|fahnen/3.jpg Types of Coax|
Types of Coax
This is the script for the Houston AMSAT Net and EZSATS. Authorization isgiven for the use of this information over any ham band. Please give credit forthe script where credit is due.
Last time we talked about what type of radios you can use for workingthe satellites. Now that you have a radio in mind, you are going toneed some coax to run to your antennas. So, tonight we will talkabout coax.
If you take a look at your total satellite station, we need to considergood antennas, good coax and a good receiver in the total solution. If youskimp on any one of these items, you could suffer tremendously by notbeing able to hear a weak DX station.
Coax is a very important part of a satellite station. It is almost asimportant as the antennas you choose. As you might know, all coax is notthe same. Some of the common types of coax that you have probably heard of areRG-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 iscalculated 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 couldpurchase that would have virtually no loss would be one where the centerconductor is suspended in air with the shield not touching the conductor.
As this is impossible, various types of materials are used to hold theconductor. These materials are normally foam and plastic. Some of theselook 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 andon 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 havesignificantly greater gain by choosing RG-8 over RG-58.
But the 2 or 3.8dB loss is not acceptable in satellite work. We wouldlike this loss to be as close to zero as possible since the signals fromthe satellites are so weak.
Now, let's look at Belden 9913/9913F. On 2m, it has a loss of 1.6 and on70cm 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 on70cm it is 1.9 dB. This -- when compared to RG-8 or RG-58 -- would be afantastic gain. Belden 9913 is a solid core coax. I use a coax that issimilar to Belden 9913 but is a stranded core that makes it easier tomanipulate and will not crack as easily when it gets cold.
Belden 9913F is their new product that was not available when I set up mystation. It is identical in properties to 9913 except that it is a strandedcore 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 andhalf 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 fromthe 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 andbasically hear nothing.
When making your selection for coax, it would be easy to select hardlinebut 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 about55 cents a foot. However, hardline might run $1.00 to $2.00 per foot, now youare talking about $200+. This might not be affordable and the 9913 cablebe more cost effective. But you should not consider using RG-8 or RG-58 foryour satellite station the losses are simply too high. They might work as astarter 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 everyconnector you add to the middle of your coax will add 1/2 dB loss. So youdo 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 sideas well. Just think how much more effective radiated power you would haveusing 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 moveyour equipment closer to the antenna and save 50 feet, you will alsoeffectively double your power and the received signal. That could makefor a very impressive station if you can get the signal loss as close tozero as possible.
Remember, you can have all the linears and all the coax and the highesttower 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 receivestation you can. You'll be surprised, if you keep this in mind, youwill be able to work everyone with minimal power and hear them evenbetter.
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