I have also replied to the inquiry on the F12 reflector so I will post this
here also to encourage other discussion. Tommy WD4K
-----Original Message-----
From: WD4K [mailto:WD4K2@Charter.net]
Sent: Sunday, September 26, 2004 9:46 AM
To: WA4MWM; Force12Talk@qth.com
Subject: RE: [Force 12 Talk] Stacking Force 12 tri-banders
HI Cliff, I have a three stack of C31's, Mag340N and 180B on one tower so I
have already been through every facet of this decision making process. I see
some issues here that need to be addressed. Are you stacking for purposes of
gain? Are all three antennas rotatable? Why are you choosing not to match
the two C31's with the third antenna? Have you addressed the problems with
matching the C31's with the C51? You mention adjusting the feedline lengths
to match for phasing but you do not compensate for differences in boom
length. You mention the length from tower center to top antenna and middle
antenna but your actual swithcing is at tower base...How does that work? How
much are you gaining with a C51 in performance vs the expense, matching and
handling difficulty? Are you contesting and need the multiple simultaneous
directions; or other purposes? If you are a contester then phasing for gain
is less of an issue than switching efficency for multiple directions. I
built a switching matrix at the tower base for all of mine so I may be able
to help you with that if your purposes are similar to what mine were. From
what I am reading, I think all of the calculations and time spent trying to
match all of this needs to take a back seat to deciding on the purposes,
mismatch of boomlengths, where and how you plan to do the switching,
distances to the actual switching, remote or ??, etc. I have not been
radioactive for a year due to other issues...but when I throw the switches,
it works :)
Reply off list to me if you want to discuss this. I posted here only to
encourage other comments and discussions on these issues. Good luck and keep
in touch. 73, Tommy WD4K
-----Original Message-----
From: Force12Talk-bounces@qth.com [mailto:Force12Talk-bounces@qth.com]On
Behalf Of WA4MWM
Sent: Sunday, September 26, 2004 5:45 AM
To: Force12Talk@qth.com
Subject: [Force 12 Talk] Stacking Force 12 tri-banders
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 from 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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