N1GY- The simple Approach to Ham Radio
and My Model Railroad Hobby
THE UNFAIR MALIGNING OF THE UHF CONNECTOR
An Edited Version of an article by Dick Knadle, KR2RIW
Editing by Geoff Haines, N1GY
A couple of years ago, I received the latest edition of a Contester E-Newsletter from the ARRL. Now I do not contest, but every once in a while, there is a gem of info in the newsletter. This was the case then, when the editor pointed us to the web site of The Savvy Microwave Group. In there was a piece by Dick Knadle, K2RIW about, among other things, the undeserved poor reputation of UHF or PL-259 Connectors. I have edited and paraphrased Dick's remarks for this page
There are many misinformed engineers and amateurs who have been led to believe that a UHF connector is the worst thing ever invented in the RF world due to it's lower internal impedance. These people believe that each UHF connector causes a 1/2 dB insertion loss and a whole lot of VSWR at 440 MHz. I have heard several amateurs claim that their 440 MHz Brick amplifier will now have 1dB greater gain since they just replaced the two SO-239s with Type N connectors. This "Old Wives Tale" has been propagated for years and years. Everyone believes it. No one challenges it. Few people have ever taken the time to actually measure it.
Here is Dick's observation. He took a 432 MHz Stripline Parallel Kilowatt Amplifier and applied 700 watts through a UHF female and a UHF Male connector, and then into his antenna feedline. After 10 minutes of 700 watts throughput power the UHF connectors were barely warm. He estimated, based on the temperature increase that 3 watts were dissipated as heat. That is an insertion loss of 0.019 dB for the pair of connectors. You say " how can this be, the internal impedance of the PL-259 is 35 ohms, it's got to cause a 1.43:1 VSWR? Well, it doesn't.
The mated UHF connector has an internal connector length of less than 0.9 inches. A free space wavelength at 432 MHz is 27.3 inches. That makes the phase length of the connector 11.9 degrees. If you plot this on a Smith chart or the mathematical equivalent, you will find that a 50 ohm antenna with an 11.9 degree long section of 35 ohm line causes an input impedance of (47.9-j7) ohms. That is an input VSWR of 1.16:1, which gives a worst case reflected power caused loss of 0.024 dB. That is totally insignificant. Now at 10 GHz where the wavelength is 1.1 inches, that 0.9 inch connector length would be much harder to take.
A type N connector can tolerate low-duty pulses of 20 Kilowatts without a voltage breakdown. However, a steady state power of more than 1 KW could cause the N-connector to fail from RF overheating of the center pin. The UHF connector has a larger center pin that can better withstand high steady state RF currents. Also an internal impedance of 30 ohms maximizes the power handling capability of a connector and the PL-259 comes in at 35 ohms.
There are probably twice as many hams out there who can properly install a PL-259 connector as those who can do an acceptable job on an N-connector. The proper installation of an N-type connector is almost an art form. The two major faults of the UHF connector are it's lack of waterproofing and the lack of a sliding finger connection for the outer conductor or shield. A proper tape and "coax-seal" job will generally take care of the moisture and checking to make sure your UHF connectors are properly seated and tightened will go a long way to take care of the second.
So, maybe it is time we stop saying such bad things about the poor old PL-259. It doesn't deserve all that flack. Properly used it gives a lot of bang for the buck. It has been around for over 60 years, and that's no coincidence.
Well, I'll get off my soapbox now. Maybe you will look at the venerable old PL-259 (or UHF connector as it is sometimes referred to) a little differently now.
Addendum: I have since had a very nice email contact with Dick Knadle, K2RIW about this very subject and he has sent me copies of several articles on this and related subjects. I will be using these in upcoming issues of the "WCF Experimenter" over the coming year as they are very interesting and may help some of us abandon the "old wives tales" about coax and connectors that we have been taught incorrectly for years.