[TowerTalk] ...and another coax adapter question

Sat Nov 25 19:44:45 EST 2023

I ran into a different failure mode on a right angle UHF M-F adapter. 
Several years back, while at Visalia, I found some new, very 
nice-looking, nicely-machined, apparently-well-built, adapters from one 
of the several vendors there; I don't recall exactly but they were 
probably less than $5 each. I snagged a handful. I've been using them 
with no problem until recently, when I was able to up my power to more 
than 600 or 700 watts. Suddenly, one night, SWR shot sky-high on a few 
dits, then seemed to return to normal. Ten or so minutes later, the same 
thing happened again. Then it began happening every few minutes; the 
high SWR condition would persist for a couple words then disappear, only 
to reappear again after awhile. Examination of my coaxes didn't reveal 
anything untoward. But awhile later, I had occasion to swap a couple 
antenna coaxes to my tuner (where I was using several of these adapters) 
and when I pulled on one that was screwed into one of those "new" right 
angle adapters, I could tell it was loose. Removing it, I was astonished 
to find the female center insulator and socket pin just pulled straight 
out of the adapter!

It turned out that this manufacturer (whoever they might have been) made 
the center conductor of their right-angle adapter by maching a male UHF 
pin from a rod; the inner part of the rod then necks down in diameter 
and feeds back into the body of the adapter to the region of the 
right-angle. There, at the end of that rod, they drilled a small hole 
through the rod. On the female end of the adapter, the UHF female pin is 
another machined rod that, on the inside of the adapter at the right 
angle, is machined down to a small diameter pin that fits into the hole 
in the rod that becomes the male pin. I don't see any solder or a weld 
joint joining the two rods and indeed, since there is no access to the 
junction of the two rods inside the adapter, you can't get inside to 
solder it, anyway. (You could, however, if so inclined, carefully drill 
your own hole through the right-angle squared-off body to stick your 
iron's tip inside to attenpt to solder them together.) It appears that 
the small-diameter pin is supposed to be a force-fit into the hole in 
the rod that forms the UHF male pin. Perhaps it is; but movement of 
either or both of any cables or connectors on either side of that 
adapter will eventually cause that force-fit to loosen up and hey, el 
presto: a broken circuit causing high SWR.

My high-SWR fault was caused by that small pin end becoming loose inside 
the hole drilled through the larger rod used to form the UHF male pin.

These are nicely-machined and very-well-fitting adapters; too bad 
they're so unreliable and virtually useless for any serious application.

I have a bunch of UHF-to-type N adapters from the same vendor; I wonder 
what failure mode they're going to present?? I have no doubt that they, 
too, will catastrophically-fail at some point.

Steve, K0XP

On 11/25/2023 12:28 PM, Ron WV4P wrote:
> Amphenol connectors are Banned at my station. They are Not what they used
> to be...
> 100% of all Up the tower jumpers are DXE RG213 and DXE Crimp ons.
>
> Ron, WV4P
>
> On Sat, Nov 25, 2023, 1:57 PM Jack Brindle via TowerTalk <
> towertalk at contesting.com> wrote:
>
>> The problem isn’t so much resistive buildup, but rather arcing and
>> flash-over. The reason the right angle connectors are such a problem is
>> that the center conductor can either  move or is bot well-placed to start
>> with. At 100 watts it is OK, but at 500 it may arc. This tends to be
>> destructive to the amplifier. Good amplifiers will protect themselves,
>> leaving you to wonder why it just shut down.
>>
>> Similarly, coax in semi-tropical environments exposed to the sun will see
>> migration of the center conductor over time, changing the coax
>> characteristics (which a TDR should see), but ending in contact between the
>> center conductor and shield, which again will be problematic to the driving
>> amplifier. I have seen this take less than a year in South Florida.
>>
>> With hamfest-variety connectors and adapters there is not much care taken
>> in assuring the center conductors are both well-placed and secure in their
>> positions. Add to that the insulators used (phenolic or something other
>> that Teflon) will weather and age with time, breaking down to
>> allow center-conductor movement.
>>
>> All this is why we use really good connectors. Most of the suggestions are
>> for UHF family connectors, but there is junk even in type-N and BNC
>> connectors as companies push to lower costs to increase sales. These are
>> easy to find, unfortunately.
>>
>> Also note that for mismatched lines, which we all use, things get far
>> worse. The scenario here is an ATU driving coax to a wire antenna. The ATU
>> will make the transmitter and amplifier happy, providing a good impedance
>> to those devices. But beyond the ATU you will have very high losses in
>> everything from the ATU components, connectors and coax. A lot of power
>> will be dissipated in these things, with surprisingly little being radiated
>> from the antenna. N6BV did some excellent work in this area, which can be
>> found in his published articles and the ARRL Antenna Book, which Dean
>> edited during his time at ARRL. His presentations show how much the
>> components in the antenna system are stressed.
>>
>> 73,
>> Jack, W6FB
>>
>>
>>> On Nov 25, 2023, at 12:51 PM, Scott Townley<scott at nx7u.net>  wrote:
>>>
>>> If that's the case, then wouldn't an insertion loss test reveal such an
>> issue?
>>> For example, I was deconstructing the power handling of a Charter
>> Engineering B5-series coaxial relay (N-female connectors).  If I map "max
>> insertion loss at x GHz" to the average power rating on their published
>> chart, at every point the dissipated power comes out to 11 watts.  That
>> tells me that "good" N-connectors can dissipate 5 watts, and for that
>> mechanical form factor I should be able to derive a power limit based on
>> measured insertion loss.
>>> Or are the physics of power handling in "not-so-good" connectors
>> different from what would be revealed by an insertion loss measurement?
>> Assuming of course that we are not in the realm of high voltage breakdown
>> of the dielectric (or maybe that's exactly the difference?).
>>>
>>>
>>> On 24/11/2023 17:29, Jack Brindle wrote:
>>>> One of the big problems with off-brand connectors and adapters is their
>> power handling capabilities. This is especially noted in the right-angle
>> adapter. We see a lot of arcing when higher power is applied.
>>>> Higher power in this case is anything above just a few hundred watts.
>> Whenever I see an amplifier log report with a lot of high reflected power
>> or PA Dissipation faults, I immediately start looking at the
>>>> connectors and feed line for problems. These are usually found in one
>> of several places - right angle adapters, and baluns that were meant for
>> low power, but have been over-stressed with high
>>>> power.
>>>>
>>>> My advice? If the right-angle adapters aren’t Amphenol, throw them out.
>>>>
>>>> 73,
>>>> Jack, W6FB
>>>>
>>>>
>>>>> On Nov 24, 2023, at 12:59 PM, Scott Townley<scott at nx7u.net>  wrote:
>>>>>
>>>>> Say I have a pile of coax adapters on my bench (we all do, right?).
>>>>>
>>>>> How do I separate the wheat from the chaff?  I have a fully equipped
>> bench...I would think the go-to would be a TDR measurement.  Any
>> better/additional suggestions?
>>>>> TIA,
>>>>>
>>>>> --
>>>>> Scott NX7U
>>>>>
>>>>> _______________________________________________
>>>>>
>>>>>
>>>>>
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>>>
>>> --
>>> Scott NX7U
>>>
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