(Reprinted from The AMSAT Journal, Jan-Feb 2006)
By Ray Soifer, W2RS
The OSCAR station described here is about as far from state-of-the-art as it is possible to get, and still have it work. Virtually everything about it can be improved upon, with better antennas, lower noise figures, etc., etc. I even wrote many of those station-improvement articles myself. So why did I write this one? I keep hearing so often, from so many people, how complicated and difficult it is to get on the linear-transponder satellites – currently AMSAT-OSCAR 7, Fuji-OSCAR 29, and VUSat-OSCAR 52 – so I want to show how easy and inexpensive it can be. The following is a true story.
During 2005 I sold my house in New Jersey and began moving W2RS into the house in Green Valley, Arizona, that I had purchased the previous year. I expect the process to go on for a long time: what ham station is ever really finished? Be that as it may, December found me with Straight Key Night on OSCAR coming up and no satellite antennas other than the Arrow hand-held beam and MFJ-1717 whip that I’ve been using with hand-helds on the FM birds. Not an acceptable situation, by any means! How to get a home-based CW station onto the linear-transponder satellites quickly and easily became the order of the day.
From the New Jersey QTH I had brought most of the station equipment I would need, all of it relatively ancient but still working: Icom IC-290A and IC-490A multi-mode transceivers for 145 and 435 MHz, respectively, as well as a Mirage B1016 2m amp and a Tokyo Hy-Power HL-90U amp for 70cm. As driven by their respective transceivers, the two amps measured 150 and 70 watts output, respectively. There’s nothing magic about any of this equipment except that I happened to have it already. Whatever you have or can beg or borrow will probably work too, if it’s even roughly comparable.
One caution concerning equipment, however: if you plan on using a single transceiver to cover both the 145 and 435 MHz bands, be sure it is capable of listening on one band while transmitting on the other. Being able to hear your own downlink is highly desirable, but not absolutely essential, on the FM satellites. It’s critical, however, to have this capability when operating CW or SSB through a linear transponder.
Now for the KISS part: the antenna. What could be simpler than a 19-inch 2m/70cm mag-mount whip? At a hamfest, I picked up a new one. It happened to be a Pro-Am MO144-440 with a matching mount. In the catalogs, the combo is listed at $47, but the hamfest was ending soon and the dealer was happy to part with it for $30. A friend# with an extension ladder (I don’t climb anything higher than two feet) put it on the roof for me – actually, he plopped it on top of my house’s rooftop heating/AC unit, which is about four feet square and provides a good ground plane. Also, my heating/AC unit is not far from the shack, so a long run of coax isn’t required. The longer the coax, the more loss.
There’s nothing magic about that particular model either. Just be sure that the one you use will handle the power you want to run. You can find lots of longer antennas, with more gain, but for reasons we will discuss shortly, I do not recommend them for this purpose. A 19-inch dual-band whip, or thereabouts, is just about right for a KISS satellite station.
Now wait a minute, you say. Don’t ground plane antennas such as that have a null at the zenith, and aren’t satellites up in the sky? Don’t you need an upward-pointing antenna? I’ll let you in on the dirty little secret of elevation rotors and all that complicated stuff: with low-earth-orbit (LEO) satellites such as the ones we’re talking about, most of the time you don’t really need to elevate your antenna at all unless it’s got a lot of gain.
The specifics will vary slightly with your latitude and the orbital inclination, but a satellite in approximately polar orbit at an altitude of 800-900 km, such as FO-29 and VO-52, will have an elevation angle of 60º or more only about 1% of the time during which it is within sight of your station. Its elevation will exceed 30º only about 10% of the time it is within range. Even AO-7, at an orbital altitude of 1500 km, will have an elevation angle of 60º or more only about 2% of the time.
I’ve had my KISS mag-mount whip up and running for a month now, and it provides good coverage of satellite passes when the elevation angle is between roughly 10º and 60º. A longer whip, with more gain, wouldn’t get coverage as high as 60º elevation.
The lower the elevation angle, the farther away is the satellite from your station, and signal strength varies inversely with the square of the distance. So, for low elevation angles, substantial antenna gain is required. To get reliable coverage below 10º elevation, you would generally want a beam of some sort: depending upon terrain, my Arrow hand-held beam can work satellite passes down to about 4º or so, while the long yagis at my old QTH could get down to 1º. You cannot point a beam from the comfort of your shack without a rotator, however, so it fails the KISS test. With our KISS antenna, you don’t even have to know in which direction the satellite is, as long as it’s within range.
What about Faraday rotation? Yes, I do notice some nulls from time to time, when the signal to or from the satellite is horizontally polarized. Most of the time, though, that’s not a problem. Satellites nowadays receive and transmit mostly elliptical polarization, so there’s usually enough vertically-polarized signal to get through. Besides, fixing Faraday rotation entirely would require you to have switchable polarization at your antennas, which also fails the KISS test.
As you’ve probably gathered by now, there ain’t no free lunch in the satellite biz either. In return for using so simple an antenna system, we are in effect giving up access to the satellite for roughly half the time when it is in sight: at elevation angles below 10º or above 60º, and the relatively few times when the polarization is purely horizontal. But isn’t 50% satellite access better than 0%?
If you’re fortunate enough to have a radio that covers both bands (145 and 435 MHz) with full-duplex operation through a single antenna connector, you may not need anything more than that dual-band antenna. However, since I have a separate transceiver for each band, I needed one more piece of equipment to bring my KISS satellite station to life: a duplexer. I saw a few at the hamfest, but they were of uncertain parentage and power-handling capacity. So, I decided to splurge and ordered a Diamond MX-72N, which is rated at 150 watts CW on 145 MHz and 100 watts CW on 435 MHz. That set me back another $50 but my KISS satellite station is now complete. The duplexer works surprisingly well, with very little desensing of the 435 MHz receiver while transmitting on 145 MHz, even with 150 watts.
At my QTH, I found the receiving preamplifiers built into the two “brick” amps to be quite adequate to hear the satellites, even through the duplexer and approximately 40 feet of coax. If you have a long run of cable, or an especially noisy receiver, you may find that you need an antenna-mounted preamp, particularly on 435 MHz. That would require mounting the duplexer at the antenna, placing the preamp in the 435 MHz line, and running two coax cables down from the roof. (Again, be sure that the transmit/receive switching system in the preamp is capable of handling your power level.) However, placing the duplexer in the shack and using just one coax cable to the antenna works well for me.
So how did my KISS station do in Straight Key Night on OSCAR? Great! I worked five SKN stations on FO-29, three on AO-7 and two on VO-52. I’ve been enjoying satellite passes with it ever since. See you on the birds!