+++++++++++++++++++++++ Date: Sat, 1 Sep 2001 10:14:06 -0700 From: "DTX" To: "Low Power Amateur Radio Discussion" Subject: [106220] Re: Random Wire Antenna Corrosion [LONG] Message-ID: <012001c13309$820b85c0$0c00a8c0 at home> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit This is a quick note I sent to Tom, figuring there wouldn't be enough general interest to post to the reflector. If no one already answered you off line, the answer is absolutely yes. Normally it takes a good strong signal, like a nearby broadcast station, to make an effective LO, but once those copper-oxide mixers get sufficiently pumped, seems like every other signal in the whole world will find an Nf +/- Mf product that will be in your passband ;-) Similar things happen when copper ground straps connect to galvenized steel towers. And in guy wires. And rain gutters. And....and......and. The thing that saves most of us is the lack of a good (STRONG) LO signal because they are actually pretty poor mixers. Going a bit farther, if you don't hear anything strange in your receiver, it's no problem, right? Usually. Because a signal strong enough to pump those mixers is most likely going to overload your receiver and cause more problems inside where there are mixers with only 6db conversion loss instead of the 60 or 70 db conversion losses in those external corrosion points. Sounds like a lot of loss until you think about it in real numbers. 80db down from a 10,000 watt BC station is still 90db above your receiver threshold, no matter how deaf a receiver you have. Thank god (budda, allah or mother nature if you're an aethiest ;-) that it takes a lot of RF to pump these things. If a few hundred watts would do it....such a nightmare I don't want to even think about. Imagine a clear channel 50kw AM station around a Mhz or so and a shortwave broadcaster with lots of power in the 8 or 9Mhz . Within a mile of each other ;-) Look at some of those 3,5 and 7th order products. 9020Khz - 2X980Khz lands where? I am not that familiar with SW allocations to know if 9020 would be a candidate for this mix, but you give me the allocations and I'll show you the mix that will kill your frequency ;-) You can hi-pass and low-pass filter and notch those fundamentals at your location, but once the mix product lands on your frequency from an outside location........... FWIW, most of my real world experience is with VHF and UHF repeaters sharing sites with multi-kilowatt FM and TV broadcast transmitters, not AM and shortwave. But the numbers game and power levels give the same results. To Tom's original question, in the absence of such strong signals I would expect he could have 90% corrosion and not notice any diffrence in receiving performance. ----- Original Message ----- From: "Trevor Jacobs" To: "Low Power Amateur Radio Discussion" Sent: Friday, August 31, 2001 11:51 PM Subject: Re: Random Wire Antenna Corrosion > You know, I've often wondered about this myself, especially since the RF > rides on the outer part of the wire where the corrosion generally is. I tend > to use solid copper insulated for antennas and seal up the ends at the > center insulator with RTV, that way I don't have to worry about it until the > insulation goes bad. I usually change antennas by that time anyway, as I > like to experiment. Good subject, let's hear more... > > 72/73 > Trev > KG6CYN > ----- Original Message ----- > From: Nick Kennedy > To: Low Power Amateur Radio Discussion > Sent: Friday, August 31, 2001 5:09 PM > Subject: RE: Random Wire Antenna Corrosion > > > > That's an interesting question. > > > > My thought on the subject has always been that if you still have some > > unaffected copper going past those bad spots, you're not going to see any > > ill effects. > > > > I'd draw the circuit this way. You've got the wire, now add your diode > > and/or capacitor in series with the run. Now, put a wire in parallel with > > the undesired component, bypassing it. That's the good stuff that's still > > left. It keeps any voltage from being developed across that rogue diode, > > so it never gets to do its non-linear thing. > > > > 72--Nick, WA5BDU > > > > > > -----Original Message----- > > From: Tom Curtola [SMTP:tcurtola at home.com] > > Sent: Thursday, August 30, 2001 11:49 AM > > To: Low Power Amateur Radio Discussion > > Subject: Random Wire Antenna Corrosion > > > > I just removed one of the random wire antennas that I no longer need. I > > made the remark that there was a lot of corrosion on it. My roommate > asked > > if that would make a difference in receiving. (He's a non-ham) I said no > > I > > wouldn't think so, then I got to thinking. If the wire was solid then I > > would suspect no. However, a stranded wire antenna; might it not exhibit > > some sort of reaction with the copper rust in-between the strands. Maybe > > even acting like tiny diodes or tiny capacitors? Could signals being > > received not heterodyne in-between these junctions? I told him it was a > > good question and that I would post it for the experts to decide. > > > > Curious, > > > > Tom > > VA3TY ++++++++++++++++++++++ Date: Mon, 03 Sep 2001 18:26:32 -0500 From: Robert Perkins To: qrp-l at Lehigh.EDU Subject: [106375] Re: Random Wire Antenna Corrosion Message-ID: <3.0.5.32.20010903182632.008d3850 at mail.gte.net> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" There was an article in QEX recently about wire composition, and corroded wires. I had been blundering along thinking that zinc plated was better than corroded copper. I found a nice price on zinc plated iron wire, intended for electric fences. It sits on a roll unused after reading the article. I wish I had heeded the advice here and elsewhere about getting THHN insulated solid from the Home Depot type places. The article never mentions IMD in stranded wires, its the measured losses that were depressing. K5IAA ++++++++++++++++++++++