bars on this.

Overall, the loop is performing better than I expected.

If there is F2 DX from the USA or South America later, I shall try again. With lower angle signals the differences might be greater.

G0LRD (26km from me) copies far more, although if he can copy signals that are 6dB weaker, I would expect this to make a considerable difference on FT8.

It will be interesting to see my results later with an external antenna. The impression I get is that the small loop is not too bad on Es signals from Europe, but it performs less well with low angle DX signals.

UPDATE 1915z: Well there's a surprise!  CX2CC (11128km) was coming through just now. I was getting him 5dB S/N better than G0LRD (26km). This may just be the momentary polarisation being optimum. Still it proves the loop cannot be too bad.

UPDATE 2101z:  At the moment I am confused: many times G0LRD is copying better than me and he certainly is spotting far more stations than me. Nonetheless the loop is not at all bad considering its small size and being indoors!

10m FT8 RX on the tiny loop in the shack

For a change I am on 10m FT8 RX with the FT817ND and the tiny indoor loop (50cm diameter) antenna. 

Plenty of Europeans being spotted by Es.  After just a few minutes, 51 stations spotted with the furthermost being 5T5PA (3791km) in Mauritania, West Africa.

UPDATE 1246z:   150 stations spotted.

UPDATE 1515z:   281 stations spotted on 10m FT8 RX today.

UPDATE 1815z:  465 stations spotted so far in 4 continents.

UPDATE 1913z:  524 stations spotted so far today.

UPDATE 2146z:  682 stations spotted today. Furthermost is PP5JD (10072km). I now have to go QRT and disconnect everything.

10m FT8 RX today (Tuesday) with the 10FT8R and the tiny indoor loop

It is now 0855z. I have been on 10m FT8 RX for about 30 minutes and have spotted 11 stations so far using the 4 transistor RX and indoor loop.

UPDATE 1601z  So far today, 98 stations spotted on 10m FT8 RX. See map.

Loops

My loop antenna works better than I expected. I stayed off 10m last year mostly as I did not have a suitable external antenna. 

At the moment I am using an old Whizz Loop on 20m-10m. It is easy to tune and just sits on the shack windowsill. I have a better loop that I have used in the past on 10m (see photo).

My guess is the loop currently used is about 10dB down on a dipole. This is an estimate based on the stations I am not spotting. Nonetheless it works quite well for something so small with just a wire inductor about 50cms across. The "proper" 10m loop has a much thicker made of central heating pipe. It is far sharper, but harder to tune, but probably several dBs better.

What has surprised me is it the Whizz-Loop has even spanned the Atlantic several times with QRP FT8 on 17m.

I hope to get a 10m dipole up outside next week. I could make it cover 6m as well.

See https://sites.google.com/site/g3xbmqrp3/antennas/magloop .

Loops

As you may know I have had a good deal of success with tiny indoor loop antennas. The photo shows the loop for 10m. 

At the moment I have no HF antennas outside, so I do not know how well my loop antennas compare with "real" antennas.   

All I can say is the loops work much better than I expected.

The tiny indoor loop

This is the inside of my loop antenna used successfully on 20m FT8 to 10m FT8. I am amazed how well it works. 

The wire used for the loop is quite small and I might try much thicker wire. Tuning should be sharper, but efficiency should be higher. My concern is higher Q means higher voltage at the high impedance point. 

Indoor loops

One of the great advantages of QRP is in the design of loop antennas for TX. With 100W, the voltage at the high impedance part of the loop can be very high, so that special capacitors are needed with low loss and a very high breakdown voltage. 

With QRP powers, ordinary capacitors may be used. Most times I use an old Wonder Wand Loop for 20m, 17m, 15m, 12m and 10m. My 10m loop, homemade with copper pipe, is sharper and more efficient, but harder to tune.

See https://sites.google.com/site/g3xbmqrp3/antennas/magloop.

Repairing the "better" 10m loop

This afternoon, I tried in vain, to mend my "better" 10m loop antenna. I failed. 

Before, there was a distinct peak when tuned and the SWR dipped significantly on resonance. Sadly, just a few minutes of soldering work left me totally exhausted and giddy. I shall have to try again another day. After about 10 minutes of physical or mental work I am just knackered. Whether this is my stroke or just old age I do not know.

At the moment I am still on 15m FT8 on the "poorer" loop.

IC-705 antennas

As the IC-705 is becoming more widely available, minds are turning to portable antennas. 

From my own limited experience, I can say that magnetic loops work and are small. They are not ideal where frequent changes of frequency are required (if efficient they are very sharp) but they are ideal for modes like FT8 or PSK where most of the time you are within a few kilohertz. 

With high power, often vacuum sealed capacitors are needed. With QRP, ordinary capacitors are fine. Even though the voltages at the high impedance points are high, they are nothing like the kVs seen with higher power.

