I have a four stack of tribanders on two different towers and one of them is
with equal length lines to the bottom and the second one is with lines that
have a bit of calculations behind them. Those terminate at to a 4:1 broadband
transformer at the middle of the tower. The length is such that it is an odd
number of electrical wavelengths on 15 m. This is because a quarter wave line
forces the current at the antenna feed point to be the voltage at the other end
divided by the line impedance. This also makes sure that the phase is the
same. I did this because the 15 m band SWR profile was the worst, on 20 and 10
the lines are about 1/2 wave or X times 1/2 wave. So, for any HF stack where
the antennas are in different environments or slightly different on their own,
this approach better guarantees proper phasing. I have an Excel sheet to
calculate these that I can share if needed.
MarkkuOH2RA/OG2A/WW1C
> From: john@kk9a.com
> To: towertalk@contesting.com
> Date: Sun, 8 Feb 2015 10:28:11 -0500
> Subject: Re: [TowerTalk] Coax lengths for phasing a stack
>
> 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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