Quoting Gary McKelvie garym@garym.org.uk:
Hi to John and the List,
I'm certainly taking onboard what people are suggesting :)
I think I also have said that I have a crossed 70 cm 19 element Yagi from before, as well as rotators to enable both azimuth and elevation control of the antennas so i'm starting to think even with the fixed polarization of the 9/19 element Tonna i will need to use the elevation rotator.
You will need to use the elevation rotor for this antenna because its beamwidth is so narrow. The length of the yagi narrows the beamwidth both in azimuth, (which, for other new folks, is the 360 deg. horizontal control) and elevation (from horizon to pointing to the sky), so longer yagis rule out comfortable fixed elevation use because they will not hear the bird as it gets too far overhead. The greatest, and most intresting, challenge I found in working with satellite antennas was to start to think in both dimensions; coming from terrestrial work we tend to think of maximizing our low-angle radiation and then narrowing the beamwidth.
Now to my next though If I have this right the LEO's use 70cm as the down link and the 2m side as the uplink, so to my way of thinking I need the most gain really on the down link side.
At one point, mode V/U -- we describe modes with this shortform Uplink/Downlink, so V/U is VHF up, UHF down -- at one point, it was the favored mode. However recently people have rethought that, in part due to the amount of rouge use of 2m around the world. Thus VO-52 was designed in mode U/V (UHF up, VHF down) and is quite popular in part because of this, and there are many upcoming birds that will be mode U/V. Finally, AO-7, which runs in mode U/V quite often, came back to life some years ago, and it is amazingly fun for DX. So I'd say that you want to be able to receive both.
So would it be better to have a small 2m Yagi crossed about 4-5 elements. The 847 output on 2 and 70 is variable 5 - 50 watts on both bands all modes. If I was to go this way I would also look at circular polarization on the antennas. I would also be looking at using a duplexer to enable me to use one feed line which again would make the XYL happier as where the antennas will have to go when up is across the back lawn. I understand that will impact on the performance of the system but I willing to accept that :) So the question is RHCP or LHCP? Which is best or do I need to be able to switch from one to another?
The polarization question is also somewhat vexed. The over-riding issue is this: the penalty for using a circ. polarized antenna on one end of the link and a linear one on the other is 3 dB. This is true no matter which sort of circ. polarization or linear polarization. The penalty for using RHCP on one end and LHCP on the other is really high, let's say 20 dB. So if we go CP, we need to get it right; whereas, if we go linear, we're just down a max of 3 dB. In many people's experience, you can still have enjoyable communications with LEO satellites and throw away that 3 dB, reducing quite a bit of the complexity of the situation.
I understand that there are two reasons you should consider not having fixed polarization of rotatable yagis. The first reason is that the polarization of a bird's signals apparently can switch during a pass, due to its changing orientation. So even if you read that the downlink of a bird is LHCP, a LHCP receiving antenna will sometimes have nulls. I've not observed this myself, mind you, but judging from the above theory, those nulls might well be worse than our fading due to CP / linear combinations. Secondly, there is no consensus among satellite builders as to what links should have what polarization. If you don't want to exclude certain satellites, you need to have both polarizations on hand.
Now that is what I understand from the theory, and on that basis I have used linear yagis. Since I homebrew, I didn't need the additional hassle of building the phasing cables, etc. And, indeed, the lost 3 dB is not a deal-breaker: you can communicate very happily with short linear yagis. However, I'm planning to build toward a 7x7 switched polarity 2m beam. Right I have a 7 element design three feet off the ground, where it will stay until the ground thaws in April.
That being said, I'm not sure I fully understand the issues involved because I note, for instance, Tony's papers on easy Lindenblad (fixed, circular pol.) antennas in the 2006 AMSAT-NA Symposium Proceedings. With the RHCP 2m design he notes that communication into the usual linear birds worked well on Field Day. I would have thought that the issues mentioned above would produce a rather spotty link. I think I've convinced myself that the only solution is to build the Lindenblads and see!
As you can see I don't have a fixed idea on the station as yet. I find it easier to talk it through with people first then go get it up and running :) So thank you all for listening to me and the help as well.
Designing and redesigning your station is some of the most fun you can have in this stuff when you're not at your operating position. Thanks for letting us join in!