Finbar EI0CF's experimental 160m magnetic loop antenna

Finbar EI0CF has been experimenting with a 160m magnetic loop antenna in his garage. He has been using powers of between 700mW and 10W and has been amazed at the results. He has had WSPR reports from several stations in North America as well as plenty of others in other directions.

For fixed frequency operation (these loops are very sharply tuned if working efficiently) these antennas can be remarkably effective.  With high powers, the RF voltages at the high impedance points can be very high (less of an issue with QRP) which is one reason people use capacitors rated for very high voltages in some cases.

It is some years since I used a loop on 10m WSPR, but recall being amazed how well these small antennas worked. The photo shows Finbar's experimental 160m loop.

472kHz

People often avoid this band thinking they will need big antennas and extensive ground systems. Whilst these would certainly help, they are not necessary to achieve useful results, especially with WSPR.

With an ERP of less than 10mW from my earth-electrode "antenna" in the ground I get spotted in Norway most nights on WSPR.

I use my FT817ND and my transverter.  The output from the transverter into 50 ohms is about 12W, but the measured ERP is only about 10mW (i.e. the "antenna" gain is about -32dB). A "proper" antenna and ground could be several dB more efficient. Most nights on 472kHz WSPR I get over 20 stations spotting me and on the better nights 30 stations! Neither my XYL or the neigbours are aware it is even being there!

This "antenna" also works pretty well on 160m. On 472kHz and 1840kHz it is an almost perfect match with a small 3C90 step-up transformer. At the old QTH I did not need this at all. I suspect it would be quite good on 137kHz TX, although I have not (yet) tried it on this band.

My theory is on LF and MF bands it works as a sort of loop in the ground. If this is correct, the loop would be bigger as you move down in frequency. It could even be quite effective at VLF.

WSPR does not really allow chats, but works with very weak signals. It is ideal for me on this band. From the old QTH I have had JT9 472kHz QSOs with stations in the UK and Belgium with a similar set-up at my QTH.

Magnetic Loops

Some years ago, and again recently, I have been using a magnetic loop antenna. These are narrow band and sharp tuning, but can be remarkably effective.

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

Magnetic loops

It is some years since I built and used magnetic loop antennas on HF. They work remarkably well, although they are narrow band and need frequent retuning unless you are using modes like FT8, WSPR or PSK31 where the activity is all concentrated in one place. I recall using a small wire loop in the bedroom and getting WSPR spots from Japan with 20m QRP.

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

I see Southgate News has a link to a video on magnetic loops.

See http://southgatearc.org/news/2018/december/magnetic-loop-antenna-current-and-voltage-distribution.htm#.XApJVvZ2u00

Magnetic loops

With high power transmitters, magnetic loop TX antennas often require special capacitors for tuning as the voltages can be high if these antennas are efficient. With QRP this is not the case and great success can be had with "ordinary" parts.

I have been amazed in the past how well they work, especially considering how small they can be. The secret is to use thick wire, ideally copper pipe or coax for the coil. For modes like WSPR, FT8 and PSK31 where big changes in frequency are not needed, these can be perfect.

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

HF Loops

This has been on before, but it is worth repeating.

HF loops are ideal in some applications. They can work surprisingly well, but are narrow band if efficient. They are ideal with QRP as quite high voltages can otherwise be created. I have used these on several bands. They can easily be made too.

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

Magnetic Loops

A few years ago I built a magnetic loop TX antenna. With QRP powers, the very high voltages that exist if the loop is efficient are less problematic. The key to efficiency is low loss. For example, the loop should be made of thick copper pipe or coax, if possible. The capacitors used to resonate the loop should also be low loss. Having said this, I have had good results with normal air spaced variable capacitors and trimmed coax as capacitors. Of course, magnetic loops tend to be very sharply tuned, but for things like WSPR, JT65 and PSK31 where frequency changes are small they are fine. In fact they work remarkably well even indoors. They can be small too as the photo shows. This loop was tuned to 10m WSPR but the same sized loop was used on 20m where I was spotted in Japan - from the back of a chair in the bedroom!

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

Loop on the ground antenna

See The Loop on Ground Antenna - the "LoG".

I was sent this link by a blog reader who has used it very successfully. It is primarily designed as a receive antenna.

