Are you sure it's the MFJ? I thought my MFJ 259 was behaving badly at some frequencies, and it turned out to be the dummy loads. One in particular was flat everywhere but 6 meters, and it took me years to notice because I don't deal with that band often.
I also have noticed the problems in high RF environments, but that's not MFJ's fault. No SWR bridge works accurately measuring an antenna with a few milliwatts at a 100 kilowatt broadcast site. Paul, AE4KR ----- Original Message ----- From: Gary Schafer To: [email protected] Sent: Sunday, May 11, 2008 1:56 PM Subject: RE: [Repeater-Builder] Wacom Duplexer Desense Problem - Mystery A couple of notes on this: Beware of the mfj to read impedance/swr. I have an mfj 259B and it is terrible on 2 meters. With a GOOD dummy load it shows a higher swr the higher you go in frequency with it. I think it will show around 1.5:1 and an impedance of around 42 ohms on the good dummy load. On lower (HF) it works fine. The problems that you found with the cable may not have been because it was not double shielded. Especially if things changed as you wiggled it around. It may have been poor connector installation. Most people that experience cable problems do not look at the connectors as being installed improperly or braid broken loose from extended use. I used to sell service monitors to two way shops and would bring the monitors in the shop to demonstrate them. I found that about 80% of the test cables that the shop had (and used on a regular basis) were bad! Usually intermittent from bad connector installation. It seems kind of unbelievable that guys that were using this equipment in day to day work would not be aware of the bad cables. Just think of how many goofy problems could be avoided with known good test equipment. After a short time I always made sure that I brought my own cables along for the demo. I used to pick up coax jumpers at hamfests for use around my shack. I can't remember the last time I got one with the connectors installed properly (always loose connectors) other than commercially made cables. Now I don't even bother to pick them up anymore. Check those connectors. If you can rotate them at all on the cable they are not installed properly and will give problems. 73 Gary K4FMX > -----Original Message----- > From: [email protected] [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of blisswheeler > Sent: Sunday, May 11, 2008 12:38 AM > To: [email protected] > Subject: [Repeater-Builder] Wacom Duplexer Desense Problem - Mystery > > I really appreciate all of the comments on my duplexer mystery. > > I made sure everything was tuned properly and took it back to my > friends home location. We put it back on the air and experienced <2db > of signal desense with an output of 15 watts. That still doesn't > duplicate the results of the dummy load test, but that's a far cry > from the 30db I was experiencing here at my home shop. Some of the > things I did learn from the experience was that: > > 1. Though an antenna may only have a 1.5 SWR doesn't mean that the > antenna is a 50 ohm match. Using my MFJ ("Mighty Fine Junk" which, by > the way work pretty darned good, but not in a high RF environment) > antenna analyzer, the antenna that gave me the most difficulty > presented an 80 ohm load. The SWR was 1.5. A discone antenna worked > the best and it presented a 55 ohm load with a 1.2 SWR. > > 2. Double shielded coax is a must in repeater operation. I > experienced this first hand. I had one short jumper I thought was > double shielded and was not which caused an intermittent such that one > time it worked into a dummy load and the next it went flaky. Moving > the coax with the repeater transmitter keyed revealed the culprit. Use > hardline or double shielded coax. Hardline to the antenna is very > important. > > 3. Bench testing duplexers into a dummy load may not duplicate the > results experienced with the antenna. > > 4. In theory isn't necessarily the same as reality. I suppose if you > know all the variables the problem can be calculated and identified, > but there are a lot of unknown variables when working with RF. > > 5. The environment your repeater is in can cause you to loose your > hair. Yes it is related to the rf generated, no not because of the > health effects but because it makes you tear your hear out trying to > identify a problem. > > 6. I learned about whiskers in GE Mastr II receivers... I experienced > their effects and how to fix the problem, though maybe for only a year > or two, but I learned to to disassemble and retune the receiver. > > 7. Your experienced Techie RF friends are a great help. I have three > good friends that gave me a lot of ideas and helped me trouble shoot > this problem. The folks here on Repeater-Builder gave me some good > tips which pointed in the right direction as well, for that I thank > you all. > > Respectfully, > Bliss > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >
