GMRS Repeater Antenna Question

[airboss20]

I have set up a repeater for GMRS on a hill overlooking the valley that I live in. I am using a Motorola RKR-1225 with a good set of cans.

My initial antenna choice does not seem to be giving me the coverage I was expecting. The elevation of the hill is roughly 400' and I am using a Workman UVS-300 on a 20' tower. It seems that I am getting great distance performance through the repeater, but not very good local coverage. I am wondering if the ground plane on the antenna is causing an umbrella affect, or maybe I am in the shadow of the hill.

Either way, I am considering upgrading to an Andrew DB408-B. It is a 4 bay dual radiator exposed dipole antenna which boasts omni-directional TX/RX pattern. I have heard great things about this antenna, but would like to hear any comments or ideas before investing another $600.

I guess I am slightly foolish for just assuming that the UVS-300 would perform well on the hill. I should have obtained a band specific antenna instead of a dual band amateur antenna for this project.

All comments are welcome, as I am looking to optimize the first GMRS repeater in our area.

 

[rfradioconsult]

Definately a band specific antenna, the DB408 is a very good performer and reliable. I would not use LMR400 for your transmission line. LMR400 is known for generating interference within the cable when used for duplex operation. How well can you visually see the antenna from the areas where you are having problems.
Your 20 ft mast may not be high enough to overcome shadowing.

 

[airboss20]

How did you know I was using LMR-400? I didn't know that it wasn't good for duplex apps.

What would you reccommend using for the tx/rx line?

I plan on installing another segment to the tower for the new antenna, perhaps another 20'. This will bring the tower to 40'.

There is a small rise between the repeater and the valley, but this is the only plot that I was able to allocate for my repeater. It has an existing 100' tower that is used for the business that I work for, and is able to utilize a hotlink @ 2 watts, however I had to set my own tower as I was not given permission to mount my antenna on the 100' tower.

The business using the 2 watt hotlink is 2 blocks from my home base radio.

 

[rfradioconsult]

For the coax length you are using 1/2" Andrew LDF would do fine, if you are planning on buying a new DB408 then purchase new LDF at the same time. There are programs available on the WEB that you can plot your transmitter site and see terrain obstructions that may be giving you problems, plus you can model differing tower hieghts to see how high you will need to go to clear any close in obstructions