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307 (307)
| | Posted on Wednesday, July 18, 2001 - 7:33 pm: | 
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Are you undecided which coax to run up to the top of your tower or up the pipe to the antenna ? Well then this may help you in figuring out which one is best for you . There are several types of coax to be purchased anywhere from RG-58 , RG-8 , Super mini 8 , RG-8X , RG-213 , 9913 and probably a good 10 or 15 others that will work in the CB world . If you are using Amateur radio equipment that goes up into the 400 MHz range and past then you need to really sit down and take a look at the LOSS factor of the coax . ALL coax has some loss to it , in other words , if you look in the back of an Engineering handbook most likely there will be a chart telling how much loss each coax has per frequency . RG-8 has little loss at 27 MHz but will not work in the UHF area . The loss of RG-8 is at least 10 dB per 100 foot at 400 MHz . So if you are running a UHF radio at say 442.100 MHz at 10 watts into the feedline , well at the antenna with a loss of 10 dB per 100 feet , you would only have about 1 watt of power . Remember that coax is passive so whatever you lose going one way you will lose the other which means that the receive will also be weak by a factor of 10 .
In most cases coax in the CB world can be just about anything except for those people running power . Larger coax will handle the power and smaller coax will heat up . If you have a 60 foot tower , consider using Beldin 9913 . This is what they call the poor man's hard-line and at 27 Megahertz it has virtually no loss . So if you put in 10 watts you will probably see about 9.9 come out the other end .
If you are unsure that your coax is working there is a simple way to figure out if it is . In most cases this applies to base station radios because in a mobile radio the feed line run is so short . Remove the antenna from the end of the feed line (coax) and hook up a power meter into a known good dummy load . Have someone key up the radio and note the power at the meter . Remove the meter from the coax and using a barrel connector place the dummy load at the end of the coax where the antenna would normally be hooked up . Go back into the house or wherever the radio is and unhook the coax from the back of the radio . Put the power meter in line . Key up the radio and note the power at the radio side of the coax . You must use the same dummy load at the end of the coax that you started with . If all goes well the power should be about 1 or 2 watts less at the far end than at the radio . I have seen coax installations even at CB frequencies that I had 20 watts at one end and 2 watts at the other . If your coax is lossy , replace it and you will notice a massive difference . |
Tech181 (Tech181)
| | Posted on Wednesday, July 18, 2001 - 7:46 pm: |
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Here is a coax loss calculator...
http://www.timesmicrowave.com/cgi-bin/calculate
Steve
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