[TowerTalk] 80m DX special dipole

[Guido Tedeschi]

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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>