|fahnen/7.jpg Types of Antennas|
Types of Antennas for Satellite Work
This script from the Houston AMSAT Net was written by AMSAT Area Coordinator Bruce Paige, KK5DO. Authorization is given for the use of this information over any ham band. Please give credit for the script where credit is due.
This should be the final step in getting your satellite station up and running. What type of antenna should I get? Well, the best type of antenna for working the OSCAR satellites are circular polarized antennas. These are antennas either for 2m or 70cm that have elements in both the horizontal and vertical plane. They are phased so that the signal normally rotates to the right. This means they are right hand polarized. Some manufacturers make relay switches which will change the polarity from right to left hand. We will talk about this in a minute.
The circular polarized antennas are the most expensive. Be prepared to spend around $500 for a pair of 2m and 70cm antennas. You do not have to purchase both antennas from the same manufacturer. If you like brand X's 2m and brand Y's 70cm, that is fine.
The antenna plays a very important role in working the satellite. It is the first part of your station that will receive the signal and the last one to transmit it. If you have a crummy receive antenna and a good transmit antenna, you will be calling CQ and lots of people will hear you but you won't hear them.
Now, let's say that these antennas are a bit too expensive for you right now. You can get away with using a single plane antenna. It does not matter if you mount it vertical or horizontal when operating the satellite. The problem you will find is that the satellite is spinning and you will hear the signals fade in and out. This is eliminated with the circular polarized antennas. There are many Europeans that use a single plane antenna.
One thing that is mandatory if you are to work the OSCARs is that you have a means to point the antenna at the satellite. This means having an azimuth and elevation rotor. If you are new and going to purchase your equipment you should think about the Yaesu 5400 rotor. This has both azimuth and elevation controls in one box and has a special connector so that you can later add computer control of the rotors. There are many good products on the market for controlling the rotors from stand-alone boxes to those that plug into the parallel port to those that have a card that goes in the computer. A simple XT computer will work just fine for the tracking, speed is not important.
I still do all my tracking by hand. I only have to work when operating FO-20 and the RS satellites. You must have the rotor controls if you plan on operating the Pacsats.
Some manufacturers make relays that will switch the polarity of the antenna. My antennas do not have this switch and I have found that in better than 85% of the time, I have no problem in working anyone. In about 10% of the time, it is difficult and would have been better if I could switch polarity. In the other 5%, I could not work the station.
The polarity switch could add about $50 to the cost of the antennas. And, if you switch the polarity while you are transmitting, you could blow the relay. There has also been talk that some relays are not properly sealed from the elements and need to be replaced. Check with some friends as to what they did to weather proof theirs and if they are happy with the relays. Since I don't have the switch, I cannot comment on any of the brands that sell them.
Some of the brands out there making good antennas for satellite use are KLM, M2, Hygain and Cushcraft. KLM (now out of business), Hygain and Cushcraft make antennas with a polarity switch. M2 now makes a polarity switch for their antennas. I have the M2 2MCP22 and the 436CP30. Rather than bore you with all the specs for these antennas, basically the 2m antennas are about 19 feet long and the 70cm antennas are about 10-14 feet long. The gain on the 2m antennas is about 11-15dB and the gain on the 70cm antennas is about 15dB.
As you can see, all the antennas are roughly the same length. You also need to make sure that you mount them high enough so that they can elevate to 90 degrees straight up and they can rotate 360 degrees. This can be a problem if you think you might have to guy the tower. The guy wires could be in the rotation path of the antennas. Check this out before you sink the tower in the ground and find it's not going to work after you have mounted the antennas. Also, you want to keep your cable runs as short as possible to minimize your signal loss.
Next we have a specialty antenna. M2 makes an eggbeater antenna. It is two loops 90 degrees apart. That is why they are called eggbeaters. They are great for working the low orbit satellites from a mobile station but do not work very well on AO-10 and AO-13. Their signal is not directional enough. I remember working a station in Alaska that was mobile and had to cut the QSO short because a policeman that pulled up next to him wanted to know what he was doing.
For working RS10 and RS15, you only need the 2m antenna for transmitting and a rotor to point the antenna. For receive you can use a dipole, vertical or beam. Each receive antenna will give you dead spots so you might need to use two different types of antennas and switch to whichever receives the best. I worked RS15 for the first time last week and I found I could hear very well from the beginning to the end of the pass with my inverted vee. Its apex is at 27 feet and one leg is 29 feet and the other is 59 feet. I was surprised how well the signals sounded during an 11pm pass. But the following morning, the signals sounded very low and there was more fading in the signals.
Carefully check out the different antennas so that you can make a good decision as to which one will suit your needs the best. I have used my antennas at Field day and of course at my home.
I have worked people using all the brands mentioned above and each has sworn how good they were. You can still get a good buy on the old Cushcraft AOP-1 antennas but they are not as good as their new line.
Satellite communications is a lot of fun. Especially that first QSO on your new station. When you get the antennas up, the coax run, the radio plugged in and you point the antennas to where the satellite is supposed to be, hear someone and call CQ. That first time when you hear your own voice echoed from the satellite is very exhilarating. You made it, you are now a satellite operator. I even felt this way last week when I made my first RS15 contact. That was just as exciting as my first one was on AO-13. It is amazing how we manage to find a 70 kilogram hunk of metal orbiting the earth, send a radio wave to it and there is someone somewhere that is doing the same thing and we have a QSO. Simply amazing.
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