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From: Henry G Chapoton[SMTP:chapoton@smtp-gw.gdls.com]
Sent: Wednesday, July 22, 1998 9:39 AM
To: kq2m@eci.com
Cc: towertalk@contesting.com; n4kg@juno.com
Subject: re: [TowerTalk] Stacking dissimilar antennas - correction
Hello All: I am sorry, but I think I have come into this thread at a late
date, but I wanted to weigh in anyway.
I have played the stacking game for many years, especially on VHF. I started
using the standard 1/4 wave RG-11 70 ohm Wilkinson power dividers for up to 4
yagis on an H frame. I have since used the M2 4 port power dividers that work
great on VHF.
When I decided to stack six tribanders on a 190 ft. rotating tower, obviously I
could not use the same old power divider technique because what is a 1/4 wave
on 20 meters is a 1/2 wave on 10 meters! First, I used the SAME yagis spaced
the same 30 feet distance apart all the way up the tower. Then I divided the
antennas into pairs; an upper, middle and lower pair. I used exactly equal
lengths of RG-213 coax from each antenna of a pair to a 25:50 ohm UNUN
physically located between the pair of antennas. I now had three of these
UNUNs on the tower. I brought equal lengths of RG-213 coax to a centrally
located "Stackmaster" switch box that allows me to select any combination of
pairs of antennas. That is accomplished using a 22 2/3 ohm UNUN for matching
all three or any two pairs to nearly 50 ohms. The UNUN is bypassed for any
single pair.
So, if you are careful to keep the feedlines the same length (either
measure the same physical length off the same reel, or better yet,
measure with an antenna analyzer) and use broadband UNUNs, you can
even stack tribanders.
My inspiration for the triband stack was an article by Dean Straw and Fred
Hopgarten (sp?). For HF operation, I simply would not consider using the
1/4 wave matching sections anymore because like someone already
pointed out, the match will be different at one end of the band from the
other.
73/Mike, N7ML
PS Does anyone have an idea of how I can reformat my Microsoft
Exchange e-mail? It looks fine on my end, but I understand that
it does not word wrap properly when it is sent out?????? Please
respond directly so that we don't take up space on the reflector.
Thanks.
Bob:
Change those references to "1/4 wave multiples" to "ODD multiples of
1/4 wave". Usually a good place to put a 1/4 w. of 75 ohm (or
whatever) for matching, along with equal lengths of 1/2w. multiple 50
ohm for the balance of length req'd to the switch box. If you don't
require the impedance transformation, just use odd 1/4 w. mults of 50
ohm.
Greg
na8v
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