Re: [Repeater-Builder] Re: Coax length between added cavity and duplexer
Nate wrote: > For the record, the interference appears as an on frequency signal > (leading me to initially suspect intermod of some type) with clear > audio of the dispatcher and officers. At first I thought I heard a > second level of audio as well that I haven't been able to identify it > whether it is background or another transmitter--partly due to > catching it at the right time, and partly because my QTH doesn't > allow me to hear it very well. KC0MLS thought receiver overload may > be the issue so that's why we're trying the Celwave cavity. Hi Nate... Just to be clear (because it helps figure it out), do you hear every transmission, or only parts of transmissions? (Mixing with something else that goes on and off air, would come and go during a longer transmission on the system you're hearing in your repeater. Often you can find the thing you're mixing with if it's on-site by watching TX LED's and taking along a few receivers... "Okay the dispatcher is on-air, and hey there's the interference, and the red TX LED just came on over here on this panel... and then... there it's gone and the TX LED went out..." You get the idea... Also seeing what's really coming down the RX side of things with a Spectrum Analyzer may be enlightening. Nate WY0X
[Repeater-Builder] Re: Coax length between added cavity and duplexer - test r...
--- In Repeater-Builder@yahoogroups.com, "George Henry" <[EMAIL PROTECTED]> wrote: > > Maybe his computer is running on UTC? > > > - Original Message - > From: [EMAIL PROTECTED] > To: Repeater-Builder@yahoogroups.com > Sent: Friday, July 27, 2007 5:16 PM > Subject: Re: [Repeater-Builder] Coax length between added cavity and duplexer - test r... > > > Jeff > > Just a dynamite presentation. Thanks for the effort. BTW, where are you (The analysis is dated July 29) > > > Bruce K7IJ > A man ahead of his time !
RE: [Repeater-Builder] Re: Coax length between added cavity and duplexer - test r...
Allen Crites correctly pointed out that I had fat-fingered the wavelength of the factory cable I had in the original version of the text - I fixed it. Also, I had previously estimated the physical length of the type N elbows to be about 1.5 inches, or 0.12 wavelength. For the heck of it, I measured the actual phase change on the network analyzer when an elbow was added and found it to be 53 degrees at 915 MHz (about 0.15 wavelength), so I changed that in the article too. Both the cubic-style and miter-style elbows measured about the same (53-54 degrees). Since two elbows produced the worst performance degradation, and two elbows would be a little more than a quarter wave (0.15 x 2 = 0.30), theoertically there is a length slightly less than the equivalent of two elbows that would produce even worse results, assuming of course that the factory cable is truly of ideal length. Revised version posted at www.broadsci.com/900.pdf --- Jeff
[Repeater-Builder] Re: Coax length between added cavity and duplexer - test r...
--- In Repeater-Builder@yahoogroups.com, "Jeff DePolo" <[EMAIL PROTECTED]> wrote: "I'm caught in a time warp in Philadelphia." Time is still unstable there 64 years later?
[Repeater-Builder] Re: Coax length between added cavity and duplexer
Knowing what is on the nearby antenna is VERY important and is most likely involved in the problem! I used to work at Sinclair as a systems engineer and dealt with solving IM problems and sources on a daily basis for customers' solutions so I know what I am asking WILL help in resolving the original problem of interference. Putting a pass can between the duplexer and receiver with a non optimized cable will help in describing the characteristic of the interference if the interference changes in RF level and then if it cures it then one can optimize the cable length. Critical length cable MEANS (to an experienced filter person) that the total response characteristics are optimized for a given response. That could be for VSWR shape or pass frequency response or even whether the notches add to more rejection than the individual notch responses or not. Also the effect of cavity interaction wrt tuning occurs with non optimized cables. Yes, I know they ran the frequencies - I can read. One cannot just look at the receive and transmit frequencies alone. If the combination isn't obvious with the differences in frequencies showing hits, you have to look at synthesizer frequencies, image frequencies, and other oscillator frequencies for the cause of the IM or spurious mixing. That is why I mentioned the 12.8MHz oscillator spur