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GMRS / FRS - Discussions related to GMRS (General Mobile Radio Service) and FRS (Family Radio Service) communications

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  #1 (permalink)  
Old 09-21-2016, 8:46 AM
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Default Antenna coax question

I currently run 20ft of lmr400 coax for my gmrs base. My swr is near flat. I have a pretuned 5 element yagi. I've been forced to relocate my antenna to the chimney, which will actually put my antenna about 15 ft higher than my previous setup. Only problem is that I'll need more coax..or is it a problem at all?

I'm wondering if I can expect to see a dramatic change in my swr. I expect there to be some loss..I don't see that being a major issue.

Thoughts?
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Old 09-21-2016, 9:04 AM
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Swr should not change , but still keep coax as short as possible , at UHF losses can add up quickly
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Old 09-21-2016, 9:46 AM
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SWR should not change with coax length, however there are specific situations where if you land on the right multiple of 1/4 wavelength your meter may show a change. In reality if you are using the proper coax and connectors, there won't be an issue.

There will be, of course, some additional loss, 0.41dB for 15 feet of LMR-400. Shouldn't be noticeable unless you are really running out on the fringes of your link loss budget.
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Old 09-21-2016, 5:10 PM
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If you are replacing the coax run with the longer one, my guess is the range benefit of being higher will more than compensate for the added loss. If you plan to splice the coax, terminate the ends and use a barrel (F-F) connecter, remember to weatherproof the splice. Actually, if done correctly the SWR should go down as the 'reflected' power is attenuated more-however for the short lengths you mentioned, I doubt the difference will be noticeable even on a meter.
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Old 09-30-2016, 5:50 PM
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Thank you for all of your replies. I'm wondering now how well, or poorly, my yagi would perform if I put it in the attic....typical home-wood frame, stucco on the outside
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Old 10-01-2016, 2:41 PM
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Whats the roof composition?

Almost all building materials will have some RF suppressing characteristics. It will vary depending on material, thickness and frequency.
If the signal is strong enough, the minor loss from an inch or two of plywood and wood shingles probably isn't going to be enough to kill your reception.
What you can run into is putting the antenna down low enough that signals get blocked by other structures. Also, you are putting the antenna closer to potential noise sources.

One sure way to test this is to just give it a try.
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Old 10-01-2016, 3:05 PM
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Coax always has some resistance, so if any signal is being reflected back from the antenna to the transmitter, the loss in the coax can "eat" some of that, so an SWR meter at the transmitter says 1:1 while the same meter moved up to the antenna might say 1:1.5, or higher.

With only 20 or 35 feet of cable, that shouldn't be an issue for you. And the SWR at the transmitter is what is important to you (so it doesn't blow up or power down) as opposed to the SWR at the antenna, which is more about antenna efficiency and output power.

If you have the usual "UHF" (PL-259, SO-239) connectors, they create impedance bumps, again not a huge issue but still a small one. Adding the extra connector and barrel to extend your cable, will add some impedance bumps and loss. Not a lot, but some, along with a potential place for water intrusion. I wouldn't worry about it unless you are really tweaking the system, but with a whole 35' of cable involved, I think I'd personally just buy one new cable run and not worry about the little stuff. Making sure the cable ends are properly terminated and the new external antenna connection Is waterproofed properly, will be more important.
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Old 10-01-2016, 4:28 PM
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Well, I'm gonna go with a 50ft run of lmr400 in white (keep the landlord happy), and use the other 20ft of lmr400 if necessary.

The house runs north to south width-wise, and my yagi needs to basically be pointed north. So the only obstruction I'd be dealing with is the usual stucco, chicken wire, and possibly plywood (not sure of the exact composition of the wall). And if I were to place it at the highest possible point in the attic, the other houses to the north are below me, making the only obstruction the wall of my house.

I saw another thread about this very thing, someone told him/her to try it, and there were no further updates to that thread......
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Old 10-01-2016, 6:22 PM
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A cynic might say, "Oh, so you want to put your yagi inside a Faraday Cage. That's good."

Stucco, with layer(s) of chicken wire, hardware cloth, or other metal screening in it, can never be a good thing to point an antenna at.

Just saying.
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Old 10-02-2016, 7:47 PM
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Quote:
Originally Posted by Josh380 View Post
So the only obstruction I'd be dealing with is the usual stucco, chicken wire, and possibly plywood (not sure of the exact composition of the wall). And if I were to place it at the highest possible point in the attic, the other houses to the north are below me, making the only obstruction the wall of my house.
So basically the antenna is going inside a Faraday cage, and it is going to work like crap. You need to get it out of the attic if you want it to work at all.
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Old 10-03-2016, 3:51 PM
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Hurry up. You're missing all the fun
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