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duplexers, antennas, repeaters
One wonders why someone isn't pushing for a digital TDMA scheme for
amateurs. You could build a very nice full duplex repeater on a single frequency that way. Sure it's totally incompatible with current FM repeaters, but then, D-star isn't totally compatible either. D-star and it's ilk are sort of half measures in that sense. No filtering, much less intermod issues in multi station at onee site systems... all kinds of good comes of it. Digital schemes on HF to replace SSB I can see having real trouble (the biggest is the lack of a "party line" capability, the other is the long propagation delay on HF paths), but on VHF and up FM, you already have a "one person talks at a time" by virtue of the standard FM demodulator. |
duplexers, antennas, repeaters
On Wed, 31 Aug 2011 09:38:51 -0700, Jim Lux
wrote: One wonders why someone isn't pushing for a digital TDMA scheme for amateurs. That's easy. Because there's no emission designator for FCC approved TDMA mobile/HT for ham radio. The ARRL is working on the problem. http://www.arrl.org/news/arrl-files-em-petition-em-em-request-for-temporary-waiver-em-with-fcc-regarding-vhf-voice-and-data-e Multiple time slot systems, such as DStar are currently approved. This is nothing new. Just approval for P25 radios. You could build a very nice full duplex repeater on a single frequency that way. Yes, except that the ARRL has decided to only petition for a waver for single time slot TDMA, which can't be used for a repeater. You're scheme would certainly work, and I too am wondering why nobody has bothered to do it. Possibly because nobody really wants full duplex (with echo, reverb, feedback, etc). Sure it's totally incompatible with current FM repeaters, but then, D-star isn't totally compatible either. D-star and it's ilk are sort of half measures in that sense. Dstar duz 5 independent simultaneous conversations through the repeater. That's not what I would call a half measure. No filtering, much less intermod issues in multi station at onee site systems... all kinds of good comes of it. Yep. Digital schemes on HF to replace SSB I can see having real trouble (the biggest is the lack of a "party line" capability, the other is the long propagation delay on HF paths), but on VHF and up FM, you already have a "one person talks at a time" by virtue of the standard FM demodulator. Yep. We did that with Amtor (Sitor) for data. The big problem was getting the switching time between T/R to less than the prop delay. If you increase the time slicing to where it could handle voice (about 200Hz) but not be audible, the occupied bandwidth increases unacceptably wide for HF. It's possible to decrease the switching time, but then the latency (delay) increases to unacceptable levels. I don't think anyone really wants a repeater with a 1 second audio delay (even though they exist). -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
On Aug 31, 11:38*am, Jim Lux wrote:
One wonders why someone isn't pushing for a digital TDMA scheme for amateurs. You could build a very nice full duplex repeater on a single frequency that way. *... No filtering, much less intermod issues in multi station at onee site systems... all kinds of good comes of it. Digital schemes on HF to replace SSB I can see having real trouble (the biggest is the lack of a "party line" capability, the other is the long propagation delay on HF paths), but on VHF and up FM, you already have a "one person talks at a time" by virtue of the standard FM demodulator. Uhhh. It's been a long time since I worked on such a system (1975 I think), but IIR the prop delay through space for moderate distances kills the idea. Rough calculations gives a round trip delay, at 10 miles from the repeater, of about 0.1 ms. For two stations at that distance that's 0.1 ms not available for sampling, bit width and processing. Keeping the BW down also needs rise and fall time as well as guard times. It added up quiclky back then. The vocoder becomes very important to reduce the data rate. Hope I got that right... 73, Steve, K9DCI |
duplexers, antennas, repeaters
You could build a very nice full duplex repeater on a single
frequency that way. Yes, except that the ARRL has decided to only petition for a waver for single time slot TDMA, which can't be used for a repeater. You're scheme would certainly work, and I too am wondering why nobody has bothered to do it. Possibly because nobody really wants full duplex (with echo, reverb, feedback, etc). I wonder whether you may not also have to be really careful with your transceiver/receiver switching design. You'll really need to be able to trust (and drive) those PIN diodes properly... goof up on even a single time-slice and you could put enough TX power into your receiver to turn its front end into a pile of smouldering char in a millisecond. This isn't a problem with normal split-frequency repeaters, thanks to the isolation in the duplexer cans. Do any of the commercial TDMA systems use the same frequencies for base-mobile and mobile-base? My recollection is that TDMA cellphone systems operate with split uplink/downlink frequencies. No filtering, much less intermod issues in multi station at onee site systems... all kinds of good comes of it. Yep. But "no filtering" comes with its own set of concerns. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
duplexers, antennas, repeaters
Noskosteve wrote:
Uhhh. It's been a long time since I worked on such a system (1975 I think), but IIR the prop delay through space for moderate distances kills the idea. That was 1975. The only people who ever were exposed to any propigation delay where those rich enough to make a long distance call that was routed over satellite. If I remember correctly it was about $5 a minute to call New York from L.A. This is 2011, everyone is used the the propigation delay of digital telephones, and VoIP. 150ms is tollerable. Geoff. -- Geoffrey S. Mendelson N3OWJ/4X1GM Making your enemy reliant on software you support is the best revenge. |
duplexers, antennas, repeaters
Geoffrey S. Mendelson wrote:
Noskosteve wrote: Uhhh. It's been a long time since I worked on such a system (1975 I think), but IIR the prop delay through space for moderate distances kills the idea. That was 1975. The only people who ever were exposed to any propigation delay where those rich enough to make a long distance call that was routed over satellite. If I remember correctly it was about $5 a minute to call New York from L.A. This is 2011, everyone is used the the propigation delay of digital telephones, and VoIP. 150ms is tollerable. Geoff. For you, perhaps. I have a mild learning disability and I can't use the phone if there's too much hesi............tation. I have a twisted pair that connects directly to an EOC at the push of a button. SMS works great for me. What's really funny is "music on hold" via mobile voice circuit. |
duplexers, antennas, repeaters
Jeff Liebermann wrote:
Yep. If the repeater is going in any of the buildings where I have equipement, it will need a cavity and ferrite isolator to prevent intermod problems. I know of one building that demands Heliax, no braided coax including LMR-xxx coax, all silver plated connectors, and other draconian anti-intermod measures. Tempest like packaging on all computahs. All I can say is that it works (if you can afford it). I've never seen that anywhere. Intermod is a math problem. No amount of silver plating will fix bad coordination. |
duplexers, antennas, repeaters
On 09 Sep 2011 19:50:43 GMT, dave wrote:
