LMR-400

[kayn1n32008]

Working on a project to add a couple of repeaters to augment the coverage of a single site.

Two companies that have responded have spec'd LMR-400 runs between the repeater and the repeater antenna. These are short runs under 100'. One or two of the three companies are saying that there should not be any desense or noise gemerated. I am looking for technical documentation detailing why LMR-400, of any length, should not be used in full duplex operations. The loss difference between LMR-400 and LDF4-50 is not enough to use loss as the deciding factor, and now the system owner wants documentation to justify spending the money on 1/2" rather than LMR-400.

 

[n5ims]

LMR-400 should NOT be used for duplex operation. When used on a repeater it will generate passive intermod distortion. Loss isn't the issue. It's the wire braid over the foil shield that causes the problem. The two different metals will cause noise on the signal. LDF4-50A has a different design and will not cause this distortion.

http://www.amsat.org/amsat/archive/amsat-bb/200808/msg00456.html

LMR-400 has also been traced to PIM (passive intermod) problems on many commercial sites when used in feeding repeaters, due to the "foil over braid" design, and many repeater owner/operators won't use it anymore. (Every little "bump" in the braid touching the foil is another tiny little junction where diode-behavior can create IM mixes.)

http://forums.radioreference.com/motorola-forum/219982-repeater-help.html
http://forums.radioreference.com/kenwood-forum/303716-tkr-750-repeter-setup.html

If you insist on LMR-400 TYPE cable, be sure to get the more expensive one that isolates the two shields and doesn't cause PIM. The "TCOM Low Loss Low PIM Coax" from Times Microwave would be the correct substitute. http://www.timesmicrowave.com/cms/products/cables/lmr/

 

[mikewazowski]

http://fxr.com/~/media/Boonton/Manuals and Software/PIM 21 Manual.ashx

Page 17 says not to use foil shielded cables such as lmr400 as part of the test configuration of the PIM test set.

 

[kayn1n32008]

LMR-400 should NOT be used for duplex operation. When used on a repeater it will generate passive intermod distortion. Loss isn't the issue. It's the wire braid over the foil shield that causes the problem. The two different metals will cause noise on the signal. LDF4-50A has a different design and will not cause this distortion.

http://www.amsat.org/amsat/archive/amsat-bb/200808/msg00456.html


http://forums.radioreference.com/motorola-forum/219982-repeater-help.html
http://forums.radioreference.com/kenwood-forum/303716-tkr-750-repeter-setup.html

If you insist on LMR-400 TYPE cable, be sure to get the more expensive one that isolates the two shields and doesn't cause PIM. The "TCOM Low Loss Low PIM Coax" from Times Microwave would be the correct substitute. http://www.timesmicrowave.com/cms/products/cables/lmr/

Thank you. I think you edited as i hit reply... apologies.

 

[mikewazowski]

http://www.mcf.amta.org.au/files/DAS.Specifications.doc#_Toc232414036

Section 5.17.5

 

[902]

Ai-yai-yai!

I would not recommend LMR-400 for any duplex operations for the reasons stated. But the other part of this thing is that more than one communications professional quoted this to you for a commercial solution. I find that alarming.

I would do the following:
1) Directly spec 1/2" LDF or similar annular copper corrugated coaxial cable,
2) Disqualify both bidders,
3) Review the bid spec so that you don't allow for exception loopholes like that, and
4) Add an itemized checklist so you don't get any surprises.

Don't forget your lightning suppression.

 

[n5ims]

Thank you. I think you edited as i hit reply... apologies.

Rereading your initial post after saving mine I thought you might need some specific details, not just what may be an opinion so I edited to add that for you. Hopefully it'll point you in the right direction and provide you with the ammo needed to make the correct decision (and get it approved).

 

[kayn1n32008]

Rereading your initial post after saving mine I thought you might need some specific details, not just what may be an opinion so I edited to add that for you. Hopefully it'll point you in the right direction and provide you with the ammo needed to make the correct decision (and get it approved).

Sorry, my first reply was a bit snippy.

I need documentation to back up why I have told them to not use LMR-400 when 2 companies have spec'd it. For a pair of repeaters, less than 100' there will be negligible loss difference, and will not affect coverage, it's the PIM, and desensitize issues. One company has said they use LMR-400 that does not exhibit these issues... I need to see a spec sheet on it... And price as well.

Why cheap out on one of the two most important parts of the repeater.

Do it right the first time!

 

[prcguy]

I would loose the company suggesting LMR400 and go with another that knows more about installing repeaters and antennas.
prcguy

 

[12dbsinad]

Re-write your bid spec to include exactly what you are looking for. For any professional shop to suggest LMR-400 for full duplex operation clearly shows they do not know what they are doing and don't understand the properties of coax cable and there intended use.

I'm shocked they would bid that out. It makes me wonder how many systems are installed using inferior products. Antenna and feed line cable is lesson 101 for any system.

Good luck.

 

[Boatanchor]

LDF4-50 would be the minimum for a 100ft repeater run on V/UHF.
LDF5-50 may be appropriate on 400-800Mhz.

And, LDF4-50 is not even that much more expensive than LMR400,

Why even argue. Just specify it in your tender documents and be done with it..

