A review of my logic (and math too) would be much-appreciated so any
mistake(s) can be caught now (easy to fix) rather than later (hard to fix).
SITUATION:
I am installing 3 stacked tri-banders. A Force12 C-51 at 132' and two F12
C-31s at 92' and 52'. (All distances are above the tower base which is
appx. 9' AGL.) I am installing the switching/phasing network near the base
of the tower for access convenience. I am feeding to the antennas from the
switching network with RG-213.
CALCULATING FEEDLINE LENGTH:
The physical distance from the center of the tower to the C-51 (top) driven
element is 68" in front of the tower (i.e. the driven element is on the
director element side of the tower). The physical distance fron the center
of the tower to the driven element of the C-31 driven element (20 mtr.) is
56" behind the tower (i.e. the driven element is on the reflector side of
the tower). Therefore the driven element of the C-51 is physically 124"
(10.33') in front of the driven elements of the C-31s. Therefore I will
have to make the feedline to the C-51 longer to keep signals in-phase.
Calculating the additional length required for the C-51 feedline, the 10.33
physical distance is reduced by the Velocity Factor of the RG-213 of .66 to:
10.33 * .66 = 6.82'. (Electrical length)
The distance from the C-51 (top) to the switching network is 132.0'.
The length of the feedline required for the middle C-31 (at 92') is 132.0' -
6.82' = 125.18'.
The length of the feedline required for the bottom (52') C-31 is the same;
however, to reduce unneeded (I think) feedline, I will reduce it by one
wavelength or: (983.6/14.2) * .66 = 45.72'. This will be 79.46'.
QUESTIONS:
1. Is logic and math correct.
2. I am using the 20 mtr driven elements as the reference. Is this
correct? Why? (I know stacked tri-banders are generally compromises and I
have read varying opinions on spacing distances (anywhere from 30' to 50+')
and feeding, but nobody tells why for example, a 30' spacing is better than
40' -- I chose 40' because that is what most opiners on the subject
(including Force 12) recommend.)
3. The middle C-31 will have 33' of excess feedline length and the bottom
one will have 27' excess length. Where is the best place to put the excess?
How tight a coil should I make (for QRO operation would heating or breakdown
be an issue with a tight coil) and should it be at the antenna or at the
switching network? (I am using baluns at the antennas.) (My thought is to
put it at the antenna to act as a lightning/common mode current eliminating
choke.)
4. I will be using LMR-600 from the shack to the switching network (appx.
300'). Can anyone see any possible reason not to do this (i.e. mix RG-213
and LMR-600)?
Thanks!
Cliff
NC4CB (formerly WA4MWM)
Winston-Salem, NC
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