Two Band Fan Dipole

[VK2CSW]

I have started to look at putting a fan dipole together for 12 and 15 metres. My 40m OCF should cover 15 but doesn't and the 20m dipole next to it is now somewhat redundant, so I will replace .it.

I have assembled the balun/choke/centre support.

At first it was RG58 wound onto a FT240-31 core. However it wasn't so effective - around -16dB rejection across the bands in question.

I rewound it with 1.25 mm diameter enamelled wire to get better than -30dB across all of the HF bands.

   

   

This afternoon on my drive home from work I realised I had no anchor points for the second dipole, so rather than anchoring off the same eyebolts I added a second set at the bottom of the case.

I was quite pleased with the end result.

[VK2CSW]

Well, there's some lessons learnt!

I had been planning to use some 13mm polypipe as spreaders. The pipe comes in rolls and needs to be laid out in the sun to straighten. I slipped it over some bamboo garden stakes and put the pieces in the sun, then I realised I should just use the bamboo, so construction recommenced.

The fan dipole went aloft just before lunch. I was blowing a gale but I figured it would be a good shakedown.

I had cut the wires long and folded them back on themselves.

On the nanoVNA the 12m VSWR lowest point was below the target frequency, the 15m lowest point was above.

So shortened the 15m it moved down and the 12 moved up. Lengthened the 12m legs. Still going in the right direction.

Then the reading started to go all over the shop. Ended up adding wire here, removing it there. After a couple of hours of lowering the wires, adjusting and raising again, the whole thing started to hang oddly. The tension was all wrong.

It was then I realised that my adjustments totalled more than the difference between a quarter wavelength on each band.

The dipoles had swapped roles...

Needless to say, the whole contraption is back in the shed where I will reconfigure it to the starting point, then reassess whether I need 12 and 15m on the same antenna...

[VK4ADC]

Colin

What is the spacing between the adjacent dipole wires ?
[It seems like the coupling between the wires is far too high and thus the interaction.]

My ray dipole legs are something like 400-500mm apart, but part of that is wire droop. 
I have two such arrays here with similar wire spacing on each and my recollection is that there was minimal interaction as I tuned each dipole set to frequency..

Doug

PS  20mm electrical conduit also works well for leg spacers

[VK2CSW]

Doug, the first spreader just next to the centre feed point is about 150mm then fans out to 500mm at the end of what was the the shorter leg.