Posted by
Richard Beerman-2 on
Jul 29, 2018; 1:27am
URL: http://elecraft.85.s1.nabble.com/Horrible-APRS-QRM-on-2-Meters-tp7643881p7643947.html
Thanks to Wayne and others who took time to reply to my inquiry. Because you all cared enough to offer help, I went back over this problem today and have some answers. First of all, Wayne, Jim and Guy were on the money regarding the NB. But first, let me go back a bit. I was listening on 144.150 mHz upper side band; no preamp on with the K-3. No other preamp in line. I am using the K-3 as an IF for a new Downeast Microwave 2 meter transverter. This transverter offers about 17 dB receive gain to the K-3 at 28 mHz, the IF frequency. The transverter is connected to a K1FO yagi with about 12 dB forward gain. The line audio output of the K-3 was patched to the mic input of a Dell PC that was running MSK 144, a program in the WSJT-X suite of software, and I was testing the system to run in upcoming meteor showers. Suddenly, I was interfered with by APRS local signals transmitting at 144.390 mHz.
Everyone was very helpful in making me understand how the two noise blankers in the K-3 work. Your comment, Jim, about calling them NB1 and NB2 is absolutely correct! This is what Kenwood does on their TS-5909SG radio. So, I tuned the TS-790S to 144.39 mHz and being a Saturday, there was plenty of activity. The local APRS station that is closest to my QTH must be really close because he pins the S meter on the 790S connected only to a mobile whip in my steel building. That is the signal that appears at 144.15 mHz on my K-3. APRS is FM and I assume that these signals are generated by a transmitter operating in Class C. Just looking at the signal on my monitor that is printing the activity on 144.15 mHz, it looked pretty dirty. And I am listening on upper side band.
There is a large signal entering the transverter input port on the K-3, given the antenna gain, the receiver gain of the transverter and the large APRS signal. Operating the noise blanker control on the K-3, I discovered that at EVERY setting level using the IF noise blanker, the APRS signal was not just leaking through but was really strong on 144.15 mHz. (Sorry, I was not able to quantify “really loud”, but you get the picture) There was no way to eliminate these APRS signals by changing AGC or eliminating AGC.
Then, I turned off the IF noise blanker and turned on the digital noise blanker. It not only eliminated the APRS QRM at 144.15 mHz, but, it also eliminated much of the power line noise. I ended up at T3-5 setting as best. So I am pretty happy now. I just need to confirm that the digital filter setting does not distort SSB signals by being too aggressive.
Here is my question to the low band crowd on the reflector. You are listening on 160 or 80 meters in the winter. You are having trouble with some kind of noise interference. Your antenna on receive is unity gain or less, but you have a 30 to 40 dB receiving preamp on your receiving antenna. You are running the noise blanker in the IF mode on the K-3. Is it not possible that you are having the same issue as I discovered? The interfering signal is off frequency somewhere relatively close by and the IF noise blanker is somehow opening up the receiver to QRM on the desired receive frequency?
Based upon what I discovered today, there is certainly a issue with the IF noise blanker in the K-3 (don’t know about the other Elecraft radios) where lots of gain is used to amplify weak received signals and where there is (are) very strong (maybe just dirty FM signals) close by. My situation could be easily duplicated.
I now recall that I first noticed this very same problem about 8 years ago with the same K-3 on 2 meters. This was at another QTH with a normal suburban lot and the same antenna. I was receiving a loud interfering signal on 144.2 mHz that I tracked down as cable channel 16 (I think) at about 136 mHz or so. This was also an FM video and audio signal and I am sure that I tried unsuccessfully to eliminate the interference using the K-3’s IF noise blanker. I had the Comcast tech come out and he explained that Comcast disconnected service to a neighbor about 100 feet away. When he said “disconnect” that is what he meant, leaving an exposed center conductor on their coax to radiate. They do not put a short or load on the disconnected line so it just radiates. Nice!
The good news is that the digital noise blanker worked well with my issue. Again, thanks for all the very helpful input! 73’s Dick W5AK
On Jul 27, 2018, at 8:59 PM, Richard Beerman <
[hidden email]> wrote:
>
> Wonder if anyone else has encountered this on 2 Meters? I am using a 12 ele K1FO yagi - about q12 dB gain on 2. That enters the shack where it is connected to a Downeast Microwave 50 Watt transverter with about 17 dB gain on receive. The output of the transverter is connected to transverter input on my old K-3. For the longest time, I was receiving loud QRM from what I initially thought were power line noise bursts. But, I became suspicious because these bursts were about a second or so in duration consistently. During this time, I was testing my setup for meteor scatter using the MSK 144 software found in the current WSJT-X suite. These loud bursts created huge red images on the monitor watching in 144.150 MHz, the MSK144 calling frequency.
>
> I got suspicious that these signals were not simple electrical pole shorting noise. So, I connected a whip to my old TS-790S rig and tuned it to 144.390 MHz, the North American APRS frequency. WALAAAAA, the signals on my monitor connected to a K-3 tuned to 144.150 MHz corresponded perfectly to the WPRS signals on 144.390 MHz, some 240 kHz away! The preamp on the K-3 was off. There were no other receiving preamps in line at the time. Later, I turned on an external 2 Meter preamp and all that happened was that the APRS signals were larger at 144.150 MHz.
>
> It appears that the front end of my K-3 simply folds up in the presence of strong signals that are WAY off the receiving frequency. I can tell you that connecting the same antenna/transverter to a Kenwood TS 590SG, there were no traces of the APRS signals. And connecting the TS-790S to the K1FO likewise produced no APRS QRM at 144.150. To me, with the presence of a number of strong APRS signals on 144.390 makes this K-3 extremely unusable on 2 Meters. I was running the K-3 noise blanker to handle the rest of the noise here but likewise, I also had noise blankers running on the two Kenwood radios as well.
>
> Any suggestions are welcome. Dick W5AK
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