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

pedro at dutrasousa.name pedro at dutrasousa.name
Thu Oct 20 11:16:22 UTC 2016

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

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