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Uhf repeater antenna question

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piper451

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I'm in the process of setting up a GMRS repeater (460 uhf) and I have a feed line question. I'm going to be putting out 50w at the repeater. I understand the importance of a quality antenna and feed line. With that being said, I have RG8X that I'm planning on using. It's a 30' run from the repeater to the antenna. With all other things taken out of the equation for this discussion, just focusing on the feed line - is this a bad idea? Obviously, using something like LMR-400 would be better, but I already have the rg8x. Am I going to see a major loss in performance with the 8x? Thanks for your input.


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zz0468

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RG8X is very lossy at 460 MHz. LMR cable works poorly in duplex service like repeaters due to the dissimilar metal shielding.
 

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piper451 said:
Any suggestions?


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1/2" hardline.

Nothing less, stay away from RG-8/RG-8x/RG-213 ect. because it is not 100% shield and is lousy at UHF. Also stay away from LMR-400 for the above stated reasons.

Also make sure to use double shielded coax jumpers between the repeater and duplexer.
 
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prcguy

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I've been a commercial UHF and GMRS repeater owner/operator for probably 30yrs and all my repeaters have been considered very high performance. For a 30ft run of feedline at UHF I would use 1/2" hardline at the minimum (Andrew LDF4-50 or 7/8" (LDF5-50) if I already had it in stock.

No RG-8X, no LMR400 or any cable with aluminum foil/braid combination belongs on any repeater due to possible IMD and noise generated from the coax, especially LMR cables when they flex. No nickel plated connectors anywhere in the system if possible due to low level IMD generation, all jumpers from hardline to equipment to antenna being Andrew 1/2" Superflex or 1/4" Heliax or in a pinch very short lengths of RG-142B/U or RG-214 coax but keep them very short.

A commercial repeater system is not something you skimp on, do it right, do it first class all the way or step aside and let someone who can put it all together for you. The components don't have to be brand new or full price either, I see perfect 30-40ft chunks of 1/2" Andrew hardline at local swap meets with N connectors installed for under $50. If you were local to me I probably have a 30ft chunk with connectors to give you.
prcguy

Any suggestions?


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DisasterGuy

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I agree, I would use nothing less than 7/8" heliax although 1/2" for 30' would be fine. For the antenna I would (and do) use a DB-424.


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902

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I absolutely concur with the comments about using LMR400 or 9913 in duplex service and for line loss, and agree with using some type of corrugated outer-conductor cable ("Heliax" is a name brand, and it's not helical anymore but annular, anyway). LMR400/9913/RG-8 is too noisy and too lossy. RG-8X would make the entire venture not even worth the doing.

I have used 1/2" cable on UHF for a 30 ft. run, but I also prefer 7/8."

I'll reserve comment about the antenna system - except for this: you can skimp on just about everything except for: 1) your cable; 2) your duplexer; and 3) your antenna. Three most important parts of any system. Cheap out on those and even the best repeater will sound like garbage.

And, don't forget about lightning suppression and grounding/bonding.
 

freddaniel

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The last consideration, that is often overlooked is connectors.

I use RG-142B/U or RG-214 coax for my jumpers as they withstand more abuse than Superflex or Heliax. Often times when someone bends a Heliax type cable close to the connector, it shorts out. With RG-142B/U or RG-214 coax using crimp connectors, this never happens.

In the last 15 years, as the price of nickel has increased, many manufacturers have shifted to other look-alike alloys, that are not good at VHF and UHF. I have also seen counterfeit connectors and adapters flood the market from China, with name brands stamped on them. For repeaters using duplexers, you must use SILVER PLATED connectors with RG-142B/U or RG-214 coax.

To prove my point, last year Motorola [finished goods dept] dumped on Ebay at least 40 new dB Spectra DB-4060 VHF duplexers, that did not meet specification. I purchased 4 units for about $95 each. They did meet spec on the bench, but in the field they failed. The problem was the cable harness and connectors. I replaced with all silver plated connectors and everything was perfect.

Not all connector plating is equal.
 

PACNWDude

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I must have ended up with some of those duplexers as well.

I have definitely seen the cheap knock off connectors flooding the market. Now, everything I use for personal use and for work comes from Tessco. However, even that has resulted in some "fake" or "knock-offs" getting through.
 

12dbsinad

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I'm still a big fan of solder on center pin connectors. These "compression" center pin configurations on newer connectors always worry me with full duplex operation.
 

cmdrwill

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I'm still a big fan of solder on center pin connectors. These "compression" center pin configurations on newer connectors always worry me with full duplex operation.
I second that. The antenna and coax/feedline are one of the most important part of a 'repeater'.
 

902

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I must have ended up with some of those duplexers as well.

I have definitely seen the cheap knock off connectors flooding the market. Now, everything I use for personal use and for work comes from Tessco. However, even that has resulted in some "fake" or "knock-offs" getting through.
I use Tessco and Hutton both.

There seems to be some quality control issues and cost-saving concessions in a lot of new products. I have a lot of problems with a particular manufacturer of interseries connector. That's a difficult one because they seem to be prevalent in the market today. Any eccentric load on them snaps the metal or causes the captivating spring to pull out of the plated brass. Just this weekend I was going from an N female to PL259, bumped the cable and the spring sproing. That was NEVER seen in the old days. Some of these things are sheer junk and if you have to use them, you'll need to hard-mount everything first and protect the connections. It's worth it to look for the old silver connectors and adaptors (especially the Amphenol adapters) at hamfests. You can recognize them because they tarnish with air exposure.
 

12dbsinad

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I use Tessco and Hutton both.

There seems to be some quality control issues and cost-saving concessions in a lot of new products. I have a lot of problems with a particular manufacturer of interseries connector. That's a difficult one because they seem to be prevalent in the market today. Any eccentric load on them snaps the metal or causes the captivating spring to pull out of the plated brass. Just this weekend I was going from an N female to PL259, bumped the cable and the spring sproing. That was NEVER seen in the old days. Some of these things are sheer junk and if you have to use them, you'll need to hard-mount everything first and protect the connections. It's worth it to look for the old silver connectors and adaptors (especially the Amphenol adapters) at hamfests. You can recognize them because they tarnish with air exposure.
I run into the same problem. Dont worry it's not just connectors...Polyphaser lighting protection, guess what, made in china is the first thing you will read on the box.
 

902

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I run into the same problem. Dont worry it's not just connectors...Polyphaser lighting protection, guess what, made in china is the first thing you will read on the box.
Lovely. Thank you for the heads-up.

I need to contact their corporate for a list of what is not made in China and very possibly sub them with something else - if there is such an animal anymore. I've made a conscious decision to pay a little more for quality control (and not have my purchases come back to me in the form of an aircraft carrier parked off our coastline someday).
 
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