Magnetic Loops on HF

10m band loop

A magnetic loop can be a very effective HF antenna, especially when the very sharp tuning is not an issue. They can be very efficient but there is a trade between bandwidth and efficiency. They are ideal for modes that do not need frequent retuning such as PSK31, JT65 or WSPR. Ideally the inductor should be made of copper pipe or thick coax and the capacitors need to be low loss and high voltage types. Tuning is usually very sharp. Having said all this. quite decent results have been obtained with loops made of quite thin wire.

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

Loop motors

Magnetic loop antennas can be very effective, small, antennas. They work well and are ideal for modes like WSPR, JT65, JT9-1 and PSK31 where frequencies don't have to be moved much. Their main disadvantage is they are narrow-band so require retuning if moving very far in frequency and they are directional.

As ever, Steve G1KQH has been tracking down bargains - in this case motors that could be used to either tune or rotate loops.

Tune up your loop remotely, only needs a coupler and a bit of ingenuity which I know your not short of?

http://www.banggood.com/N20-DC12V-100RPM-Gear-Motor-High-Torque-Electric-Gear-Box-Motor-p-972986.html

They also have 6 volt too

http://www.banggood.com/N20-DC6V-100RPM-Gear-Motor-Miniature-High-Torque-Gear-Box-Motor-p-972984.html

Just needs a simple adjustable Voltage regulator.

20RPM model on eBay

http://www.ebay.co.uk/itm/3mm-Shaft-20RPM-DC-12V-Electric-Speed-Reducing-Geared-Motor-for-DIY-Toys/190949265575?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D30111%26meid%3D459df1ca462c456e82318d0415d429da%26pid%3D100005%26rk%3D5%26rkt%3D6%26sd%3D190896856301&rt=nc

Wonderwand loop on HF

http://www.wonder-wand.co.uk/WonderWand/WW_Photos_files/TCL-ad01.jpg

Rather than go QRT because of the approaching storms, I decided to try my Wonder Wand Tunable Compact Loop antenna indoors on 20m WSPR. The loop is about 0.5m diameter. In my case, more for convenience, it goes to the rig via my Z817 ATU. SWR is perfect 1:1 on the tuned frequency. No TX spots seen yet at 2W, but no shortage of RX spots across Europe so far. The loop is just above the rigs in a far from ideal, indoors location.

WSPR spots so far today on 20m with the indoor Wonder Wand Compact Loop

UPDATE 0916z: My first TX spot on the indoor Wander Wand Compact Loop came from DJ6LB at -28dB S/N.

UPDATE 0944z:  Second TX spot was by DJ7KA at -26dB S/N.

UPDATE 1010z: I QSYed to 15m and am getting lots of spots on TX at just 2W.  Best DX report on 15m is from SM6WZI (1007km) at -6dB/-9dB S/N (i.e. strong) - it is amazing this tiny indoor antenna works so nicely. So far 6 TX spots of my 2W in just 8 minutes on 15m. Incredible. Sorry about the poor photo of the loop, but you get an idea of the set-up. It will work even better on 10m and 6m as the loop is larger electrically. Set-up is totally un-optimised: the loop just happens to be where it is.

UPDATE 1230z:  Now QSYed to 10m and the best DX on the Wonder Wand Compact Loop indoors is now LB9YE (1489km) at -25dB S/N with 2W.

 

10m WSPR spots with 2W today with indoor loop

On 472kHz again this evening and future plans

In the next few days I need to buy some more wire and coax to continue my antenna experiments. The first thing I shall be trying is the larger "in the air" TX loop for the LF and MF bands - probably 136kHz , 472kHz and 1.8MHz coverage. The lower part of the loop can be several strands of PVC covered wire or coax in parallel to reduce resistive losses but the "up in the air" part will have to be 1mm diameter to avoid sag and being visible to the neighbours. I think I should be able to increase the loop area by 50% and reduce the wire losses by about 50%, so there should be a useful increase in ERP. This could be some weeks away though.

In the meantime I am continuing to TX and RX on 472kHz WSPR with the odd shape loop I currently have running along the garden fence and back along the surface of the lawn. It works, but as I said yesterday, it is some dB below what I can achieve in my small back garden whilst maintaining low visual impact to avoid neighbour issues.

