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

pedro at dutrasousa.name pedro at dutrasousa.name
Thu Oct 20 14:48:52 UTC 2016

```Hi Rico,

Thank you for that explanation.

The main problem here is that the original antenna has the UHF
elements just to help the VHF tune into UHF. It's not a true dual feed
VHF/UHF.
So if I get this concept in circular polarisation it'll always be for
the VHF leaving UHF to the mercy of a low gain.

The simulation confirms that the gain will bend into an elliptical
polarisation, but not that significant.

It's tricky alright.

Here are the calculations from 4nec2.

https://dl.dropboxusercontent.com/u/6751494/single_VHF.png
https://dl.dropboxusercontent.com/u/6751494/single_UHF.png

https://dl.dropboxusercontent.com/u/6751494/3inch_VHF.png
https://dl.dropboxusercontent.com/u/6751494/3inch_UHF.png

https://dl.dropboxusercontent.com/u/6751494/16inch_VHF.png
https://dl.dropboxusercontent.com/u/6751494/16inch_UHF.png

73s Pedro
CU2ZG

Quoting Rico van Genugten <rico.van.genugten at gmail.com>:

> Hi Pedro,
>
> 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
> PA3RVG
>
>
> On Thu, Oct 20, 2016 at 1:16 PM, Pedro via AMSAT-BB <amsat-bb at amsat.org>
> wrote:
>
>> 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:
>>
>> VHF
>> Both antennas 1.03SWR
>> Combined gain 9.25dB
>>
>> UHF
>> Both antennas 1.12SWR
>> Combined gain 8.67dB
>>
>> 16 inches separation:
>>
>> VHF
>> Both antennas 1.03SWR
>> Combined gain 9.24dB
>>
>> UHF
>> 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?
>>
>>
>> 73s de Pedro
>> CU2ZG, HM77
>>
>>
>>
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