PL-259 and impedance

[ghz24]

I just discovered this.
The SO-239/ PL-259 connector on the back of virtually all CB's suffers from an impedance problem?
In the center of the connector the impedance is ~30-40 ohms.
Is this true at 26-30 MHz ?

How is this not ~equal to a SWR of ~1.6:1 before the signal even gets to the coax? (Simplified math)

Yes I know the definition of SWR involves the transmission line and connectors and antenna/load but impedance mismatches cause reflected signals where ever they happen right.

This has cause me to ponder a few things like:
Should performance seekers change the pl-259 type connectors on our 10-11 meter radios to N type connectors?

And:
Could this be why built in SWR meters are "notoriously inaccurate" ? Maybe they are just as or even more accurate than external SWR meters at least at measuring what the radio actually sees as it's SWR before the reflections of the 2-4 connectors required for an external SWR meter? (I know ... BLASPHEME !)

 

[BOOTY MONSTER]

"In the center of the connector the impedance is ~30-40 ohms."

how did you come to this conclusion ?

 

[W5LZ]

While you're at it, how about that 50 ohm coax thjat's very seldom ever exactly 50 ohms? Or how about the 'shape' of the antenna determining it's input imp[edance, very seldo ever 50 ohms exactly? How about any number of things not being exactly what they are supposed to be?
And just for grins, what are you using for the 'other' terminal to determine what that center conductor's impedance is? Sorry, it always takes two terminals for impedance. So what's the other one's impedance?
See where that's headed?
- 'Doc

 

[Captain Kilowatt]

First off, I have seen and heard it a million times about internal SWR meters being "notoriously inaccurate" and could never see. It seems to be mainly a CB thing especially among he golden screwdriver crowd that this perception exists. I have had many radios in my life and all the internal meters were quite close to any external meter. Two problems exist with this myth. One, the golden screwdriver and peaking and tuning throws the meter circuitry out of whack and two, people do not understand what happens when the antenna impedance is not 50 ohms at the feedpoint. The SWR will vary depending on the placement of the meter along the transmission line. They take a radio with a built in meter and then place another meter a few feet down the line using a patch cable. The two meters read different values. They beg, borrow, or steal another meter and put it in the same place and it reads the same as the one they used the first time and conclude that the built in meter is junk. What is really junk is their knowledge, or rather lack of it, about what the real cause is. OK 'nuff said about meters.

The PL-259 is what is known as a non-constant impedance connector. It's actual impedance varies somewhat depending on the frequency range it is used on. In short the PL-259 is not bad and with the exception of possibly VHF and and much moreso on UHF, changing out the connector will not make a hill of beans difference in performance neither observed nor measured.

Also having what is called an impedance bump in a system, which is a sudden change in impedance in a very short space such as a kink in the cable or an odd connector etc., will not throw off the entire system SWR. It can however be measured using other measurement systems than simple SWR such as a time domain reflectometer.

 

[psycho]

I just discovered this.
The SO-239/ PL-259 connector on the back of virtually all CB's suffers from an impedance problem?
In the center of the connector the impedance is ~30-40 ohms.
Is this true at 26-30 MHz ?

How is this not ~equal to a SWR of ~1.6:1 before the signal even gets to the coax? (Simplified math)

Yes I know the definition of SWR involves the transmission line and connectors and antenna/load but impedance mismatches cause reflected signals where ever they happen right.

This has cause me to ponder a few things like:
Should performance seekers change the pl-259 type connectors on our 10-11 meter radios to N type connectors?

And:
Could this be why built in SWR meters are "notoriously inaccurate" ? Maybe they are just as or even more accurate than external SWR meters at least at measuring what the radio actually sees as it's SWR before the reflections of the 2-4 connectors required for an external SWR meter? (I know ... BLASPHEME !)

They make effective pills for impedance problems.

 

[W5LZ]

... and it's a fact that bowels age and result in the possessor becomes intercontinental. Just depends...
- 'Doc

and I'll leave it at that!

 

[ghz24]

Hi Booty, thanks for the wecome BTW.

how did you come to this conclusion ?

And just for grins, what are you using for the 'other' terminal to determine what that center conductor's impedance is?

I didn't measure anything. I have no way to measure the reactive part.

I got the info from the wikipedia page on pl-259 connectors.

 

[Se7en]

Don't know about all that CB jive, but PL259s SO239s are an issue with VHF - SHF & up....

I could go on another LSD rage and Edit all of Wikipedia's info... READ with an OPEN mind when eye sighting the internet and its info.

