The Dummy Load. Newsletter of the San Juan County Amateur Radio Society December 2008 Rev A

Transcription

1 The Dummy Load Newsletter of the San Juan County Amateur Radio Society December 2008 Rev A Next Monthly Meeting: Friday, December 12, 11:30A Second floor, Friday Harbor Firehouse, Friday Harbor Summary of N7JN Repeater "Buzz" Tests of November 11, 2008 By Ed Kelm Introduction: The "buzz" is a raspy low frequency audio signal which arises when the input signal to the 2M repeater is weak. The buzz frequency measured by Ed K. and Wayne some days before the trip to the hill was ~111Hz. It has been described by some as an arc welder sound. The buzz is similar to a problem encountered in the past, which was fixed by replacing the feedline. The present problem did not arise until fairly recently, while Wayne was in CA. (We do not believe his absence caused the problem.) Observations: 1. The buzz can be re-created on-site by radiating a signal of the appropriate strength (pretty weak) from the garage. 2. Replacing the antenna with a dummy load at the antenna location eliminates the buzz. 3. Replacing the antenna with a new one of the same make and model does not reduce the buzz. 4. Replacing the coax to the antenna with new LMR-400 does not change the buzz. 5. Removing the 2M preamp reduces the buzz. 6. Shutting off the 440 repeater does not change the buzz. 7. Removing the triplexer and running the antenna straight to the 2M duplexers does not change the buzz. 8. Operating the 2M transmitter into a dummy load while the receiver is still connected to the antenna through the duplexers eliminates the buzz. 9. A warm body on a ladder near the base of the antenna reduces/eliminates/changes the buzz. 10. The 2M duplexers were checked by Wayne and found to be tuned correctly except for one of the 2M receive notches. Wayne touched it up, increasing the total receive notch depth by ~8db. When done, both notches were ~89db down. This improvement did not change the buzz. 11. Transmitting on the 2M repeater output frequency from Peter's van with the 2M repeater receiver squelch open, and no input signal produces no buzz. 12. Disconnecting the 120V switching power supply does not change the buzz. 13. Adding a 1-cavity bandpass filter to the 2M receiver input does not change the buzz. 14. Adding the same filter to the 2M transmitter output does not change the buzz. The Post Card Project By Bryan Hoyer Mailed 272 Returned by Post Office 17 Responded to date 3 1 deceased 1 already member 1 potential new member Cost for printing the four color glossy 1 side with black on back and addressing per our list $90 plus postage $73 total $163 Annual Holiday Potluck Friday, December 12, 11:30A Friday Harbor Firehouse Bryan Hoyer will be coordinating the preparations for this fine annual event. Join the weekly Wednesday net for all the details and information. An attempt will be made to coordinate contributions. Turn Your Radio On Please send Dummy Load contributions to: drathmarine@rockisland.com

