Page 1 of 1

Wideband shielded loop antenna project

Posted: 19 Apr 2018 20:22
by daveB
I've been working on a wide-band shielded loop antenna based on a design by PA3GZK. ... %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.

RE:Wideband shielded loop antenna project - Part 2

Posted: 23 Apr 2018 21:42
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: ... 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.

Re: Wideband shielded loop antenna project

Posted: 29 Apr 2018 19:05
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.

Re: Wideband shielded loop antenna project

Posted: 29 Oct 2018 20:28
by daveB

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


* A fancy way of saying I pegged the wire to a curtain rail.