Wideband shielded loop antenna project

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daveB
Posts: 1562
Joined: 07 Oct 2014 16:49

Wideband shielded loop antenna project

Post by daveB »

I've been working on a wide-band shielded loop antenna based on a design by PA3GZK. https://pa0fri.home.xs4all.nl/Ant/Activ ... %20eng.htm
I've used 10 mm soft drawn tube for the shield and multi-strand insulated copper speaker cable.

The beta version is under test and is producing some reasonable results. The problem is that I've no proper test equipment and all I can only compare it to the short dipole at 7 m agl. However the spectrum display of the SDR allows comparisons of noise floor / signal to noise ratio.

Using a ready-made board from E-bay it works well from long wave all the way up to the FM band.

With the loop at a median height of 2m agl and lined up in roughly the same orientation as the short dipole (7m agl) it was evident on the 80, 40 and 20 m band that the loop was doing a lot better with signal pick up (where did all those signals come from?). [But note: the short dipole is cut to 70 MHz and is a straight feed via 75 ohm coax - so there are bound to be all sorts of mis-matching losses - worse on some frequencies than others].

One thing (of many!) that I don't understand is why, if a magnetic loop is supposed to symmetrical, the design directly feeds the unbalanced input of the pre-amp. So in my ignorance I've wound a 1:1 balun on a binocular core and used that.

Next thing to do is properly mount it - away from the FM aerial mast - but there was no point it doing the extra work if the loop didn't give at least an indication it was worth following the project. up.
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

daveB
Posts: 1562
Joined: 07 Oct 2014 16:49

RE:Wideband shielded loop antenna project - Part 2

Post by daveB »

Plodding on I've now made the mounting plate so that it can be clamped to a pole. It is temporarily lashed up with the bottom of the loop about 6 ft off the ground and a reasonable distance from the 2" mast for the FM band.

I found another article which was useful - especially for the balun instructions - link: https://www.george-smart.co.uk/wordpres ... W-News.pdf

He has done some interesting experiments with baluns at different ratios. I've tried the 1:2 version and it to give slightly inferior results to a 1:1 balun when using the pre-amp board with the INA-02186. Grounding the centre tap of the balun to the shield made it slightly worse still.
Shielded loop balun connection.jpg
Rotating the loop reduced the wideband VDSL interference in the shortwave bands by some 10 dB making many signals visible. Mid evening I was listening to a CW QSO between a CT station and JK1P on the 80m amateur band! The Japanese station was weak but audible. (I was using a decoder program as my CW is rusty by some 35 years). I even found cw on Top Band - just now DF1TL andDK1WI coming in nice and clearly.

And for the last half-hour, until it went off-air I was listening to what was probably a pirate on 1625 kHz.

Added: The preamp gives over 30 dB and I find that it is essential to use 10dB attenuator to avoid overloading the SDRPlay - even with maximum gain reduction and with the LNA switched off. This appears in the form of background intermodulation/mixing signals.

I am impressed and my interest in HF is renewed - I shall have to seriously consider buying an SDRPlay RSP1A with the improved filtering so I can do some serious MW DXing. That will be after I've found a permanent home and cabling - at the moment I swapping thebetween the feed to the FM compact yagi at the bottom of the garden ad the loop - and I can't have them when the SpE season is about to start.
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

daveB
Posts: 1562
Joined: 07 Oct 2014 16:49

Re: Wideband shielded loop antenna project

Post by daveB »

Where are the nulls? I'm used to quad loops on VHF. So it took quite a while before I actually 'googled' the radiation pattern of a balanced loop antenna. Doh! Nulls are at right angles to the plane of the loop - the opposite of a VHF quad loop.
Shielded loop radiation pattern.jpg
So having got that in my head it took but a few moments, armed with a Great Circle Map to completely null RTE on 252 kHz and find Algeria weak but clear. So it IS working properly.

It is now mounted such that it can be left outside and I can start getting familiar with MX DXing and leave it recording overnight - at least until the SpE season starts up.
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

daveB
Posts: 1562
Joined: 07 Oct 2014 16:49

Re: Wideband shielded loop antenna project

Post by daveB »

UPDATE:

I have finally constructed the balanced pre-amplifier using MPSH10G transistors as per the Fig 22 this article:

https://www.george-smart.co.uk/wordpres ... W-News.pdf

The most difficult thing was the winding of the tri-filiar transformer for the output. The pre-amp is once again fed via the coax and the power is simply fed to Vcc by connecting one leg of the output coil to Vcc instead of to the ground - as per M0AYF's original design.

I've only tested it indoors on an un-shielded asymmetric* loop with a total length of around 3.75 m / 12 ft in order to be able to balance the voltage to the two transistors. Reasonable signals on 80m and 40 m along with the 49m and 41 m bands were received. The next step is to replace the MMIC pre-amp in the shielded loop itself and see how it compares. The ideal would be to construct a second shielded loop so that A-B tests could be carried out.

I'm going to wait until there is a relatively warm, day, windless day; which hopefully won't be too long.

David

* A fancy way of saying I pegged the wire to a curtain rail.
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

daveB
Posts: 1562
Joined: 07 Oct 2014 16:49

Re: Wideband shielded loop antenna project

Post by daveB »

Time for another update. The loop is now "permanently" installed at a mean height of round 3m/10 ft. It is manually rotatable.
In addition to working well from 100 KHz to 30 MHz it is giving a good performance on the OIRT band - similar to my dipole mounted at 7m/23 ft. The key point is that at this frequency the peak direction is now at right angles to the plane of the loop - so it is acting like a quad loop. It also performs well on Band II with a gain about 5 dB less than my 5 ele compact - which is to be expected.
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

daveB
Posts: 1562
Joined: 07 Oct 2014 16:49

Re: W/band shielded loop antenna project - latest

Post by daveB »

The home built balanced loop amplifier has died with no obvious problem - it first started operating intermittently. Testing gave no obvious answer, so to save time I replaced it with the original e-bay circuit board yesterday when there was a clear spell and no wind.

