What coax cable for repeater.

[WSHH555]

Hello everyone.  Slowly working to put up my own repeater and currently in the coax stage.  I understand LMR400 is not the best for repeaters as the metals don't play well with it.  I am running approximately 50ft so nothing crazy.  What have y'all been using that works well?

[tweiss3]

You want hardline, like Heliax (Comscope/Andrews) or whatever RFS calls it. At 50', you are looking for 1/2" or 7/8", unless you can find a deal for larger somewhere. 

[WSHH555]

20 minutes ago, tweiss3 said:

You want hardline, like Heliax (Comscope/Andrews) or whatever RFS calls it. At 50', you are looking for 1/2" or 7/8", unless you can find a deal for larger somewhere. 

Thank you very much!

[BoxCar]

4 hours ago, WSHH555 said:

Hello everyone.  Slowly working to put up my own repeater and currently in the coax stage.  I understand LMR400 is not the best for repeaters as the metals don't play well with it.  I am running approximately 50ft so nothing crazy.  What have y'all been using that works well? I

Heliax is the best, but a lot depends on the repeater itself. How much power does the repeater have? What antenna are you planning to use? A low power repeater (less than 25W) can usually get by with LMR 400 or better coax for that length run. Coax loss can often be made up with antenna gain.

Where the repeater is located, terrain, and intended use also factor into the need for Heliax over coax. Repeaters only cover the area from the antenna to the radio horizon and increased power only means the signal is stronger in the same area determined by the antenna type and placement.

[tcp2525]

23 hours ago, BoxCar said:

Heliax is the best, but a lot depends on the repeater itself. How much power does the repeater have? What antenna are you planning to use? A low power repeater (less than 25W) can usually get by with LMR 400 or better coax for that length run. Coax loss can often be made up with antenna gain.

Where the repeater is located, terrain, and intended use also factor into the need for Heliax over coax. Repeaters only cover the area from the antenna to the radio horizon and increased power only means the signal is stronger in the same area determined by the antenna type and placement.

A lot of people make that common mistake of assuming one can overcome the loss of using that crappy coax by brute force and throwing more watts at the problem. The main issue they overlook, whether intentional or ignorance, is how much the receive is attenuated. You can pump the same amount of water through a 1/2" pipe compared to a 2" pipe by turning up the pressure on the pump. A wasteful exercise in futility.

[nokones]

There is no doubt that a known brand helix is the best way to go. For my GMRS repeater, my LMR400 coax run is just shy of 50 feet, with an arrestor, but it is a known quality brand coax used by commercial radio shops.

Also, I am using a Laird FG4605 Omni 5 dB antenna with the tip at 30'. On relative flat terrain, I am able to hit the repeater with a portable approx 15-17 miles away.

[WSHH887]

6 hours ago, tcp2525 said:

 You can pump the same amount of water through a 1/2" pipe compared to a 2" pipe by turning up the pressure on the pump. A wasteful exercise in futility.

Not exactly true.  In order to pump the same amount of water you need to increase the flow rate through the 1/2" pipe.  As you do this the friction loss increases and eventually regardless of how high the pressure the flow rate will actually decrease.

[WRUE951]

if you are running a 40-50 W Repeater and your antenna is within 150-200',  go with LMR 400.   If you are pushing over 200' then consider Heliax Coax.  With the latter, be prepared to shell out a lot of money, coax, connectors, tools etc....... 

[tcp2525]

44 minutes ago, WSHH887 said:

Not exactly true.  In order to pump the same amount of water you need to increase the flow rate through the 1/2" pipe.  As you do this the friction loss increases and eventually regardless of how high the pressure the flow rate will actually decrease.

Maybe so, but to get the higher flow rate in the 1/2" pipe your pump will require more power to overcome the resistance of the smaller pipe. Resistance goes up and so does the pressure. No matter how all of this interact with each other, it's not the best and most efficient way to put the same amount of whatever to it's destination. 

[LeoG]

1 hour ago, WRUE951 said:

if you are running a 40-50 W Repeater and your antenna is within 150-200',  go with LMR 400.   If you are pushing over 200' then consider Heliax Coax.  With the latter, be prepared to shell out a lot of money, coax, connectors, tools etc....... 

If you are running a base station, simplex or repeater communication I'd say that might be so.  But if you are running a repeater I wouldn't go much more than 50' with LMR400.  Plus the dissimilar metals in the coax can cause issues long term.  I'd use 1/2" Heliax minimum for a repeater for the whole run and when it got farther than 125' I'd switch over to 7/8" or larger.

