Hi Allan,
Coax line impedance is determined mainly by the size of the center conductor and its spacing to the outer shield of the cable and somewhat by the dielectric in the cable. The same is true for a connector. Any time you change the size of that center conductor or the spacing to the shell the impedance changes slightly. In a PL type connector the ratio of center conductor to shield changes from what the ratio of the cable to shield is so that gives an impedance change. A connector with a constant impedance may have different sizes of center conductors (center pin) than what the cable size is but that change in size maintains the same ratio of pin size to shield as the ratio of the cable center conductor to shield. That keeps the impedance the same through the connector as what the cable is. This is why a connector for a 75 ohm cable will be slightly different in size than one for 50 ohms. Note that 75 ohm cable has a smaller diameter center conductor than a 50 ohm cable that has the same outer shield size. 73 Gary K4FMX _____ From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of allan crites Sent: Thursday, March 22, 2007 10:57 PM To: [email protected] Subject: RE: [Repeater-Builder] LMR feedline revisited and revised! I sure would like to hear what you all mean by " constant impedance ". Allan Crites, WA9ZZU Gary Schafer <[EMAIL PROTECTED]> wrote: > -----Original Message----- > From: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com [mailto:Repeater- > [EMAIL PROTECTED] <mailto:Builder%40yahoogroups.com> .com] On Behalf Of Ken Arck > Sent: Thursday, March 22, 2007 7:42 PM > To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com > Subject: Re: [Repeater-Builder] LMR feedline revisited and revised! > > At 04:33 PM 3/22/2007, you wrote: > > > > > > > >doesn't make it right though considering the potentual losses. > > <---Psssst...don't tell anyone but a PL259 ain't as horrible (loss > wise at least) at VHF & UHF as "folklore" would have you believe. The > real issue is not of loss but rather that of a PL259 not being a > constant impedance connector. This is where BNC's and N's shine. > > Ken > Ken is exactly right! I would venture to say that there are probably few to none on this list that have any equipment that could measure the difference in loss between a PL259 and a good N connector. That loss thing is an old myth. Now as Ken said they are not a constant impedance and you will get a bump in the impedance with one that can give a mismatch in a fixed tuned circuit and you can have what is called "mismatch loss" which results from the circuit getting detuned slightly because of the impedance difference. There can also be swr losses caused by the impedance mismatch but a direct loss from the PL259 at VHF and UHF is almost non existent. At least not measurable. 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. Case in point: The Motorola 2600 service monitor uses an N connector in order to meet the flatness spec over the entire range even though most people want a bnc for the rf connector on it. So they supply an N to bnc adaptor with each unit for those that insist on bnc connectors. 73 Gary K4FMX
