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Commercial Radio Antennas Please keep discussion related to professional, commercially used antennas and antenna systems for the two-way radio industry. Topics for the use of these antennas on amateur bands are accepted here.

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Old 07-08-2012, 3:14 PM
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Default Can I Use this Feedline on VHF/UHF

Someone gave me about 15-20ft of half inch Feed Line from which used to be used on a Telephone Pole i guess for Cable or some sort of data.

It looks similar to Heliax Line but it its shield is solid Aluminum and the Center conductor looks to be copper plated aluminum or somthing like that. I do not have anything to identify it by, no numbers or markings.


Is this useable as feedline for a Transciever on VHF/UHF?

How bad would the Loss be, Or would it be ok seeing as the center conductor is larger than regular cable?
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Old 07-08-2012, 3:55 PM
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That would be Cable TV distribution cable and 75 ohm. While it would be rather low loss I wouldn't recommend using it for your transceiver. The impedence wouldn't match either the transceiver or the antenna and the connectors would probably be more than some good 50 ohm cable would be and you'd still need some adapters to make things work right. Too much trouble and expense for a 20' run.

Thinking about it further, you may be able to modify the cable and PL-259 connectors to make them work, but you'd need to reduce the center connector on the cable to match the size of the PL-259s and getting the solid aluminum shield to mate with the PL-259 would also take some doing. Still, quite a bit of work for only a 20' run and you'd still have the impedence mismatch issue.

Last edited by n5ims; 07-08-2012 at 3:59 PM..
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Old 07-08-2012, 4:10 PM
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Is the line you were given flexible or very stiff? Does it have raised "ribs" along the length of it?

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Old 07-08-2012, 4:16 PM
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Probably 500 hardline, and as said, for cable tv, 75 ohm, especially if it was on a pole. 20 feet of it is pretty useless, unfortunately. You can use some impedance matching tricks to make it work just fine on VHF/UHF 50 ohm service, but like was said, the hassle of the connectors for such a short length would be pointless, other than just for the exercise.

We've pulled thousands of feet of that stuff out at work, and just recycle it. Every now and then a ham operator will want it and they'll use it for HF work, but usually for long runs where the effort to do the connectors pays off. Also, the mismatch between 75 and 50 ohm cable is usually negligible when concerned with SWR.
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Old 07-08-2012, 6:12 PM
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It is Very Stiff.. You can bend it but its very difficult compared to any regular cable.

It has no "Ribs" in it as something like LDF450A would have. Its a smoothe Outer Shield, Coated with black plastic

the entire line itself is slightly bigger than 1/2 Inch but the center conductor is about the same size of what a 1/2 Heliax line would be.


I can try it out and look at the power and SWR...
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Old 07-08-2012, 6:34 PM
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Quote:
Originally Posted by centuryvrproductions View Post
It is Very Stiff.. You can bend it but its very difficult compared to any regular cable.

It has no "Ribs" in it as something like LDF450A would have. Its a smoothe Outer Shield, Coated with black plastic

the entire line itself is slightly bigger than 1/2 Inch but the center conductor is about the same size of what a 1/2 Heliax line would be.


I can try it out and look at the power and SWR...
Try again, to find some data or info on the cable itself?

as other have said it's a rather short length, but it might be useful as a jumper on a duplexer or somthing?

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Old 07-08-2012, 7:09 PM
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Talking I Guess I was rong about the 25ft thing,

I took the roll of line out to look, I mesured it to be 47 feet long. I never got a good look at it until now.
Its not 25ft as i remembered it to be, its that i have 25ft of LDF450A for my repeater.


Im actually ready to put up a Dualband 8db base antenna. Would this be worth feeding it or is it better to buy some Half inch 50 ohm line?

And there is nothing written on the sides of the line... just black

as for the "Duplexer jumpers" Would it be better to use that rather than RG174 im using now? even with the 75ohm impedence...

Last edited by centuryvrproductions; 07-08-2012 at 7:12 PM.. Reason: add in about the Duplexer jumpers
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Old 07-08-2012, 10:18 PM
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The bend radius (or lack there of) would make it difficult to use for much other than long straight runs. If you bend it too far it will kink, and then it'll be useless as this will mess up the impedance and that point.

