This is a really nice review... thanks Bruce...
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Bruce Robertson Sent: Thursday, December 07, 2006 8:08 AM To: [email protected] Cc: [email protected] Subject: [amsat-bb] Re: Beginer Antenna Selection
Quoting Steve Raas [email protected]:
Well here is my 1st question.
In researching what is needed for working sat's , I have found that I am looking for an antenna that has RHCP, although selectable RHCP & LHCP is avail.. I just cannot justify the added expense for a 5% probability of an improved signal.
Steve:
What an excellent question! My answers and advice below are based on some experience and on reading this list for the last couple of years. I hope those who are more knowledgeable will correct any errors.
Antenna selection is partly determined by your rotation scheme. If you are using azimuth rotation alone, which I and many others would recommend and you suggest below, the following advice pertains.
I would strongly recommend against *any* circular polarization to start. Use a linearly polarized antenna of modest length instead. If you do get a circularly polarized ones for 2m and 70cm, they should have switchable polarization. This is because different birds use different polarization for their antenna array, and in LEO they can end up reversing polarization.
However, adding switchable circ. polarization adds enormously to the cost of the array or, in the case of homebrew, to the risk of muddling the thing up.
That being said.. Are 'Typical' terrestrial SSB Yagis RHCP? I am looking at the following antennas for known performance vs. price:
No, they're not. Yagis can produce circularly polarized signals if two of them at right angles are fed in proper phase. Thus, a yagi-like antenna that is circularly polarized will look like a combined vertically-oriented and horizontally-oriented yagi.
2m - DPM144-5LVA (http://www.directivesystems.com/lva.htm)
430-440 - DSFO432-15RS (http://www.directivesystems.com/DSFO432-15.htm)
Tuning on this band can be very touchy. If it says it is for 432, I'd give it a pass: you're going to be up at 435 MHz.
You may wonder why I'm looking @ such small antennas and I will answer that My main coaxial run from rig to preamp may be at most 30'.
These are not small antennas for LEO work. They are, in fact, too large if you are planning to use an azimuth-only rotation setup, which is really all you need (esp. with preamps on the mast). I'd go much, much smaller.
As a sort of point of comparison, I'm upgrading from homebrew 8-el on 70cm and 4 el on 144 with fixed elevation (az-only) only in order to work the bottom 5 degrees of elevation and DX into W. Europe. That modest setup has garnered me hundreds of contacts and a world of fun. Like many on this list, I used the WA5VJB 'Cheap Yagi' design. It works. A recent pdf is at: http://portal.ucpel.tche.br/py3vhq/home/antenas/uhf-sw.pdf
My guess is that any commercially-built array will be much more than the cost of a 2m/70cm SWR meter (you can even use a FT-817 as a poor-man's version!) plus the materials needed for a Cheap Yagi array. You might try some of the other designs that people have had success with, such as the Texas Potato Masher II.
All of these are predicated on the fact that a longer boom yagi will make your (fixed az.) station *less* fun to work with. Greater gain comes at the cost of a narrower beamwidth, both vertically and horizontally. You'll therefore find worse nulls as the bird goes overhead, and you'll find the adjustment of the az. rotor will be unpleasantly touchy.
There is a long-running debate about the optimal gain pattern for fixed el., but I'd say my 4-el. 2m antenna has a shade too little directionality for my liking. Still, I've used it for over a year now. The 70cm antenna is a champion in my books.
I'm
probabaly going to feed them both with an LMR-400 type cable, possibly LMR-600 on UHF but with such a short run and loss under 1.0db I don't know nor have decided if it would be worth it. Also I plan on mounting these ( or similar yagis ) @ a 30deg angle with a simple rotor as an AZ/EL rotor is not an option at this moment due to cost. So to sup up my initial statement I don't need another 3dB of gain to make up for line loss, So in theory I can keep the antennas smaller.
30 deg. is pretty high, even with the short yagis (brutally so with long ones). Unless you will not get a signal to the lower elevations, due to foliage, houses, etc., I would experiment with 10 deg. to 20 deg.
You've probably read that you need to put together the 'best receiving station you can'. You may also, as I did, fear that people will label you an alligator if you can't hear very well. In practice, folks on the birds are very understanding if you use modest power and are beginning. What's frustrating are the people who have been around for ages, put booming signals into the bird and don't seem to be able to hear anything but a signal equally booming. Because one can hear one's own signals, the problem is quite obvious to others.
A last thought, which doesn't respond to your questions, but might be as much fun for you as it was for me: one of my first satellite antennas was a 70cm 1/4w groundplane. I just soldered some house hookup wire to a bnc connector: two for a groundplane, one for a vertical element. With a 6' run of RG-58 and a wooden roof, I can hear many birds in CW mode: LO-19 (booming!), the FM of AO-51 (listening in CW mode), GO-32. No rotating, no nothing. Granted, in my experience such an antenna doesn't quite suffice for two-way communication. But it reduces the number of variables in setting up a computer-aided station, if that's the way you want to go, and AOS prediction in any case. It is so cool to just listen to the bird come up, explore the elevation at which it is heard, listen to fading patterns, and observe doppler. Recreate the OSCAR 1 experience in about 5 minutes of antenna building!
Later on, when something goes wrong with your outdoor array, you can always go back to this antenna as a baseline. In fact, when I lost my 70cm antenna to moisture in the feedline, I used this antenna a) to confirm the problem; b) to transmit up to VO-52! It's also a great way of testing an unknown receiver on 70cm: if LO-19 doesn't come in LOUD with the 1/4w groundplane, I'd say that receiver is useless for LEO work.
Similarly, a three-element yagi on 2m will do a great job of picking up VO-52. I lashed one together, and pointed it by hand within my wood-frame house. Not R5 copy, mind you, but great fun and cheap. It will even copy AO-7.
73, Bruce VE9QRP ----- End forwarded message -----