Jeff Liebermann wrote: Yep. If the repeater is going in any of the buildings where I have equipement, it will need a cavity and ferrite isolator to prevent intermod problems. I know of one building that demands Heliax, no braided coax including LMR-xxx coax, all silver plated connectors, and other draconian anti-intermod measures. Tempest like packaging on all computahs. All I can say is that it works (if you can afford it). I've never seen that anywhere. Intermod is a math problem. No amount of silver plating will fix bad coordination. Sounds like you've never had to deal with transmitted intermod. Consider yourself lucky as getting rid of it is a PITA. Also, not all intermod comes from mixing in the receiver. TX intermod is real and preventable. The xmit cavity and ferrite isolator prevent any RF from adjacent antennas from going down the antenna, into the power amp, mixing there with the xmit signal, and having the power amp amplify the intermod. Google for "intermod suppression panel". http://www.google.com/search?hl=en&q=intermod+suppression+panel&um=1&ie= UTF-8&tbm=isch http://www.repeater-builder.com/antenna/ant-sys-index.html http://antennasystems.com/product/sinclair-PC3/PC3113.html http://www.telewave.com/pricelist/impanels.html http://www.sinclairtechnologies.com/catalog/group.aspx?id=32 http://birdtechnologies.thomasnet.com/item/components/intermodulation-suppression-panels/81-series? http://www.ferrocom.com/products.htm http://www.taradios.com/IM_Supression_Panels.html etc... Incidentally, my Rotomola UHF MSF-5000 repeater has *THREE* isolators, connected in series, in addition to a band pass cavity filter. Lossy, ugly, but clean. Next, silver plating is not a magic fix. Getting rid of bright nickel plated cheap connectors *is* a magic fix. I've cleaned up several systems by simply getting rid of cheap connectors and adapters and replacing them with silver plated brass connectors (or just brass as in Heliax connectors). Google for "Passive Intermod Distortion": http://www.amphenolrf.com/simple/PIM%20Paper.pdf Materials: Ferromagnetic materials such as nickel or steel must be eliminated from the current path due to their non-linear characteristics. Brass and copper alloys are generally accepted as linear materials. Tests have shown that nickel plate under gold on the center contact will typically result in a 40 to 50 dB increase in PIM. Stainless Steel in the body will usually give a 10-20 dB increase in PIM. Mo https://engineering.purdue.edu/IDEAS/PIM.html (nice video) In case you've seen magnets taped to coax connectors, this might explain why. More on PID. http://aeroflex.com/ats/products/prodfiles/articles/8814/Intermod.pdf -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com # http://www.LearnByDestroying.com AE6KS |
duplexers, antennas, repeaters
Jeff Liebermann wrote:
On 09 Sep 2011 19:50:43 GMT, dave wrote: Jeff Liebermann wrote: Yep. If the repeater is going in any of the buildings where I have equipement, it will need a cavity and ferrite isolator to prevent intermod problems. I know of one building that demands Heliax, no braided coax including LMR-xxx coax, all silver plated connectors, and other draconian anti-intermod measures. Tempest like packaging on all computahs. All I can say is that it works (if you can afford it). I've never seen that anywhere. Intermod is a math problem. No amount of silver plating will fix bad coordination. Sounds like you've never had to deal with transmitted intermod. Consider yourself lucky as getting rid of it is a PITA. Also, not all intermod comes from mixing in the receiver. TX intermod is real and preventable. The xmit cavity and ferrite isolator prevent any RF from adjacent antennas from going down the antenna, into the power amp, mixing there with the xmit signal, and having the power amp amplify the intermod. An intermod study compares transmit freqs as well as input freqs. Circulators are accepted practice. The intermod study will tell you if you need more traps, BPF, etc. There is a lot of pseudoscience in technology (and a comparable amount of "overkill"). By far, the worst problem I encounter is XM radio on 2.5 gHz and ground radar from airplanes (they like to use tower sites as benchmarks). I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John Hancock building, the router room at Channel 4, etc.) and I have never seen a blanket ban on LMR because it leaks. |
duplexers, antennas, repeaters
All it takes is ONE user who believes himself to be above
any technical standards in his quest to maximize profit. These are the same clowns that strip a site of any and all hardware that isn't nailed down or currently connected to something. Or throw together a "repeater" out of junk bought at the swap meet and nailed to a piece of plywood. Jeff-1.0 wa6fwi -- "Everything from Crackers to Coffins" |
duplexers, antennas, repeaters
On 10 Sep 2011 12:57:07 GMT, dave wrote:
I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John Hancock building, the router room at Channel 4, etc.) and I have never seen a blanket ban on LMR because it leaks. It's not leakage. The problem is the plated steel wire used over the foil wrap on the shield. The steel is non-linear and subject to PIM (Passive Intermod) problems. The aluminum foil to steel junction can easily become a diode if the mylar coating is penetrated. I've seen it with LMR-400 on a lab test similar to the YouTube video that you apparently didn't watch. The problem was bad enough that Times had to conjure a special mutation of LMR-400 with low PIM: http://timesmicrowave.com/products/lmr/downloads/126-129.pdf I think (not sure) that the only difference is that the braid over the foil is now aluminum. The initial reaction of most techs is that the PIM is sufficiently low level that it would not have an effect on receiver performance. Wrong. In cell sites, where squeezing every dBm of sensitivity out of the receiver is necessary to deal with perpetually marginal cell phone handset signals, that install cryogenically cooled front ends and tower mounted preamps to do this, can definitely see the effect. Look at a cell site install and try to find anything other than Heliax. "SITE MANAGEMENT EQUIPMENT INSTALLATION RULES" (sample) http://www.repeater-builder.com/antenna/site-stuff/radiositerules.html "C. All cabling from the building to tower including on the tower to the antenna, shall consist of a minimum of 1/4 inch jacketed corrugated copper "Heliax" type cable. Semi-rigid "LMR-400", "LMR-600", etc. cable and non-rigid cable, such as RG8, RG, 213, RG-214, RG8X, etc. will NOT be used as transmission cable exiting the building." In most cases, this has been extended to include internal coax cabling that carry transmit RF. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
On Sat, 10 Sep 2011 08:12:23 -0500, Jeffrey Angus