After confirming the performance with the larger loop in the coming weeks, I shall try a short Marconi vertical to compare results - better omni coverage expected, but may not be as effective as the loop in the best directions?

After that, I shall probably be returning to 481THz and infra-red frequencies for more over the horizon scatter tests. I particularly want to try some IR scatter tests in daylight as I find going out on dark nights unpleasant.

TXing tonight with a 472kHz "in the air" loop

As the next stage in my experiments from the new QTH at LF and MF, this afternoon I converted my earth-electrode antenna into a complete "in the air" loop by removing the earth connections and joining the wires with a length of wire along the grass. The DC resistance of the loop is 0.9ohms and the enclosed area somewhere around 60-100 sq m although the loop is far from vertical and the bottom wire actually runs along the grass rather than up in the air. The measured loop current is around 0.9A and tuning very sharp with around 1.7nF in series and matched with a 3C90 step down transformer. According to my calculations of radiation resistance, the ERP should be in the 20-40mW region, considerably greater than with the earth-electrode antenna.

630m WSPR spots of G3XBM so far this evening

My first report this evening was from PA3ABK/2  at -28dB S/N but I am hopeful of a few more 472kHz spots tonight. I expect that the results with the loop will be better than with the earth-electrode system, but the loop can be further improved by raising the upper wire (bigger vertical enclosed loop area) and thickening up the lower wire to reduce resistive losses further.

I am doing things one step at a time so I can assess the difference.

UPDATE 1800z:  M0PPP has spotted me several times this evening but had failed to copy me in the last 2 weeks, so this is encouraging. Also reports from G8HUH and G6AVK.

More loops

HF TX loop in the bedroom

Small TX magnetic loop performance continues to astound me. This evening I erected my little loop antenna made with 6mm diameter copper pipe and tuned with a standard 365pF air-spaced variable as used in my regen receivers. The loop has a diameter of just 65cm (about 2m circumference) and is mounted on a piece of PVC pipe as a support. The 365pF is attached to the hi-Z part of the coil with a couple of crock-clips. Matching is with a coupling loop about 1/5th the diameter of the main loop.

The simple tuning unit - a 365pF airspaced variable

Without any effort, apart from the tuning sharpness,  it tunes from 10-28MHz and got loads of spots on WSPR. I then tried with 2W CW on 14MHz and received several European reports via the reverse beacon network. The loop is just resting against a wall in the shack. Overnight on 20m WSPR the best report was from 8338km with 2W indoors. On 10m this morning (July 4th) it is already exchanging WSPR spots with CN8LI despite being orthogonal to him i.e. in the worst orientation, 2W and indoors.

According to my loop calculator the efficiency should be around 12% or 9.3dB down on a "proper" dipole antenna on 14MHz. This is less than 2 S-points difference. Results bear this out. On 28MHz the efficiency is better.

Purists will point out that the crocodile clip connections will have a few milliohms resistance and this will reduce loop performance. At 2W there is no sign of arcing across the plates. The point is with QRP one can get away with simple circuits that would not be possible running 100W.

Loopy results on 15m

WSPR results today with 2.5W to 80cm loop in the bedroom on 15m

Well never mind the theory - my loop WORKS well. On 15m, with the loop taped to the chair in the bedroom shack, I have already been spotted on WSPR in Japan when using just 2.5W from the FT817 at  9474km.  So, my first attempt with an indoor HF magnetic loop in the bedroom has left me very impressed.

Loop antenna in the bedroom (notice the grandchildren's toys!)

Current set-up is about 80cm diameter 6mm copper pipe inductor tuned with a small 365pF air spaced variable cap. Matched with a coupling loop 1/5 the size of the main loop made with RG58 coax. With the 365pF variable it tunes from 40m-10m, but the variable cap needs padding out as tuning is extremely sharp.

I should have tried magnetic HF loops long ago.

UPDATE 1600: Just QSYed to 14MHz with the loop in the bedroom and power reduced to 500mW and still getting plenty of WSPR reports out to 2600km or so so far.

