LMR400/LMR600 for repeaters?

[DAL-COM]

In another thread it was stated that LMR400 is unsuitable for repeater use. Why is this, and does it apply to LMR600 etc? Is it the fragile nature of the foil shield, breaking on tight bends?

[kcbooboo]

For the most part, once a piece of coax is put up for repeater use and tied to the support structure, there's little or no future bending. I don't think that breaks in the foils shield are the problem.

I believe it's due to dissimilar metals being used in the outer braid that rides along the foil shield, and when the coax heats up, things expand and slide, causing noises. Same thing happens when the temp goes down. Since most repeater antennas are outside and in the sunlight, this problem is a common one.

Coax such as Heliax Superflex or Cellwave 7/8 inch use an internal copper pipe and an outer corrugated copper conductor. These are the only metal components in the coax, so you don't get rubbing of a braid material against a foil. You do pay a premium price for such hard line, but once installed, there's not much that can go wrong with it.

Bob M.

[Nand]

The link below points too a page at Time Microwave’s web site. It is an excellent cable, but even Time Microwave does not mention repeater use in their brochure where they point out many advantages for other applications. They also do not explicitly say that LMR cable is better than corrugated cables like Heliax etc.

http://www.timesmicrowave.com/telecom/pdf/engdisc.pdf

Nand.

[DAL-COM]

While I am not a shill for Times Microwave, the web page that Nand pointed to does claim on page 4 that it may be used for tower feeder runs. On page 5 they describe grounding kits. This does not rule out repeater use.

[Nand]

In the above literature, they refer to base stations not repeater stations. The reason is that noise can and most likely will be generated in this cable when used for a repeater. This may not be a problem initially but will often show up after some time. You really need perfection in a cable for repeater use. The same is true for the antenna if the construction is not really solid.

In case of a 100 Watt repeater and ignoring duplexer losses, the transmitter produces at least 70 Volts across the feed line while it is looking for a signal as low as .25 uVolt. That is a 280 million times smaller signal. Any imperfections in the cable will show up as noise when this cable is only slightly moved or vibrated. This noise can easily be higher than the signal you want to receive.
When used in multi-coupled systems this problem also will show up even if no repeaters are on the system.

In base station systems, these problems do not show because the base is never receiving while transmitting. In these cases LMR cable is excellent because of its other great characteristics.

Using it for repeater use with separate TX and RX antennas and separate feed lines should not be a problem though.

Nand.

[DAL-COM]

I agree that a 100W transmitter must deliver 70V across 50 ohms, and that receivers often have a 12dB SINAD sensitivity of 0.25uV. However they are usually operating at different frequencies, with the duplexer supplying a great deal of isolation.
Maybe LMR causes such intermod that the transmit power gets mixed with something else and ends up on the RX frequency?

What I am really asking is whether there are documented cases where a repeater has suffered desense, and it was cured by removing the LMR and replacing it with Heliax? That is the acid test.

[Nand]

It’s just that the not-really-solid connection between the foil and the braid causes fast rise time wideband noise that is picked up by the repeater receiver. I know for a fact that a local ham group did replace the LMR cable for their repeater with Heliax because of this noise problem. It was a bit difficult to convince them that the relatively new LMR400 wasn’t good enough for repeater use, particularly in a close spaced system as used on 2 meters. The problem isn’t much different from banging two tools together in front of the antenna. It also creates wide band noise that is heard in the repeater input. Another example is the C clips that Sinclair used in the female type N connector center pins on duplexers at one time. Vibration in the floor would cause these to vibrate as well and cause noise in the system. Or rub one of the coax connectors on a duplexer to the duplexer cavity. It will often cause noise if a week signal is received.

Nand.

[DAL-COM]

Does the cable have to be moving to do this? I have heard noise on a local repeater during wind-storms which can be 100mph on the Oregon coast. I thought maybe it was a loose connector but perhaps it was LMR-type cable. If so it seems that other foil-and-braid types should have this issue, eg 9913.
I did not know about the phenomenon you mentioned of banging tools together and getting rx noise, while in the near-field of an energized antenna. Probably not a good place to work.

