You can have fairly substantial phasing errors and not have a huge effect on
the forward gain. (45 degree difference results in less than 1dB reduction
in forward gain) (10 deg is less than 0.13 dB)
Where mismatched phasing lines will bite you is:
a) If there is significant mutual coupling between the elements, changes in
the length of the transmission lines can lead to markedly different changes
in the radiated phase from the element. That is, a 45 degree difference in
length of the feedlines "might" result in a more than 45 degree difference
in phase. This is probably not an issue with stacked beam antennas,
because, by design, they don't have much coupling in the vertical direction.
b) the back and side lobe performance. Small changes in phases can have
huge effects in sidelobe levels, with low gain elements. Say you've phased
for a 20 dB null with omnidirectional antennas. If misphasing results in a
0.3 dB decrease in forward gain, and the power that formerly went to the
front goes to your null, that -20 dB null is now a -10dB null. In a
stacked beam situation, the antennas are going to define the back and
sidelobe response. If, instead of omnis in the above example, you have
antennas with a 20dB F/R, what was a 40 dB null before is now a -39dB null.
c) Mismatched phasing lines will put the main lobe at different take off
angles than you expect. Since you'll probably be figuring out which steering
to use empirically (i.e. you try the various combinations at a given time,
and see which one works best, right now) the absolute takeoff angle isn't
all that important. The philosophical benefits of stacks are that it gives
you some flexibility and is easier to implement mechanically (as oppsed to,
for instance, just making a single antenna that's twice as long).
----- Original Message -----
From: "Bill Tippett" <btippett@alum.mit.edu>
To: <towertalk@contesting.com>
Sent: Tuesday, March 15, 2005 6:23 AM
Subject: [TowerTalk] phasing antennas
> I wrote:
> >If you look at a model of a stack,
> the pattern is fairly insensitive to slight differences in phase.
> I recall seeing little difference for ~10 degree phase difference,
> which is far more error than you would get by simply cutting
> lines to the same physical lengths, even on 6 meters.
>
> I did a quick check of this using an old (pre-YO7)
> Eznec model when I was modeling a stack of my KLM 610's.
> This is for a two antenna stack, as you will have on 6m.
> I used 1 wavelength spacing:
>
> Phasing Gain (over flat terrain)
>
> In-phase 16.85 dBi (all at 8 degrees TOA)
> Bot +10 16.83 dBi
> Top +10 16.81 dBi
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