Going loopy on 10m

Prototype 10m TX loop using 6mm copper pipe

Today I've made a small TX loop for 28MHz CW/WSPR using an odd piece of 6mm copper pipe I had around the place. As I don't want to move the frequency much and the maximum power out will be 5W (usually far less) I have used a small length of twin core mains cable, trimmed to resonance, as the capacitor to resonate the antenna. The loop is matched to 50 ohms with a small coupling loop made out of RG58 coax. Bandwidth is quite sharp suggesting the loop is working as expected.  At the time of writing (early evening)  I've only copied one signal EK6RSC at 3586km earlier in the day and am still awaiting some reports on TX. The loop is mounted on a PVC pipe taped to the back of a chair in the shack.

If successful, the idea will be to put this up in the loft at the new bungalow and use it with WISPY and a small dedicated netbook PC to run 10m WSPR.

Mobile 8.977kHz VLF loop tests started

Today I started to do my tests on 8.977kHz using my 5W earth mode transmitter at home but using a mobile loop antenna on the car connected to my PC via a tuned preamp. The idea is to be able to drive around and measure signal levels with Spectran software whilst actually on the move.

30t 80cm loop mounted behind the car

The loop was mounted behind the car in such a way that it would detect any ground propagated signals. The loop is about 10cm off the ground.

A drive test to Swaffham Bulbeck was carried out and signals were detected more or less continuously out to 3.5km from home before they disappeared in the noise. Bandwidth used was 0.18Hz with a continuous carrier. Although coverage was as I anticipated, signal levels were not as great as when the loop was actually laid directly on the ground. At one of my usual test sites 3.5km out in the fens there was no copy with this loop arrangement yet there was a decent signal copied with the loop on the ground a few days earlier. I need to do some direct comparisons between the loop on the ground, the loop mobile mounted 10cm above the ground and with the E-field probe on the car roof. Initial indications are that the difference between the EFP and the mobile loop is probably no more that 5-6dB.

Signal received with Spectran and the mobile loop

An INDOOR loop on 472kHz TX?

Over the last few days I have been receiving quite strong WSPR signals on 472kHz from Richard G4KPX who is in Ely, some 14km north east of me.  Looking on the WSPR database I see he has been copied up to 573km on the band using WSPR. Now, in a email from Richard today he revealed his secret: he is using just 1W RF into an indoor 3m x 2m single turn loop made of 10mm pipe leaning against the wall of his spare bedroom!  The EIRP is at most 100uW and probably a lot less because of the losses with mounting it where he does and the capacitors used to resonate it. This is a pretty amazing set of results.

Richard has been spotted across the Atlantic on 160m using the same indoor antenna and has similar magnetic loop antennas for all HF bands I believe.

So, if you believe you have no space for an MF band TX antenna, and you don't even have a garden to put a couple of earth-electrodes in the ground, you could always follow Richard's example.

Some months ago I believe one of the German 136kHz stations (DF6NM?) tried something similar using a large coax as a big indoor loop on 136kHz and also got some quite decent results. Never say you cannot erect an antenna!  What you need is a bit of ingenuity.

LF/MF Loop antennas on transmit

For the last few winters I've used a wire loop antenna on 136 and 500kHz transmit with moderately successful results. Well, considering the extremely low ERP it is better to say excellent results. My wire is not that thick: around 1mm PVC covered multistrand wire and the loop area around 80m square, with the lower part of the wire either very close to, or actually on, the ground.  Using the TX loop calculator at http://www.66pacific.com/calculators/small_tx_loop_calc.aspx the efficiency is around -30dB at 500kHz, so that 5W from the PA would result in around 5mW ERP radiated. My calculations and measurements, with less than ideal loop geometry, suggest the ERP is some 3dB lower at around 2-3mW. At 137kHz the loop is considerably less efficient at around -50dB.

For a small plot with nearby trees I am not sure how well this compares with a small Marconi with a short vertical section (around 6m max) and a horizontal section of around 6m. Anyone know the answer?

Easy mistakes

On Monday night I was looking for Uwe DJ8WX again on 8.97022kHz.  To my great surprise I copied nothing when G3ZJO and M0BMU were getting good copy. I'd copied Uwe several times before. After suspecting my PC sound card was faulty, I found out why I copied nothing tonight: on Sunday I'd retuned my loop ATU to copy M0BMU on 500kHz and I'd forgotten to retune it back to 8.97kHz. Although Spectrum Lab locked onto GBZ OK, the sensitivity was about 20dB down! We learn by our mistakes.