I suspect loops are down on full-sized antennas, but they work pretty well. Being narrow-band probably helps keep QRN down too. In the absence of external antennas they can be used in the shack. If making the loop yourself use the thickest material you can for the loop inductor: I use central heating copper pipe, but I suspect thick coax would be fine. You don't need much.

I see Southgate News has a piece on a new commercially available loop although it is quite easy to make your own.

See http://www.southgatearc.org/news/2020/october/al-705-magnetic-loop-antenna-for-the-icom-ic705.htm#.X5qZKLuTK00

See also https://sites.google.com/site/g3xbmqrp3/antennas/magloop

Magnetic loops on HF TX

For the last few months I had been unable to get on 10m as my external antenna was down. In the end I rebuilt my loop antenna which I last used on 10m many years ago. This is just taped to a small table in the shack downstairs in our bungalow. I have no idea of the "gain" of this compared with a dipole or beam at a reasonable height.

Where lots of frequency changes are needed they are far from ideal as they are very sharply tuned if efficient. At 100W or more the voltages at the high impedance points can be very high, so special capacitors are needed. At 10W or less the voltages are nothing like as high. For modes like WSPR, FT8 or PSK31 they can be ideal as frequency movements are small.

I have been very pleasantly surprised how well my loop works. It cannot be more than 1 S point down on a dipole. If you are stuck, I can recommend them.

See https://sites.google.com/site/g3xbmqrp3/antennas/magloop

10m loop improvements

Yesterday, I modified my 10m loop to make it easier to tune. The measured capacitance at resonance was just under 40pF. I replaced the 365pF variable with a fixed 27pF in series with a fixed 27pF capacitor in series with a 5-65pF trimmer. Tuning is now much less difficult and the SWR on-tune very good. A quick test was spotted by 3 locals.

That 10m indoor loop (again)

Later today, all being well, I intend to finish the rebuild of my loop for 10m.

I have still to decide on WSPR or FT8. Before doing it, I shall try to (roughly) cut the coax capacitor to close to the correct length, having found out what capacitance I need from the online loop calculator.

My capacitance meter that has not been used for about 7 years! The battery probably needs replacing! I hope it has not leaked.

UPDATE 1035z: The gods are with me!  Some time ago, I must have removed the battery to use it somewhere else. I just put a new battery in and the capacitance bridge is working fine. According to my calculation I need about 43pF with a 0.6cm thickness pipe with a circumference of 2m. I shall recheck the pipe cross-section. It is thin bore copper pipe used in central heating systems. If I cut the length slightly long, I can snip a bit off until it hits resonance. The very most I can run is 20W (most times 500mW WSPR or 2.5W FT8) so I am unlikely to have problems with high voltages. In the past I even used a dead ordinary 365pF air spaced variable without issues in a loop antenna.

UPDATE 1535z: The loop is now finished and operational. In the end I decided on FT8 and used a 365pF airspaced variable to get to frequency. It is very sharp! This is a good sign. It seems a few dB up on the other loop judging by local reports. Using the airspaced variable C means that the loop antenna can be used on other bands. The photo shows the completed loop. It was much harder than I expected. Years ago it would have been trivial.

UPDATE 1749z: Just 2 locals have spotted me so far today on 10m FT8 TX with my 2.5W to the better indoor loop antenna.

More on loops

At our old house, I did some tests with a loop antenna using central heating pipe.

More recently I have been using a Wonder Wand loop that was a gift many years ago. As described yesterday, this loop uses a different way to match the loop and much thinner wire for the loop. Thicker wire results in better efficiency and sharper tuning. As I recall, my loop of some years ago was surprisingly effective.

For 10m FT8 the loop could be brought to resonance with a small length of coax. This needs to be carefully cut to tune to frequency, but once cut, it should not need further adjustment. A short length of coax acts as a capacitor and should be able to cope with the high voltages at the top of a resonant loop.

See https://sites.google.com/site/g3xbmqrp3/antennas/magloop

Indoor loop matching

Normally, loops are matched with a primary winding about 1/5th the size of the main loop. In my case the matching is done using a tapped toroid tuned circuit. This matches the loop on 20m, 17m, 15m, 12m, 10m and 6m.

The wire used in the loop is about 1mm diameter and the loop diameter about 45cm across. It just sits on my shack windowsill. It works far better than I ever expected.

As I mentioned before, if you cannot use an outside antenna and you enjoy modes that don't require lots of retuning, consider a loop!

Another loop

Jay, W5OLF sent me a photo of the loop antenna he has been using on 10m WSPR. The loop is quite small, but the wire diameter quite large, so it should be pretty efficient. For modes like WSPR, FT8 and PSK loops can be a good choice as they are small and narrowband.