2 November 19 Repeater Work Party Report By Ed Kelm On site: WA6MPG, K2SPR, KD7TUN, KG6GEU Primary goal was supposed to be looking for the source of the "buzz", but recent wild excursions in 2M sensitivity (changing from one minute to the next), plus some 440 problems, made the first item of the day looking for possible loose connections and antenna problems. 'GEU climbed a ladder and wiggled the antenna, while 'MPG watched the SWR meter. We also tried whacking the antenna with a handy piece of PVC pipe. After much prodding and poking, the only thing found was a possible loose connection in a cable/connector from the triplexer. The triplexer was replaced for good luck. With that out of the way, we turned our attention to the "buzz", and did some tests proposed by 'TUN in an of 11/18. (The test designations are from that .) Results: TEST A: Verified that keying up the 2M repeater transmitter caused the buzz to appear on the 2M repeater receiver, with no other input signal required. The appearance of the buzz on weak signals was thus explained as having to have a signal strong enough to key up the repeater, but not strong enough to mask the buzz signal. In fact, keying up the transmitter is all it takes. TEST B: Used Wayne's spectrum analyzer to look at the input presented to the repeater receiver input while the transmitter was operating. Two things became immediately obvious: A broadband spectrum of closely spaced lines appeared when the transmitter was keyed (at least 500 KHz wide, but possibly limited by the bandwidth of the duplexers), and when demodulated by the spectrum analyzer, the characteristic buzz sound was evident. The buzz was especially strong when demodulated as AM. An expansion of the spectrum showed peaks at roughly every 200 Hz, not inconsistent with the 110 Hz observations written up previously. Conclusion: The buzz was almost certainly NOT being generated in the receiver. The transmitter was now looking more like a possible source of the non-linear mixing. TEST C: Looked at the signals coming in to the 2M transmitter from the antenna. Nothing obvious. TEST D: Looked closer at the receiver input signals during transmit, recorded levels of the transmitter harmonics. TEST E: Tried transmitting on from Wayne's van and listening on the repeater receiver. The buzz is still being generated, but more weakly. Buzz amplitude varied with transmitter power. Concerned that the inactive 2M transmitter in the repeater was still the source of non-linear mixing, we broke the connection between the transmitter and duplexer and terminated both ends with dummy loads. The buzz was still present. This indicated that the non-linearity is not the transmitter, but is outside the repeater, somewhere in the immediate vicinity. Possibilities are corroded joints on the metal roof of the garage, electronics like TV receivers in nearby houses, etc. Surveyed the RF spectrum from about 30MHZ to 750 MHz, looking for the same spectral signature seen in Test B. None found. As a further experiment, two different antennas were tried on the repeater. First, a mobile mag mount was slapped on the metal garage roof, resulting in minimal buzz. Then a Comet tri-bander of the exact same model as the one in current use (in fact, the one removed at the last visit) was mounted on a temporary pole. At the first location chosen, the buzz was essentially gone. Moving the antenna to a more advantageous location for semi-permanent mounting brought the buzz back. Bryan and Wayne found that moving the antenna along the back wall of the garage resulted in the buzz coming and going roughly every half wavelength at 2M. TEST F: Not done, due to lack of time. TEST G: Not done, due to lack of time and apparent irrelevance. Note that one thing we haven't ruled out is that the non-linearity is in the repeater rack somewhere, but other than in the transmitter or receiver. Perhaps the RF gets into the unshielded wires on the interface board and gets mixed in the audio circuits of one of the repeaters or the controller. Wish we'd done "G". CONCLUSION: There is some non-linear element nearby which mixes the transmitter signal (or one of its harmonics) and a possibly nearby buzzy signal, maybe some data transmission, to the repeater input frequency. The exact location of the repeater antenna affects the strength of the interference. One might guess that the data transmission is originating from Mt. Constitution where a lot of powerful transmitters are located, but there is in fact no evidence to indicate the location of the original buzz signal. Page 2

3 POSSIBLE SOLUTIONS: 1. Find the non-linear element and "fix" it. If it's a corroded screw on the roof, clean it up and reinstall it. If it's in someone's house, who knows? Problem: How do you find it? Sniffing around with a receiver seems like a good idea, but only a very unusual receiver could detect the weak signal in the presence of the 50W signal being radiated by the transmitter just 600 KHz away. 2. Move the antenna away from the non-linear element. This reduces the amount of transmitter power available to drive it, and reduces the strength of the spurious signals sent back to the antenna. Problem: What direction is "away"? Into the woods away from the houses, etc., seems like a good guess, but that is off the property. Bryan's and Wayne's observations suggest that moving along the back wall of the garage away from the main house may be good. 3. Reduce the transmitter power. There may be a threshold below which the non-linear element behaves much less non-linearly. This could compromise the ability of the repeater to overcome Cable TV channel 18 in certain areas. San Juan Country Amateur Radio Society Minutes of Regular Meeting Friday, Nov. 14, 2008 Friday Harbor Fire Station Submitted by Nancy Lindenberg Attending: W7BXH, KD7IRD, KE7TLE, KG6GEU, K9MRQ, KE7KRO, K2SON, K2SPR, KD7CIN, KD7TUN, WA6MPG, KE7KJM, N6AU Meeting called to order 11:30 A.M. Minutes of October 10 meeting were published in the Dummy Load. Accepted as published. Treasurer: Balance of $5766 was announced by treasurer D. Vandaveer. There are a few accounts payable. Publicity: Brian and Dani Hoyer reported on the 272 postcards sent out to all hams in San Juan County, at a cost of $163. They were congratulated on a very attractive and effective mailing. First result was our visitor, Gregg Dietzman, KE7KJM, gregg@wps2.com, of Friday Harbor. Repeater: Source and analysis of the buzz on the 2 meter band is still under investigation. The work party of last week has placed additional batteries in service, and has changed the antenna. More work is yet to be done. Ed Kelm requested a new repeater controller command that would set the clock to 9AM, for example. Measurements made in late October to November of this year on the repeater controller clock indicate that it is running slowly, about 30 seconds a month. Summer readings may be different. Discussion followed regarding a voice announcement of Battery Low at the end of the ID message. Emergency Coordinator: Peter McCorison reported on a District EC/RO meeting he attended. This includes four counties. It seems there is some conflict with our DEM. Wayne observed that this has little relevance to San Juan County. Frequencies: A PRELIMINARY Frequency List was passed out for review. Reminders to use the Tactical ID. After about 2 months more of review, we will adopt the list at the January meeting. Christmas Potluck: Brian Hoyer will serve as party coordinator and will invite local special guests. Host Gift: Peter McCorison will investigate and recommend a suitable holiday gift for our repeater host. Equipment: Jim Crutchfield, W7OI, has a tribeam and 55' tower antenna for sale on San Juan Island. Brian Hoyer is the club contact person. Technical discussion followed on the automatic shutoff for the battery. Adjourned 1:10 P.M. Notes by D. Drath and N. Lindenberg Respectfully submitted Time Set Command Available Submitted by Dan Drath Command #308 is now available to trusted users. It will set the repeater controller clock to 9AM. Usage: 1. log-in 2. enter key the mike, enter #308, then unkey at the precise moment you want the clock set to 9:00:00AM. 3. log-out The controller response to this command is: "Set time 9 AM" then after 5 seconds, a female voice will respond reading out the controller clock. Page 3

