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Author Topic: 12m spiderbeam doublet support  (Read 453 times)

Offline G0BVZ

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12m spiderbeam doublet support
« on: February 14, 2014, 14:40:53 UTC »
My workhorse antenna is an inverted V doublet long enough for the bottom end of 80m fed by good quality 300 ohm windowed twin feeder. It is supported by the 12m spiderbeam telescopic vertical but I left the top two sections inside, so the top is around 10m.  I did that because I obtained it last autumn and I didn't know the product so I kept it shorter and stiffer to see how it survived the winter. So far so good, so I'll probably go to 12m when I replace the present wire with something thicker.

I drove around a metre of 50mm aluminium scaffolding tube into the ground. When I'd finished with the sledgehammer the top was well mushroomed so I cut the top 1cm off and rounded the edges well for fear that a sharp edge would cut into the tube as it moved about in the wind. I intended to use bungees to secure the mast to the tubing. That was good enough to keep the mast up while I guyed it.

I secured the centre insulator to the top of the mast with cable ties and then the 300ohm feeder was taped to the mast just above each joint.  If the joint slipped down then that tape would stave off disaster and a big bulge in the feeder would signal a problem before complete failure. I hoped.   ;D

I was able to hoist the mast upright and bungee it to the scaffolding tube single handed without incident. Guying it was easy. For guys I used hard to see military paracord. £10 buys a 100m reel. I used a round turn and half hitch to tie the guys about 2.5m above ground and ran out three equally spaced lengths horizontally to conveniently placed trees, where I tied them off using tarbuck knots.   These are good because they will slide slowly under extreme tension rather than snap under snatch loading.  I can walk about under the guys without problem.

I had to link the mast and house wall with another length of paracord so I attached it to the mast at around 3.5m height at one end and to a masonry bolt embedded in the house wall at the other.  I used this as a fourth guy but its main task was to support the twin feed which was tied to this catenary support every metre.

So the mast was up, the doublet tunes on all bands (being much too short for 160m it is more like a dummy load there but I can make local contacts on it, out to maybe 100km.)  There is little else to say about this antenna, other than point out that one photograph shows I was careful to take the feed away from the mast in a curve rather than a sharp right angle. Oh yes, one further thing: when I took coax out of the equation by finding a way to lead the twin feed through the house wall this doublet started hearing much better.

When the weather is nicer I will drill and tap the scaffolding tubing so that I can bolt a static earth bleed to it and I'll make a nice boxed spark gap above the catenary out of silver wire. The trick will be to keep it at 300 ohms. Click the pictures to see bigger versions.

Vic