Thanks for the input. I missed a couple critical pieces of information. The FM Voice repeater is running through a 6-cavity duplexer. The APRS setup had a single cavity, tuned to the 144.39 APRS frequency. I am assuming this single cavity is acting as a low pass filter to prevent the voice repeater from overpowering the APRS, but I honestly don't know. You are correct in your assumption that APRS is running on a rack-mounted mobile rig.
I should elaborate a little more too. We realize these antennas are way too close. We are going to be replacing the APRS antenna with something with quite a bit less gain, and we're going to put as much vertical spacing between it and the Voice repeater's antenna as we can. I guess my curiosity is in regards to trying to understand what is causing the affects were are seeing/hearing. Noise and desense are one thing, but the popping and crackling when trying to hook the antenna to a powered off radio, and the weird noise that is initiated by keying the APRS, but won't leave until the voice repeater is unkeyed has me scratching my head. Thanks again for the feedback. David wrote: > > > First of all you do not have enough isolation between antennas with only > two wavelenghts horizonal seperation. A single bandpass cavity will not > be enough. I would try 2 bandpass/bandreject cavities. Reject set for > the TX freq and the other set for the RX freq. I believe that RF from > your repeater is exciting the RF amp in your aprs tranceiver, see if the > problem is still present with the aprs hooked up to the antenna but with > your power supply disconnected. Most people used a simple mobile for > aprs which creates alot of headaches where several transmitters are > used. They just dont have the filtering needed for this application. You > should also use a circulator on your aprs radio. This will help keep RF > out of your aprs transmitter. > > David Epley, N9CZV > Winchester, In > > --- In [email protected] > <mailto:Repeater-Builder%40yahoogroups.com>, "bullmus" <kf7...@...> wrote: > > > > Here's a weird one. I have my ideas, but I need to bounce it off a > couple brains bigger than mine. > > > > APRS on 144.39, and FM Voice repeater at 147.24/147.84. Antennas are > identical Telewave 4-bay stacked dipole. Both antennas are horizontal to > each other, and roughly two wavelengths from each other center loop to > center loop. > > > > The problem... When folks are using the voice repeater, then APRS > transmits a packet, it generates noise on the voice repeater the > listener hears. However, when APRS stops transmitting, the noise > continues on the voice repeater until the voice repeater is unkeyed. > > > > In our effort to troubleshoot, we found a couple extra oddities. For > example, with APRS antenna unhooked and the APRS radio turned off, when > we attempt to reattach the APRS antenna, we get all kinds of popping and > crackling on that turned off radio. > > > > We've checked grounding until we're blue in the face, and everything > appears to be in order. > > > > We have tried different cables on everything, one at a time, > bypassing the polyphaser, rack bulkhead connectors, etc... No fix. > > > > We discovered the problem when we began noticing APRS wasn't getting > out. Troubleshooting identified that the single band-pass can we had on > the APRS was reflecting 100% of the transmit power. We do not know the > history or condition of this can in the first place. It is very possible > it has been faulty for years, and only recently began to degrade to > worthless. We have temporarily bypassed the can, and expected possibly a > little noise on the voice repeater because of it, but we didn't expect > the weird things we're getting now like the noise continuing even when > APRS isn't transmitting, and the level of desense it creates. APRS is > transmitting 5 watts. The Voice repeater is I believe 25 watts. > > > > Thanks for the help... > > > >