on the Mastr 3. Any non-linear junction in the presence of multiple RF signals will generate intermodulation. Even ferromagnetic materials like nickel will generate intermodulation. Characterizing the interference level changes with the application of a pass cans or attenuators can help in determining whether the source is internal to the receiver or external (cables, filters, antenna and beyond) Harold, VA3HF --- In Repeater-Builder@yahoogroups.com, Ron Wright <[EMAIL PROTECTED]> wrote: > > I believe the inital posting stated no freq combination/mixing could be found. For the most part this would illimate an intermod problem. However, since there are so many txs in a typical city part of the problem could be from a distant unknown tx. > > 73, ron, n9ee/r > > > > >From: Harold Farrenkopf <[EMAIL PROTECTED]> > >Date: 2007/07/26 Thu PM 01:50:26 CDT > >To: Repeater-Builder@yahoogroups.com > >Subject: [Repeater-Builder] Re: Coax length between added cavity and duplexer > > > > >What are the frequencies of your repeater and the frequencies on the > >nearby antenna > > > >What are the radios used on the nearby antenna? > > > >Important information in determining the problem - I had one source of > >noise caused by a Mastr III that had a spur of considerable strength > >at 12.8MHz down from the transmitter carrier - synthesizer oscillator! > >It was received with the correct transmitter modulation as the carrier > >frequency and a pass on it or a notch at -12.8 MHz solved our problem. > > The level of the emission was within spec of the Mastr 3 but was > >still too strong for our antenna at 500' away. > > > >What is the configuration of their filters? > > > >4' horizontal separation is not much at all so filters should be > >designed to all the frequencies used to work together. > > > >Then, it only takes one transmitter to cause noise in an antenna if > >the antenna is "noisy". > > > >What does the interference sound like? > > > >Harold, VA3HF > > > >--- In Repeater-Builder@yahoogroups.com, Ken Arck wrote: > >> > >> At 11:06 AM 7/26/2007, you wrote: > >> > >> > >> > >> Intermod means there is mixing in YOUR PA with some outside RF energy. > >> > >> <I should probably clarify that "intermod" is an often misused > >> term that has become a generic terms used to describe just about any > >> type of received interference. Ya know, kinda like Klenex has become > >> the name for any tissue. > >> > >> Intermod is a very specific type of phenomenon and for accuracy, > >> should only be used to describe intermodulation issues and not mixes, > >> adjacent channel interference, etc. > >> > >> (more of my 2 cents) > >> > >> Ken > >> > > > > > > > Ron Wright, N9EE > 727-376-6575 > MICRO COMPUTER CONCEPTS > Owner 146.64 repeater Tampa Bay, FL > No tone, all are welcome. >
Re: [Repeater-Builder] Re: Coax length between added cavity and duplexer
I believe the inital posting stated no freq combination/mixing could be found. For the most part this would illimate an intermod problem. However, since there are so many txs in a typical city part of the problem could be from a distant unknown tx. 73, ron, n9ee/r >From: Harold Farrenkopf <[EMAIL PROTECTED]> >Date: 2007/07/26 Thu PM 01:50:26 CDT >To: Repeater-Builder@yahoogroups.com >Subject: [Repeater-Builder] Re: Coax length between added cavity and duplexer > >What are the frequencies of your repeater and the frequencies on the >nearby antenna > >What are the radios used on the nearby antenna? > >Important information in determining the problem - I had one source of >noise caused by a Mastr III that had a spur of considerable strength >at 12.8MHz down from the transmitter carrier - synthesizer oscillator! >It was received with the correct transmitter modulation as the carrier >frequency and a pass on it or a notch at -12.8 MHz solved our problem. > The level of the emission was within spec of the Mastr 3 but was >still too strong for our antenna at 500' away. > >What is the configuration of their filters? > >4' horizontal separation is not much at all so filters should be >designed to all the frequencies used to work together. > >Then, it only takes one transmitter to cause noise in an antenna if >the antenna is "noisy". > >What does the interference sound like? > >Harold, VA3HF > >--- In Repeater-Builder@yahoogroups.com, Ken Arck <[EMAIL PROTECTED]> wrote: >> >> At 11:06 AM 7/26/2007, you wrote: >> >> >> >> Intermod means there is mixing in YOUR PA with some outside RF energy. >> >> <I should probably clarify that "intermod" is an often misused >> term that has become a generic terms used to describe just about any >> type of received interference. Ya know, kinda like Klenex has become >> the name for any tissue. >> >> Intermod is a very specific type of phenomenon and for accuracy, >> should only be used to describe intermodulation issues and not mixes, >> adjacent channel interference, etc. >> >> (more of my 2 cents) >> >> Ken >> > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