[WRYZ926]

6 minutes ago, LeoG said:

If you are running a base station, simplex or repeater communication I'd say that might be so.  But if you are running a repeater I would go much more than 50' with LMR400.  Plus the dissimilar metals in the coax can cause issues long term.  I'd use 1/2" Heliax minimum for a repeater for the whole run and when it got farther than 125' I'd switch over to 7/8" or larger.

The maximum run of LMR400 I am willing to use on 70cm or GMRS is 100 feet. And you definitely have more loss once you get to 80 feet or more. I kept the LMR400 run to my base GMRS antenna at 35 feet and 80 feet for my dual band antenna.

Just personal opinion here. But if I had a run that was over 100 feet between a GMRS repeater and its antenna(s) then I would look at using 7/8 hard line. there is not a whole lot of difference between LMR400, 1/2 heliax, and 7/8" heliax when talking about 50 feet or less. But there is a definite difference once you get to 100 feet or more, especially once you go up to 7/8" heliax.

[WRUE951]

19 minutes ago, LeoG said:

If you are running a base station, simplex or repeater communication I'd say that might be so.  But if you are running a repeater I wouldn't go much more than 50' with LMR400.  Plus the dissimilar metals in the coax can cause issues long term.  I'd use 1/2" Heliax minimum for a repeater for the whole run and when it got farther than 125' I'd switch over to 7/8" or larger.

you are welcome to use whatever you want, its your money..  I can tell you i have operated repeaters in the spec range i specified and had no problems talking over a range of 100+ miles.  I've had more issues with Antennas effecting my range than the coax and length. Just my two cents.  

[LeoG]

1 minute ago, WRUE951 said:

you are welcome to use whatever you want, its your money..  I can tell you i have operated repeaters in the spec range i specified and had no problems talking over a range of 100+ miles.  I've had more issues with Antennas effecting my range than the coax and length. Just my two cents.  

The price difference between LMR400 and 1/2" Heliax isn't really that large.  The dB loss between the two is pretty significant.  Going up to 7/8" is a big jump in price and the gain in signal isn't that huge in shorter runs.  But pretty significant when distances grow.

[WRUE951]

Just now, LeoG said:

The price difference between LMR400 and 1/2" Heliax isn't really that large.  The dB loss between the two is pretty significant.  Going up to 7/8" is a big jump in price and the gain in signal isn't that huge in shorter runs.  But pretty significant when distances grow.

right at this moment, I'm running about 130' LMR 400, lightning protected and  my repeater talks 100+ miles every day.  Everyone that uses it is happy, There is no need to throw anymore money at it.    Which would be a good chunk of change if i did.  I did go through an ordeal with Antennas, I now have that sorted out and back in business.  Sharing my experience, hopefully people can benefit from that.  

[LeoG]

12 minutes ago, WRUE951 said:

right at this moment, I'm running about 130' LMR 400, lightning protected and  my repeater talks 100+ miles every day.  Everyone that uses it is happy, There is no need to throw anymore money at it.    Which would be a good chunk of change if i did.  I did go through an ordeal with Antennas, I now have that sorted out and back in business.  Sharing my experience, hopefully people can benefit from that.  

Sounds like you have a good location which is probably better than any coax or antenna.  I need more punch since I am limited in height and live in hilly terrain with lots of trees.  Height is might, and that's something I'll always lack.  Highest up I'll get is 80' above ground at 166 FT above sea level.  To the east and west of me 5 miles away are 300-400' mountains, dead cutoff

[tcp2525]

1 hour ago, LeoG said:

If you are running a base station, simplex or repeater communication I'd say that might be so.  But if you are running a repeater I wouldn't go much more than 50' with LMR400.  Plus the dissimilar metals in the coax can cause issues long term.  I'd use 1/2" Heliax minimum for a repeater for the whole run and when it got farther than 125' I'd switch over to 7/8" or larger.

Sound advice for someone that wants their repeater to work. Now, if you're building a dummy load, by all means use a nice long piece of LMR400.

[WRUE951]

58 minutes ago, LeoG said:

Sounds like you have a good location which is probably better than any coax or antenna.  I need more punch since I am limited in height and live in hilly terrain with lots of trees.  Height is might, and that's something I'll always lack.  Highest up I'll get is 80' above ground at 166 FT above sea level.  To the east and west of me 5 miles away are 300-400' mountains, dead cutoff

My antenna is 52’ abgl and 2700’ absl and sits about 300’ above the valley floor.  It has as far as the eye can see range at 270 degrees and about 29 miles in south 90 degrees,   The valley floor is all desert, no trees or tall buildings, just distant mountains with exception to one big one just south.   It gets most use by 4 wheelers/off roaders in nearby hills.  

