If I understood correctly the antenna, the shorted coax should be ok
(15.07' same electrical lenght to have the same inductance), the
shortest coax is wrong and must be 4.3' (it should be of the same
lenght, independently from the cable type) and the open coax is also
wrong and should be 38.5' instead of 36.7' like I did (about 869 pF).
Probably the impedance is too high, because with the shortest coax 5,5'
long the feedpoint is farther from center.
I'm correct or wrong?
73 de Guido, ik2bcp
Il 11/09/2018 22:37, Guido Tedeschi ha scritto:
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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