Checking duplexer

[tweiss3]

20 miles seems like a bit of a stretch at 35 feet above ground, unless its the tallest point in the area. 80' of coax isn't the end of the world at VHF, though that is more than twice your antenna height.

Still sounds like you should get the repeater receiver checked and the duplexer. Could be one cavity is out of tune slightly. You might be able to see that with the NanoVNA if you have some experience tuning duplexers and take your time, but it sounds like you would be better off talking to a professional on site to get things checked out.

Also, LMR400 is not ideal for repeater use, hardline should be used when possible.

 

[FreddyM]

20 miles seems like a bit of a stretch at 35 feet above ground, unless its the tallest point in the area. 80' of coax isn't the end of the world at VHF, though that is more than twice your antenna height

The plan is to install a tower to get the antenna to 70 feet.

Is there a formula that will tell me what to expect if all things stay the same except I'll double the height of the repeater antenna?

 

[tweiss3]

You can model it online: Radio Mobile Online
Like every computer model, garbage in equals garbage out, but it can show you the difference between 30 feet and 70 feet. It may not be as much as you are thinking.

 

[FreddyM]

OK thanks to all the guys who answered my questions and tried to steer me in the right direction.

 

[MTS2000des]

Proper receiver sensitivity testing on a duplex system:

Tools required: tracking generator
ISO-T if one desires to get true idea of sensitivity

This ASSumes antenna, feedline, grounding is all proper install. A bad antenna/feedline WILL affect duplexer tuning. Need to know the type of duplexer so one knows what performance level to expect.

Transmitting into the same antenna one is receiving from requires precision tuning, correct installation, the right test gear and skill set. Lots of people can throw repeaters up. A few can actually make them work well.

 

[prcguy]

Proper receiver sensitivity testing on a duplex system:

Tools required: tracking generator
ISO-T if one desires to get true idea of sensitivity

This ASSumes antenna, feedline, grounding is all proper install. A bad antenna/feedline WILL affect duplexer tuning. Need to know the type of duplexer so one knows what performance level to expect.

Transmitting into the same antenna one is receiving from requires precision tuning, correct installation, the right test gear and skill set. Lots of people can throw repeaters up. A few can actually make them work well.

I would think a service monitor and ISO-T or directional coupler then test sensitivity with transmitter off then on, should be no change in sensitivity. A tracking generator will show resonant peaks and nulls on the duplexer but won't test sensitivity.

 

[buddrousa]

35 foot you are very lucky to talk across town

 

[FreddyM]

35 foot you are very lucky to talk across town

Why is it than that we can talk 30 miles if we are talking on our mobiles and not going through the repeater?

Talking from mobile to mobile.

Not always but most of the time.

 

[WA0CBW]

Most would suggest to not use LMR400 for repeater use because of the combination foil/shield.
BB

 

[mmckenna]

Why is it than that we can talk 30 miles if we are talking on our mobiles and not going through the repeater?

Talking from mobile to mobile.

Not always but most of the time.

Topology plays into it quite a bit.

VHF is ~mostly~ line of sight, with some flexibility. If the antennas can "see" each other, chances are you can communicate.
The mobile radio doesn't have to deal with a duplexer that may or may not need tuning.

Also,
Not sure how it works in Canada, but down below here, the FCC license tells us what power and antenna height we can use. Does your license have a limitation on how high your antenna can be?

 

[FreddyM]

Most would suggest to not use LMR400 for repeater use because of the combination foil/shield.
BB

All the equipment for the repeater was bought as a package with the ends crimped onto the cable already.
I did not specify LMR400 that is what the company spec'd.

I said our antenna would be at 70 feet that is why the coax is 80 feet.

Are you saying just because the swr on the antenna is around 1.3, because of the LMR400 it might not be working as it could with a different coax?

 

[FreddyM]

Not sure how it works in Canada, but down below here, the FCC license tells us what power and antenna height we can use. Does your license have a limitation on how high your antenna can be?

Our radios are limited to 30 Watts.
I cant find anything on my license that limits the height of the antenna.

