Bit of Top Band nostalgia

On 27/12/2016 13:09, alan@g3xaq.net [rsgbtech] asked about my experiences with phased receive loops.  I rather over-answered his query with a raft of nostalgic reminiscence about when I was a proper Topbander.

With an output around -35dBi I’m wondering if one could get away without outdoor preamps? I wouldn’t have thought you really need the buffering of the preamps if a decent hybrid combiner was used. Or would that just exchange the noise pickup in the preamp that you suffered for noise pickup on the less-than-perfect coax back to the shack?

I certainly found that in-shack and near-house noise sources were a problem, so adding a fair slab of gain helped to raise the wanted signal over the local noise floor.  With the loops alone, I couldn’t detect any increase of band noise compared with a screened dummy load at the far end (or near end) of the coax.

I also tried a pair of loops a quarter-wave apart to see if I could steer the nulls. Results were a bit inconclusive, and by then the sunspot numbers were on the way up and I rather lost the fanatical urge for 160.

One thing for certain is that you must detune the transmitting antenna or you get a lot of induced noise and pattern distortion from that.

My tx antenna was a 70ft tiltover pole, pivoting on top of a vertical 20ft piece of 4 x 6 inch steel box section using a 1 inch steel pin for the bearing.  The base section of the mast pole is made from schedule-80 steel water pipe with 2 inch ID, then steel scaff pole, then thickwall and then thinwall aluminium tube.  On top of the thinwall section is a 30mm fibreglass tube about 2ft long.

The base of the pole is in a 5ft hole with a concrete collar top and bottom, and brick fill between.  The pivot pole is earthed at the base to 6600ft of buried radials and a 20ft square mesh mat, plus about 30 long ground stakes and was also bonded to every sheet of the 30x30ft corrugated iron roof of a low outbuilding.  In the configuration I was using for 160, I ran a 2.5mm copper wire from the top of the ally mast out about 70ft, to one end of a 2ft spreader, which was tensioned by a braided Dacron lanyard round a pulley fixed to a tree at about 60ft, with bungees to keep a steady tension.  The wire then went along the spreader and from there back to the top of the 2ft fibreglass pole.  From there the wire went to another piece of fibreglass tube at right-angles to the mast, then down to about 2ft above ground, where it was fixed with a barrel tensioner to a section of alkathene water pipe bolted fixed to a steel anchor plate set into the concrete. I connected the tilting section to the main pole with a section of 1 inch wide braided copper strap.

I fed it with a huge and heavy 4 inch diameter ferrite ring autotransformer to match the approx 110 ohm impedance of the folded hairpin to 50 ohms.  To detune it I used a vacuum relay to switch in a motorised 500pF vacuum capacitor (left over from an earlier version where I just tuned the thing as a simple inv-L using an omega match to a cage of wires from the 45-ft level to a ring just about ground)

To balance the tension of the horizontal section, I ran a long lanyard to haul up a 160m dipole fixed to a 35ft pole back on the house.  I fed DC up the dipole feeder to a vacuum relay to switch in a 330pF mica cap in one leg to detune the dipole on receive.  I used a simple 2 inch diameter ferrite transformer at the feedpoint to match the approx 20 ohm feedpoint impedance to the coax, with a 10k carbon resistor from each leg to the coax outer to bleed off any static.  That dipole was WAY better for ZL and VK6 and 9M2 greyline, also for those rare QSOs with KL7 from inside the auroral oval.  The vertical was best for west coast US, JA and just about everything else.

I welded up a conduit made of thirty or so scrap 6ft sections of 1/4 inch wall 4 inch steel tube and ran the feeders and control cables through it, and buried it about a foot deep.  That is also bonded to the earth system.

I dropped the big pole in summer 2000 with the idea of doing a bit of maintenance, but never got round to it.  Now lot of trees have grown around it.  It is currently stuck at about 30ft agl, so without major tree climbing and chainsaw work, it can’t go up or down.  Also I replaced the corrugated iron roof with a painted version last year and I couldn’t raise enough enthusiasm to sand a corner of every sheet, drill it and do all the stainless bolts and crimp tags all over again.

I had a huge amount of fun messing about with antenna systems on 160.  Looking back, I must have had an awful lot of spare energy then.  I sound more and more like an OT with a bad case of nostalgia.  To cure that malady, I have gone right to the bottom of the learning curve to have a proper bash at all this Four Metres and Down nonsense.

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