Hi Skip and Joe, Yes, placing attenuators in line can tell you a lot about IM and interference problems.
In Joe's case it would be interesting to know if placing an attenuator in the antenna line (assuming a shared receive/transmit antenna) produces an equivalent reduction in the NOAA signal, or a two or three fold reduction. That could provide some more clues as to the location of the actual mixing. BTW, You could also use a lower gain antenna instead of a power attenuator. High gain antennas are not always a good idea :-) 73, Mark vk3byy > -----Original Message----- > From: [email protected] > [mailto:[EMAIL PROTECTED] On Behalf Of skipp025 > Sent: Tuesday, 12 February 2008 4:56 AM > To: [email protected] > Subject: [Repeater-Builder] Re: Interference on a 6 meter repeater > > Re: Interference on a 6 meter repeater > > You are living one of my past nightmares... Let me give you > the probable fix first. > > If you have them, try a number of attenuator pads in series with > the Yaesu Radio. First might be a 10 dB pad and back the values > down while testing for grunge. > > I ended up using a 3 dB pad in series with the Yaesu Radio... the > problem generator radio still worked just fine in the intended > operation and the 3dB pad fixed the problem. Of course the pad > will have to take the attenuated transmit power value. > > Yes I tried cavities and shook my head when they didn't work (even > multiple cavities and really high insertion loss settings) > regardless of configurations. > > The front end of some Yaesu Radios can be a real serious problem > maker... been there, done that, coffee mug and tee shirt. You can > tell the Yaesu Owner their radio is probably causing other problems > besides your issue and they should deal with it as best possible. > My fix was the pad option short of replacing the radio. > > Try the high power pad... and hope the 3dB value is enough to > knock the problem out of the ball park. Let us know if it works > for you. > > cheers, > s. > > > Joe <[EMAIL PROTECTED]> wrote: > > > > I've been working on an interference problem on a 6 meter > repeater and > > would like to pass it by the "brain trust" for some input. > > > > The repeater is on 53.85Mhz with the input on 52.85Mhz. When the > > repeater is keyed up, NOAA weather radio comes through the repeater > > output loud and clear. (Decode PL turned off) I identified > the problem > > as intermodulation in a Yaesu VX-2500V transceiver at the > site used for > > telemetery on a simplex frequency of 173.3375Mhz. The mix is: > > > > 4(53.85)-162.55=52.85 (repeater input) > > > > The VHF transceiver frequency is not involved in the mix, > but the PA > > stage of the Yaesu is where the mix is being created. I > proved this by > > disconnecting the coax to the Yaesu and the IM goes away. > Also, when > > the Yaesu keys up on 173.3375, the interference goes away on the > > repeater. The IM is only being caused when the Yaesu > transceiver is in > > the receive mode. No cavity is on the Yaesu, it goes > directly to the > > antenna. > > > > The site is on a water tank, so there is only about 10 feet of > > horizontal separation between the telemetry antenna and the 6 meter > > repeater antenna. The NOAA station is running 500 watts > 1.6 mile away, > > line of site. I added a VHF cavity tuned to 173.3375 to the Yaesu > > telemetry radio, but it did not fix the problem. (The can > had about > > 25dB rejection at 162.55Mhz and about 40dB rejection at 53.85Mhz.) > > Prior testing showed that reducing the 6 meter repeater > output from 25 > > watts to 2 watts solved the problem. > > > > My next thought is to put a highpass filter and the VHF cavity in > series > > with the telemetry radio antenna. I am thinking of using a > 6/2 meter > > diplexer, terminate the 6 meter port with 50 ohms, and connect the > > telemetry radio to the 2 meter port. The diplexer should give good > > rejection to the 6 meter signal going into the telemetry > radio (along > > with the additional isolation of the VHF cavity) and the VHF cavity > > would give rejection of the NOAA radio signal. If this > works, I will > > contact TX/RX and see what they can provide to make the > installation > > professional. We are guests at the site and need to > provide something > > professional to the water company. > > > > Any ideas? We already thought of changing frequency on the 6 meter > > repeater, but that would be difficult to coordinate. > > > > 73, Joe, K1ike >
