At 6/7/2008 10:24, you wrote: >Hi Guys, > >My question is have you ever put up a downtilt antenna to replace an >antenna of the same configuration, i.e. gain, etc. and have been able to >say with 100% certainly that the downtilt worked? My 911 center went to >high band for fire. Within a few days Industry Canada was on the phone >saying that our cross band repeater from low band to high band was >severely causing interference to a fire department in Canada. While I was >on the phone, I would hear our units coming in loud and clear at the >Industry Canada Office. Make a long story short, I had many conference >calls between the FCC and Industry Canada and I agreed to Canadas request >to mount a down tilt antenna at the same location of the existing antenna. >A week later the antenna was installed and there was NO difference in >signal quality from the Alma Hill New York Tower 2,558 to the location in >Canada some 125 miles away. I cut the amp out and used the six watt >exciter and I could still hear the signal over the phone from Canada just >fine. We finally negotiated a frequency change and I walked away knowing >that downtilt in this application didnt work. I might add that this was >not inversion or ducking, the signal was there 24 X7 day after day.
What was the antenna gain & how much downtilt? If you starting with only 7 dBi & then put 3° of downtilt on it, you're not going to drop the on-horizon gain very much. Since dropping the power from (amp) watts to 6 watts (let's say that's 10 dB?) didn't make a difference, you'd almost have to add enough downtilt to put the antenna's 1st null on the horizon for downtilt to work in this case. Unless you're dealing with a very high gain antenna, doing that would also cut out some of your desired coverage. Bob NO6B P.S.: you will definitely get "ducking" if your repeater is in their migratory path ;)
