While this does one really answer your question of using different feedline
lengths, I use equal lengths of coax with no particular length in regards to
wavelength. Mine all run to the base of the tower. You could save a lot of
coax by having them meet in the middle however if you use switching then
your relays are 150' in the air in your example. If you used four yagis,
you could group them on the tower and bring only a pair of feedlines down
the tower for switching and save some coax. FWIW, LDF4 is much easier to
coil up on the tower than LDF5 and it is inexpensive if you shop around.
John KK9A
To: TowerTalk@contesting.com
Subject: [TowerTalk] Coax lengths for phasing a stack
From: Grant Saviers <grants2@pacbell.net>
Date: Sat, 07 Feb 2015 18:48:10 -0800
I'm ruminating about what lengths I need for 5L over 5L stacks to equalize
the antenna feedpoint phase. The literature seems thin and the common answer
is "make the feed-lines the same length", which I am certain will work.
However, when the stack separations are sufficient, there may be more than 1
wavelength of coax needed to equalize the lengths. e.g. 60' separation on
20m at 0.89 vf (LDF5) is just about 1 wl. It seems to me there is little
downside with ham radio modulation systems to phase at 0 (equal lengths) or
+/- n * 360 degrees (where n is 1 or 2). Perhaps some cw waveshaped
attack/decay envelopes will change a tiny bit, but that seems like a slight
downside.
So it may work out that no extra phasing line is needed for certain
separations. I am planning that all misc lengths of coax's and types are the
same for both antennas (rotator loops, boom and choke lengths).
Then there is the slightly counter-intuitive result that making the longer
(higher antenna) feedline a bit longer minimizes the coax used in the
phasing at some separations. And one more trick would be to reverse the
outputs of one side of the TLT matching transformer for BIP to eliminate 1/2
wl of phasing line. It wouldn't matter for Top or Bottom only use.
So, is this analysis correct? Any other options for using less coax?
Grant KZ1W
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