[amsat-bb] Crossed polarity dual band single feed yagi

Rico van Genugten rico.van.genugten at gmail.com
Thu Oct 20 14:09:22 UTC 2016

Hi Pedro,

I have thought about this subject a lot. How to build an antenna that is
single boom, dual band, circularly polarized. I have come to the conclusion
that it is pretty difficult due to the following considerations:

 - To create a circularly polarized antenna a phase shift of 1/4 wl is
needed. A phase shift can be obtained through physical distance on the
boom, or a difference in feed line length.

 - When a phase shift of 0 is produced, for instance when the elements are
on the same location on the boom and there is no feed line difference, you
will end up with a diagonal linear polarized antenna. Think about it: You
have two perpendicular linear antennas in the same plane, the resultant
will be the summation of the two antennes: a diagonal antenna.

- When a phase shift of anything in between 0 and 1/4 wl (or any multiple)
is produced, the antenna polarization is somewhere between linear and
circular: it will be elliptically polarized. This will mean that it is not
completely deaf in one plane like a linear antenna, but will have a gain
optimum in a particular plane and a gain minimum in a plane perpendicular
to it.

- 1/4 wl spacing on UHF is (obviously) not the same as 1/4 wl spacing on
VHF, but if you use open-sleeve feeding (2 feed points) there is no way to
manipulate either the position on the boom or the feed line difference for
VHF and UHF independently

- If you use 4 feed points you have full control of the placing of the four
antennas on the boom and the feed line differences and are probably able to
produce a true single boom, dual band, circularly polarized antenna, but
the VHF and UHF antennas will influence each other so strongly that you
will probably end up placing the UHF antenna after the last element of the
VHF antenna, leaving you with either a very long boom or very little gain.

My two cents. :)

73, Rico

On Thu, Oct 20, 2016 at 1:16 PM, Pedro via AMSAT-BB <amsat-bb at amsat.org>

> Hi all,
> The best of ham radio is that you can build and experiment.
> My current portable yagi is a dualband single fed (VHF element) home
> built, from DK7ZB designs, 4+5 elements version. VHF has 9.25dB gain
> 1.03SWR, UHF 8.67dB 1.12SWR.
> It works like a charm.
> Now that I'm on the final stages of condo permits for a tower - although
> it's a tower on top of the building there are still space limitations - my
> goal is to build a crossed polarity set of my portable antenna. Something
> similar to what Gulf Alpha had, but instead of 4 feed points, one for each
> band/polarisation, I'll have only two, combined into one impedance matched
> feed line to the shack.
> Due to the required materials for the feed points and the elements
> positions I came up with 2 versions where the vertical antenna is ahead of
> the horizontal by 3 inches and 16 inches. My understanding is that circular
> polarisation would require a specific position for the vertical antenna
> plus the proper cable circuit for it.
> I'm aware that RHCP would give me a stable signal reception, rather than
> my setup even tilted 45 degrees instead of plain horizontal/vertical. I saw
> PU3GUO report on his VHF only antenna while receiving ANDE.
> I've ran the model in 4nec2 for the two options, 145MHz and 435MHz, and
> using 5W. Here are my findings.
> 3 inches separation:
> Both antennas 1.03SWR
> Combined gain 9.25dB
> Both antennas 1.12SWR
> Combined gain 8.67dB
> 16 inches separation:
> Both antennas 1.03SWR
> Combined gain 9.24dB
> Horizontal antenna 1.12SWR; Vertical antenna 1.49SWR
> Combined gain 8.27dB
> My conclusion is that advancing the vertical antenna by so many inches
> will result in interference from the horizontal one in such way that the
> gain is lower and SWR increases.
> Gain in UHF is lower, expected as same happens in my current portable
> antenna, and that's because it's being fed in the VHF element.
> Has anyone built something similar that can share any comments?
> Would there be any other configuration possible for a single boom?
> Thanks in advance.
> 73s de Pedro
> CU2ZG, HM77
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