RE: [Repeater-Builder] Re: Coax length between added cavity and duplexer
> 50 ohms is ONLY at the pass frequency. The concern here is > that the wrong > cable length between the pass cavity & duplexer can cause > undesired effects > at reject frequencies. 'zactly. Although a bandpass cavity (or multiple cavities) is always a good idea in front of any receiver, the original poster may want to take a look at what's actually getting to the receiver in the current configuration. Lack of attenuation of your own repeater transmitter carrier due to inadequte duplexer isolation can result in overload of your receiver's first mixer (or first RF gain stage, if it has one), aggrevating receiver-induced intermod when other nearby transmitters key up. I've seen a number of repeaters that seemed to play just fine without a hint of desense when running in a quiet environment, but when other signals came down the hose, even ones that would typically be considered "harmless" in terms of their low amplitude, suddenly intermod was realized. In such a case, adding a pass cavity could help by keeping the other signals out, but the crux of the problem is really too much of your own transmit carrier hitting the front of the receiver. --- Jeff
[Repeater-Builder] Re: Coax length between added cavity and duplexer
> > Intermod IS the result of mixing. > ...like a rusty joint on a tower... > ...or a guy wire... > ...or a fence... Or the rf power amplifier or preamp down the way that doesn't have protection from the outside world. In an earlier time I know how a 30 watt kit-built Ramsey 2 meter repeater power amplifier hosed up a few carrier squelch ham repeaters across the entire mountain top for a day or so before someone noticed. A nice 25 watt mixer... cheers, skipp
[Repeater-Builder] Re: Coax length between added cavity and duplexer
--- In Repeater-Builder@yahoogroups.com, "Gary Schafer" <[EMAIL PROTECTED]> wrote: > > Intermod IS the result of mixing. The mixing can take place in your receiver > (commonly called receiver intermod). > Mixing can take place in your own transmitter, which generates a product > that falls on your receive frequency(or on someone else's) or the mixing can > take place in someone else's transmitter with the resulting product falling > on your receive frequency. It is all intermodulation. i.e. the result of > mixing of two or more frequencies in a non linear device. And, if I understand correctly, even a loose/poor/corroded connector or other diode type of junction in the RF field can be at fault. I am admittedly inexperienced in this sort of issue. For the record, the interference appears as an on frequency signal (leading me to initially suspect intermod of some type) with clear audio of the dispatcher and officers. At first I thought I heard a second level of audio as well that I haven't been able to identify it whether it is background or another transmitter--partly due to catching it at the right time, and partly because my QTH doesn't allow me to hear it very well. KC0MLS thought receiver overload may be the issue so that's why we're trying the Celwave cavity. As much as I'd like to go in and rebuild the site properly, hire climbers to reposition the antennas, and send them the bill, it's not within the realm of possibility. So, let's just toss out any ideas of working with anything but the amateur radio repeater. - Nate >>
[Repeater-Builder] Re: Coax length between added cavity and duplexer
> Intermod means there is mixing in YOUR PA with some > outside RF energy. I'm not sure if one of the intermod sources has to be YOUR or the local/specific device PA. > term that has become a generic terms used to describe just about any > type of received interference. Ya know, kinda like Klenex has become > the name for any tissue. An "Amen" from the crowd... > Intermod is a very specific type of phenomenon and for accuracy, > should only be used to describe intermodulation issues and not > mixes, adjacent channel interference, etc. And of course there will be many different opinions... hence the reason I prefer to stab, jab and run fast for cover when the topic comes up. > (more of my 2 cents) > Ken I've got a 3 cent coin here... but it's not for sale. cheers, s.