wrote: All it takes is ONE user who believes himself to be above any technical standards in his quest to maximize profit. Yep. However, sometime they actually have a clue. One of my friends recently orchestrated a site cleanup and purge, emphasizing coax cables and isolators. After the complaining, yelling, and screaming stopped, so did the intermod. On the other foot, the county decided to do the same things on a crowded tower that we were sharing. All the LRM-400 came down, and was replaced by Heliax. Much of the intermod went away, but the mixes generated in the receiver front ends remained. These are the same clowns that strip a site of any and all hardware that isn't nailed down or currently connected to something. They're probably the same clowns that steal my scope probes that I leave plugged into the scopes at various sites. I had a weird problem related to unused equipment. There was an unused "smog alert" receiver at one site, connected to an external ground plane antenna half way up the tower. It was turned off as the system was obsolete. Someone noticed that if they unplugged the antenna connector, some of the intermod would magically disappear. The outside antenna was picking up RF from the tower, delivering into the building, and the badly shielded receiver front end was re-radiating it all over the rack. The building manager immediately instituted a reign of terror, demanding that all unused equipment and antennas be removed, resulting in most of the junk exiting the building and tower. There was a slight but noticeable decrease in intermod. Oh well. Or throw together a "repeater" out of junk bought at the swap meet and nailed to a piece of plywood. Ahem... You must have been looking at my photos. Please don't do that. Here's our unfinished plywood APRS weather station, built on a plywood (with ash veneer) bookshelf: http://802.11junk.com/jeffl/WR6AOK-WX-Station/ The 2m bottle is not in the picture. I use screws, not nails. There was a good (political) reason to use plywood. Also, the rack in my living room has plywood shelves, as are the radios in my Subaru. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
Jeff Liebermann wrote:
On 10 Sep 2011 12:57:07 GMT, dave wrote: I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John Hancock building, the router room at Channel 4, etc.) and I have never seen a blanket ban on LMR because it leaks. It's not leakage. The problem is the plated steel wire used over the foil wrap on the shield. The steel is non-linear and subject to PIM (Passive Intermod) problems. The aluminum foil to steel junction can easily become a diode if the mylar coating is penetrated. I've seen it with LMR-400 on a lab test similar to the YouTube video that you apparently didn't watch. The problem was bad enough that Times had to conjure a special mutation of LMR-400 with low PIM: http://timesmicrowave.com/products/lmr/downloads/126-129.pdf I think (not sure) that the only difference is that the braid over the foil is now aluminum. The initial reaction of most techs is that the PIM is sufficiently low level that it would not have an effect on receiver performance. Wrong. In cell sites, where squeezing every dBm of sensitivity out of the receiver is necessary to deal with perpetually marginal cell phone handset signals, that install cryogenically cooled front ends and tower mounted preamps to do this, can definitely see the effect. Look at a cell site install and try to find anything other than Heliax. "SITE MANAGEMENT EQUIPMENT INSTALLATION RULES" (sample) http://www.repeater-builder.com/antenna/site-stuff/radiositerules.html "C. All cabling from the building to tower including on the tower to the antenna, shall consist of a minimum of 1/4 inch jacketed corrugated copper "Heliax" type cable. Semi-rigid "LMR-400", "LMR-600", etc. cable and non-rigid cable, such as RG8, RG, 213, RG-214, RG8X, etc. will NOT be used as transmission cable exiting the building." In most cases, this has been extended to include internal coax cabling that carry transmit RF. Cell sites are a different animal . We were talking about 2-way, point-to-point, VHF/UHF broadcast type sites. YouTube is pretty intense. |
duplexers, antennas, repeaters
In article ,
Jeff Liebermann wrote: I have worked some of the premiere sites (Cedar Hill, Mt. Wilson, South Mountain in Phoenix, Mt. Harvard, Senior Road in Houston, the John Hancock building, the router room at Channel 4, etc.) and I have never seen a blanket ban on LMR because it leaks. It's not leakage. The problem is the plated steel wire used over the foil wrap on the shield. The steel is non-linear and subject to PIM (Passive Intermod) problems. The aluminum foil to steel junction can easily become a diode if the mylar coating is penetrated. I've seen it with LMR-400 on a lab test similar to the YouTube video that you apparently didn't watch. The problem was bad enough that Times had to conjure a special mutation of LMR-400 with low PIM: http://timesmicrowave.com/products/lmr/downloads/126-129.pdf I think (not sure) that the only difference is that the braid over the foil is now aluminum. That same diode-like effect also seems to be capable of causing the cable to generate a nontrivial amount of broadband noise, when energized by a sufficiently strong transmitter signal. In simplex applications this seems not to matter, but in repeater applications it tends to cause enough of an increase in the noise floor at the receiver to appreciably de-sensitize the receiver. The system I work on, was originally build with LMR-type feedlines within the cabinet, and didn't "hear" particularly well. When the chief hardware guru threw out all of those (well-constructed) pigtails, and replaced them with 1/4" heliax... the problem went away and has not returned. Heliax is good. Double-braid shielded cable (with silver-plated copper braid, not aluminum) seems to be almost as good. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
duplexers, antennas, repeaters
"Jeff Liebermann" wrote in message ... I had a weird problem related to unused equipment. There was an unused "smog alert" receiver at one site, connected to an external ground plane antenna half way up the tower. It was turned off as the system was obsolete. Someone noticed that if they unplugged the antenna connector, some of the intermod would magically disappear. Such is the case on USN ships of my acquaintance. If the ship buys a commercial transceiver and throws the antenna any old place, the front end becomes a mixer. There's a reason why (most) military gear is pricey. It's been engineered not to do that. Aside: It's not always an active device that causes problems. I had one ship that was getting massive interference on UHF comm circuits between about 325 MHz to 399 MHz. from a radar operating around 430 MHz. Normally not a problem. The cause was a tangled hunk of wire I found in the field of the radar. It had been used to secure scaffolding during the ship's previous inport period. Every time the radar lit up that bailing wire, the resulting arcing and sparkling generated broadband RF pulses at the radar's rep rate. I was climbing around on the mast, looking for just something of the sort. When I saw that wire, I actually spoke out loud to it. I said, "Well, hello there!" True story. It's one of several reasons we preached "Topside Housekeeping" to our Sailors during ship visits. Leave nothing on the mast that doesn't have to be there. "Sal" |
duplexers, antennas, repeaters
Dave Platt wrote:
In article , Jeff Liebermann wrote: That same diode-like effect also seems to be capable of causing the cable to generate a nontrivial amount of broadband noise, when energized by a sufficiently strong transmitter signal. In simplex applications this seems not to matter, but in repeater applications it tends to cause enough of an increase in the noise floor at the receiver to appreciably de-sensitize the receiver. The system I work on, was originally build with LMR-type feedlines within the cabinet, and didn't "hear" particularly well. When the chief hardware guru threw out all of those (well-constructed) pigtails, and replaced them with 1/4" heliax... the problem went away and has not returned. Heliax is good. Double-braid shielded cable (with silver-plated copper braid, not aluminum) seems to be almost as good. Interesting. I always hear people bragging about LMR. Are we using the term "Heliax" generically? Is semi-flex no good too, (It's all aluminum and brass metallically isn't it?) Did your guru make a profit on the replacement cables? How do you know a $15 can of Cramolin wouldn't have helped just as much? Tinned copper braid is OK, no? http://www.corrosionist.com/Corros1.gif |
