I've seen dual stage isolators (have one sitting here for 440 just in case I get a UHF pair to work with), and am actually planning to use them as exactly that on each transmitter... and I've proposed your idea if worded slightly differently by suggesting a circulator between the combined output of my transmitter chain, the antenna, and the input to my receiver chain.
I even suggested sticking a circulator between the RX and TX halves of a standard duplexer.. get you an extra 20-30db of isolation at very minimal cost if your antenna is reasonably matched to the transmit frequency. You might need a harmonic filter between the circulator and antenna, but that shouldn't be a huge problem. Total additional insertion loss.. perhaps 1db, and a lot less transmitter getting into your receiver :) -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Nate Duehr Sent: Tuesday, October 23, 2007 7:52 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: duplexer isolation and reciever noise bud John Barrett wrote: > Re: selectivity and rejection: I'm looking at the Icom V-8000, which > specs 75db for all of spurious/image/rejection. I have no data on > dynamic range or 3^rd order intercept. (haven't seen data like that on > any of the radios I've been considering for this system) > > Is there any way to estimate the missing info from > spurious/image/rejection data?? Jeff was recommending that you actually MEASURE it, if you're curious. I agree. As one of my RF mentors says, "Are you a thinkin' man, or are you a knowin' man?" With that said, if you're attempting all of this without a spectrum analyzer, you're nuts. You are building a system that would ultimately have multiple devices in an "odd" configuration (meaning that your methods of combining don't appear to be practiced by anyone here, nor in any documentation anyone can find -- probably for good solid engineering reasons?) and one device (the isolator) is known to throw harmonics. Additionally, now you're mentioning that you want to put high-gain pre-amps into the mix (which may or may not be inherently stable, depending on who's you use -- also your setup will put a LOT of RF from your transmitting side directly into them, which is likely to blow them up or otherwise make them very unhappy)... > Is there anything meaningful that can be determined from the info I have > available?? No, this is why Jeff pointed out it'd make a lot more sense to measure and KNOW than use the manufacturer's paperwork (specs) and GUESS. You're going to have to see this whole thing with a spectrum analyzer at some point anyway, if you have any chance of it being "clean" ... so might as well start measuring things up-front if you're dead-set against using traditional tried-and-tested combining and filtering methods. Right? > No trade secrets now!! Spill all the dirty details J Wow, the only "trade secrets" being "hidden" might be that the rest of us *usually* use completely different best practices for combining and splitting that look nothing like what you're attempting. If you already realize that, then there's nothing being hidden here other than perhaps a few cringing people hiding behind something sturdy waiting for the big "boom" when something blows up over in your general direction. (GRIN) Your idea has sucked me in SLIGHTLY, however... you do know you could get a FOUR port isolator, right? (Ha... that's bound to give you some REALLY wild ideas.) Look for "dual-stage" isolators. Port 1 TX Port 2 Antenna Port 3 RX Port 4 Load to protect TX and keep stuff from going around forever by getting into the PA and mixing/amplification. LOL! Frankly, I can't believe I just posted that, though -- I think you're missing something here... > That's for carrier attenuation only. Aside from the passive losses in the > system, you've done nothing to attenuate transmitter noise, which is going > to be a bigger enemy in your close-spaced frequency assignments. If you're having trouble getting an image of what Jeff's talking about here... look a the output of any FM transmitter with a spectrum analyzer. It's not a single vertical line. It's a "mountain" shape with a slope on either side. SKIRTS. You have to deal with all that RF energy that's NOT directly on-channel too, not just the "directly on-channel" RF. That's some of the "noise" Jeff is referring to... or at least it's part of the energy in the skirts... If you put that into an isolator (which will create harmonics not just of the main frequency, but possibly of ALL of those frequencies in the skirts!!! MESSY!), and than pass it directly to a very sensitive receiver, possibly directly into a PRE-AMP?! -- Well, the receiver not only needs to be able to handle the large amount of RF directly on the TX's frequency, but the SKIRTS of that signal too... and all those harmonics... Without desensing the receiver. Not gonna happen. (I think you'll find it won't find a receiver that's even going to come close to being able to handle this, at the spacings you're talking about. You can try... we can't stop you.) I think it's interesting that you're experimenting, but an awful lot of folks have combined transmitters and split receivers and done this, and they're all saying your spacing is really tight to pull it off without a LOT of filtering -- if you want a *clean* system. Something to think about. The original comment that it might be best to split the antennas and get some distance between these systems... or move the repeater to UHF... seemed a lot easier to me than what you're trying to do there. But ultimately, you get to decide how you spend your time. I really don't know your motivations. No one here can stop you, but I think there are a number of us holding up big orange day-glo warning signs... as you're whizzing by. :-) Nate WY0X Yahoo! Groups Links