 

[kayn1n32008]

I would loose the company suggesting LMR400 and go with another that knows more about installing repeaters and antennas.
prcguy

I fully agree. They are respected company, and I have used at least one DMR system they sold and installed. Works great. Not sure why they are spec'ing LMR-400... Me thinks it is cause it is under 100'.

Re-write your bid spec to include exactly what you are looking for. For any professional shop to suggest LMR-400 for full duplex operation clearly shows they do not know what they are doing and don't understand the properties of coax cable and there intended use.

I'm shocked they would bid that out. It makes me wonder how many systems are installed using inferior products. Antenna and feed line cable is lesson 101 for any system.

Good luck.

I want them to spec the bid for LDF4-50, but need the technical documentation to back up why I'm saying not to use it. I agree not using LMR-400 is feed line 101... First day kinda ****.

LDF4-50 would be the minimum for a 100ft repeater run on V/UHF.

LDF5-50 may be appropriate on 400-800Mhz.



And, LDF4-50 is not even that much more expensive than LMR400,



Why even argue. Just specify it in your tender documents and be done with it..

Please reread my OP I'm looking for technical documents to back me up... It's not my first day building repeaters. I am well aware of what LDF4-50 is good for. I learned by doing, and had 'no LMR-400 for full duplex' hammered in along time ago... Just don't have anything on paper to back up the learning by doing...

Mike, that may help my cause. Thank you.

 

[jim202]

There always has been an on going finger pointing rage for a number of years. You will find people that have used the LMR-400 and it worked well for about a year until the moisture managed to creep into the cable. That was when the noise started to show up.

You will find some who say that if you do a good job weather sealing the connectors you won't have a problem. Why take the chance. Most of us that have been around a long time take pride in our work. Then there are others that build in service calls with the work they do.

I won't steal this thread, but want to point out the quality of work makes a big difference. When I was building cellular sites, we had one tower crew that would do a good job installing and weather sealing the coax cables. But before they left the top of the cable run, they would take a knife and slice open the top of a cable connector to let in the rain water. We got suspicious and sent up a different tower crew right after the first one got finished. The took photos of the slashed weather sealing that was done the day before. The first tower crew was just looking for more work.

So if your contractors are pushing the LMR coax, be cautious. Like others have already mentioned, your much better off paying slightly more for the Heliax type cable and don't have to worry about the noise problems developing down the road.

 

[Boatanchor]

http://www.repeater-builder.com/antenna/andrew/andrew-braid-over-foil-imd.pdf

This is for new cable, under lab conditions.

As mentioned, if any moisture gets into the LMR400 cable, expect a world of pain.

 

[wirelesssamurai]

Unfortunately, there are many clueless people (and entire companies) in this industry who proclaim themselves to be experts.

Try and find someone who recommends http://www.timesmicrowave.com/documents/resources/TCOM-400.pdf as a minimum (or who at least knows the difference between TCOM and LMR!).

 

[PACNWDude]

This is so true on many levels. My previous employer used LMR-400 and 240 for portable repeaters. This was because it was cheap and we could get it in spools that fit in a small trailer shelf easily. (Heliax can be a pain once unrolled.)

For portable, meaning taken down and set up many times, LMR-400 worked well. Cable runs were kept at 50 feet or less and antennas were mounted on a tripod on a building roof for instance.

Real world, permanent, or close to it, repeater installation, just go with heliax. Make sure it is written in the contract, even down to which part number, supplier and connector type. Good lawyers and contract writing can make for a lot less problems later on.

The person who mentioned the tower crew slashing the cable on the way down, that happens more often than people think.

 

[kayn1n32008]

Thanks to all that replied. I think I have found, on and off list what I need to make the case to not use LMR-400.

 

[mformby]

Working on a project to add a couple of repeaters to augment the coverage of a single site.

Two companies that have responded have spec'd LMR-400 runs between the repeater and the repeater antenna. These are short runs under 100'. One or two of the three companies are saying that there should not be any desense or noise gemerated. I am looking for technical documentation detailing why LMR-400, of any length, should not be used in full duplex operations. The loss difference between LMR-400 and LDF4-50 is not enough to use loss as the deciding factor, and now the system owner wants documentation to justify spending the money on 1/2" rather than LMR-400.

The antenna and coax are the two most important parts of a system so never cut cost there. Also take into consideration the cost of the labor to install those components and use the best you can afford.

 

[12dbsinad]

The antenna and coax are the two most important parts of a system so never cut cost there. Also take into consideration the cost of the labor to install those components and use the best you can afford.

Agreed. I'd use at minimum 1/2 inch heliax (probably even 7/8) regardless if LMR-400 worked well or not for full duplex for a 100' run. I like the least amount of loss as possible, and if your going to go through the trouble of installing a system, why not make it the best it can be.

 

[kayn1n32008]

Agreed. I'd use at minimum 1/2 inch heliax (probably even 7/8) regardless if LMR-400 worked well or not for full duplex for a 100' run. I like the least amount of loss as possible, and if your going to go through the trouble of installing a system, why not make it the best it can be.

At under 100 feet there is little difference between LMR-400 and LDF4-50. Not enough to impact coverage. It is the braid over foil, and the PIM potential, that is the bigger issue, and why I was asking for technical documentation.