Finding connectors is possible, but they will likely be more standard cable tv type. Not a big deal if you don't mind feeding it with F connectors. You may be able to find some BNC type depending on the exact type.

Using it for your duplexer jumpers would be a pain as you won't be able to bend it very well. Since it isn't very flexible, you will risk damaging the connectors on the tops of the cans.

It'd be great if you had a tower you could use it on.
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Old 07-08-2012, 10:43 PM
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So i guess i can make some connectors or look around for some. I am going to run it through the celing, through the attic and out the side of the house up to an antenna. I will make sure the bends in the line do not kink it. ( I will bend it over a large section) for a curve, not a kink.

Would it peform any close to somthing like LDF450A?

Im guessing that the Impedence mismatch is the only issue here...? (besides connectors)


I Want to use it as feedline for my repeater (UHF) because my LDF450A wont reach the height i want to place my antenna at, I figured if i can get this to work, i wont have to buy more heliax but it sounds like a bad idea to use because of the mismatch and i do not want any issues with the signal...
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Old 07-08-2012, 10:54 PM
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Quote:
Originally Posted by mmckenna View Post
The bend radius (or lack there of) would make it difficult to use for much other than long straight runs. If you bend it too far it will kink, and then it'll be useless as this will mess up the impedance and that point.

Finding connectors is possible, but they will likely be more standard cable tv type. Not a big deal if you don't mind feeding it with F connectors. You may be able to find some BNC type depending on the exact type.

Using it for your duplexer jumpers would be a pain as you won't be able to bend it very well. Since it isn't very flexible, you will risk damaging the connectors on the tops of the cans.

It'd be great if you had a tower you could use it on.
Wow, yeah if that's what it is, is LDF 450A then that's not gonna fly as a patch for a duplexer. If you can obtain a "N" style crimper, then there are compression fittings available allowing you to pair a flexible piece of RG8U to it.
Congrats on determining it's better than just CATV line!

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Old 07-09-2012, 1:49 AM
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Quote:
Originally Posted by centuryvrproductions View Post
Would it peform any close to somthing like LDF450A?

Im guessing that the Impedence mismatch is the only issue here...? (besides connectors)
Operating under matched conditions, it would work just fine for a repeater.

Quote:
Originally Posted by centuryvrproductions View Post
I Want to use it as feedline for my repeater (UHF) because my LDF450A wont reach the height i want to place my antenna at, I figured if i can get this to work, i wont have to buy more heliax but it sounds like a bad idea to use because of the mismatch and i do not want any issues with the signal...
Finding connectors for that will be a bit of a challenge. That's the first problem. The second is the mismatch. While the mismatch is not anything that's going to hurt a transmitter, and it would be just ok (but less than optimal) for a ham station, using it for a repeater could present a few more problems.

Operation of the duplexer is dependent on matched conditions on all the ports. A mismatch on the antenna port will reduce the isolation, and could be the source of desense you otherwise wouldn't have. Most people who have used 75 ohm line for a repeater (because that's what they have) have made some effort to match it to 50 ohms on each end of the cable.
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Old 07-09-2012, 2:37 AM
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Ah, ok so Im better off using it as just a Feedline for a Transciever or a scanner antenna.
I guess I can just use it as a base station feedline.

Impedence match shouldnt decrease performance that much right? (on a two way radio 25W)
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Old 07-09-2012, 8:03 AM
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I think your best bet would be to find out exactly what you do have. Have the thing 'swept' (tested) at the frequency of use. There's no real answer to your questions until you do know what you have.
Can an impedance mismatch be all that bad a thing? It can be, especially at UHF.
The one thing you said that could make your unknown coax profitable is your using RG-174 on your duplexers. That's just not a good thing at all if their lengths are more than -very- short (inches, not feet). Duplexers are a source of loss to start with and adding the use of RG-174 really makes them a 'loosing proposition'.
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Old 07-09-2012, 1:19 PM
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As someone who was worked with many types of 'hardlines', it sounds like you have a section of either
'500' or '625' hardline from the cable company. You can find a local cable line tech and have them conferm.

