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 Canada's 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 didn't work. I might add that this was not inversion or ducking, the signal was there 24 X7 day after day.
73 JIM KA2AJH Wellsville, NY ----- Original Message ----- From: [EMAIL PROTECTED] To: [email protected] Sent: Saturday, June 07, 2008 12:35 PM Subject: RE: [Repeater-Builder] Re: antenna suggestions for 440mhz At 6/7/2008 08:22, you wrote: >In fear of moving off topic... I'd like to ask how one can determine the >electrical downtilt of an antenna? > >I just put into service a RSF/Celwave Super StationmasterR Model 10017-6 >that is designed for 925-960 MHz on my 927.5250 repeater. The added gain >factor of the antenna (an additional 4dBd over what was previously in place >- a Decibel DB586-Y) does not seem to benefit the receive (at 902 MHz). I >gained what seems like a little extra receive range, but not equal to what I >seem to have gained in transmit coverage. Downtilt shouldn't vary too much between TX & RX freqs. What's probably happening is that the increased gain is resulting in increased noise pickup from the horizon as well as signal. Changes in gain directly affect your transmit ERP, but they don't necessarily translate directly into increased RX range depending on where the noise is. Slightly related: I once maintained a repeater at a residential mountain site with lots of elevation but no clear view to the ground (trees in the way). The site RF characteristics on 2 meters were somewhat like an RF "black hole": RF could get in but was hard to get out. We needed ~200 watts of TX power on the repeater to balance TX & RX with a 50 watt mobile user. I believe the reason was foliage absorption combined with a high noise floor down below. Around here antenna noise temperatures on 2 meters are typically ~3000 K. However this site had much lower noise - it's the only site around here where adding a preamp to a G.E. receiver resulted in actual system sensitivity improvement. So with biological attenuation surrounding the site, both signal & noise approaching the site were attenuated. With the low noise RX, the net reduction in S/N due to the attenuation was minimal. However, the attenuation directly reduced the TX signal leaving the site. So the net effect was the site "heard" OK but TXing out was difficult. Bob NO6B
