Sergio -
The ideal electrical length of a phasing line is an odd multiple of 1/4
wavelength. On 20 meters (14.150 MHz) preferred 9913 lengths (89%
velocity factor) would be odd multiples of 15 ft 6 inches. In your case,
46 feet 6 inches would be an ideal choice.
Feedlines that are an odd multiple of 1/4 wavelength have been the
preferred length for phased Yagi arrays by many experienced builders over
many years. The reason for this preference is that it helps to force
equal currents in the driven elements of each Yagi.
Let me make an additional point about air dielectric cables like 9913. I
never use 9913 cables in my HF phasing lines, because it is very difficult
to keep water out of the line since it is not pressurized. Water tends to
get into 9913 either from direct entry through imperfect water proofing or
thru condensation. Water has a particularly negative effect on a phased
array, because it will change the velocity factor of the cable, as well as
increasing cable loss! For this reason, most builders prefer solid
dielectric coaxial cable, such as RG-213; or for improved resistance to
water entry -- hardline cables such Andrew LDF4-50A Heliax or impregnated
cables such as Decibel Products VB-8.
I'd recommend that you N O T use 9913 in phasing lines, but that you use
RG-213 or better. If you use RG-213, you will probably need to use a 5/4
wavelength cable, which would be 57 feet 4 inches long. A 3/4 wavelength
piece of RG-213 solid dielectric cable would be 34 feet 5 inches --
probably a little too short for your installation. If you used Andrew
LDF4-50A Heliax, you could use 3/4 wavelength cables without reverting to
undesirable unpressurized air dielectric phasing lines.
73
Frank
W3LPL
donovanf@sgate.com
On Thu, 7 Aug 1997 pp5jr@pp5jr.ampr.org wrote:
> i'd like some help about which is the ideal length of coax i need
> between the antenas and the phased box, in a 20m phased yagis, 5 over 5,
> with 49 ft boom, and with 50 feet of distance between them. The coax is going
> to be Belden 9913 RG 8/U.
> SERGIO PP5JR ALSO ZX5J
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