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Author Topic: LMR-400 For Repeater  (Read 12623 times)

KI6DYR

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LMR-400 For Repeater
« on: December 01, 2009, 01:16:04 PM »

I have read the concerns that in some cases LMR-400 may cause intermod when used in a duplex operation. Some, including some here, disagree. Our system has been working very well with excellent mountain coverage for 8 months and no intermod reported.

But over a period of time we're getting a desense loop. The repeater is at 30 watts to the antenna, which is located 30 feet above the shack. The WaCom 6-cavity duplexer sits next to the TKR-750 repeater and is attached with 3 foot LMR-400 cables using silver plated connectors. None of the equipment has been moved; the duplexers haven't been bumped. I have been running the TKR in narrowband mode

I cannot get much more separation between the antenna and the TKR repeater, so this leaves only changing the cable to hard line. Will this make much difference? I had to decrease the repeater to 5 watts out of the duplexer to stop the loop. Snow is on the ground with 4-5 more days coming this weekend, so I need to do something or do nothing.

The only variable that has changed is occasional HF operation with antennas that are on the same 1/4 acre lot. Running at 500 watts on 40 meters often causes the RC-210 controller to completely reset -- even with ferrite cores on all exposed leads. I expected more from a controller that is designed to allow HF cross-link, but we'll leave that for another time.

Although I have other VHF equipment that is not affected by HF, is there anything about the TKR that makes it more susceptible so that when the horizontal HF field hit it I angered it?
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WB2WIK

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RE: LMR-400 For Repeater
« Reply #1 on: December 01, 2009, 02:24:33 PM »

The difference you're experiencing may be due to the snow, or a large drop in temperature.

Such is not so unusual with mountaintop repeaters in cooler places.

In some such installations, even hardline may not help unless it's pressurized with dry gas to keep condensation out of the cable and connectors.  I've been through something similar several times: Summertime heat and dry WX, all is great; winter comes along with huge temp changes, and condensation screws it all up!
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KI6DYR

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RE: LMR-400 For Repeater
« Reply #2 on: December 01, 2009, 02:49:41 PM »

Thanks for the quick reply -- I knew that you'd be in to offer sage wisdom. I think that you're at least partially right because this started near the end of the short summer that we have up here, however;

The repeater and duplexer are in a temperature controlled room -- my upstairs bedroom and workshop. Although humidity is seasonally low, the room is kept at a constant ~68 degrees. I have the gas and electric bill to prove it 8=-)

I may try moving the repeater downstairs to my radio room -- actually -- into a closet that I can line with foil. That will give me more vertical separation. Oddly, unplugging the coax from the receiver causes the desense loop to break and the controller resets. I guess this suggests adequate isolation between the TX and RX sections of the TKR. Since lowering the power caused the desense loop to go away I think it is a fair assumption that the TX is getting into the RX through the antenna system. Even at 5 watts the 6,400 feet high repeater does OK -- but not so good for portables.

One other thing to try, I guess, would be to swap out the cavity tees. They are PL-259 (SO-239) and not N connectors. I also don't like the BNC connector on the receiver of the TKR and think I'll change that out to a N connector as well. But all the connections are clean.

FWIW, I use LMR-400 with silver-plated connectors on everything and the HF line has a RFI filter that cuts off at 30MHz. The filter comes off of the Ameritron downstairs in the radio room, so it is physically isolated as well. Just before it is a LDG AT-1000 autotuner. I'm guessing that the issue with the RC-210 resetting is unrelated to the desense issue insomuch that the RX section is probably fine on the TKR.
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WB2WIK

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RE: LMR-400 For Repeater
« Reply #3 on: December 01, 2009, 02:55:54 PM »

The cold temps and thermal change I was referring to is actually all "outside," where the feedline and antenna are.

Condensation forms in the antenna connector, and if that connector is really well sealed, it has no way to escape so it can stay in there a long time, even after the sun comes out to warm things up a bit.