[007]

My experience was that the cable does not have to be "moving". We had a run of LMR-400 as a jumper from our Dipole to the 1/2" hardline on the tower, and we could see noise on it with the HP analyzer as it expanded and contacted with heating/cooling depending on how long the repeater was up. We were shooting 375w up the pipe, and were seeing about 160w at the jumper (1/2 hardline run is 350')

The problem was worse in the summer because the feedline was already hot and the braid was moving on the foil as it heated/cooled.

[Jim202]

This may not be the information your looking for, but bear in mind that cable on a tower will vibrate in the wind. Most manufacturing companies make a recomended spacing between supports.

I have seen 1 5/8 coax come loose from towers due to wind vibration. This was caused by the cable clamp brackets to coming loose. There were several runs on the tower. One run had been installed with locktite on the threads of the mounting hardware. The other run was installed by a different tower crew that took the short cut and didn't treat the threads.

Guess what, the second tower crew had to go back and re do all the mounting hardware. As well as replace all the missing hardware that had vibrated off the tower.

My point here is, that if 1 5/8 coax can do this, how much movement do you think the cable with the foil shield will have. Any movement in this foil will cause noise and deteriation of the shielding quality.

Use the cable you want, but just remember that your long term installation will bite you if you take any short cuts. Use the best cable you can find and install it correctly the first time. Tower crews make a living on the short sighted people that think they know how to build a radio site.

Jim

[Richard Collier]

I use it between the tower to the antenna as a jumper from 1/2 heliax to the antenna about 6 ft. long with no problem in a repeater setup.

[radiomidwest]

It’s just that the not-really-solid connection between the foil and the braid causes fast rise time wideband noise that is picked up by the repeater receiver. Nand.

Finally. Do you know how hard it is to convince people that stranded/braided cable generates noise when used in the transmit path! Having built over 60 800mhz, 50 450mhz and 50 150mhz repeater systems in the last 16 years, I know for a fact that braided cables generate noise. If at all possible, I avoid RG142/213/214/LMR ect....
Been there. Done it.

1/4", 3/8" and 1/2" superflex is my prefered cable. The transmitter, duplexer, combiner tune up better and easier.

Also, NO ADAPTERS!!!! Buy the right connector.

Thanks for letting me vent.

[DAL-COM]

I am surprised to hear that RG142 is a problem, it seems to be the choice of many duplexer makeres like TXRX, db/andrew etc. One would think the double silver -plated braids would make excellent contact with each other.
What are the symptoms of the problem, radiomidwest?
I will start checking for it if I know what to listen for.

[radiomidwest]

I belive the problem is low level broadband noise when power is applied to the cable. It can be difficult to see on a typical service monitor, but if you have access to a good spectrum anylizer you should be able to see it. If you have a receiver with a sensitive limiter test point, you should be able to see a rise in limiter current when transmitting.

The spectrual purity of the transmitter can play a role as well. If the tx is a little rich in undesired product, that just compounds the problem.

For those experimenters out there that want to test this idea, take 2 mobiles (maxtrac/GM/1225ect, because they are wideband rx and a little noisy on tx) setup a quick little repeater. first use RG58, program the radios for close spacing (open pl) until the tx starts to get into the rx. Change cable to 142, see how close of spacing you can get with that. (retuning the duplexer of course). Now change the cable to 1/4 superflex and see how close you can get. Remember, NO adapters!

Also, if using a tracking gen to tune the duplexer, note the differences in tuning with the different cables. you should be able to see a shift in the notch or pass with the different cables. This goes back to NAND's other poll on cable length.

I have noticed that cable length is less critical when using superflex. I've seen big swings when using rg-142. Especially when using tx equipment that doesn't seem to have a very stable 50ohm output.

Manufacutres use 142 because it's quick, easy, cheap and for the most part it works OK. But if you want to squeak that extra out of your system or want to minimize the RF noise floor at your site, use superflex.

My two cents. I'd like to hear if anyone performs the above experiment.