duplexers, antennas, repeaters
Dave Platt wrote: That same diode-like effect also seems to be capable of causing the cable to generate a nontrivial amount of broadband noise, when energized by a sufficiently strong transmitter signal. Yeah, I've seen some of that. I spent several days finding the culprit on a UHF repeater where the over the air rx sensitivity varied substantially and erratically. Watching the IF noise level, showed it going up and down with the sensitivity changes. In frustration, I grabbed a broomstick and beat on the accessible coax cables. I eventually found a length of 9913 coax that was apparently involved. I replaced it, and the noise problem disappeared. Inspecting the coax carefully, the outer jacket was slightly corroded and white dust was visible. In simplex applications this seems not to matter, but in repeater applications it tends to cause enough of an increase in the noise floor at the receiver to appreciably de-sensitize the receiver. Yep, but the mechanism isn't obvious. All transmitters belch some level of synthesizer or oscillator noise. The notch type duplexer does a great job of getting rid of the noise in the receiver bandpass produced by the transmitter. However, when there's a diode present, the very low level tx synthesizer spurs, or other signals picked up at the antenna, mix with the tx synthesizer noise, and land on the receiver frequency. It's intermod, but instead of dealing with a collection of individual frequencies, it deals with broadband noise. The same mechanism is a problem in broadband mux, broadcast, and cellular systems. The system I work on, was originally build with LMR-type feedlines within the cabinet, and didn't "hear" particularly well. When the chief hardware guru threw out all of those (well-constructed) pigtails, and replaced them with 1/4" heliax... the problem went away and has not returned. Requiring Heliax is a good but expensive solution. Requiring Heliax on initial installation makes is somewhat less expensive. Heliax is good. Double-braid shielded cable (with silver-plated copper braid, not aluminum) seems to be almost as good. Sorta. I have problems securely attaching connectors to RG-213/u. Unlike the rigid and semi-rigid cables, crimp type connectors are problematic. In addition, much of the RG-213/u floating around is NOT silver plated, but bare copper. That will corrode, and form diodes. I also don't like the attenuation of RG-213/u. 5.1dB/100ft at 450MHz, while LMR-400 is 2.7dB/100ft at 450MHz. On 11 Sep 2011 13:32:15 GMT, dave wrote: Interesting. I always hear people bragging about LMR. Are we using the term "Heliax" generically? Is semi-flex no good too, (It's all aluminum and brass metallically isn't it?) A picture is worth 1000 words: Heliax: http://www.google.com/search?tbm=isch&q=heliax LMR type coax: http://www.google.com/search?tbm=isch&q=LMR+coax I've never heard of semi-flex. Perhaps you mean semi-rigid coax, which includes aluminum outer jacket coax as used in the CATV industry? The coax is fine, but where it transitions to a brass or silver plated connector, there's a problem. In general, it's a bad idea for reducing PIM (although I use CATV coax because I'm cheap). Did your guru make a profit on the replacement cables? Guru's do not stay in business very long unless they're profitable. Even the glorified poverty style of guru has to eat. How do you know a $15 can of Cramolin wouldn't have helped just as much? Cramolin is now DeOxit and has been reformulated. http://store.caig.com/s.nl/sc.2/.f It's a total disaster on RF connectors because it contains oleic acid, which is great for removing oxides from electrical contacts, but equally good at rotting off the plating from connectors over long time periods. The reformulated DeOxit allegedly contains a different anti-oxidant, which allegedly has the same effect. Tinned copper braid is OK, no? Dunno. I never use tinned braid except for some semi-rigid microwave coax, which is quite stiff. Most often I see tin plated braid. Since tin is not magnetic, there's no PIM problem. http://www.corrosionist.com/Corros1.gif -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
Jeff Liebermann wrote:
Cramolin is now DeOxit and has been reformulated. http://store.caig.com/s.nl/sc.2/.f It's a total disaster on RF connectors because it contains oleic acid, which is great for removing oxides from electrical contacts, but equally good at rotting off the plating from connectors over long time periods. The reformulated DeOxit allegedly contains a different anti-oxidant, which allegedly has the same effect. Cramolin is still alive and well. It was and still is made in Germany, DeOxit is made in the US, Caig used to be the US distributor of Cramolin products, but went their own way, with a different formula. I'm not sure which one is the one that you call "a total disaster", but AFIK neither is to be used for anything except cleaning. Caig sells solutions (pardon the pun) for use on connectors. I have them because I can only buy DeOxit in small tubes off of eBay and still get it shipped here, and that was the only way I could get fader lube, but I have never used them. Don't go looking it up and show me auctions of just fader lube, after I ordered the sets, which did not show I up, I commented to the vendor that I wanted just the fader lube and now they list it. They also replaced the missing packages. Geoff. -- Geoffrey S. Mendelson N3OWJ/4X1GM Making your enemy reliant on software you support is the best revenge. |
duplexers, antennas, repeaters
In article ,
dave wrote: Heliax is good. Double-braid shielded cable (with silver-plated copper braid, not aluminum) seems to be almost as good. Interesting. I always hear people bragging about LMR. Are we using the term "Heliax" generically? Yes... I meant "cable with a solid or near-solid outer conductor". Is semi-flex no good too, (It's all aluminum and brass metallically isn't it?) I think that the key is avoidance of (1) contact of dissimilar metals and (2) ferromagnetic materials. And, you want a cable where you can get a really good electrical contact between the outer braid, and the connector shell. I believe that solder beats crimp in this application, since you end up with a connection which will reliably remain gas-tight and won't oxidize. A good semi-flex would probably be fine, I'd guess. LMR-400 seems to be dodgy (for repeater use) over several of these issues. I do like it for simplex applications. Did your guru make a profit on the replacement cables? How do you know a $15 can of Cramolin wouldn't have helped just as much? Nope... he's one of the volunteers in the repeater group. He donates his time, we use donated materials (e.g. the heliax cable, ex-cell-site) when possible, and any supplies the repeater needs are bought from independent commercial suppliers (e.g. the connectors, in this case). He was annoyed at having to go to the trouble of replacing the existing pigtails, but *very* pleased at the result... it turned the system from a "basket of snakes" into one in which the three repeaters in the cabinet can all operate simultaneously and independently without any cross-band interference that we can detect in any way. We're also sharing the hospital-roof site with at least one cellphone system, one pager transmitter (for several years - gone now), and several public-safety LMR repeaters... and so doing things carefully is quite important! Both sets of cables (the original LMR and the replacement hardline) had high-quality commercial-grade connectors... no cheap nickel-plated imports. Hence, I do think it's accurate to ascribe the difference in performance to the cable itself. Cramolin is nice stuff for dealing with connector-to-connector contact issues, but it doesn't do a thing for problems internal to the cable. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