Pressurized line and connectors take care of the whole problem, if this is actually the problem.

One way to find out would be to climb up and disconnect the transmission line from the antenna and see if the connector is wet inside.  If it is, use compressed air to blow it out so it's completely dry, do the same thing with the connector on the antenna, and then re-connect and see if it makes a difference in the noise/desense/etc.
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KI6DYR

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RE: LMR-400 For Repeater
« Reply #4 on: December 01, 2009, 05:28:09 PM »

I'll check them but I need to back-burner it. There is still snow on the roof. More snow is on the way. As it stands, my daughters cancer has returned and the Doc just called and wants to admit her. Priorities. We aren't in fire or camping season, so we'll make due.

I'll plan on replacing the LMR with hard line, which has been the plan all along. I'll also move the repeater downstairs for more vertical separation. I intend to add a dual cavity pass filter that will tighten the frontend up as well. Until the club actually has members, this is a LOL :)

Bob - AF6D
http://rimoftheworldarc.net
Wide-Area Coverage Repeater
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K9KJM

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RE: LMR-400 For Repeater
« Reply #5 on: December 01, 2009, 10:58:56 PM »

If you are forced to use PL 259 type connectors, There is a product on the market called "Stuff" that is an inert "foam" product designed to keep moisture out of such connectors. Just squirt some of that inside them when putting the connectors together. (Make sure they are dry to start with)

I am not a real strong proponent of the "stuff", but I have used it for such applications, With overall not real bad luck.

We have never had any problems with the TIMES LMR coax itself, When connectors have been properly installed.
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KI6DYR

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RE: LMR-400 For Repeater
« Reply #6 on: December 01, 2009, 11:15:35 PM »

I'll look into that. But first we need to change the antenna that up until recently has worked great. It is a Hustler G7-144. It has a SO-239 at a 45 degree angle with specific instructions not to weather seal the connection. I have always been troubled by this.

Candidly, what started out as a group effort turned into a one man bank account insufficiently funded by a 7 year old with cancer. I have had to wait or compromise but now neither is acceptable. What I won't do is give up the coordination over a minor technical snag solved by turning the power down. A Kreco Coaxial might be affordable more so than a Station Master.

It dawned on me the daytime to nightime changes up here year round. Daytime may be 80ish and nightime in the 40's. Right now we have snow on the ground, daytime temps of ~60 and currently 27. These wild swings sure can cause condensation but I did not associate this with desense. Live and learn. With help from fellow hams it won't be as painful.
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KI6DYR

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RE: LMR-400 For Repeater
« Reply #7 on: December 02, 2009, 04:19:06 AM »

Kreco has offered me a fair price on a stacked collinear, the SC-151a. Does anyone have experience with stacked collinears? I've built a couple of singles that work great on 6 meters.

http://krecoantennas.com/stackedcaxial.htm
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WB2WIK

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RE: LMR-400 For Repeater
« Reply #8 on: December 02, 2009, 10:34:02 AM »

>RE: LMR-400 For Repeater       Reply
by KI6DYR on December 1, 2009    Mail this to a friend!
I'll look into that. But first we need to change the antenna that up until recently has worked great. It is a Hustler G7-144. It has a SO-239 at a 45 degree angle with specific instructions not to weather seal the connection. I have always been troubled by this.<

::Your G7 has an SO-239?  Wow.  Mine has a type N female on it, I *thought* that was "standard" with the G-7.  Maybe it isn't.  Might be a different P/N to order or something.  I haven't bought one in about ten years, maybe they made a change.  About the WX sealing, it's a double-edged sword.  Great WX sealing seems like a good idea, but unless everything is pressurized, condensation will form inside the connector when the temperature drops, anyway (unless the humidity outside is always zero), and when you perfectly seal a connection, that condensation will remain in there pretty much forever.  Letting the connection "breathe" will potentially let more stuff in, but also let it back out and condensation will be able to evaporate.

