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Author Topic: 12m vertical centre fed dipole  (Read 481 times)

Offline G0BVZ

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12m vertical centre fed dipole
« on: February 14, 2014, 16:04:18 UTC »
"Take one garden fork, cut off the D shaped handle and push into the ground with the shaft vertical until the shoulders are flush with the ground." -That's my recipe for an effective, quick telescopic fishing pole support. If you unscrew the bottom cap of the telescopic fishing pole you'll be able to slide it over the shaft of the fork and in normal weather, that's it. A 10m fishing pole will just stand there all by itself, the fork shaft invisible!!  In my experience this is perfect for antenna experiments.  If the weather is windy, a few simple paracord guys will keep the thing up, even if it bends alarmingly in the storm.

For guying I use 1m mild steel pins as used by roadmenders to fence off their roadworks.  They're cheap, easy to drive into the earth and to remove whilst standing upright (-a little comfort for my old age!!  ;D  ) They're also great as attachment points for elevated radials....  I hit one too hard and the hooked end broke off (they are probably made brittle by brutal cold bending) That was when I discovered they will keep a 7m or shorter fishing pole upright unsupported.   I now use a heavy rubber mallet and there have been no more breakages.

OK, the antenna. In the photograph there are two vertical fishing poles.  The one on the right is 9.1m long and) it has a 12m half wave dipole taped to it.  (The lower end of the dipole is about 3m above the ground. 

The pole on the left is around 7m long and I fixed the balun at the same height as the dipole centre so the electrical half wave of lightweight 72 ohm twin feeder connecting dipole centre and balun would be horizontal when the poles were upright.. 300 ohm would be more durable but I didn't have enough.  It was cold, windy and raining.  The pvc tape was brittle and the stuff would not stick to anything wet so I tied the balun in place with a length of rubber bungee rope. I used a non inductive knot.  ;D

The low loss feeder was taped to the pole -the tape would stick to itself.  I first put up the long pole, then the shorter one. The sky turned dark and there was an increasing wind so I arranged a few hasty guys and retreated indoors.

By some miracle the resonant frequency was 24.9MHz at an swr of 1.4.

I'm sorry the pictures are so bad, the weather was shaping up for another storm. In the photo of the assembled antenna you can see a thick black line above my blue kick bag. That's the horizontal feed for  the doublet.

I enjoy using these miraculous long fishing poles. They're light, very flexible and so they're strong. I've learned accept that they move in the wind.  I shudder when I think back to my long aluminium mast,  It was hard to put up, dangerous single handed, and it needed 3 sets of 4 guys.  It was strong, it didn't move in the wind but if it had fallen over it could have caused severe injury.

Click the pictures to see a slightly larger version

Vic /content to be indoors today