[LeoG]

1 minute ago, WRUE951 said:

My antenna is 52’ abgl and 2700’ absl and sits about 300’ above the valley floor.  It has as far as the eye can see range at 270 degrees and about 29 miles in south 90 degrees,   The valley floor is all desert, no trees or tall buildings, just distant mountains with exception to one big one just south.   It gets most use by 4 wheelers/off roaders in nearby hills.  

See, that's cheating.

[SteveShannon]

Some people have concerns about the mixed metals in the shield of LMR 400 for actual full duplex use.  They claim that the contact points between the two different metals become a semiconductor material that causes problems.  I don’t know whether that’s true or not but one simple solution is to use M&P Broadcom 10 or some other cable that doesn’t use mixed metals in its shield.  The size and performance is almost identical to LMR400 in terms of attenuation but all of the shielding and the center conductor is copper.  They also have even larger cables, such as their 13 mm cable.

[LeoG]

2 hours ago, SteveShannon said:

Some people have concerns about the mixed metals in the shield of LMR 400 for actual full duplex use.  They claim that the contact points between the two different metals become a semiconductor material that causes problems.  I don’t know whether that’s true or not but one simple solution is to use M&P Broadcom 10 or some other cable that doesn’t use mixed metals in its shield.  The size and performance is almost identical to LMR400 in terms of attenuation but all of the shielding and the center conductor is copper.  They also have even larger cables, such as their 13 mm cable.

That's what I'm using at home M&P Hyperflex 13.  It's been water infiltrated.

[SteveShannon]

21 minutes ago, LeoG said:

That's what I'm using at home M&P Hyperflex 13.  It's been water infiltrated.

I know you were documenting the changes in SWR as freezing temperatures happened. Were you able to determine where the water damage occurred?

Of course water infiltration is possible with any coax if the connectors are exposed to the elements. You might be able to salvage some of that by cutting the last few feet off. 

[WRUE951]

2 hours ago, SteveShannon said:

Some people have concerns about the mixed metals in the shield of LMR 400 for actual full duplex use.  They claim that the contact points between the two different metals become a semiconductor material that causes problems.  I don’t know whether that’s true or not but one simple solution is to use M&P Broadcom 10 or some other cable that doesn’t use mixed metals in its shield.  The size and performance is almost identical to LMR400 in terms of attenuation but all of the shielding and the center conductor is copper.  They also have even larger cables, such as their 13 mm cable.

for the most part, these dissimilar metals only become problems when they are introduced to environments where corrosive effects can cause a reaction such as moisture.  Sealing and protecting any metal connection is mandatory.  Even gold can cause corrosive effects on electrical connections under certain environmental conditions.  Install your connections with simple electrical common sense and connect away..   

[LeoG]

11 hours ago, SteveShannon said:

I know you were documenting the changes in SWR as freezing temperatures happened. Were you able to determine where the water damage occurred?

Of course water infiltration is possible with any coax if the connectors are exposed to the elements. You might be able to salvage some of that by cutting the last few feet off. 

No not yet.  Either Tuesday or Friday I think I'll be doing the swap out.  Borrowing a bucket truck and I will be swapping out the antenna from a generic 7.2dBi to the Comet CA712EFC and the 25' run of M&P Hyperflex 13 to 1/2" Heliax.  Also going to be putting up some guy wires because I'm always worried when the wind whips up.  I have 15' of mast with a 5' antenna and now I'll be putting a 10' antenna up there so more torque.  And I don't want to go through all that work only to have it blown down.

Not sure what will become of that cable.  Not even sure that's the problem, could be the antenna itself.

[SteveShannon]

On 4/18/2025 at 11:02 AM, tcp2525 said:

A lot of people make that common mistake of assuming one can overcome the loss of using that crappy coax by brute force and throwing more watts at the problem. The main issue they overlook, whether intentional or ignorance, is how much the receive is attenuated.

This is exactly right. Increasing the power results in more ERP, you never completely make up for the losses. And increasing the power does nothing for reception. 

[LeoG]

One of the repeaters I use has better reception than transmission which is rare.  I was able to contact my wife with my 20 watt mobile and when she responded I wasn't able to hear her from the 50 watt repeater.

[tcp2525]

7 hours ago, SteveShannon said:

This is exactly right. Increasing the power results in more ERP, you never completely make up for the losses. And increasing the power does nothing for reception. 

Yep, and wait for it. Someone just might come in and say, "Well, I'll just install a receive preamp and brute force the receiver" not taking into account they are also raising the noise floor with reduced selectivity and adjacent signal rejection. Of course there are some very tight frequency specific preamps, but they are expensive. On a repeater it might create more issues that will drive up the costs to get your Band-Aid working. Definitely a bad move if your repeater is located in an oversaturated RF environment.