duplexers, antennas, repeaters
In article ,
Jeff Liebermann wrote: Sorta. I have problems securely attaching connectors to RG-213/u. Unlike the rigid and semi-rigid cables, crimp type connectors are problematic. In addition, much of the RG-213/u floating around is NOT silver plated, but bare copper. That will corrode, and form diodes. Yeah, you have to make sure you're getting the "good stuff", with silver-plated copper braid... and then (I think) solder, rather than crimp. Interesting. I always hear people bragging about LMR. Are we using the term "Heliax" generically? Is semi-flex no good too, (It's all aluminum and brass metallically isn't it?) A picture is worth 1000 words: Heliax: http://www.google.com/search?tbm=isch&q=heliax LMR type coax: http://www.google.com/search?tbm=isch&q=LMR+coax I've never heard of semi-flex. Perhaps you mean semi-rigid coax, which includes aluminum outer jacket coax as used in the CATV industry? The coax is fine, but where it transitions to a brass or silver plated connector, there's a problem. In general, it's a bad idea for reducing PIM (although I use CATV coax because I'm cheap). See http://www.isoconnector.com/cableassemblies.html - "semi-rigid" and "semi-flex" are different constructions. Cramolin is now DeOxit and has been reformulated. Well, sorta and sorta not. Cramolin is still made by the original (German) manufacturer, but isn't easily acquired in the U.S. DeOxIt is made by Caig, who used to import Cramolin but are now making a similar product themselves. It's a total disaster on RF connectors because it contains oleic acid, which is great for removing oxides from electrical contacts, but equally good at rotting off the plating from connectors over long time periods. The reformulated DeOxit allegedly contains a different anti-oxidant, which allegedly has the same effect. My recollection is that the instructions which came with Cramolin said that you should clean the remains of the "red" Cramolin (the oxide remover) off of the contacts after de-oxidizing. They made another "blue" product which was intended to provide some residual anti-re-tarnishing protection... I think it was based on palm oil. When I use Cramolin (I still have a bit of the concentrate) or DeOxIt on contacts, I follow it with a final cleaning using isopropyl or denatured alcohol (or acetone if I'm feeling really thorough), to remove the remains of the anti-oxidant. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
duplexers, antennas, repeaters
On Sun, 11 Sep 2011 17:29:04 +0000 (UTC), "Geoffrey S. Mendelson"
wrote: Jeff Liebermann wrote: Cramolin is now DeOxit and has been reformulated. http://store.caig.com/s.nl/sc.2/.f It's a total disaster on RF connectors because it contains oleic acid, which is great for removing oxides from electrical contacts, but equally good at rotting off the plating from connectors over long time periods. The reformulated DeOxit allegedly contains a different anti-oxidant, which allegedly has the same effect. Cramolin is still alive and well. It was and still is made in Germany, DeOxit is made in the US, Caig used to be the US distributor of Cramolin products, but went their own way, with a different formula. Got it. All I can find it DeOxit in California. More detail: http://siber-sonic.com/electronics/caig.html I'm not sure which one is the one that you call "a total disaster", but AFIK neither is to be used for anything except cleaning. Caig sells solutions (pardon the pun) for use on connectors. Different stuff. Most of it is just grease to keep the connectors from oxidizing. The total disaster was what I found on a tower where someone had used Cramolin to clean an assortment of 7/16 DIN and N connectors. The 7/16 DIN connectors were stainless. They turned black and had a rough pitted surface. The N-connectors were silver on brass. The silver was intact, but would flake off in small pieces, exposing the underlying brass, which rapidly turned dark green. Everything had to be replaced. I don't recall how long it took to get to this point but I think it was at least 2-3 years. If you Google for corrosion characteristics of oleic acid, you'll find that it attacks steels, but is compatible with copper. Not true. Pure oleic is compatible, but add some water and the stuff becomes corrosive. Oleic acid is a major component (70%?) of peanut oil. You could probably use peanut oil mixed with some organic solvent in place of Cramolin. (No, I haven't tried it). I have them because I can only buy DeOxit in small tubes off of eBay and still get it shipped here, and that was the only way I could get fader lube, but I have never used them. Don't go looking it up and show me auctions of just fader lube, after I ordered the sets, which did not show I up, I commented to the vendor that I wanted just the fader lube and now they list it. They also replaced the missing packages. I mix many of my own chemicals. Most of the tuner lube type stuff is fairly simple. Organic solvent, ionic solvent (water soluble), oxide remover, and some oil: http://store.caig.com/s.nl/sc.2/category.293/.f -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
Jeff Liebermann wrote:
Got it. All I can find it DeOxit in California. More detail: http://siber-sonic.com/electronics/caig.html I believe on of the reasons that Caig stopped carrying it was that it is too corrosive to put on an airplane and they wanted something UPS/FEDEX would carry. Different stuff. Most of it is just grease to keep the connectors from oxidizing. The total disaster was what I found on a tower where someone had used Cramolin to clean an assortment of 7/16 DIN and N connectors. The 7/16 DIN connectors were stainless. They turned black and had a rough pitted surface. The N-connectors were silver on brass. The silver was intact, but would flake off in small pieces, exposing the underlying brass, which rapidly turned dark green. Everything had to be replaced. I don't recall how long it took to get to this point but I think it was at least 2-3 years. Ok, good thing I only use it to clean contacts and that's a drop on the end of a q-tip or toothpick. I mix many of my own chemicals. Most of the tuner lube type stuff is fairly simple. Organic solvent, ionic solvent (water soluble), oxide remover, and some oil: http://store.caig.com/s.nl/sc.2/category.293/.f For me those days have passed. Between space limitations, lack of working area, inability to order those chemicals, etc, I'm going to have to make do with little tubes sent airmail. Luckily I don't need much and have all I need for a long time. :-) Geoff. -- Geoffrey S. Mendelson N3OWJ/4X1GM Making your enemy reliant on software you support is the best revenge. |
duplexers, antennas, repeaters
"Dave Platt" wrote in message ... snip When I use Cramolin (I still have a bit of the concentrate) or DeOxIt on contacts, I follow it with a final cleaning using isopropyl or denatured alcohol (or acetone if I'm feeling really thorough), to remove the remains of the anti-oxidant. Isn't there any concern for plastic deterioration from the acetone? When I read what you wrote, I cringed, having seen acetone's effect on (some) plastics. Keeping a few four-leaf clovers on the bench? 73, Sal |
duplexers, antennas, repeaters
