GU4YOX Shielded Loop for 160M
I have suffered from high noise levels on 160M for years, with 9+20 being usual. The source of noise is unknown but I think it is just an electrically noisy location on topband. I'm around 10 wavelengths from the Atlantic Ocean on the west coast of Guernsey and it is largely a rural area with some small growing businesses nearby. About 4 years ago, I managed to run two Beverage antennas to the south and west which have enabled me to work many new countries on 160M. I specifically recall in October 2004 being called by KH6AT and KH6ZM for new countries and States for me. This is a difficult shot from Western Europe and I really enjoyed my time at work that day. The volume was very high and the noise just bearable on the Beverage antennas. I use a Yaesu FT1000MP Mk V and a VL1000 Amplifier usually, and my transmit antenna is an Inverted L. This can be anywhere between 45' and 86' high, dependant on weather conditions. There is around 4500' of ground radials connected to the base of the antenna assembly.
I have read with great interest many articles and heard several stories regarding loops and I thought I would try my own out as an extra string to my bow for CQWW 160M in January 2006. Well, I finished it in time and it worked a treat for me with good results. I managed to log 963 QSO's of which over 300+ were with US stations (Thanks!). So how is a loop like this constructed ? Let me explain how I built mine for simplicity and solid construction.
Firstly, I was given some ex-GSM antenna coax LDF5-50 This is a heliax antenna type coax of 50 ohm impedance and it is solid in construction. Its loss characteristics are excellent and it is not that difficult to obtain a short length to make a loop like the one I describe. I used a 20' length in total and formed it into a circle, taking care not to get the shape un-circular. The diameter is just over 6' when the loop is closed. The loop was measured exactly for its centre point and at the top of the loop a 1" section cut out of the outer conductor only.
The loop of coax was then secured to a timber 2" x 2" cross frame assembly and clipped with plastic cable clips (available from your electrical wholesaler or horticultural stockist). I screwed the clips using stainless steel screws as the salt air in Guernsey can corrode galvanised components easily. The bottom of the loop is comprised of a 6" x 6" galvanised box which can accommodate a pre-amplifier in the future if required. I used some 28-42 mm plastic weather sealing glands to stop the rain getting in. Across the two inner cores of the coax is around 1000pf of capacitance at setting value. I used two 300-1000pf trimmer capacitors in parallel and tuning was carried out later. The outer sheaths are connected together and to the coax back to the Rx. The inner core is also connected to the inner of the coax back to the Rx as can be seen in the photos below.
When your loop has been constructed and you are ready to tune the loop, I found it easier to take a very long 2 core cable out plugged into the headphones whilst tuning the loop for a peak "noise" in the middle of the day. You could also use hand held or GSM phones if you have "Free" minutes on a couple of mobiles! Anyway, it will tune for maximum noise and that is where to finally leave the setting. I used 50 ohm coax back to the shack and I did not have any broadcast interference problems with the loop. I select the Rx button on the Mk V and select between Beverages and the Loop. The loop is by far the quietest and the signal levels only drop around 20dB. I showed my friend and great DXer Mike GU4EON and he could not believe the difference from the Inv-L. I also mounted a rotator on the ground post and it did not seem to make any noticeable difference except for reducing noise. Noise was loudest end firing and this tended to be towards the house area. I left it perpendicular to the house end firing to the US and EU.
There is nothing original here, it just works for me .Good Luck with your new loop!
73 Bob, GU4YOX
...will be continued