extract from vk-repeater mail list: (10/04/2002)
discussing intermod diagnosis resolutions (or lack of) A good example of bureacracy gone WRONG !
Hi Grant (& All), Your points are well noted & are spot on, however in both cases the problem was the intermod was being mixed in external commercial transmitters & being re-radiated on our RX frequency. In the first case with the 2a - b Intermod, the problem was TOTALLY due to Telstra's irresponsibility in adhering to basic commercial practice on high RF sites. Initially Telstra had just one pager (I think it was on 148.0375MHz.) When they started their second service, ALL their main pager sites then had the two Pagers on 148.0375 and now also on 148.1875MHz. They would feed 500 watts plus up their sticks, but the sticks (the antennas) were mere feet away in a horizontal plain. What did they think was going to happen? Well we found it..... 2 x 148.0375 - 148.1875 = 147.8875, 12.5kHz above our input frequency (12.5kHz below the Blue Mtns 147.300 input). So then every Telstra site was producing this Intermod ILLEGALLY on the 2m Amateur band. SMA weren't convinced that it was really the problem. After many phone calls, a site meeting was planned for 2am (yes 2 in the morning) to test that this was REALLY the problem. First they again fully tested our Rptr. Apart from it's sensitivity "problem", it actually had FAR better specs than a commercial unit. As the Rptr was home-made, their perception was that it would be sub-standard. What they didn't know was that it was built by a 2-way tech with 30+ years in the communication industry who built the receiver to be "bullet-proof". Anyway, after the testing was complete (& the SMA reps finished checking the calibration of the eqpmt because our Rptr's RXer & TXer were well beyond their previous measurements), they phoned Telstra and ALL pager transmissions from Telstra were shut down for 15 minutes. That was the quietest our 7275 Rptr had been since Telstra had started their second Pager service. Then when the word was given to Telstra to recommence transmissions, the Rptr blurted back to life with pager intermod. It was pretty convincing. The two SMA reps were now convinced of what we had known for weeks (probably months). The following day I received a phone call from SMA. Their directive to us was that we had to reduce the sensitivity of our Rptr's RXer. We were "picking-up" intermod transmissions that we shouldn't have been hearing! It was OUR fault!!!! Well after I picked myself up from the floor, I made lots of phone calls to inform others of SMA's decision. To cut a long story a bit shorter, this directive was not accepted & fortunately we had a friend in a "high place" who convinced SMA that this was not the correct conclusion. Telstra was then directed to install filtering (ring circulators/isolators, etc) at their Pager sites. A big win! (or so we thought). During the following months, I made countless phone calls chasing Telstra. You know what they did? They only bought ONE set of filters & moved it about from site to site!!!! (I didn't find this out to months later.) The run-around went like this. I would ring the Telstra RF Dept & inform them of the changed SMA directive & that there was an intermod being produced at a particular site (including Sydney sites). After about 5 more follow-up phone calls they would take the filtering system from say Hurstville to Wollongong. Wollongong would now be quiet, but of course Hurstville would now be bad. I would inform them of this & they would say "oh, now that's a surprise, we'll look into it". Weeks & more follow-up phone calls would ensue, then they would ring & say Hurstville is now fixed! But of course Wollongong was now bad again!!!! This went on for months & months. I would ring SMA & inform them of the Telstra "run-around" & they of course would tell me that they would look into it. I was losing my sanity. My phone bills were huge (being left on hold for ages was the killer), but finally I saw the light. I realised what most of us now know - the Govt Depts don't give a F**K about us. We were just a damn nuisance to them. We finally fixed the problem by changing the frequency (not an easy fix as it turned out & a whole new story in itself!). Of course some of you are now saying - "well that's what I would have done in the first place". Well hindsight is a truly wonderful thing. Two things didn't allow that to be an earlier solution.... 1) - There were no spare negative offset 2m Rptr freqs pairs left in this Sydney region (still a problem) and, 2) - I believed in the "system". We were in the right. It wasn't OUR fault. The Govt Depts would do the right thing by us. I of course learnt a very valuable lesson from this experience. The "icing on the cake"??? Years later I met an RI who is also an Amateur at one of the VK2 WIA Conference of Clubs getogethers. He had heard via the grapevine of our previous problem & asked about it. When I told him the sad & sorry story (expecting "sympathy" from him), I received a dressing-down for not (in his opinion) doing enough to fix the intermod problem!!!!!!!! (More exclamation marks really apply here!) I walked away in total disgust & despair. This whole saga was one of those "life lessons" we often hear about. The experience truly had an effect on me & to this day I have a lot of cynicism when it comes to Govt Depts, in particular ACA/SMA/DOTC/DOC, whatever they're calling themselves this year. (Do they change their name regularly hoping we'll forget the previous Dept's indisgressions???) Here endeths the sermon. Hope it was interesting to somebody! Kind Regards, Rob McKnight - VK2MT Rptr Officer Illawarra Amateur Radio Society inc > -----Original Message----- (email addresses removed to avoid spam-bot address trawlers) > > > Folks, > > Of course, the correct answer in this senario of being told "the > repeater is too sensitive" depends on exactly where the > intermodulation is occuring. If it is in the PA stage of the > transmitters and being radiated by the faulty transmitters, then the > ACA is obliged to have the offending transmitters cleaned up. > > However, unfortunately many of the IMD products are generated in > receivers who's IP3 point (or dynamic range as it is often quoted) is > not high enough for the signals hitting the front end - and as a > result, the receiver front end mixer stage will generate the > intermods. One solution to this, run higher power into the Rx mixer > stage from the Local Oscillator. Also, design the front end to be > hard to drive into class C operation due to overload and have good > filtering on the front end. Bullet proof receivers are not easy to > design mind you, especially when you consider that some > commercial ones around have resorted to running Watts of power into > the LO rather than the more typical tens of milliwatts used by > amateur gear (and most "mobile based" commercial gear too) to raise > the IP3 capabilities of the equipment to cope with very noisy > environments. > > Regards > VK5ZWI > >
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