From the November 2018 issue of the Big Island Amateur Radio Club Newsletter
Big Island Hawaii
The Pepeekeo repeater is back to normal, reports BIARC Repeater Committee Chair Gary Schwiter, WH6EPS. He updated the membership on the recent trip he and Darrell Asuka, KH6RDO, made to the site recently to install the filter created by Hank Kaul, KH6HAK. “Thanks to Hank, there’s a nice coverage area of Hilo town,” Gary said at the October meeting. The repeater is at 146.88, with a PL of 100 in and out.
Hank offers this description of the new filter and how it works: “The repeater antenna is installed on an FM broadcast tower in very close proximity to TWO FM broadcast antennas. As such, it picks up an extraordinary amount of RF from these high power antennas. Even though they are not on the 2-meter FM frequencies, the receiver front end is saturated and desensitized at the desired operating frequency. To put it simply… the receiver is ‘deafened’ by two very loud shouts while it is trying to hear a distant whisper of a signal. The solution to the problem is to eliminate as much of the FM Broadcast frequencies RF as possible, right at the receiver input.
“The filter(s) I constructed are each a 1/4 wave coaxial stub (open at the far end) that are tuned to each FM broadcast frequency. An ‘Open’ 1/4 wave line will reflect a dead short at the opposite end, so by carefully tuning the length of the coax we can effectively short out the FM Broadcast RF back at the coax junction. Note that there are two coax stubs coming off the line. Each slightly different in length, each one is tuned to the specific FM broadcast frequency of one of the interfering transmitters.
I made these by calculating the approximate length of the Coax, ( ¼ wave at frequency X Velocity factor of coax) and cutting it slightly long. Then I hooked it up to a network analyzer that sweeps thru the frequency and draws a graph of the attenuation. It shows me a nice notch, about 40dB and all I have to do is carefully trim the coax open end until the notch moves up to center on the exact frequency I want.
“So each of the FM transmitters has been attenuated by approximately 3540dB at the receiver front end, and that is enough to eliminate the desense problem. I am still looking at another filter that might be even more effective, and possibly a preamp to boost weak incoming signals. We strive for perfection!”