Re: [Repeater-Builder] Re: Coax length between added cavity and duplexer
At 7/26/2007 10:54 AM, you wrote: >Skipp said: > >>I don't have time to debate or argue the point... but I will write >>that I don't agree with the above statement. We can go back and >>forth about that later on if you like... > >cruising7388 said: > >Yes, indeed it is a critical length if if is your desire to superimpose >the bandpass curve properly on the >pass curve of the duplexer. It should be an electrical 1/4 wave that >accounts for the velocity propagation >of the cable plus the electrical length of the coupling element in the >Celwave cavity > ><---I suspect Skipp is referring to the same issue BUTNate >specifically said he had an extra Celwave BANDPASS cavity. > >Yes I know that if we were dealing with a Bp/Br cavity, coax length >between it and other Bp/Br cavities IS a factor. But I stand by my >comments that all things being equal - coax length absolutely does NOT >matter if the impedances at each end match the coax's characteristic >impedance (ie 50 ohms is maintained). 1/4 wave, odd multiples thereof, >1/2, etc would make absolutely no difference whatsoever is impedance's >matched throughout 50 ohms is ONLY at the pass frequency. The concern here is that the wrong cable length between the pass cavity & duplexer can cause undesired effects at reject frequencies. Specifically, for maximum off-frequency rejection the electrical distance between the pass can & duplexer needs to be a 1/4 or 3/4 wavelength. You can use any other length you wish but then the off-frequency rejection characteristics of the pass cavity may suffer. Bob NO6B
[Repeater-Builder] Re: Coax length between added cavity and duplexer
What are the frequencies of your repeater and the frequencies on the nearby antenna What are the radios used on the nearby antenna? Important information in determining the problem - I had one source of noise caused by a Mastr III that had a spur of considerable strength at 12.8MHz down from the transmitter carrier - synthesizer oscillator! It was received with the correct transmitter modulation as the carrier frequency and a pass on it or a notch at -12.8 MHz solved our problem. The level of the emission was within spec of the Mastr 3 but was still too strong for our antenna at 500' away. What is the configuration of their filters? 4' horizontal separation is not much at all so filters should be designed to all the frequencies used to work together. Then, it only takes one transmitter to cause noise in an antenna if the antenna is "noisy". What does the interference sound like? Harold, VA3HF --- In Repeater-Builder@yahoogroups.com, Ken Arck <[EMAIL PROTECTED]> wrote: > > At 11:06 AM 7/26/2007, you wrote: > > > > Intermod means there is mixing in YOUR PA with some outside RF energy. > > term that has become a generic terms used to describe just about any > type of received interference. Ya know, kinda like Klenex has become > the name for any tissue. > > Intermod is a very specific type of phenomenon and for accuracy, > should only be used to describe intermodulation issues and not mixes, > adjacent channel interference, etc. > > (more of my 2 cents) > > Ken >
Re: [Repeater-Builder] Re: Coax length between added cavity and duplexer
Skipp said: I don't have time to debate or argue the point... but I will write that I don't agree with the above statement. We can go back and forth about that later on if you like... cruising7388 said: Yes, indeed it is a critical length if if is your desire to superimpose the bandpass curve properly on the pass curve of the duplexer. It should be an electrical 1/4 wave that accounts for the velocity propagation of the cable plus the electrical length of the coupling element in the Celwave cavity <---I suspect Skipp is referring to the same issue BUTNate specifically said he had an extra Celwave BANDPASS cavity. Yes I know that if we were dealing with a Bp/Br cavity, coax length between it and other Bp/Br cavities IS a factor. But I stand by my comments that all things being equal - coax length absolutely does NOT matter if the impedances at each end match the coax's characteristic impedance (ie 50 ohms is maintained). 1/4 wave, odd multiples thereof, 1/2, etc would make absolutely no difference whatsoever is impedance's matched throughout Now if one wants to argue that coax is really 52 ohms as opposed to 50 ohms, that's another story... Ken
Re: [Repeater-Builder] Re: Coax length between added cavity and duplexer
* skipp025 <[EMAIL PROTECTED]> [2007 Jul 26 11:49 -0500]: > Depending on how often the problem comes back I would first check > for possible mixing, receiver blocking, receiver & preamp 3rd order > problems. You've said the mix numbers were already figured with > some type of computer program... but that's not a 100% tell all. We found the TX frequencies of the site and everything with a few miles (this is a rural site with no other towers nearby) Using the FCC database and ran those numbers into a program found online. I won't vouch for the program. We've only been involved for about two weeks so we're still in the information gathering stage for the most part. > Taking a first shot in the dark with cavities... I'd probably try > to insert a notch or suck out cavity on the other repeater/base > station tx frequency... in your/my repeater/receiver antenna line/ > path. I