One "problem" with the G7 and many other antennas is the connector's on the bottom and running a coaxial line straight down a mast from that point is not a good idea.  You should *always* use a "drip loop," a 360 degree circle formed of the coax itself, immediately below the antenna connector and bound to the mast to hold its form.  This can serve two useful purposes: A way to prevent water from ever migrating down the transmission line any farther than the bottom of that loop; and a strain relief if you use "heavy" cable below the loop and it starts to pull a lot.

When using hardline/heliax, you normally don't make the loop out of that, but out of double-shielded flexible line like RG-214/U (or LMR-400), and then connect to the hardline at the end of the loop.


>Candidly, what started out as a group effort turned into a one man bank account insufficiently funded by a 7 year old with cancer. I have had to wait or compromise but now neither is acceptable. What I won't do is give up the coordination over a minor technical snag solved by turning the power down. A Kreco Coaxial might be affordable more so than a Station Master.<

::Sorry about your daughter's cancer, our best wishes are with you and your family.

Note that desense/noise/junk in duplex repeater systems can have a lot of sources.  Could the antenna itself; the connection to it; the transmission line; the duplexer and all the obvious things.  Could also be from intermod mixing in the repeater's transmitter (mixing the repeater's TX signal with some other signal source out of your control, such as another transmitter located LOS to you and quite strong, not necessarily on your property but close by), or another TX on your property, or sometimes even an AM BC station operating near 600 kHz -- lots of stuff.  Turning down the TX power would often relieve the problem almost no matter what its cause.

>It dawned on me the daytime to nightime changes up here year round. Daytime may be 80ish and nightime in the 40's. Right now we have snow on the ground, daytime temps of ~60 and currently 27. These wild swings sure can cause condensation but I did not associate this with desense. Live and learn. With help from fellow hams it won't be as painful.<

::Temp changes make repeater operation more challenging, but obviously not impossible.  I had repeaters in northern NJ (beginning in 1973!) at locations where the temp could be 100F in the summer and -20F in winter, and sometimes the day-to-day or day-to-night changes were 40F or more, so we were presented with lots of challenges!  One by one, you learn to overcome all of them and make the damned stuff work LOL.
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KI6DYR

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RE: LMR-400 For Repeater
« Reply #9 on: December 02, 2009, 12:55:22 PM »

Thanks for the great input. Before buying the Kreco Coaxial I am going to do some more detective work. My neighbor, who's had repeaters on his tower for years, has added a new one of unknown frequency. The last I knew their repeater was down on 144 and has been a constant source of issues with TASMA and other repeater owners. They keep moving from one location, then another, then back.

I discounted this because I found that by turning my TX power down the desense goes away. This suggests to me that my transmitter is getting into my receiver rather than intermod. He is one house away and even with 100mW I could potentially key up his repeater with my repeater and if I am getting into him, he into me, and a looping on my input.

Am I missing anything?

I need to go outside and see if enough snow has melted off the roof to safely work up there. I'm on a roof-mounted guyed tower 30 feet above -- at 6,400 feet I really don't need more than a few feet.

Am I mising something?
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WB2WIK

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RE: LMR-400 For Repeater
« Reply #10 on: December 02, 2009, 02:20:16 PM »

*Everything* can cause noise in a duplex repeater system.  Turning your own TX power down will almost always reduce intermodulation caused in the TX itself, regardless of the source, that's perfectly normal.

"Desense" covers a lot of ground.

A single loose connection anywhere "up on top" (antenna, antenna sections/tubing/radials; antenna to mast connection; mast to tower connection; guy wire connection; turnbuckle connection -- anything.

Remember in duplex we're transmitting with real power and using a receiver with submicrovolt sensitivity using the same set of hardware.  A single oxidized connection can become a "noise generating diode" when excited by a local RF field.  It won't generate any noise until you transmit, but when you do, the noise generated can be exponentially proportional to the TX power output.  That can be a major cause of what sounds like "desense."

