> -----Original Message----- > From: Repeater-Builder@yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] On Behalf Of Nate Duehr > Sent: Thursday, March 22, 2007 11:04 PM > To: Repeater-Builder@yahoogroups.com > Subject: Re: [Repeater-Builder] LMR feedline revisited and revised! > > On 3/22/07, Gary Schafer <[EMAIL PROTECTED]> wrote: > > > If you really want to get picky use only N connectors and not even bnc. > A > > bnc is not a constant impedance connector either although somewhat > better > > than a PL259. Yes I know you can plug an N connector into a bnc but the > bnc > > still is not a constant impedance device. > > Could you site a credible source that claims this? I can't find any > references to impedance regarding BNC connectors. > > I've stayed out of the debate so far, but even Amphenol claims > "nominal 50 ohm impedance" for their 50 ohm BNC connectors up to 4 > GHz. On their N-connector they drop the "nominal" but they don't > elaborate. > > They also say the BNC has "low reflection" below 4GHz... and their N > connector specifications show nothing at all regarding that. The > assumption here would be the N is slightly better again. > > And they show that you have to switch to a TNC for anything above 4GHz > through 11GHz. Or the N again, of course. > > I'll agree that maybe a BNC is not as good as an N-connector overall, > but similar to your argument -- I'll bet no one here can measure the > difference at VHF/UHF. > > Amp shows (much!) more RF leakage from a BNC than an N (which would be > a detriment in some installations!), insertion loss numbers that are > within .05 dB of each other, and the center conductor also has a > slightly higher resistance on the BNC vs. the N. If I missed anything > there, here's their links: > > http://www.amphenolrf.com/products/bnc.asp > > http://www.amphenolrf.com/products/typen.asp > > But again, just like your comment about the PL259's... I would love to > see a credible source on impedance measurements of BNC's that's > available to the public. > > Don't worry, I'm not defending the BNC -- I'd rather not use them > myself. Just playing devil's advocate on the statement, since it > doesn't have any more credibility or science behind it than the PL259 > comments did. > > I *can* find credible sources that show PL259's generally stink with > real measurements at VHF/UHF... but I haven't been able to find such > data for BNC's (yet). And judging by Amphenol's site, TNC's keep up > pretty well with the N connector. > > Nate WY0X >
I don't know if I have seen this stuff in print either. I have gotten most of my info from engineers at Motorola and Wavetek. I used to sell their service monitors. Jeff pretty well sums up what the problems are with the bnc connectors. In most applications it doesn't matter much. But like I noted with the service monitors they could not maintain the output level and return loss spec with the bnc connectors and this was probably mostly due to the bnc not being a solid connector. Don't get me wrong, I am not saying that a bnc connector is junk. The comment was that "if you want to worry about connectors then use an N rather than a bnc". This was for the benefit of those that worry about PL259's not being any good. The bnc is not too bad as HP used it on the front end of their older spectrum analyzers. They latter switched to a type N though. I guess they saw the light. :>) A PL259 serves the purpose well in most cases and will have almost immeasurable loss but it will not have a constant impedance. Some places that matters and some places it does not. If you start sticking several PL type adaptors in line with your wattmeter you may see the wattmeter read different to what looks like loss but it is not loss in the adaptors or connectors. The change in impedance fools the wattmeter and it gives a different reading. If that change in impedance is reflected to a point where it upsets the tuning of a tuned circuit, like back to the output of a transmitter, then it can detune the transmitter slightly and actually cause it to put out less power. But the less output power is not due to connector loss. Jeff mentioned that non Teflon connectors get warm. I am not so sure that is due to poor dielectric or poor contact of the mating surfaces. Dielectric does not come into play much until you get into uhf. Coax cable dielectric makes little difference in cable loss thru vhf as most loss is attributed to resistive losses. 73 Gary K4FMX
