Thread: 151Mhz transmitter - range issues - help needed.

Hi all, I am hoping that we have some radio experts out there and that someone can help me with a work related issue I have. I recently set up a 151Mhz transmitter/reciecver (Actually running at 151.600Mhz) to turn a genset on from a distance of about 1.2Km. It is just a basic single channel setup that operates a relay to start the machine, it then runs on a timer and shuts down - all good, and it has been running fine for several months, but recently it has started to play up. The signal strength has dropped right off, and it only starts occationally which is ticking the customer off. Both ends have 3.5 Db gain antennas on them, and they are almost line of sight. The maximum line of sight distance is supposed to be around 4KM, so I'm well within range - even with the small obstacle in the way.

So, for the experts out there - any ideas what's going on? There is no interferance on the frequecy, so I can count that out. All I can think of is the antennas, and the connections/cables. The transmitter is running through about 10 metres of 50 ohm coax, and the reciever has about 12 metres of the same stuff.

Do I need to ground both ends of the coax? On some CAN leads I use (to load share large gensets), we only ever ground one end of the screen so as not to get any induced current flow through the shield - is this the same for an RF system or not? It is currently grounded both ends on both antennas.

Also, could it be the connections deteriorating in the weather? They are all supposed to be weather proof, but they are all factor fittings - would I be better off remaking these myself with new connectors - they just have the standard CB style connectors, which are easy to fit.

I don't want to fit new antennas, as these ones did the job well at first, and are obviously up to the task, Also, the higher gain units get large and expensive for this frequency range.

Any ideas or thoughts on fixing this and improving the range would be appreciated.

Cheers - Gav

My first thought would be to get hold of a dummy load,and check the output of the transmitter,to eliminate output power transistor failure.I would also disconnect the coax and check for a short in the PL259 connectors.
There should be no continuity between the centre connector and the outer shell of the PL259.
If you can get hold of a VSWR bridge,this will let you know of any high impedence problems and they invariably contain a power meter,and you will be able to check the output power.If you can get hold of an RF meter,you will be able to check the effective radiated power of the antenna at various distances from the antenna.
Colinear antennas can be prone to damage from extreme vibration,especially if it is covered in a fibreglass radome that has no vibration insulating product inside.Helical loaded antennas may be a better choice in this type of environment.
Just a few ideas,hope that some of them are of assistance