Thanks to all for the comments.
I did model pattern change for feedpoint phase differences and found
that at even with one feed offset +/- 45 degrees the lobe angle is
unchanged at 9 deg for my 20m stack (127' + 67') and the peak gain
deteriorates by about 0.6db. (17.75 to 17.15dbi). For 20m the edge
to edge wavelength delta is 0.025wl (9 degrees) which will have
negligible affect on gain and pattern if I used the 1wl offset in
feedline lengths, one half that if I used 14.175Mhz as 1 wl. A 4.5
degree band edge feed phase error models as 0.00 db change in Pro4.
For 15m and 10m (28 >29Mhz) there are single digit degree wl change.
The antennas are identical OWAs so swr variation is not a concern. btw
60' of ldf4 is 0.12db loss at 14Mhz. So it seems either "same length"
or 360 deg phase shifts both work, with more loss with added coax to
make equal lengths.
Each antenna is set up on its own ring rotator and a transformer/relay
box is at the tower base for azimuth diversity / selection of BIP/T/B.
I have a 1200' spool of LDF4, but adding a phasing length and coiling it
up at the tower base when it isn't really needed seems like a waste of
db's, $, and makes clutter.
73,
Grant KZ1W
"a few tenths of a db here and a few tenths there and pretty soon there
is -3db"
On 2/8/2015 7:56 PM, Matt wrote:
Spot on John.
As the phase angle between a single stacked pair increases, the high angle
lobe starts to grow, and the low angle lobe starts to deteriorate... but not
at all in a linear manner - quite negligibly for all practical purposes even
at +/-45 degrees phase angle. At 90 deg phase angle the degradation
between the main and second lobe is only down by about -3db. I will note,
however, that the deep nulls between the main lobes do start to fill in.
At about 150 degrees phase angle or so, the high angle BOP lobe really
starts to really take over.
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
john@kk9a.com
Sent: Sunday, February 08, 2015 9:12 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Coax lengths for phasing a stack
Phase in antenna stacking is not that critical, you can be out 20 degrees
and not see much difference in the pattern. Using equal lengths of feedline
is so much easier and in most systems it is not cost prohibitive. According
to my modeling software even if the feedline to one antenna is 100 WL longer
the pattern does not change. Does this seem correct?
John KK9A
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Coax lengths for phasing a stack
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Sun, 08 Feb 2015 10:23:01 -0800
On Sat,2/7/2015 6:48 PM, Grant Saviers wrote:
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.
Yes.
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.
It's important to understand that PHASE is NOT a two-valued function, it is
a CONTINUOUS function, measured in degrees, and it is a function of
frequency and time. That 1 WL line will only be 1 WL at the frequency for
which it is cut; either side of that, the drive to the two Yagis will be
increasingly out of phase. The difference may or may not be enough to
matter. The effect would be to tilt the vertical pattern, and would be most
visible in the nulls. Equal length lines has the advantage of making them in
phase across the band.
73, Jim K9YC
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