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Re: [TowerTalk] 80m DX special dipole

To: John Simmons <jasimmons@pinewooddata.com>, towertalk@contesting.com
Subject: Re: [TowerTalk] 80m DX special dipole
From: Guido Tedeschi <guidoted@gmail.com>
Date: Tue, 11 Sep 2018 22:37:04 +0200
List-post: <mailto:towertalk@contesting.com>
Many thanks for the quick answer John.

I thought that the coax cables were used as stubs, so only the impedance and velocity factor were important; but if capacitance per meter is also important the issue is explained, because the RG213 has 100 pF per meter and the BROAD-PRO 50/C has 74 pf per meter.

I was mislead because I was remembering to have seen a low power version made with RG58, but RG58 has the same capacitance per meter of RG213.

Now I have to understand better the antenna details and see how to recalculate correctly the cables lenght of BROAD-PRO 50/C

73 de Guido, ik2bcp


Il 11/09/2018 22:06, John Simmons ha scritto:
Guido,

This is an interesting article- thanks for forwarding. In the construction notes: "Don't substitute anything other than RG8U for the RG213. The lengths of the coax stubs are calculated based on the dielectric properties and velocity factors of these cables."

Carefully compare all the specs of the two cables. It may be that the capacitance per foot is causing your difficulties.

73,
-John NI0K

Guido Tedeschi wrote on 9/11/2018 2:27 PM:
I built an 80m DX special dipole.

http://1vc.typepad.com/ethergeist/2011/01/a-broadband-contest-antenna-for-80m.html

I used the Messi & Paoloni BROAD-PRO 50/C cable instead of RG213 (https://messi.it/dati/immagini/BROAD-PRO50-All6_EN.pdf).

The cable lenghts are recalculated for a velocity factor of 0.85 instead of 0.66 and the overall lenght (cables plus wires) is the same of the original project, but the result is not satisfactory.

The SWR pattern is correct but it is like having an offset of about 2: the two lower dips have an SWR of about 2 and the highest SWR on band reach 3

What could be wrong?

Is it necessary to use a cable with 0.66 velocity factor?

Thanks and 73

Guido, ik2bcp


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