Jeff DePolo, Gary Schafer, Nate Duehr, and Burt Lang,
It is certainly interesting to read the widely diversified lack of
authoritative information on the subject of "constant impedance" and other
descriptions of the impedance characteristics of connectors used for RF, namely
PL259, BNC, TNC, type N, SMA, SMB, and the ubiquitous RCA (shortened) phono
connector which has or have been presented in these forgoing discussions.
There are however in my observation, several individuals who by their
experience or knowledge, have contributed factual and important descriptions of
the impedance characteristics of RF connectors. They are Jeff DePolo, Gary
Schafer, Nate Duehr, and Burt Lang. Burt was right on when he said that a
PL-259 was a 35 Ohm connector (because of the Bakelite insulator dielectric)
and that with air as a dielectric it results in a 50 Ohm characteristic
impedance. The only saving grace for "UHF" connectors is in the use of Teflon
dielectric in place of the Bakelite, and use in lower frequency applications.
As an engineer employed in the Engineering Dept.of Amphenol-Borg
Electronics Corporation in Broadview, IL (Amphenol) during the 1950's and
1960's I was personally involved in the evaluation of many of the RF connector
products manufactured by Amphenol and others.
Along with my engineering supervisor appointed by the company, Norbert J.
Sladek, I was the co-author of many engineering reports which contained the
measurements of RF characteristics of which I have performed.
The most note-able here of which is "Engineering Report on the VSWR of
Series 83 UHF Connectors for the Frequency Range 20-500 Mc/s" by N.J. Sladek
and A.B. Crites, dtd September 14, 1955.
In it there are many graphs and tables of the measured impedance
characteristics and RF signal losses of the PL 259 (Amphenol # 83-1SP) mated to
other Series 83 connectors. For those desiring a copy I would suggest
requesting a copy of the report done for E.P.A. 340-25-1913 from Amphenol now
located in Danbury, Connecticut. It should be noted here that the 83-1SP and
mating connectors all used either a Bakelite or Teflon dielectric. (There are
"UHF" connectors in the marketplace which use white Nylon or other substances
and are lower cost and unsuitable for high frequency use.)
At the conclusion of the project Mr. Sladek made the following rule of
thumb regarding the use of "UHF" connectors. "Don't use UHF connectors in the
UHF band".
There were also 25 other reports generated during this time, on projects
that were related to RF connector development at Amphenol, in which I
participated.
The term "constant impedance" used to describe a parameter of an RF connector
is to my way of thinking a misnomer as there are no "constant impedances" in
the construction of an RF connector, only nominal 50 ohm sections or
structures, line sections known as discontinuity capacitances, and higher
impedance inductive compensations. All these discontinuities are the result of
gaps in the mating of the center conductor and outer conductor, the step up
(undercuts in) or down in the dielectric (so called insulator supporting the
center conductor) and/or the center conductor, and the difference in center
conductor and outer conductor diameters of the junction of the coaxial cable
interface to the connector dielectric (sometimes referred to as the bead).
Another factor in the design of RF connectors is the reflection from the
faces of the dielectric which is the transition which departs from or to
another dielectric section ( air to Teflon and back, Teflon to Polyethylene,
etc.) These reflections are more important however, in the GHz frequencies.
For those who are interested, a discussion of the methods used to achieve a
low reflection RF connector for use at higher radio frequencies can be found in
" Development of 50 Ohm Low Reflection, Captivated Contact TNC Connectors" by
A.B. Crites, R.P. Honn, and N.J. Sladek, dtd February 19, 1958. E.P.A.
340-25-2163.
Also see " The Interrelationship Between Dielectrics and The Electrical
Properties of Radio Frequency Coaxial Connectors" by N. Sladek, dtd June 1,
1959, for USAF Contract AF33(616)-6243 for the fundamental considerations in
the design of RF connectors.
I agree with the comments of the BNC connector being inferior to the type
N, and I prefer the use of the type N connector where ever and when ever
possible for the reasons given.
The SMB connector was developed by Amphenol for use on RG-58/U and other
similar size Teflon coaxial cables as a lower cost, quick connect / disconnect
connector.
The SMA was developed by another connector manufacturer as a screw on
version. The Motorola, 800 MHz, 100W., Quantar Transmitters uses these SMA
connectors.
One connector not mentioned is the type C which was developed as an
alternative to the type N, having a Teflon dielectric which encompassed the
mated center conductor contacts, leaving no air section, having improved power
handling, high voltage breakdown, and VSWR, along with an improved quick
connect / disconnect feature.
One last item. I own an HP 8714A N/A and I'm sure I could make measurements
of any parameters relating to the impedance and insertion loss of RF
connectors. We did not have the equipment in 1955 to do the wondrous
measurements available today.
Allan Crites WA9ZZU
Gary Schafer <[EMAIL PROTECTED]> wrote:
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: [email protected] [mailto:Repeater-
> [EMAIL PROTECTED] On Behalf Of Ken Arck
> Sent: Thursday, March 22, 2007 7:42 PM
> To: [email protected]
> 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
