Double SO239 Coupler impedance

needairtime

Member
Joined
Apr 9, 2018
Messages
357
Location
CO, USA
Granted PL259/SO239 are bad at UHF, has anyone got ideas how to identify at least somewhat "acceptable" SO239 F-F couplers by visual inspection?

I got a coupler and well the first thing I tried doing is feeding it through a VNA to see what its characteristics are like. It seems that while it's okay at HF, it's bad at VHF, and plain awful at UHF. Smith chart spirals out. In fact if I take the coupler out and just hand hold the PL259 center contacts and shields together, it gives me a better SWR. Granted I have a lot of adaptors on the signal path, but it's strange that the Smith chart spiral is much more packed in the center when just hand holding the two PL259s together than when using the coupler.

The behavior of the SO239-SO239 coupler is worse than a CB SWR meter with two SO239s that I did the same experiment on - the CB SWR meter was good at HF, passable at VHF, and bad at UHF.

Would be nice to get a coupler that works acceptably at UHF - Perhaps it's just impossible to tell by looking at them?
 

W9BU

Lead Wiki Manager
Super Moderator
Joined
Jul 18, 2004
Messages
6,176
Location
Brownsburg, Indiana
I remember many years ago, the ARRL Lab cut a UHF elbow in half to expose the connection between the male and female ends. That connection was a spring like you'd find in a ball-point pen. Limited power handling capability and questionable high frequency characteristics.

You may have to grab a few adapters from different vendors and check them to see how they pan out. Not an efficient process, but it'll give you some piece of mind if you find a good one. Oh, care to say who was the manufacturer/vendor of the one you tested?

What VNA are you using? I'll assume that you redid the calibration process with your adapters in the signal path. ;)
 

needairtime

Member
Joined
Apr 9, 2018
Messages
357
Location
CO, USA
I got it at a hamfest and it really has no markings on it. I actually had my NanoVNA with me so I tested it after I got it, I assumed it would be at least ok for HF even if it turns out crappy for UHF, so it's allright, will work for HF once I get HF gear.

I assume these things are built by a die cast or press straight tub, the pre-molded center insulator/pin shoved in -- and then the other end is swaged to hold the center inside. So I suspect the dielectric is pretty leaky or too capacitive at UHF or something to this extent. As I don't have the capability to recalibrate with the SO239/PL239 (don't have SOL and this would have been the T, though T cal constants probably doesn't matter for S11) so I only can take relative measurements, which so far seems somewhat consistent with what I've learned from playing with the device. Yes the adapters definitely make a difference and hoping the relative offset of the measurements give a general but inaccurate idea of how good the cables are behaving.

Oddly enough I didn't look very carefully or remember the insertion loss of the coupler versus just shorting the PL259 pins and shields together by hand.

For reference, what is the typical smith chart like over the band of two PL259's held together by hand (both connectors facing each other straight on, center connectors parallel and touching, shields mostly overlapping but of course have a gap the width of the center conductor.) What I got wasn't good but I was surprised it wasn't as bad as I thought either-- again I can't calibrate so I don't have a valid number...
 

W9BU

Lead Wiki Manager
Super Moderator
Joined
Jul 18, 2004
Messages
6,176
Location
Brownsburg, Indiana
The RF Connection has female to female UHF connectors ranging from $1.00 each (import) to $11.00 each (Kings with silver body and silver center conductor). Maybe you get what you pay for. ;)

 

Firekite

Member
Premium Subscriber
Joined
Apr 2, 2019
Messages
329
Too bad can't see any difference between them other than price... (so much for stock photos...)
Out of curiosity, what would you expect to see different in photos to clearly indicate differences in impedance?
 

needairtime

Member
Joined
Apr 9, 2018
Messages
357
Location
CO, USA
If the good ones at least brand their units, that would be helpful. Of course a lot can be faked but need something, anything to go on.
 

eorange

Secret Agent Man
Joined
Aug 20, 2003
Messages
2,336
Location
Cleveland, OH
I almost bought an inexpensive SO-239 at a hamfest from a vendor who had like 100 red parts bins, but upon closer inspection...the hole was just a little offset from center, and there was a slight uneven gap between the white insulator and the inner ring. But was it ever shiny!
 
Top