I use both these types of cable in my radio systems to include both a ham and commerical repeated and the differance between the 50 ohm and 75 ohm is so very small that things have been working for over 10 years.

I have a 100w ham repeater on 2mtr with 125ft of the 500 and the swr is less then1 watt reflected and my commerical 150mhz is the same, my ham 440 100w using the '625' also only has 1.5w reflected and the 440 commerical has 1w reflected.

I also use RG6 and RG11 (both 75ohm) for my regular ham radios to include HF, VHF,UHF and have very low to no reflected.

If you know someone at the cable company, you can get them to prep and install connectors that are called 'F' ends and then you can do a 'F' to uhf or N for the radio and the antenna. In preping this type of cable, you need a special 'coring tool'.

And yes, you also do not what to use over 4 feet of RG174 on ANY frequency in any install because of the line loss.
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Old 07-09-2012, 1:37 PM
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Quote:
Originally Posted by cabletech View Post
I have a 100w ham repeater on 2mtr with 125ft of the 500 and the swr is less then1 watt reflected and my commerical 150mhz is the same, my ham 440 100w using the '625' also only has 1.5w reflected and the 440 commerical has 1w reflected.
If your antennas are actually a 50 ohm load, your long feedline runs are masking a 1.5:1 SWR. Now, that's not bad at all, really.

Quote:
Originally Posted by cabletech View Post
I also use RG6 and RG11 (both 75ohm) for my regular ham radios to include HF, VHF,UHF and have very low to no reflected.
I've used 75 ohm cable before, as well, and in practical use, it works just fine. In applications where a 50 ohm impedance is critical, such as when operating through a bench tuned duplexer, it can be a problem. Touching up duplexer tuning will cure any problems, but that requires that someone know what they're doing. The OP isn't there yet, but he's learning.
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Old 07-09-2012, 2:39 PM
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Ah Each jumper on my duplexer is about a foot.. i guess i have to change that to like 4-5 inches

and i will have to run some frequencys through it and see what the SWR is and the power output at the end of the line...

I am going to try and make some connectors for the ends. probably UHF type
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Old 07-09-2012, 9:18 PM
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Quote:
Originally Posted by centuryvrproductions View Post
Ah Each jumper on my duplexer is about a foot.. i guess i have to change that to like 4-5 inches
Why? The jumpers from the radio to the duplexer are non-critical. The critical length cable is the jumpers to the tee where the antenna connects.

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Originally Posted by centuryvrproductions View Post
and i will have to run some frequencys through it and see what the SWR is and the power output at the end of the line...
Um... How is this duplexer going to get tuned?
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Old 07-09-2012, 9:40 PM
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Wirelessly posted (Mozilla/5.0 (BlackBerry; U; BlackBerry 9780; en-US) AppleWebKit/534.8+ (KHTML, like Gecko) Version/6.0.0.600 Mobile Safari/534.8+)

Century, go back to the other thread about what kind of cable to use for duplexer jumpers and reairead it again. We(all of us that replied) 'suggested' that you NOT use rg-174 for jumpers, for a range of reasons. I am starting to get a goose egg on my forehead from banging it on a wall. Why ask for advice and then do the exact opposite?
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Old 07-09-2012, 9:54 PM
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I think I have a box of N connectors for that stuff. I could send you a couple since I will probably never use them.
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Old 07-09-2012, 11:50 PM
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(Um... How is this duplexer going to get tuned?)

Its already tuned for my 440 Freq, What i ment was i was going to transmit a range of frequerncys and see how they perfom through it. As i would use it as a feed line. then maybe use it as a feedline for my repeater...
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as for the jumpers radio to duplexer, I do have the RG174 for each side.... it is a Mobile Duplexer, jumpers between cavaties comes with built in cable.

For the antenna side of the duplexer, I have about 6 inches of the 174 wich goes to the current 25' of 1/2 Heliax for my feedline.

Quote:
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I think I have a box of N connectors for that stuff. I could send you a couple since I will probably never use them.
Thanks, yes I could use them, send me a private message or somthing, let me know...

Last edited by centuryvrproductions; 07-09-2012 at 11:52 PM..
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