4 From the beginning of the TV days in the early 1950s, ham clubs have been providing TVI committees to evaluate interference problems. These committees provide a combination of analytic and diplomatic services to hams and neighbors. If the committee operates properly, corrective action can be identified and good neighborly relations can be maintained. This is a story of one such success. Ed. A Little TVI Tale By Ed Kelm At the end of the recent repeater seminar conducted by Dan, N6AU, Athlene, KD7TUO, mentioned that she had a TVI problem; her transmissions on 2M were interfering with her neighbor s TV reception. Dan and I agreed to form a two man TVI committee. We finally got a date set when the neighbor and Athlene would be home, and Dan and I were available to go take a look. The neighbors were watching a DishTV channel, coming in on channel 3 on their set. Sure enough, when Athlene transmitted, even on low power, the TV went nuts with loss of color, herringbone lines, etc. Dan and I then did a bunch of experimenting. We found: 1. Operation of two different handhelds at 5W in the immediate vicinity of the receiver causes slight interference. 2. Operation of either hand held directly beneath the dish causes very slight interference. 3. Operation of N6AU s handheld next to the RG-6 down lead outside the house causes very slight interference. 4. Operation of either handheld about half way between Athlene s house and the dish causes substantial interference. 5. Operation of either handheld near Athlene s antenna caused less interference. 6. Installing two ferrites on the RG-6 where it entered the house did not improve things. 7. Installing two more ferrites on the downlead RG-6 where it connected to the sat receiver did not improve things. 8. Tightening the connection of the RG-6 to the sat receiver did not improve things. Why would the interference be worse farther away, but not too far away? Maybe there was a nonlinearity at that point that caused mixing with the 2M signal and something else. The dilapidated TV antenna on a garage nearby (TV antenna #1) looked suspect. But how did the interference get into the TV? Dish is way up at 10 or 12GHz, so mixing from 2M up didn t seem real likely. On the other hand, the dish was pointed at that spot, albeit skyward. KD7TUO 25 watt 2 meter Transmitter MHz J-Pole TV Antenna #1 Log Cabin w/ TV Sat Dish Sat Receiver Neighboring House Channel 3 Distribution Throughout house Distribution Amplifier Area of worst interference using handheld TV Antenna #2 Simplified Block Diagram Prepared by Dan Drath Strong Channel 12 signal MHz Page 4