One of the big reasons the old Stationmaster design is so popular with repeater owners is that internally there aren't any opportunities for a bad connection.  Everything in sight is copper, and soldered.  Nothing's clamped and there isn't any hardware except for the extremely robust mounting clamps that grab the enormous (2-1/2" OD or so) aluminum pipe at the antenna base.  The connection made to the antenna is hidden up inside a recessed area at its base, which is completely hidden and protected by the aluminum pipe, so once installed it's impervious to everything, and the supplied "stub" for your hardline connection is mil-spec RG-214/U (double shielded, silver plated), which is pretty good low-noise cable.

Even then, placement on a "guyed" tower, unless special precautions have been taken with all guy clamps, turnbuckles and hardware, can be bad news.

Of course, there are definitely remedies for all of this, but it can take a lifetime of installing repeaters to figure out about half of them!

I had one hilltop repeater back east that would go into an intermod loop (just sounded mostly like noise) as soon as the transmitter came on, caused by the local 610 kHz AM BC station that was a few miles away but very line of sight and very strong.  The BC station mixed with the TX signal from the repeater and created a huge signal only 10 kHz off the repeater's input frequency.  Had to go to a circulator in that case, nothing else worked.

Aren't repeaters fun?
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KI6DYR

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RE: LMR-400 For Repeater
« Reply #11 on: December 04, 2009, 01:06:34 AM »

Excellent advice!

The snow melted off the roof while I was in Orange County today listening to people complain that it was "cold." We left for the hospital at 6am and it was 18 degrees. Before I left I changed the kerchunk setting to 350mS from 250mS and the problem *appears* to be gone. I do not believe that the RX is desensing, per se. I tested from a marginal area, the 57 freeway through Diamnond Bar, I was able to hit the machine. That satisfies me that the RX hasn't taken a hit from a strong HF field.

Don't get me wrong, I like and have used Station Masters and for all of the reasons that you mentioned. Our original plan was a Telewave exposed dipole array with 5 degrees downtilt, which is difficult to get from any manufacturer these days. As a emergency services repeater serving the national forest as the primary coverage area we need that downtilt without sacraficing gain. You know the area -- Running Springs straight down to San Bernardino is a 4-5 degree downtilt according to my software.

But some issues beyond my control caused the antenna to be ordered but divereted to another machine. The G7-144 would have to do. Kreco has given me a great price on a collinear stacked coaxial that also has a concealed antenna connection.

7 days of snow heading into town. The repeater is back at full power and loud and clear in north San Diego down to Mexico and west to Santa Monica-ish. To the east we drop at about Banning. 147.705- 167.9
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K5MBV

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RE: LMR-400 For Repeater
« Reply #12 on: December 15, 2009, 11:48:32 PM »

Although everything was working before with the
PL259 connectors, it would be a good idea to replace
them with type N connectors as a starting point and
eliminate their leakage. This might be enough
to get the system back on line with full power especially
if moisture has accumulated in any of them.

73  Ken  K5MBV
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KI6DYR

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RE: LMR-400 For Repeater
« Reply #13 on: December 15, 2009, 11:54:49 PM »

Thanks Ken. But I'm locked in the shack until the snow melts off the roof. The repeater is at my home at 6,400 feet and the antenna is up 30 feet (tripod mounted; dacron rope guyed.) It isn't safe up there. Mountain living is awesome but comes with drawbacks. You can actually iceskate on my driveway -- we call it the tennis shoe shuffle HI HI
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N8EKT

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RE: LMR-400 For Repeater
« Reply #14 on: December 16, 2009, 08:48:28 AM »

Don't use LMR for duplexer jumpers.

Replace them with either RG-400, RG-142, or 1/4 inch
superflex jumpers.

If your duplexer was properly tuned on a tracking generator, these jumpers may be compromising your isolation.
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