On Sun, 11 Sep 2011 18:39:04 +0000 (UTC), "Geoffrey S. Mendelson"
wrote: I believe on of the reasons that Caig stopped carrying it was that it is too corrosive to put on an airplane and they wanted something UPS/FEDEX would carry. Duh... They carry peanut oil, which is 75% oleic acid, but won't carry Cramolin, which is 5% oleic acid. I just checked the 49 CFR list at: http://www.ups.com/content/us/en/resources/ship/hazardous/download/chemical.html and Oleic acid isn't listed. The MSDS sheet shows slightly flamable, but otherwise safe. My guess(tm) is the business relationship fell apart and that the UPS/Fedex story isn't true. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
On Sun, 11 Sep 2011 10:57:31 -0700, (Dave Platt)
wrote: See http://www.isoconnector.com/cableassemblies.html - "semi-rigid" and "semi-flex" are different constructions. Thanks. I've been calling both types semi-rigid, which is wrong. They made another "blue" product which was intended to provide some residual anti-re-tarnishing protection... I think it was based on palm oil. No clue on the oil, but my guess would be cheaper mineral oil and some solvent to wash off any residue from Cramolin red. I've never used (or seen) Cramolin Blue. When I use Cramolin (I still have a bit of the concentrate) or DeOxIt on contacts, I follow it with a final cleaning using isopropyl or denatured alcohol (or acetone if I'm feeling really thorough), to remove the remains of the anti-oxidant. I use 409 cleaner, which has a pH of about 11, or ammonia window cleaner, both of which should neutralize the acid. Acetone will make a mess of anything plastic and really is overkill. For open frame relays, I sometime use electroless silver to replate the contacts. I also have a diy silver brush plating kit, which I use large contacts. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
On 10 Sep 2011 18:27:58 GMT, dave wrote:
Cell sites are a different animal . We were talking about 2-way, point-to-point, VHF/UHF broadcast type sites. Perhaps you didn't notice but several of the examples of PIM and rotten coax induced intermod were for non-cellular systems. The problems are much the same with any service type. If you have moderate TX power, magnetic materials in the connectors, and sensitive receivers, PIM might be a concern. Also, it may not be obvious unless you've worked on a cell site, but the state-o-de-art in radio is currently defined by cellular. The current generation of cellular radios, pre-distorting power amps, tower mounted amps, cryogenic front ends, steerable antennas, channel loading, codecs, and environmental protections, are far superior to what I've seen in commercial radio, and light years ahead of the 3rd hand garbage commonly found in ham repeaters. Yes, cellular is different, but it's also quite superior. If the cellular people think they have a PIM problem, it's probably quite real. Unfortunately, cellular can't always get everything right: http://802.11junk.com/jeffl/SCCARC-talk-2010-06-18/Burning-Towers.htm http://www.cellsiteanalysis.net/cell_site_analysis_images/Cell_Site_Mast_Loaded.jpg YouTube is pretty intense. Duz that mean that you still haven't watched the video clip? -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
When I use Cramolin (I still have a bit of the concentrate) or DeOxIt
on contacts, I follow it with a final cleaning using isopropyl or denatured alcohol (or acetone if I'm feeling really thorough), to remove the remains of the anti-oxidant. Isn't there any concern for plastic deterioration from the acetone? When I read what you wrote, I cringed, having seen acetone's effect on (some) plastics. Keeping a few four-leaf clovers on the bench? It depends on what sort of thing I'm cleaning. You're correct... I don't use acetone on anything plastic. Depending on the plastic, either an alcohol, or a light petroleum solvent (Stoddard solvent, naptha, etc.) is a safer "wash". -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
duplexers, antennas, repeaters
In article ,
Jeff Liebermann wrote: When I use Cramolin (I still have a bit of the concentrate) or DeOxIt on contacts, I follow it with a final cleaning using isopropyl or denatured alcohol (or acetone if I'm feeling really thorough), to remove the remains of the anti-oxidant. I use 409 cleaner, which has a pH of about 11, or ammonia window cleaner, both of which should neutralize the acid. Acetone will make a mess of anything plastic and really is overkill. True - it's for metals-only applications. For open frame relays, I sometime use electroless silver to replate the contacts. Is that Cool-Amp (powder), or some form of liquid electroless silver? I've made up (and use) a homebrew substitute for Cool-Amp - silver chloride and salt, finely ground, and then rubbed in firmly. Makes a nice, solderable surface for DIY PC boards. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
duplexers, antennas, repeaters
On Sun, 11 Sep 2011 16:57:02 -0700, (Dave Platt)
wrote: For open frame relays, I sometime use electroless silver to replate the contacts. Is that Cool-Amp (powder), or some form of liquid electroless silver? http://www.cool-amp.com/cool_amp.html That's silver chloride and calcium carbonate powder. $25/oz. Ouch. I've used Cool-Amp immersion plating (it's not really electroless). It works well for PCB and contact plating. You might find this of interest: http://yarchive.net/electr/pc_board_silver_plating.html What I have is: http://www.transene.com/ag.html I think (not sure) it's a silver nitrate, ammonia, sodium hydroxide, and who knows what else mix. One catch is that it has to be applied warm, which is somewhat of a challenge. I'm sure easier to use stuff exists, but I didn't pay much for the 1 oz bottle which seems to be lasting forever. When I run out in about 10 years, I'll look at alternatives. Mo http://www.finishing.com/faqs/silverathome.shtml I've made up (and use) a homebrew substitute for Cool-Amp - silver chloride and salt, finely ground, and then rubbed in firmly. Makes a nice, solderable surface for DIY PC boards. Nice, but don't use salt. Use calcium carbonate instead. Salt (sodium chloride) might precipitate the silver out at silver chloride. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
On 9/8/2011 10:43 PM, Dave Platt wrote:
You could build a very nice full duplex repeater on a single frequency that way. Yes, except that the ARRL has decided to only petition for a waver for single time slot TDMA, which can't be used for a repeater. You're scheme would certainly work, and I too am wondering why nobody has bothered to do it. Possibly because nobody really wants full duplex (with echo, reverb, feedback, etc). Not so much full duplex, but single frequency half duplex, with negligible time delay (implying 100ms frame time) between Rx and Tx. I wonder whether you may not also have to be really careful with your transceiver/receiver switching design. You'll really need to be able to trust (and drive) those PIN diodes properly... goof up on even a single time-slice and you could put enough TX power into your receiver to turn its front end into a pile of smouldering char in a millisecond. This isn't a problem with normal split-frequency repeaters, thanks to the isolation in the duplexer cans. Do any of the commercial TDMA systems use the same frequencies for base-mobile and mobile-base? My recollection is that TDMA cellphone systems operate with split uplink/downlink frequencies. Sure.. 802.11 is half duplex on a single channel, for instance. Lots and lots of radars have fast and reliable T/R switching at pretty much any frequency you care to name from DC to light. TDMA cellphone uses split bands probably because it was on top of existing AMPS systems. There is also a frequency allocation issue (e.g. no need for new licensing). Having separate forward and reverse bands also helps with frequency reuse and near-far issues. I hardly think that hams are going to carpet the country with repeaters to the extent that cell sites do. |