 

[prcguy]

All the equipment for the repeater was bought as a package with the ends crimped onto the cable already.
I did not specify LMR400 that is what the company spec'd.

I said our antenna would be at 70 feet that is why the coax is 80 feet.

Are you saying just because the swr on the antenna is around 1.3, because of the LMR400 it might not be working as it could with a different coax?

LMR400 and similar coax with dissimilar metal inside that contacts each other like the tinned braid and aluminum foil can act as a diode junction when they get tarnished/corroded and create intermodulation products. This is not a problem in a half duplex system but in full duplex like a common repeater feed line the repeater transmitter can cause passive IMD inside the coax resulting in a raised noise floor or ghost signals that interfere with the repeater receiver.

Any coax with tinned braid and aluminum foil shield should be avoided in the repeater antenna path but its ok to use from the duplexer to the transmitter or receiver since that is a one way path.

 

[FreddyM]

LMR400 and similar coax with dissimilar metal inside that contacts each other like the tinned braid and aluminum foil can act as a diode junction when they get tarnished/corroded and create intermodulation products. This is not a problem in a half duplex system but in full duplex like a common repeater feed line the repeater transmitter can cause passive IMD inside the coax resulting in a raised noise floor or ghost signals that interfere with the repeater receiver.

Any coax with tinned braid and aluminum foil shield should be avoided in the repeater antenna path but its ok to use from the duplexer to the transmitter or receiver since that is a one way path.

So why the hell was it spec'd than?

So we added the repeater to increase range and reliability.
It seem's they are very finnicky.

Would there have been something we could have added to just bounce the signal of versus repeating it.

 

[buddrousa]

Let me guess you bought online and not from a local BRICK AND MORTOR STORE to save a few dollars.

 

[mmckenna]

So why the hell was it spec'd than?

Low bid?
Shop that doesn't care and just wants to make a buck?
Seller that has no idea what they are doing?
:no offense: Buyer that has no idea what they are doing?

So we added the repeater to increase range and reliability.
It seem's they are very finnicky.

They are. They are not a consumer product. While there are lots of places on the internet that will sell you the parts, it takes a knowledgeable tech to actually set one up correctly. It is absolutely not a "plug-n-play" sort of thing.

One of the things I've run into in the last 10 years is that thanks to YouTube and Amazon, everyone thinks they are a radio expert. I've had a lot of issues at work where someone will randomly start buying equipment off the internet and then try to make it work. I eventually get called in to "fix" it. I usually end up having to replace a lot of equipment to make it work correctly. Ultimately it ends up costing more than if they'd just asked us to do it right the first time.

But, you are here asking good questions, and getting good answers. You are ahead of most. But I do think you are getting to the point where a professional tech is required. Like I said, this isn't consumer oriented equipment.

Would there have been something we could have added to just bounce the signal of versus repeating it.

This is exactly why Push to Talk over Cellular is becoming very popular for non-public safety users. It's cheaper, way better coverage, less headaches, less up front costs. Even high end public safety radios are starting to include Push to Talk over LTE as an option.

 

[mmckenna]

You guessed wrong.

So, like I said, no offense intended.

What was the answer?

You can't always trust sellers to know what they are doing.

 

[mmckenna]

LMR-400 can work if it's kept perfectly dry, zero moisture, never gets the jacket damaged, never has any water/moisture intrusion. But in real life, that's unlikely, or unlikely for very long.

You can easily fix that by using some 1/2" Heliax. It's the right stuff for the job:

Commscope LDF4-50A - 1/2 Heliax Hardline

50 Ohm Low Loss 1/2 Inch Heliax Hardline Coaxial Cable

theantennafarm.com

 

[FreddyM]

Let me guess you bought online and not from a local BRICK AND MORTOR STORE to save a few dollars.

You guessed wrong.

Low bid?
Shop that doesn't care and just wants to make a buck?
Seller that has no idea what they are doing?
:no offense: Buyer that has no idea what they are doing?

It was the lowest bid.
But in hindsight the buyer didn't know what he was getting into.

 

[buddrousa]

It happens but you are being given good advice from people that have done this or are doing this for a living.