agree. But, we don't have one handy. > Relative to the grand scheme of things... tis probably better to > suck than to bandpass (in this case). You'd probably get more bang > for your buck... (result for your effort). This is simply an idea to try and will help us work toward the next step which will surely involve some $$$. One aspect of this is the poor construction of the site. The site is owned by the county and located on county land. The local (to that county) 2-way shop maintains the site. To be fair, I'm not sure if the county doesn't wish to spend any money or if the 2-way shop is simply incompetent as I've not seen any of their other sites. In short the site is a disaster with not even the basics of proper grounding or installation adhered to. The feedlines from the tower hang through the wall and dangle to the floor, if they reach that far, or just hang by their own weight (the weight of one was being held by the RG-8 jumper between it and the radio). No lighting protection or ground kits. One ground rod on the opposite side of the hut from the AC service entrance and the two grounds aren't tied together. There does appear to be a ground rod for the tower, but the ground wire spirals up the leg, wrapped around it, no less, to its attachment point. As I mentioned in my first post, I tightened several coax connectors that weren't even finger tight, which gives an idea of the sloppy work at the site. The reputation I'm hearing of the 2-way shop is that a service call to fix one problem usually leads to more problems and additional billable follow up service calls. Sadly, they contract for our county as well as they are the only shop within an hour of here. It is in this poor environment that their repeater lives. Accessability is an issue as the site is not accessable after 4:30 PM and I've noticed the interference most during the evening hours. For the record, I work for a transportation company and our land mobile radio system is part of my job. Our sites are independent and we have no other services co-located with or even near us. So, I've not had to address a case of interference like this yet (call me lucky!). Lost in all of this, though, is whether the coax length between the receiver port of the duplexer and the Celwave cavity I will loan them is critical. I don't think it is. BTW, we checked the Celwave cavity with 50 ohm pads inline and there was no change in the tuning. I don't have access to the duplexer at the moment. 73, de Nate >> -- Wireless | Amateur Radio Station N0NB | Successfully Microsoft Amateur radio exams; ham radio; Linux info @ | free since January 1998. http://www.qsl.net/n0nb/ | "Debian, the choice of My Kawasaki KZ-650 SR @| a GNU generation!" http://www.networksplus.net/n0nb/ | http://www.debian.org
Re: [Repeater-Builder] Re: Coax length between added cavity and duplexer
Lots of times those BpBr cans will pass stuff not too far from the desired pass freq - A 2 loop BP cavity could very well help with receiver overload from a 155 Mhz signal - a simple test would be to hook a receiver to the REC port of the duplexer and see how strong the 155 Mhz signal is. As far as the placement of the BP cavity in line with the receiver, first, kill the transmitter and see what the receiver sensitivity through the duplexer might be. Next, add the BP cavity and see how much it impairs the receiver - then you decide - Me thinks the cable lengths won't make much difference, but as my friend Jack Daniel at RF Solutions sez, but I might be wrong. Best 73 and let us know how the extra can works out. Steve NU5D skipp025 wrote: > > I don't have time to debate or argue the point...
[Repeater-Builder] Re: Coax length between added cavity and duplexer
> ... if cavities are properly tuned to 50 ohms, the length of > 50 ohm coax between them doesn't make one bit of difference. I don't have time to debate or argue the point... but I will write that I don't agree with the above statement. We can go back and forth about that later on if you like... > IMHO, the additional cavity is a waste of time and effort until > you identify the source of the interference (and made sure your > duplexer is first properly tuned) I would agree the band-pass cavity is probably a popcorn fart until you really figure out what is going on. I've recently spent a bit of time tracking down a number of similar mix problems and I will say that none of them were from the same source/reason. The lastest similar gremlin I sourced was a real beast, which was cured simply by putting the covers completely back on the various rf boxes. And much to my surprise the mix problem did not decrease in value until the 3rd from the last (out of 27) box cover screws was inserted back in place. Depending on how often the problem comes back I would first check for possible mixing, receiver blocking, receiver & preamp 3rd order problems. You've said the mix numbers were already figured with some type of computer program... but that's not a 100% tell all. Taking a first shot in the dark with cavities... I'd probably try to insert a notch or suck out cavity on the other repeater/base station tx frequency... in your/my repeater/receiver antenna line/ path. Relative to the grand scheme of things... tis probably better to suck than to bandpass (in this case). You'd probably get more bang for your buck... (result for your effort). cheers, s.
