RE: [Repeater-Builder] Re: Quasi-Simulcast?
There's good reason a hot air alarm probably went off for many of you when reading the article. I'm trying to figure out what his closing paragraph is trying to say: A duplexer is working correctly when the sensitivity of the receiver is not degraded when the transmitter becomes active. There are test procedures to check this out, but the explanation of these tests is beyond the scope of this article. [OK, I'll agree with him so far] However, should you hear a slow oscillation of the transmitter when it turns on and off (a rate of about 1-2 Hz rate on weak signals), then you do have duplexer desensitization. What is this 1-2 Hz oscillation he's talking about? His use of quasi-simulcast is what we used to call sloppycast. Basically the transmitters are not time-aligned as far as the launch time of the audio from each site. Furthermore, the transmitters may or not be locked to a high stability reference (UHSO TCXO/OCXO, GPS, Rubidium, etc.), so there may be carrier frequency errors as well resulting in audible beat products. Several of my co-channel ham repeaters are sloppycasted, but with accurate carrier frequency matching (using Rubidium reference oscillators). There is some minor distortion in areas where the signal levels from two transmitters are comparable due to the lack of AF time-alignment, but none of the users have ever commented on it. Still bugs ME though - will have to do something about that eventually... I question whether or not the Icom radios they used for links are certificated for use as fixed station equipment. We've had local instances of field agents from the Commission shutting down Part 22 and Part 90 base/repeater stations operating using mobile radios. In one case, a Mitrek had such bad LO leakage that it was getting into another receiver several miles away, which is what prompted the FCC interference investigation. --- Jeff
Re: [Repeater-Builder] Re: Quasi-Simulcast?
Quasi .. that just means virtual. Some manufacturers, in the interests of meaningless product diferentiation, choose words for effect. Looking at the article it seems that the quasi part is a precision offset to each base stations frequency so that a stationary mobile will not stay in a null. All the other simulcast rules must be followed and having personally worked on a tait quasi system the automatic transmitter training routines are cool and fast. Hope this isnt too much hot air... Ian G8PWE - Original Message - From: skipp025 To: Repeater-Builder@yahoogroups.com Sent: Friday, May 11, 2007 4:56 PM Subject: [Repeater-Builder] Re: Quasi-Simulcast? The article should not be taken as gospel. In fact at first look it appears have a lot of pontification plus potential smoke mirrors. [paste text] For a voter to be working properly, all of the receivers must sound the same. This includes the audio level, and the overall sound. Not really... might be desired but it's not a must-have. You do not want anyone to be able to say that is the north receiver because it sounds different from the rest. Might be something desired but not always an easy do, nor is it actually a requirement. The initial system lineup requires that all of the receivers be perfectly matched to each other in every parameter. Don't know what planet this guy is from... I guess he's never had multiple brands of remote receivers through combinations of phone lines, microwave and/or radio links. You will need high quality testing equipment to accomplish this. You mean I can't use low quality gear? shucks... There's good reason a hot air alarm probably went off for many of you when reading the article. cheers, skipp While reading this article: _http://mrtmag.com/mobile_voice/radio_big_signals_small/_ (http://mrtmag.com/mobile_voice/radio_big_signals_small/) I came across the term quasi-simulcast. Anybody heard of that term? How does it differ from normal simulcast.
Re: [Repeater-Builder] Re: Quasi-Simulcast?
Hi Jeff, A duplexer is working correctly when the sensitivity of the receiver is not degraded when the transmitter becomes active. There are test procedures to check this out, but the explanation of these tests is beyond the scope of this article. [OK, I'll agree with him so far] However, should you hear a slow oscillation of the transmitter when it turns on and off (a rate of about 1-2 Hz rate on weak signals), then you do have duplexer desensitization. What is this 1-2 Hz oscillation he's talking about? A weak signal brings the repeater up, desensitization causes the receiver to no longer hear the weak signal, the repeater drops, and the cycle repeats. 73, Bob, WA9FBO AOL now offers free email to everyone. Find out more about what's free from AOL at AOL.com.
RE: [Repeater-Builder] Re: Quasi-Simulcast?
At 5/11/2007 09:45 AM, you wrote: There's good reason a hot air alarm probably went off for many of you when reading the article. I'm trying to figure out what his closing paragraph is trying to say: A duplexer is working correctly when the sensitivity of the receiver is not degraded when the transmitter becomes active. There are test procedures to check this out, but the explanation of these tests is beyond the scope of this article. [OK, I'll agree with him so far] However, should you hear a slow oscillation of the transmitter when it turns on and off (a rate of about 1-2 Hz rate on weak signals), then you do have duplexer desensitization. What is this 1-2 Hz oscillation he's talking about? In the context of the above, he's probably referring to the repeater transmitter cycling on off due to loss of input signal when the TX is on due to desense. His hang time is probably only 1/2 second, hence the 1-2 Hz TX cycling period. BTW, I quasi-simulcast 2 UHF TXs about 70 miles apart; one is 1000 ft. AMSL the other is 7000 ft. Some low hills between them but not much dirt otherwise. Only TX stabilization is temperature-controlled xtals on both. I try to keep them within 50 Hz but one xtal is aging a bit so right now they're about 200 Hz apart. Still, the heterodyne in the overlap areas is not bad the system is perfectly usable everywhere. Only thing missing is the 2-channel voter. I have the hardware to put it together but there isn't enough usage of the system to justify the time spent to do it, so for now the 2 RXs are selected by CTCSS freq. Bob NO6B
