UHF Covergae

[W0AJA]

Hello All!

Recently my fire department has started a private repeater (Yes, they have the appropriate FCC Public Safety License) and they seem to have an issue with their UHF system.

Some background,

Location:
Map Coordinates of Selected Location
Latitude:N 40° 52' 36.628"
Longitude:W 79° 51' 56.3307"
Latitude:N 40° 52.610466'
Longitude:W 79° 51.938844'
Latitude:40.876841°
Longitude:-79.865647°
Elevation: 1,256 Feet
Tower: 80 Feet (Ontop of 1,256)
Antenna: New Diamond X50C2
Frequency Split: 453/458
Reciver and Transmitter: CDM1250 (Tried at 25 and 40 Watts)
Duplexer: Tried 2 mobile ones, think Sinclair is the current one
Coax: Says Times Microwave but its MUCH thicker than LMR400 and its a hard shield, not braid and foil. (got used)

They are lucky if the can only get 2 miles out, and I know they should be getting more, its almost LOS to our 911 center tower's antennas.

Any suggestions to what would be making it so hard to get out? They are one of the highest elevations in the county, and with the extra tower and such it puts it about 30-40 feet above the nearest tree line. Coax is fine, I check it. Antenna is new (When they were replacing their old Decible Unity with the new one, the firefighter got a RF zap or burn while doing it, could that have been static discharge?)

Any help is appreciated.

[KG4NEL]

The duplexer part would worry me a little - what do you mean when you say "mobile one"? Even at 450, cavity duplexers are still pretty big,

Assuming you've already TDR'ed the line, can you try running the transmitter with no duplexer in-line, and see what your coverage is like in the field?

[W0AJA]

I will see if we can do that Monday and hardware it I tot he antenna and simplex it to check.

Now, me and the chief were thinking the Duplexer is shot because sometimes the guys TX on a 470 Frequency with the Duplexer in line.


http://www.eyou.com.au/published/pub...ipexer_pic.png

[W0AJA]

*hardwire

Also, theirs is Celwave, not Sinclair. Sorry about that

[K7SGJ]

Out of curiosity, what is the receive side like for reception? Has anyone tried looking at the SWR? And I agree with NEL to bypass the duplexer and see what kind of range you get with the transmitter going directly to the antenna; and this would be a good time to check the SWR.

As far as the duplexer goes, you mentioned you tried some from the mobiles. Keep in mind they will be tuned just the opposite of the one for the transmitter since they transmit on the repeater receive frequency, and receive on the repeater transmit frequency. You need to make sure the repeater duplexer is swept and tuned for the proper transmit/receive frequencies. I really think this is where you will find the problem.

[N8YX]

...You need to make sure the repeater duplexer is swept and tuned for the proper transmit/receive frequencies. I really think this is where you will find the problem.

This right here.

With a vector network analyzer.

Hint: An MFJ-269 ain't gonna cut it.

[kd6nig]

I know the Ham repeater I use regularly has both 2m and 440 on it, and its a good 50 miles away though up on a hillside facing the Central Valley. Oftentimes, the 440 side is stronger than the 2m side.

Something is way off like everyone else is saying, especially at the wattage its supposedly running. If its not the duplexer, I'd be checking the output of that radio for sure.

[W0AJA]

Thanks for all the suggestions guys!

Im thinking its the duplexer or TX radio because RX is fine, TX is the issue.

[NQ6U]

FWIW, you can download instructions to build your own 70cm band duplexer here.