5 Maybe the interference was getting in on Channel 3, but how? The TV shouldn t be that poorly shielded. Maybe the coax from the sat receiver to the TV was bad, but it would have to be really bad. Besides, the neighbors said it affected all three of their TVs in the house. We left, moderately confused, promising to think about it. The turning point came a few days later. Dan found a web site describing TVI on a Dish system. It pointed out that if the interference is the traditional herringbone stuff (and it was, plus the traditional loss of color) the problem is AFTER the satellite system. In effect, 2M is interfering with Channel 3. If it were interfering with the satellite signals, the picture would break up into big pixels or any of the other weird things that MPEG2 digital TV does when messed with. That got us back on track. The interference had to be getting into the TV on Channel 3. If 2M got mixed with something, it would have to be a fairly strong signal. Channel 12 is exactly 144 MHz above Channel 3, and it s strong, so that would work. (Channel 3 goes from 60 to 66 MHz; Channel 12 goes from 204 to 210 MHz) It couldn t be generated by the 2M signal getting directly into the TV itself, because the handhelds right near it caused little or no problem. But if the interference was getting generated out in the yard, how did it get in? On our first visit we noticed a TV antenna high up in the neighbors tree, but they said they didn t use it. We had a couple of theories now. Maybe they didn t have three sat receivers, but instead piped Channel 3 all over the house to the other TVs. That would give lots of places for bad connections to let the interference in. Or maybe that antenna in the tree, TV antenna #2, was still connected somehow. With something to go on, we excitedly scheduled another visit. The first thing we tried was feeding the TV with direct video instead of Channel 3. Interference gone, but what about the other TVs? After doing a little looking, it became apparent that there were no other sat receivers, and Channel 3 was indeed piped all over the house. If that system had a leak, we should hear the video carrier of Channel 3, and possibly even the sound carrier pretty strongly. No luck. Next, we turned the TV to channel 2 nice clear picture! And we got a fair picture on 4, 5, and 7. That antenna in the tree was still hooked up! The neighbor wanted to just cut the coax, but we were concerned with fixing this properly. Looking under the house revealed several little boxes with coax all over the place. I finally bit the bullet and crawled under there. There was the antenna lead, going to a pre-amp power supply. From there, the output went to a combiner. It had Antenna in, Channel 3 in, and TV out ports, with the TV out port feeding a 4-way distribution amplifier that must have supplied the various TVs around the house. Mystery solved! That antenna was feeding any interference on Channel 3 right in. In fact, while looking at things earlier, we had noticed some non-ham interference on the Dish picture. We disconnected the combiner, and fed the sat generated Channel 3 directly to the distribution amplifier. Just for good measure, we unplugged the preamp power supply. I crawled back out. Dan dusted me off, and we went back inside for a test. Problem fixed! What generated the interference in the first place? Who knows. There might be something nonlinear out there, perhaps in the cabin, but it could also be the preamp on the antenna, or distribution amplifier. It wasn t likely to be in one of the TVs, because all three had the problem. If it wasn t in the cabin, why was there a magic spot near there where the interference was strongest with a handheld? Standing close to the house, one would be pretty much under the TV antenna, in a pretty weak part of the antenna pattern. Halfway to Athlene s, we were out in the main lobe of the antenna, but at Athlene s, we were farther away. Athlene has a better antenna than a handheld, and it s mounted higher up, so she could light up that TV antenna pretty well. There s a lesson in there for the dish installer, whoever he is. Next time I meet one, I m going to mention a good way to cause a big problem! G, g, M, m, K or k GHz is the common abreviation for a gigahertz, or one billion hertz. MHz is the shortened form of the word megahertz or million cycles per second. mhz, by common acceptance would be a millihertz or one thousandth of a hertz. On the other hand, the engineer's society, IEEE, and most other sources prefer khz to KHz. This apparently makes it less likely that users will confuse kilo (decimal 1,000) with the computer K (1,024). Isn t science wonderful? Page 5

6 From the Minutes of the November 2009 Club Meeting: Frequencies: A PRELIMINARY Frequency List was passed out for review. Reminders to use the Tactical ID. After about two months more of review, we will adopt the list at the January meeting. The list is below. Page 6

7 This page is intentionally blank to facilitate two sided printing. Page 7

8 The ARES Net meets each Wednesday at 8PM on the San Juan County Amateur Radio Society Repeater MHz; -600 khz offset; Hz tone and/or , MHz; +5 MHz offset; Hz tone Net Control Schedule Please arrange a substitute if needed. Dec 03 Ed Kelm, KD7TUN, Dec 10 Nancy Lindenberg, KD7IRD, Dec 17 Jerry Swalling, WA7ZTT, Dec 24 Mark Dawber, N6CSM, Dec 31 Jerry (2) Gonce, KD6CWE, Jan 07 Dan Drath, N6AU, Jan 14 Babs McCorison, KD7CIN, Jan 21 Dave Vandaveer, K9MRQ, Coming Events December 12, Christmas Potluck and Monthly Club Meeting December 22, Deadline for material for the next Dummy Load. Let the Editor know if you wish to be added to the Net Control rotation This issue of the Dummy Load was prepared with the help of Jim and Jan McCorison. The Dummy Load c/o Drath POBox 1460 Friday Harbor, WA First Class Mail