KC0CSE
Member
need your help : VHF business band radio rated at 25 watts..antenna going up some 15 to 20 feet in the air..what would you recommend for coax with pl 259 ends?
Thanks Mike
Thanks Mike
The right Coax
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n5usr
Member
How far does it have to be able to get? If you're only hitting a repeater somewhere nearby, you could use just about anything you wanted. If you're trying to make those 25 watts go to the horizon, you'll want to be a little pickier. Especially if you will be working simplex instead of through a repeater.
VHF isn't terribly bad about loss, but for a 30 foot run (allowing a bit inside to hook to the radio) at 155 MHz:
RG-58A/U has 1.71 dB loss
RG-8X has 1.27 dB loss
RG-8 has 0.727 dB loss
9913 has 0.481 dB loss
LMR400 has 0.463 dB loss
Pick your price point!
None of those loss figures are particularly awful - even with the RG-58, for 25W in you're going to get 17W at the antenna. But LMR400 will have 22.5W at the antenna! (This also works the other way for receive, of course.)
Personally, I'd use LMR400 if available just because I do a lot of weak-signal work, but RG-8 wouldn't be objectionable.
Edit:
If you want to run some different numbers for yourself, the loss calculator I use is here:
http://www.ocarc.ca/coax.htm
It lets you pick different types of coax, and plug in the frequency, line length, power input.
VHF isn't terribly bad about loss, but for a 30 foot run (allowing a bit inside to hook to the radio) at 155 MHz:
RG-58A/U has 1.71 dB loss
RG-8X has 1.27 dB loss
RG-8 has 0.727 dB loss
9913 has 0.481 dB loss
LMR400 has 0.463 dB loss
Pick your price point!
None of those loss figures are particularly awful - even with the RG-58, for 25W in you're going to get 17W at the antenna. But LMR400 will have 22.5W at the antenna! (This also works the other way for receive, of course.)Personally, I'd use LMR400 if available just because I do a lot of weak-signal work, but RG-8 wouldn't be objectionable.
Edit:
If you want to run some different numbers for yourself, the loss calculator I use is here:
http://www.ocarc.ca/coax.htm
It lets you pick different types of coax, and plug in the frequency, line length, power input.
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KC0CSE
Member
Coax
Simplex right now...no more than 5/6 miles.....thanks...I found some RG8U 15 or 20 feet of it..what do you think?
Simplex right now...no more than 5/6 miles.....thanks...I found some RG8U 15 or 20 feet of it..what do you think?
N
N_Jay
Guest
MBURGE said:
RG8 is usually only good outside in the sun for a year or two.
It is a good start.
n5usr
Member
That should be just fine.
I've not had only a 1-2 year lifespan from RG-8 (or any similar coax for that matter) - not sure what N_Jay has experienced. I've seen it outside on antenna towers for many years and do just fine. (Just took a bunch of it down earlier this month, at the house of a ham who had died.)
Do be sure to seal the outdoor connection well. Any moisture that gets into the connector will readily wick down the braid, rendering the coax useless. At the very least tape it well with electrical tape (I've gotten away with that in the past) but best is to get some "conformal" tape - it's putty-like, when you wrap it on you kind of mold it with your fingers to create a solid seal. Then cover that with a layer of electrical tape as well. The stuff I have is just called "Coax Seal".
I've not had only a 1-2 year lifespan from RG-8 (or any similar coax for that matter) - not sure what N_Jay has experienced. I've seen it outside on antenna towers for many years and do just fine. (Just took a bunch of it down earlier this month, at the house of a ham who had died.)
Do be sure to seal the outdoor connection well. Any moisture that gets into the connector will readily wick down the braid, rendering the coax useless. At the very least tape it well with electrical tape (I've gotten away with that in the past) but best is to get some "conformal" tape - it's putty-like, when you wrap it on you kind of mold it with your fingers to create a solid seal. Then cover that with a layer of electrical tape as well. The stuff I have is just called "Coax Seal".
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temode
Member
What about RG11/u?
Hey All,
After doing some reading here, I saw that rg11/u is right up there with 400. I did not see it listed on the charts in your program. I bought some IBM "military grade" off the inter net. What do you think about using a 100 ft. run of this RG11/u from a 50ft high homemade multi-band dipole to a scanner? Over Kill?, It'll Work or Bad Idea? Oh, and I'm at the base of Mt. Si. so, do yu think I'll pick up much?
You guys are God (kinda),
Temo
Hey All,
After doing some reading here, I saw that rg11/u is right up there with 400. I did not see it listed on the charts in your program. I bought some IBM "military grade" off the inter net. What do you think about using a 100 ft. run of this RG11/u from a 50ft high homemade multi-band dipole to a scanner? Over Kill?, It'll Work or Bad Idea? Oh, and I'm at the base of Mt. Si. so, do yu think I'll pick up much?
You guys are God (kinda),
Temo
n5usr
Member
RG-11 would be okay for a receive-only application. It is 75 ohm coax, so there's an immediate mismatch with a 50 ohm antenna and radio system. I used to think that mismatch would be significant, but ran the numbers once and it's really not that much.
However, if the system was intended for transmitting, I'd probably be more concerned about it. Even if everything else was *perfect*, use of 75 ohm cable means the lowest SWR you can get is 1.5:1. While that's not too terrible, things are seldom perfect, so the transmitter could see a SWR of 2:1 on an antenna that would otherwise be "okay". Many transmitters now fold back their output to protect the finals when they encounter high SWR, so not only is there inefficiency loss, now the radio simply doesn't put out as much power in the first place.
At least, many HAM transmitters now do that. I don't know if commercial systems do, or they may be built more rugged and just go for broke...
The one exception, where I've seen many people go ahead and use 75 ohm coax for transmitting antennas, is in the case of CATV hardline. It is (was?) often possible to get "ends and pieces" from the cable company that they would otherwise toss, that are up to 75 or even 100 feet long. Many people have gotten that to run out to and up their towers, as the expense for 50 ohm heliax or hardline is otherwise quite high.
However, if the system was intended for transmitting, I'd probably be more concerned about it. Even if everything else was *perfect*, use of 75 ohm cable means the lowest SWR you can get is 1.5:1. While that's not too terrible, things are seldom perfect, so the transmitter could see a SWR of 2:1 on an antenna that would otherwise be "okay". Many transmitters now fold back their output to protect the finals when they encounter high SWR, so not only is there inefficiency loss, now the radio simply doesn't put out as much power in the first place.
At least, many HAM transmitters now do that. I don't know if commercial systems do, or they may be built more rugged and just go for broke...
The one exception, where I've seen many people go ahead and use 75 ohm coax for transmitting antennas, is in the case of CATV hardline. It is (was?) often possible to get "ends and pieces" from the cable company that they would otherwise toss, that are up to 75 or even 100 feet long. Many people have gotten that to run out to and up their towers, as the expense for 50 ohm heliax or hardline is otherwise quite high.
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