duplexers, antennas, repeaters
On 9/8/2011 10:05 PM, Noskosteve wrote:
On Aug 31, 11:38 am, Jim wrote: One wonders why someone isn't pushing for a digital TDMA scheme for amateurs. You could build a very nice full duplex repeater on a single frequency that way. ... No filtering, much less intermod issues in multi station at onee site systems... all kinds of good comes of it. Digital schemes on HF to replace SSB I can see having real trouble (the biggest is the lack of a "party line" capability, the other is the long propagation delay on HF paths), but on VHF and up FM, you already have a "one person talks at a time" by virtue of the standard FM demodulator. Uhhh. It's been a long time since I worked on such a system (1975 I think), but IIR the prop delay through space for moderate distances kills the idea. Rough calculations gives a round trip delay, at 10 miles from the repeater, of about 0.1 ms. For two stations at that distance that's 0.1 ms not available for sampling, bit width and processing. You don't need that big a guard time if you keep track of how far you are from the repeater and adjust your timing appropriately (that's what a lot of systems do, and it's what was used for coarse position finding in the phase 1 E-911 systems). That was an ordeal in 1980s to implement, but today, it's in the piece of cake area, at least from an implementation complexity and hardware standpoint. There is probably off the shelf IP for it, too. Keeping the BW down also needs rise and fall time as well as guard times. It added up quiclky back then. The vocoder becomes very important to reduce the data rate. Yes, but on the other hand, standard cellphones use 8kbps and while the quality isn't great, it's good enough. Of course, that gets us into that whole "any decent codec is tied up with licensing problems" rat's nest. |
duplexers, antennas, repeaters
On Mon, 12 Sep 2011 09:19:29 -0700, Jim Lux
wrote: Not so much full duplex, but single frequency half duplex, with negligible time delay (implying 100ms frame time) between Rx and Tx. Most of the delay will not come from the flight time or mux switching. It will come from the necessary audio compression. It won't work without audio compression, which means that some types of uncompressible pre-randomized data is not going to work (no big deal). Also, the more compression, the longer the latency. TDMA cellphone uses split bands probably because it was on top of existing AMPS systems. Yep. The problem was that TDMA (IS-54/IS-136) had to be compatible with the then existing analog cell systems. Therefore, all early TDMA phones had to offer analog compatibility. Digital only phones weren't available until about 2003. There's also little justification for making the change. It will not double the number of available channels as some pundits have suggested. Since the return audio now has to be sqeezed into the previously transmit only channel, the number of users per channel is cut in half. The result is no capacity change. There is also a frequency allocation issue (e.g. no need for new licensing). Having separate forward and reverse bands also helps with frequency reuse and near-far issues. I hardly think that hams are going to carpet the country with repeaters to the extent that cell sites do. I've looked into butchering cellular handsets into something usable on ham radio. I have internals on some of the old Motorola flip phones and bag phones and could probably modify the firmware sufficiently to turn it into a conventional radios. The fatal flaw was the fixed 45MHz T/R offset. There were simply too many components that would need to be replaced in order to operate on the smaller offset available to hams, or on simplex. In addition, it's usually fairly easy to go down in frequency, but the phones would require going up from the 850MHz cellular bands to the 915MHz ham band. I gave up on the idea. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
On Mon, 12 Sep 2011 09:24:00 -0700, Jim Lux
wrote: You don't need that big a guard time if you keep track of how far you are from the repeater and adjust your timing appropriately (that's what a lot of systems do, and it's what was used for coarse position finding in the phase 1 E-911 systems). That was an ordeal in 1980s to implement, but today, it's in the piece of cake area, at least from an implementation complexity and hardware standpoint. There is probably off the shelf IP for it, too. Yep. The problem was that GSM had a built in distance limit at about 35 km. Any furthur and the timing would get mangled. That was changed with adaptive timing: http://en.wikipedia.org/wiki/Timing_advance Keeping the BW down also needs rise and fall time as well as guard times. It added up quiclky back then. The vocoder becomes very important to reduce the data rate. Yes, but on the other hand, standard cellphones use 8kbps and while the quality isn't great, it's good enough. Of course, that gets us into that whole "any decent codec is tied up with licensing problems" rat's nest. Codecs are incredibly important. A 1% increase in channel capacity translates to adding thousands of additional users to a system. Nobody uses fixed rate codecs these daze. The current fashion is variable bandwidth schemes, such as EVRC, SMV, 4GV, etc (for CDMA). The challenge is to get something that sounds decent with low latency, but doesn't blow up with weak signals, high error rates, lousy SNR, etc. When someone succeeds, it's immediately patented, creating the predicable licensing mess. Drivel: I once worked on a codec that required the receiving end to have a library of the speakers phoneme sounds in storage. That drastically reduced the amound of information that needed to be sent. It would have worked and possibly sold, except that it was far too easy to impersonate someone by simply switching phoneme libraries. It was a fun project while it lasted. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
On Sun, 11 Sep 2011 16:05:44 -0700, Jeff Liebermann wrote:
On 10 Sep 2011 18:27:58 GMT, dave wrote: Cell sites are a different animal . We were talking about 2-way, point-to-point, VHF/UHF broadcast type sites. Perhaps you didn't notice but several of the examples of PIM and rotten coax induced intermod were for non-cellular systems. The problems are much the same with any service type. If you have moderate TX power, magnetic materials in the connectors, and sensitive receivers, PIM might be a concern. It's still a math problem. You can predict intermod products from known frequencies whether the non-linear device is active or passive. |
duplexers, antennas, repeaters
On 9/19/2011 12:48 PM, dave wrote:
[ dribble snipped ] I see you're over here trying to sound impressive too. Jeff -- "Everything from Crackers to Coffins" |
duplexers, antennas, repeaters
On Mon, 19 Sep 2011 12:48:09 -0500, dave wrote:
On Sun, 11 Sep 2011 16:05:44 -0700, Jeff Liebermann wrote: On 10 Sep 2011 18:27:58 GMT, dave wrote: Cell sites are a different animal . We were talking about 2-way, point-to-point, VHF/UHF broadcast type sites. Perhaps you didn't notice but several of the examples of PIM and rotten coax induced intermod were for non-cellular systems. The problems are much the same with any service type. If you have moderate TX power, magnetic materials in the connectors, and sensitive receivers, PIM might be a concern. It's still a math problem. You can predict intermod products from known frequencies whether the non-linear device is active or passive. Yep. And after I've done the math, I still have to get rid of the intermod. The problem is not the math. That's well known and easy to do. The problems a 1. Finding which of the hundreds of signals found on a typical mountain top is causing the problem. 2. Finding where the likely culprits are located (i.e. which building). 3. Finding any and all sources of non-linearity that are producing the mixes. That could be anything from a gold on nickel connector to insufficient reverse power protection on a broadband power amp. 4. Site management and politics. It's no longer single "known frequencies" causing the intermod. In these days of broadband everything, it's fairly wide swaths of digital noise that's causing the intermod. For example, CDMA phone is 1.25Mhz wide, WCDMA is 5Mhz, and CDMA2000 is up to 25Mhz wide. The worst part is that most of the culprits can't be decoded on my service monitor, so I can't tell for sure if they're causing the intermod. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
duplexers, antennas, repeaters
On Mon, 19 Sep 2011 14:59:53 -0500, Jeffrey Angus wrote:
On 9/19/2011 12:48 PM, dave wrote: [ dribble snipped ] I see you're over here trying to sound impressive too. Jeff I don't need to sound "impressive". I have been paid to conduct many intermod studies using proprietary software. |
duplexers, antennas, repeaters
On Tue, 20 Sep 2011 00:16:29 -0700, Jeff Liebermann wrote:
Yep. And after I've done the math, I still have to get rid of the intermod. The problem is not the math. That's well known and easy to do. The problems a 1. Finding which of the hundreds of signals found on a typical mountain top is causing the problem. 2. Finding where the likely culprits are located (i.e. which building). 3. Finding any and all sources of non-linearity that are producing the mixes. That could be anything from a gold on nickel connector to insufficient reverse power protection on a broadband power amp. 4. Site management and politics. It's no longer single "known frequencies" causing the intermod. In these days of broadband everything, it's fairly wide swaths of digital noise that's causing the intermod. For example, CDMA phone is 1.25Mhz wide, WCDMA is 5Mhz, and CDMA2000 is up to 25Mhz wide. The worst part is that most of the culprits can't be decoded on my service monitor, so I can't tell for sure if they're causing the intermod. Cellular phones are a different animal. I worked on fixed and mobile, mostly analog, mostly FM radios. Theory and practice are quite different. The tower owner should have an inventory of every transmit and every receive frequency, plus all the standard I.F., plus nearby external high powered sources. The owner should have cleared each frequency before it went on the air, and should not add a tenant if doing so would create a harmful spur to existing users. This is site management 101. I don't care how the WL people run their data streams. Cellular folks don't like high mountains (except for backhaul). I know they use very advanced techniques to hear signals below the noise floor; keeping that noise floor as low as possible is of paramount importance when you are looking at 100 mW devices in people's pockets 5 miles away. FWIW, Tek has a real nice analyzer that will reverse engineer TDMA spurs. make time-lapse spectrum analysis, and can even write on a map for you. |
duplexers, antennas, repeaters
On Tue, 20 Sep 2011 07:16:40 -0500, dave wrote:
Theory and practice are quite different. One day, you're going to eat those words, when you have to decide whether to follow theory or practice. When I find that they're different, it's usually because I'm doing something wrong. Also, if you understand the theory, you can probably figure out the practice (what to do). However, if you know the practice (i.e. seat of the pants engineering), you're highly likely to fumble somewhere. The tower owner should have an inventory of every transmit and every receive frequency, plus all the standard I.F., plus nearby external high powered sources. The owner should have cleared each frequency before it went on the air, and should not add a tenant if doing so would create a harmful spur to existing users. This is site management 101. You almost made me spill my hot chocolate. You're correct. Site managers should do all that. The problem is that all but one of the site managers that I know of are business types, not engineers. They hire engineers, tower jockeys, construction crews, and generally run the business. It's not unusual for me to get a call or email with "I just signed on to have [insert name] company put their radios in the building. I'll let you know if anyone complains". This translates to "Don't burn any billable hours doing calculations until AFTER someone experiences interference. In short, I get paid to clean up the mess, not to do the planning. If I want to enforce any engineering standards, it's also done post mortem. At best, I would get an email asking where in the building and tower I would guess the new radios should be installed, usually without telling me the frequencies or equipment. Interrogating the prospective new customer is something I try to do, but often they contract out the repeater service to a comm shop, which claims that they don't know anything because they're afraid I might steal the customer. I don't wanna talk about licensing, HAAT calcs, and coordination. Hopefully, your operation is a bit closer to theory than practice. I don't care how the WL people run their data streams. Cellular folks don't like high mountains (except for backhaul). Generally true. The CDMA crowd doesn't like high mountains for the same reason they don't like CDMA operation in airplanes. The noise floor is much higher up high and there are not enough channels available to handle all the potential users if in a metro area. However, they do like medium high mountain tops with fairly well controlled coverage areas. They also like to share site ownership and management with public agencies to reduce costs. I know they use very advanced techniques to hear signals below the noise floor; keeping that noise floor as low as possible is of paramount importance when you are looking at 100 mW devices in people's pockets 5 miles away. 100mw is about the maximum that a cell phone can belch. Power control will usually keep that down to about 30-50mw. FWIW, Tek has a real nice analyzer that will reverse engineer TDMA spurs. make time-lapse spectrum analysis, and can even write on a map for you. Well, the 20+ year old P25 radios are finally being forced into service by FCC edict, along with various incompatible TDMA implementations. Meanwhile, cellular is heading towards various CDMA spread spectrum technologies (CDMA200, WCDMA, LTE, etc), which makes TDMA look kinda dated. Anyway, I can't afford much in the way of expensive test equipment and usually borrow or rent what I need. I haven't actually seen a spur, mix, intermod, or noise on a spectrum analyzer for many years as the receiver sensitivities are well below the analyzer noise floor. Same problem with PIM (passive intermod). It takes quite a bit of power to produce PIM making it almost impossible to measure PIM while the xmitters are in operation. Trying to see PIM on a spectrum analyzer is futile. -- # Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 # 831-336-2558 # http://802.11junk.com # http://www.LearnByDestroying.com AE6KS |
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