 

[KM3F]

I can a-sure you that the impedence 'looking into or out of' the radio through the connector is not set by the connector.
Reading something from a source is one thing, understanding it is another.
You just can't run with something you read, unless you do.
It's not that simple.
Consider that there are any number of coax cables of different sizes but all are rated at 50 ohms 'in this case'.
The inside diameter and spacing to the center conductor, center conductor size plus the insulating medium sets the nominal impedence.
Such that many different combinations for these dimensions can result in a nominal 50 ohms impedence.
For example on size, you would not want to use small dimensioned hardware on very high powers, so the size of the hardware and it's insulating medium is important in handling high RF voltages and curents.
The best of the 259 series is rated up to 300 mhz but not good to use much above 144 for the very best performance.
If one builds a 50 ohm dummy load flat to 450 mhz and does match testing, you would get a good education on what the various types and sizes do at 144 mhz and above for match and loss.
Lesson is to watch using such connectors as 90 degree ells and barrel connectors.
They are not all that good.
Below 50 mhz just about any junk will work as long as there are no obvious shorts, breaks, or faults.

 

[BOOTY MONSTER]

i wouldn't trust wikipedia as a source for definitive statements about anything in the RF hobby myself . FWIU the 259 is perfectly fine for the CB band .... as long as you aren't exceeding it's power ability and its attached (soldered) to the coax correctly .

 

[mackmobile43]

About the only thing to be done with those are to make sure the pl-259's shielding is brought into the connector right up to the insulator that centers the center conductor as not to upset what tolerances the coax has.

As for the pl-239 in the radios most cb's have a single piece of un insulated wire connecting the 239 to the radio's circuit board and that may be the deciding factor of how the radio see's the antenna so I suppose if you're anal to the point you could remove that wire and replace it with a short piece of coax such as that found in HF radios and make sure the shielding is connected to a board and chassis ground.

 

[ghz24]

First thanks for your advice/input

changing out the connector will not make a hill of beans difference in performance neither observed nor measured

This seams reasonable and the apparent general consensus (even if stated slightly differently)

Does anyone know what the impedance is at 27MHz?
Just curious.

Part of the reason I considered this option is I have a 40 foot piece of LMR-400 with N male connectors and a dozen or so N-female bulkhead plugs.

So least of all evils is to just replace one of the N males with a PL-259 vs. an adapter PL-259 to N-female or changing the connector on the radio.
Even though I'll have to wait for it to be shipped from China via evil-bay.

That's how I ended up on the wikipedia page in the first place (double checking to make sure I ordered the right connectors before ordering)

 

[KM3F]

You asked, what the impedence is at 27 mhz?
Of what, the connector or coax?
The question is NA.
A feedline made for nominal 50 ohms impedence is 'not frequency sensitive'.
To prove this, take a feedline and terminate it with a flat 50 ohm resistive load.
At any frequency below the coax and connector design range, apply power with an SWR meter in line at the driving source or at the load end and you get no SWR other than 1 to 1 at either end proving the load is a flat 50 ohms non reactive, proving neither the feedline or the load is frequency sensitive up to a reasonable limit where the connectors begin to show impedence upsets at very high frequencies.
At this point, changing over to better suited hardware such as N type becomes apparent when the 'upset' becomes objectional for the application.
The upset begins to show up above 144 mhz for uhf hardware.
Reason is the upset becomes a' larger part' of the wave length freqency when inductance, hence' inductive reactance' starts to become 'dominant'.
I'm sorry to explain in these terms but it has to be said something like this to be accurate and fit the laws or it become meaningless and you would still not understand.
Good luck.

 

[ghz24]

Don't confuse ignorance and stupidity

The PL-259 is what is known as a non-constant impedance connector. It's actual impedance varies somewhat depending on the frequency range it is used on

So my question would be regarding the connector.

and you would still not understand

I was insulted at first by this and had to restrain my impulse to post a scathing retort, but as I reread I don't think it was meant as an insult.

Maybe it is meant more as a statement of unneeded accuracy that could obscure the issue and the you is plural.
So I'll take it that way.
Personally I'd prefer if you (plural) answer or explain to the best of your ability at what ever technical/mathematical level is required to answer and let me decide what I'm capable of understanding.


In return I'll try to carefully consider/research all of your responses before asking for clarification, disagreeing , or pointing out apparently contradictory information.


If you (plural again) perceive my ignorance of some concept is to deep to address in the thread we are on feel free to just post/PM a pertinent link and I'll try to catch up. I'm in constant research mode any way.

 

[Beetle]

The question of the impedance (of a PL-259?) at 27 MHz is meaningless. The length of an impedance "bump" depends on the physical size of the "bump" versus the wavelength, generally expressed as a percentage. A PL-259 is what - two inches long? And an eleven meter wavelength is ... well, you do the math.

The impedance of 50-ohm coax is -- wait for it -- 50 ohms, plus or minus a couple. RG-8, back when it was a Military Specification item, had a 52-ohm characteristic impedance.

The PL-259 is fine at HF and up through 2 meters. Notice, however, that the two-inch length is now getting somewhere near a "significant percentage" of the wavelength. I have a fairly short run of RG-213 for my 2M/70cm antenna, and there are PL-259s at both ends. No problems in 20+ years.