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 Dual beam pro, opinions please 
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Post Dual beam pro, opinions please
I have been looking for a beam / rotating dipole for our clubs portable use so it needs to be simple. The Dual Beam Pro http://www.proantennas.co.uk/dualbeampro.htm looks on face value to be a likely candidate but..........

On closer inspection of the manual and the reviews in the press it seems in my opinion to be a bad antenna with a cunning device intended to make it 50 ohms. The manual calls an item a 'Transformer' which sounds horribly like the UNUN on many of the broad band verticals, then it states minimum coax length 20 meters. Hmmm.

So has anyone any hands on experience, or indeed know what their talking abot (I say that because I have yet to get my hands on one)
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dual beam pro  my thoughts 1.jpg
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thanks in advance Andy


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dual beam pro  my thoughts 1.jpg
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Andy - GD1MIP
Using decidedly average equipment from Mwyljyn Moddey, Ellan Vannin or IO74ti to you and I.
Mon Oct 31, 2011 2:56 pm
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 Dual beam pro, opinions please 
Silent Key

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Post Re: Dual beam pro, opinions please
Andy,

Your instincts are right!

It's basically a multiband doublet that uses end-capacity loading to allow it to be somewhat shorter than the 3/8 wavelength you would normally consider to be the minimum length required.

With a conventional doublet you would typically use 450 Ohm ladderline to limit the VSWR to around 10:1 between the various bands, and to reduce losses. The Dual Beam does something similar by using a 9:1 impedance transformation (my guess) at the feedpoint to limit the VSWR on the coax to 10:1 - it's the "dipole equivalent" of feeding a non-resonant end-fed wire through a UnUn. But, crucially, the losses in coax with a 10:1 VSWR will be significantly more than in ladderline with a 10:1 VSWR.

For best efficiency you would want the coax to be as short as possible; but if you're trying to market it as a multiband antenna that doesn't need a good external tuner, you would want coax losses. Maybe that's where the minimum coax length requirement comes from ;)

I think the basic idea has merit: it's reasonably compact; it's light enough that you ought to be able to get it pretty high; and it has useful dipole-like directivity, so it's probably worth putting it on a rotator. But if I were making one I'd be using ladderline to feed it from a good tuner; you only need rotate it through +/-90 degrees so I think the mechanical issues should be surmountable.

My biggest objection to it, apart from the coax loss, is the name: it's not a "beam" in the usually-understood meaning of the word, and I don't see anything that's "dual" about it.

73,
Steve G3TXQ


Mon Oct 31, 2011 3:52 pm
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 Dual beam pro, opinions please 
Silent Key

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Post Re: Dual beam pro, opinions please
Just remembered I had an EZNEC model of the Dual Beam Pro. For 20m of RG213 feedline, here are the VSWRs on the coax at the antenna end, the VSWR at the radio end, and the cable loss for each band:

20m 16:1, 8:1, 2.7dB
17m 9:1, 5.8:1, 2dB
15m 7:1, 4.8:1, 1.7dB
12m 6:1, 4.2:1, 1.7dB
10m 6:1, 4.1:1, 1.8dB

Looks like I underestimated the VSWR on 20m!

Nothing there looks too dire, but you might begin to worry if you had twice as much feedline or were using RG58.

Steve G3TXQ


Mon Oct 31, 2011 4:09 pm
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 Dual beam pro, opinions please 
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Post Re: Dual beam pro, opinions please
Thanks Steve, Plan B then it is.

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Tue Nov 01, 2011 4:21 pm
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