Batboard

I need your recommendation for 1/2" coax. This will be for a 45W commercial repeater, 450Mhz, located 2 floors below the roof so estimate about 50-70 feet of length. Cable run will be in dedicated conduit, so it doesn't have to have plenum rating.

From my very limited research, I'm thinking about going with Andrew LDF4-50A. It seems to have lower loss than FSJ4-50B which is what we're using for a shorter run in another building. I looked at Times LMR-500 but its loss is greater than LDF4. Any others I should consider?

Go with the LDF4-50A. It is the "standard" for 1/2" coax. You can also use the RFS, Commscope, or Eupen equivalents. I don't really like the Commscope though because the outer conductor is smooth and not corrugated.

The FSJ is a Superflex coax designed primarily for small jumpers and transitioning from something 1-5/8" to the radio equipment, etc. It's not really intended to be used for the main feeder.

The LMR-anything is good coax, but it isn't good for duplex operations because the differing coefficients of expansion between the aluminum foil shield and copper braid cause noise in the system.

Go with the LDF4-50A and be happy.

Let me know if you need a quote on cable, connectors, etc.

Jeff

Thanks Jeff!

The LMR-anything is good coax, but it isn't good for duplex operations because the differing coefficients of expansion between the aluminum foil shield and copper braid cause noise in the system.

Really? I've never heard anyone mention this, there is repeaters around here using LMR-600 for duplex use without problems or noise.

I have never personally experienced it, but I have heard people tell stories about it. It makes sense from a theoretical standpoint. I would suspect it happens mostly in climates where there is a large temperature differential between night and day.

Personally, I wouldn't use LMR for a repeater. Not because it's LMR, but because I prefer Heliax. LDF4-50A is about 20% more expensive than LMR-600, but LDF4-50A has slightly less loss, and the accessories such as hangers and grounding kits are more readily available.

I think LMR is great stuff, and I'm not knocking it, I just prefer the Heliax product for repeater installs.

Jeff

I pretty much agree with the arguement for heliax.

Don't get too twiddled about miniscule differences in loss, either. Remember, it doesn't matter if you're talking about a tenth of a watt, or 1000 watts, a change of 3 dB is still "1/2 S unit" at the other end.

There's a good reason why there's a lot of LDF-50 type stuff used.

Every 1/2" installation I've ever seen has been LDF4-50A Heliax. That says something to me.

I've also heard the noise issue with LMR for repeaters. Haven't wittnessed it but enough reputable people have said that I believe it.

I give another LDF4-50 VOTE.
Its really hard to go wrong with that.

The Heliax is a good choice. You can put the connectors on with ordinary hand tools. Connectors, hangers, grounding kits are all readily available (in stock everyewhere).

I'm not adverse at all to the Commscope "Cellreach" stuff. Excellent quality, very rugged. Some customers require it, so, I've used it. You really need their tool to do the best job with connectors.

I listened a lot to the raging debate about LMR-400 and duplex use. Lots of opinion, very little data. So, I asked the application engineer at Times. He said

"For the UHF system you describe, LMR-400 is not the best choice. A much better choice is Times TCOM series that has much better passive intermod performance." He went on in another part of the email to say "the long-term performance of LMR-400 relative to IMD performance has not been characterized or studied".

I know lots of people that use it, though.

Rob
Northeast Massachusetts

Throw me in the pile with those who've heard LMR is not good for repeater use.

I know people who swear by LMR-400 for their installations but not when it comes to repeater work.

Well, LMR-400 is not 1/2" anyway. LMR-500 is.


I forgot about that earlier, but what Jeff mentioned about LMR's expansion causing noise I have also heard the same thing long ago. Haven't read any case-studies supporting the claims though.

Anyway, thanks everyone for your supporting comments for LDF4. The work order has been submitted.

Only time that I've heard of problems with LMR400 is when the connectors are not put on properly.

I've used it tons of times on UHF and VHF repeaters and it always works fine. One thing to note though, I've always used crimp N connectors on it. If you're using PL-259's and or soldering, it may change your results.

Kurt

an fellow repeater builder was setting up his 2mtr repeater i told him not to use lmr 400 for jumpers but his dad said it would be just fine .. well now that they got it up and running and the poping and crackling has started he came over and got a few jumpers[superflex] from me to test with they wanted to buy my jumpers after trying them so i gave them a hand full and now his repeater is noise free ..mixed metals dont work.