I hate to throw a monkey wrench into this discussion.
There is a very importent property of transmission lines that is
essential to the proper functioning of phased arrays, and especially
phased verticals where the impedance of vertical elements is almost
always significantly different than the characteristic impedance of the
transmission line.
The phase delay of any transmission lines is N O T the same as its
electrical length except when either of these two special conditions is met:
1. if the line is terminated is its characteristic impedance, its phase
delay is the same as its electrical length, or
2. if the line length is any multiple of exactly 90 degrees, its phase delay
is the same as its electrical length and the phase delay
is not dependent on the VSWR on the line.
The two special cases are very significant to this discussion. In a phased
array, a line that is 135 degrees in length when terminated in
its characteristic impedance will N O T produce a 135 degree delay
when it is used in a phased array because both of the two special
conditions are violated. The actual phase delay when the line is
installed in the phased array will depend on the impedance of the
element it is feeding. That impedance will be significantly different
that the characteristic impedance of the line.
For these reasons, most phased vertical arrays used by hams use phasing
lines that are a multiples of 90 degrees in length. If any additional
phase control is needed, its usually done with phase control networks
that require specialized test equipment for adjustment.
73!
Frank
W3LPL
---- Original message ----
>Date: Tue, 17 Apr 2007 07:36:24 EDT
>From: NPAlex@aol.com
>Subject: [TowerTalk] 135deg cable
>To: towertalk@contesting.com, w7why@verizon.net
>
>Tom,
>Two methods, determine what length a 1/4 wave (90 deg) @ 7mHz is, and then
>add one half that value for the total length ( 90 deg. + 45 deg= 135 deg). Or
>measure at 2X the 40m frequency (20m), and in that case a 135deg line at 40m
>will look like a 270deg or 3/4w line at 14 mHz.
>
>Norm W4QN
>===========================================================
>
>Hi TT'ians
>
>I got my 3 vertical array up and running Sunday and it seems to be working
>pretty slick, switchable in 6 directions.
>
>I am using a 1/4 wave phasing line. I have misplaced my ON4UN Low Band
>DX'ing book (anyone got one for sale) here, but I seem to remember that a
>135 degree phasing line would be better.
>
>What I was wondering, can I measure the 135 degree line with my MFJ 259B
>just like I measured the 1/4 wave line? Will I get the same reading, being
>it is a 1/4 wave on the end of a 1/2 line.
>
>Or, what frequency is a 135 degree line for 40 meters a quarter wave for?
>That would make it simple to figure out. Thanks and 73
>Tom W7WHY
>
>
>
>
>************************************** See what's free at http://www.aol.com.
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