This is the script for the Houston AMSAT Net and EZSATS. Authorization is given for the use of this information over any ham band. Please give credit for the script where credit is due.
|Originator:||Bruce Paige, KK5DO|
|AMSAT Area Coordinator|
|Date Posted to Internet:||1995 MAR 26|
|Updated:||1997 JUL 05|
|Updated:||2000 OCT 08|
|Segment:||Types of Coax|
Last time we talked about what type of radios you can use for working the satellites. Now that you have a radio in mind, you are going to need some coax to run to your antennas. So, tonight we will talk about coax.
If you take a look at your total satellite station, we need to consider good antennas, good coax and a good receiver in the total solution. If you skimp on any one of these items, you could suffer tremendously by not being able to hear a weak DX station.
Coax is a very important part of a satellite station. It is almost as important as the antennas you choose. As you might know, all coax is not the same. Some of the common types of coax that you have probably heard of are RG-8, RG-58, RG-213, Belden 9913/9913F, LMR-400, and "hardline".
The biggest problem you will have with your coax is line loss. The loss is calculated based on a 100 foot piece of coax and then the type of dielectric, the size of the conductor and other variables. The best coax you could purchase that would have virtually no loss would be one where the center conductor is suspended in air with the shield not touching the conductor.
As this is impossible, various types of materials are used to hold the conductor. These materials are normally foam and plastic. Some of these look like spider webs and some are thin ribbons that spiral wrap the conductor.
Using 2 meters for downlink, RG-8 would have a line loss of 2 dB and on 70 cm a loss of 3.8 dB. RG-58 has a loss of 7 dB and 15 dB respectively. As you can see between these two common types of coax you can have significantly greater gain by choosing RG-8 over RG-58.
But the 2 or 3.8dB loss is not acceptable in satellite work. We would like this loss to be as close to zero as possible since the signals from the satellites are so weak.
Now, let's look at Belden 9913/9913F. On 2m, it has a loss of 1.6 and on 70cm it is 2.8 dB. LMR-400 has a loss at 70cm of 2.7 dB and at 1 GHz 4.0 dB. Half inch hardline has a loss on 2m is only .9 dB and on 70cm it is 1.9 dB. This -- when compared to RG-8 or RG-58 -- would be a fantastic gain. Belden 9913 is a solid core coax. I use a coax that is similar to Belden 9913 but is a stranded core that makes it easier to manipulate and will not crack as easily when it gets cold.
Belden 9913F is their new product that was not available when I set up my station. It is identical in properties to 9913 except that it is a stranded core with foamed gas injected interior.
If you are going to work 1.2 gig, Belden 9913/9913F loss would be 5.8 dB, once again, the LMR-400 loss is only 4.0 dB and half inch hardline loss is 3 dB. RG-58 would be useless, its loss is 25 dB.
You can see that the better the coax, the more signal you will save from the antenna to the receiver. If you were to add a preamp to your antenna, and you had a high signal loss, you would wind up amplifying the noise and basically hear nothing.
When making your selection for coax, it would be easy to select hardline but the cost might be more than you could afford. I have two 75 foot runs, one for 2m and one for 70cm. If I spent 50 cents per foot for 9913 type cable, that is $75. I checked the price of LMR-400 (2000 Oct 8) and it was about 55 cents a foot. However, hardline might run $1.00 to $2.00 per foot, now you are talking about $200+. This might not be affordable and the 9913 cable be more cost effective. But you should not consider using RG-8 or RG-58 for your satellite station the losses are simply too high. They might work as a starter coax to get you on the air while you decide which coax to purchase.
Choose the best coax you can at a price you can afford. And remember every connector you add to the middle of your coax will add 1/2 dB loss. So you do not want to purchase 4 pieces and hook them together with connectors.
Don't forget that 3 dB is also a significant loss on the transmitter side as well. Just think how much more effective radiated power you would have using coax that has 3 dB less loss than another.
One final thing to keep in mind. The shorter your coax run, the less loss. As you can see, if you have a 6 dB loss on a 100 foot run and you move your equipment closer to the antenna and save 50 feet, you will also effectively double your power and the received signal. That could make for a very impressive station if you can get the signal loss as close to zero as possible.
Remember, you can have all the linears and all the coax and the highest tower in town. But, if you can't hear the other guy it's all useless. Keep the coax short, keep the power low and build the best receive station you can. You'll be surprised, if you keep this in mind, you will be able to work everyone with minimal power and hear them even better.
Updated 8 October 2000. Article courtesy of Bruce Paige, KK5DO (firstname.lastname@example.org). Feedback to KB5MU.