So now I've been able to carry out a reasonable test on MW with this cheap amplifier board, as shortly after I made the loop back in 2018 my MW reception was totally wiped for the whole winter season and by the time I got clear MW again I'd swapped to the home-built board.

On 9/10 March 2020 CRI 1521 was received reasonably well at 20:00. Before midnight VOCM was coming in at good strength. Other stations recorded up to 03:00 (in no particular order) were weak signals from PJB Shine on 800 and Harbor Light on 1400, CKVO and WOR on 710, Family Radio on 1560, WBBR 1130. And others.

It also works very well on the FM Band - apart from lack of directivity - so I expect it will work as well as the home-built circuit board on the OIRT band. In the 2019 Es season on the OIRT band it easily out-performed the horizontal dipole - acting as a full band loop - so I've raised the height by a few feet. (it's only 6 weeks before I start seriously monitoring for the start of Es!)

So if anyone wants to try a shielded loop antenna and without spending £250 it is well worth the effort. That is quite apart from the satisfaction of a home-buit project. Earlier posts have details of the source material for the design. Here is the main link again https://pa0fri.home.xs4all.nl/Ant/Activ ... %20eng.htm

The LNA to buy on e-bay is the one that has a small black button for the active device -you may be able to see N02 id on it. Attached is a photo of the board I have - and how it modified for line power plus the small capacitors from the pa0fri details - which I presume are a rudimentary low pass filter.
LNA preamp for loop antenna.jpg
And if anyone would like to know how it compares to a commercial loop I'd be happy to try comparative testing - just supply me with the loop :D
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

simbeav
Posts: 1158
Joined: 14 Oct 2014 15:00

Re: Wideband shielded loop antenna project

Post by simbeav »

Interesting Dave, but way beyond my competencies, I'm afraid.

Any idea what makes this loop effective, to such high frequencies ? If I remember correctly, the Wellbrook antenneas don't go much above 30mhz if that. At sunspot maximum it was fine for the signals at 26 - 30 mhz. Perhaps I should have tried higher.

Anyway - Let's have some medium wave logs from you !

daveB
Posts: 1562
Joined: 07 Oct 2014 16:49

Re: Wideband shielded loop antenna project

Post by daveB »

Hi Simon,
It is the pre-amplifier characteristics. I would expect the Wellbrook and similar loops are designed to avoid pick up of strong VHF signals - to avoid messing up the inter-modulation figures.
At VHF it acts more like a full wave loop and with the LNA board from ebay if has a surprisingly good performance - apart from being bi-directional.
On LW I can get something like a -15 dB null which quite sharp and the peak signal is in the plane of the loop. On VHF I can also get up to -15dB null on the lower half of the FM band, but it deteriorates at the high end and the peak signal is broadside to the plane of the loop - i.e like a quad antenna.
I don't have the resources/patience to do any further optimisation - it will do as it is.It will be most useful on the OIRT band.

As for MW DX logs - I just can't get 'into' MX DXing - it is more a case of casual dabbling. But at 23:25-23:00 12/03/2020 I'm getting weak TA on 1010 which as it's a YL is CRFB and VOCM on 590 weak but it can be understood. WBBR on 1130 is listenable.
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

daveB
Posts: 1562
Joined: 07 Oct 2014 16:49

Re: Wideband shielded loop antenna project

Post by daveB »

And "finally" - my friendly aerial contractor is still working and so I got him, among other things' to run another length of coaxial cable so I could have both the FM aerial at the bottom of the garden and the loop antenna permanently connected. I decided to use the rotator for the magnetic loop - which hasnow ended up 3 m off the ground.

With the rotator I can now measure just how good the nulls are.
Using the HDSDR software with measurements taken between 1500 and 1600 UTC (no sky waves):
198 kHz (BBC R4) -30 dB (S9+30 down to S9+0)
1242 kHz Smooth Radio (Kent) -38 dB. This station is only 16 km away. (S9+38 dB down to S9+0)
1458 kHz Lyca Radio -21 dB (S9+28 to S9+7)
1548 kHz Gold (London) -21 dB (S9+22 to S9+1)

The null is very sharp - the rotator only goes in 6 deg steps and +/- 1 step makes a significant difference in strength. I suspect the deepest null is where the step points directly at the station and the lower nulls are where the rotator is off by 3 or 4 degrees.

On the FM band there is still a reasonable null
90.0 MHz BBC R2 (Dover) -13 dB (S9+25 down to S9+12)
102.8 MHz Heart -17 dB (S9+32 down to S9+15) this is line of sight at the top of Boughton Hill nr Canterbury

The depth of null on MW/LW is equal or better than that reported in the articles I based the loop antenna on - so I'm pleased with the results.

Note that these measurements are on the version with the 'cheap 30dB preamp from e-bay preceded by a 1:1 balun. They are still on offer at a low price - look for the one with a round chip and N02 printed on it.
Location: Sittingbourne, Kent JO01ji
SDRPlay, AirSpyMini, RTL dongles, ST311, Yamaha TX930. 5 ele yagi 20ft agl - 70 deg. FM5 - 80 deg, 5.1 Yagi 16ft agl - 120 degFM5 -180 deg. Vert 3-ele Moxon - 80 deg. Shielded magnetic loop 0.1-200 MHz - 10 ft agl

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