APRS Transmittion causing interferance?

[W9DTC]

I have a VX-8DR HT and a FT-7900 mounted in my vehicle. Today I was experimenting with APRS and experienced something strange. My FT-7900 was scanning the local repeaters and I heard data come over on one of them (which is strange). Then I looked at my HT (which was running the APRS) and noticed channel "A" (the non-aprs channel) was set to the frequency where I noticed the problem. I took the following steps.

1. Changed channel "A" to 144.39, to eliminate the possibility that my radio (or my understanding) was messed up and sending APRS to the wrong frequency somehow.
2. locked my FT-7900r to 146.700 (the repeater frequency where I heard the data).
3. manually transmitted a beacon from the HT on 144.39. In doing so, the FT-7900 "heard" the transmittion on 146.700.
4. I changed the FT-7900 to listen to the input frequency on that repeater (146.100 if I'm not mistaken)
5. manually transmitted my beacon again. The FT-7900r didn't hear this transmittion.

SOOOO.... any thoughts on why I am hearing transmittions from my HT on 144.39 through my mobile radio set to 146.700?

This evening I'll try again and verify that the repeater isn't actually capturing my beacon, but I'm pretty sure it wasn't since I didn't hear it on the input frequency. I guess the possibility exists that I heard somebody else's beacon on the repeater at exactly the same time I sent mine (however unlikely).

 

[K9WG]

Probably just overloading the front end of the FT-7900. If your rigs/antennas are close enough, the transmitted RF will get into the front end of the receiving rig to be demodulated. Also depending on harmonics/spurs/IF frequencies, two radios in close proximity can have strange results.

 

[W9DTC]

Thanks for the quick response.

The antenna for the 7900 is on the roof of the truck (so up about two feet and back about 3 feet from the HT). The HT was in my hand inside the truck using the rubber duck antenna. Sounds like it must be some sort of interference, but I don't really understand how that happens, how to correct it if possible, and how likely it is to happen with any two radios on any two frequencies. I did see in the vx8dr manul some discussion about intereference between the two channels on the same radio (and have experienced that issue as well)

Hopefully I wasn't causing a problem on the repeater, but it sounds like the issue is probably issolated to the close proximity of the cab of my truck... weird.

 

[KD0LDK]

Probably just overloading the front end of the FT-7900. If your rigs/antennas are close enough, the transmitted RF will get into the front end of the receiving rig to be demodulated. Also depending on harmonics/spurs/IF frequencies, two radios in close proximity can have strange results.

Agreed. I'd say this is a safe bet as to what's going on. Same deal with my Spectra, if you key it up just about every radio within 10 feet of my truck finds a frequency and receives.

Thanks for the quick response.

The antenna for the 7900 is on the roof of the truck (so up about two feet and back about 3 feet from the HT). The HT was in my hand inside the truck using the rubber duck antenna. Sounds like it must be some sort of interference, but I don't really understand how that happens, how to correct it if possible, and how likely it is to happen with any two radios on any two frequencies. I did see in the vx8dr manul some discussion about intereference between the two channels on the same radio (and have experienced that issue as well)

Hopefully I wasn't causing a problem on the repeater, but it sounds like the issue is probably issolated to the close proximity of the cab of my truck... weird.

Sounds to me like you may have been too close as K9WG suggested. Try moving away from your truck a bit and see if you get the same results.

Something kind of out there, but I always ask. Are both of them hitting repeaters ok? Reason I ask is I have a couple radios that became un-aligned and received off frequency from what they display. Pretty rare, but not something to rule out.

 

[W9DTC]

Something kind of out there, but I always ask. Are both of them hitting repeaters ok? Reason I ask is I have a couple radios that became un-aligned and received off frequency from what they display. Pretty rare, but not something to rule out.

The 7900 is definitely hitting repeaters well.

I've had a little trouble with the xv8dr, but I think I'm just asking too much of it. I'm out in the sticks a bit and can hit some repeaters with the rubber duck but not very well. I did try a directional antenna to one of those same repeaters and got in 5x9. I've also been able to hit a couple of satellites with the HT using the yagi, so I believe the transmitt to be on... but clearly there are some spurious transmittions (assuming I'm using that terminology correctly in this case)

 

[LtDoc]

The simple answer is just 'over load'. The same thing is noticeable in my mobile (APRS and non-APRS radios on the same band and proximity). Absolutely normal. I seriously doubt if your interfering with anything else not in about the same proximity, but a simple way of finding out is to ask anyone also listening to that repeater. (And then hope 'they' aren't mistaken, or pulling your leg.)
- 'Doc

 

[AK9R]

2. locked my FT-7900r to 146.700 (the repeater frequency where I heard the data).
3. manually transmitted a beacon from the HT on 144.39. In doing so, the FT-7900 "heard" the transmittion on 146.700.

I have experienced this, too. While listening to 146.700 with one radio and transmitting APRS beacons on 144.390 with another radio, I hear my APRS transmissions on the 146.700 radio.

 

[W9DTC]

Thanks for the help all. I did confirm this morning that while I am hearing the interference (from overloading I guess), the repeater is not receiving anything and I'm not interferening with its opperation at all.