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Tonna 2x19 70 cm + polarity switching



Hi All,

First of all thank you to all who responded to my question
about the Tonna 2x19 element 70 cm antenna.

Most are quite negative about this antenna. A few are happy with it.
One of the problems is a rather high VSWR in our part of the 70 cm band. 
In my case about 1.9:1 in the worst case. 
Another problem is that the driven element (a folded dipole) is not designed
to suit a cross yagi. You have to modify the housing yourself in order to get 
the cable out in parallel to the antenna boom. If you do that it works better - 
but not as good as one could expect.
This makes it very difficult to achieve a true circular polarization.  
In my case, I made a few simulations using EZNEC and found that the driven 
element impedance changed a lot from one end of the band to the other. 
Another result of the simulations was that the rear driven element and the front 
driven element did not change impedance the same way.
I found that I could correct the last problem by making the distance between the two long parts of the 
rear driven elements a bit smaller. In my case 3 - 5 mm did the difference. 
After all this experimenting I a not happy with the antenna - but it works OK now.

I have asked Tonna to comment on the results - but they have NOT answered to my questions.

The switching between Right Hand Circular Polarization and Left Hand is useful at times. I have 
tested it on FO-20 and FO-29. In both cases it has a clear advantage to fixed Right Hand Circular 
antennas. 
On some of the UoSATs it does not make much difference. This is a good indication since they are 
linearly polarized.
On AO-27 it can make all the difference between a very good signal and no signal - much to my 
surprise. The antennas on this satellite are linearly polarized.
On AO-10 it sometimes makes a lot of difference to change. This is in good agreement with results I 
have seen before.

After a couple of month experimenting with the switch I feel it is safe to say that it is an advantage to 
be able to switch - but I think it is much better to use linearly polarized antennas if you are a 
newcomer to satellite work. The disadvantage is not very big.
FO-29 change polarity very quickly - at least when it is to the West of me here in Copenhagen. That 
makes the operation a bit difficult if you use switching.

  73 de OZ1MY/Ib

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