[amsat-bb] CP vs. LP and all that implies
n4ufo at yahoo.com
Thu Jan 22 19:24:38 UTC 2015
> What do others think?
This last fall, I put up a couple of linear yagis... 11 elements on UHF and 7 elements on 2m. I actually started with 10 elements on 2m and cut it down. I don't appear to have any problems with not being CP and/or not being dual polarity except for maybe SO-50... "the fade". It doesn't last but under a minute and it just isn't worth the expense or hassle to me to bother changing over to CP. Because if you do CP, you need to have it switchable between RHCP and LHCP... if you are on the wrong CP, the loss is just as bad as the bird being horizontal when your antenna is vertical. In short, no switch... same "fade". And to be honest, I'm not sure that fade is from polarity problems, I've only been told it is... I actually think it could be the antenna being blocked by the bird as I've also experienced it with a handheld yagi that I could adjust polarity on. The 1/4 wave whip on SO-50 is canted inward towards the spacecraft and would block the signal on the 'far side' of the craft.
As for my antennas, I think I'd add switchable dual polarity rather than circular polarity if anything. I've seen a number of stations do that. Even before that, I might add some more elements to the UHF. I have trouble penetrating the house next door when the pass gets low. But then again, more transmit power would do the same, but either way, it might not matter. I've never used CP antennas, so I cannot speak 'absolutely' but other than the possible issue with SO-50, I can't tell that I am missing anything. I have preamps and can hear most of the linear birds down to 0-1 degrees (in my clear horizon directions). Even SO-50 I hear down to 5 degrees and sometimes as far as 3 degrees with noise. I spent about $100 on a couple of used linear yagis... and they work. Can't ask for much more than that. My extra money was FAR BETTER spent installing preamps and diplexers for filters; the preamps make a huge difference overcoming coax loss and I have no desense at the antennas. Pics of my installation can be seen here: http://n4ufosat.blogspot.com/
I'm not the only sat op that was going to go CP at one time and opted to just do linear. And the cheaper antennas seem to work just fine... Diamond makes a 10 and a 15 element UHF antenna I've seen used a lot. I opted for Cushcraft... there are others. This isn't EME or terrestrial weak signal work. You are essentially talking through a repeater... a flying repeater, but a repeater. And anyone with a 2m HT can understand that you need enough to clearly 'hit' and hear the repeater... beyond that, little payoff for the extra effort.
Just my two cents, your mileage may vary, no warranties expressed or implied and batteries not included...
73, Kevin N4UFO
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