[Elecraft] SO-239 Concerns -- Another Perspective

[Brian Alsop]

Joel,

How do you respond to this blurb from a "DX Engineering" guru?

I have been bitten by the non-captive center pin migration problem 
(almost impossible to find) and won't use N connectors again for any 
non-UHF high power application.  This migration if enough creates an 
open circuit.  It is hard for me to tell the difference in captive pin 
and non captive pin N connectors.  Also I prefer the larger center 
conductor surface area of the UHF center pin connectors.  They are 
proven to have very little heat generation beyond UHF.

73 de Brian/K3KO
----------------------------
For example, DX Engineering does not offer the RR-8 series switches with 
N connectors. Using UHF type connectors, this system of remote relays 
offers superior port-to-port isolation and power handling from 1.8 
through 54 Mhz at low loss and low SWR. Only due to superior engineering 
is this series of Remote Switches usable at 144 Mhz with slightly 
elevated SWR and reasonable loss of only 0.25 dB. However, these units 
are not intended and cannot be used on UHF, so N connectors are 
intentionally not available.

There is no question that the constant impedance characteristic of the N 
connector make it a superior choice for low loss VHF and UHF applications.

However, it is a fact that on HF frequencies, very high power at high 
duty cycle modes will heat the small diameter center pin of the N 
connector. Heating can cause the center pin to migrate, ruining the 
impedance characteristics of the connector and, at continuous high 
power, potentially cause a short circuit. If this problem were to happen 
during testing or operations, one might mistake high SWR or high loss as 
a defective of the switch, rather than a problem with the connector. 
This cannot happen with PL-259 and SO-239 connectors that are properly 
installed. At low SWR they can handle several kilowatts of HF RF.

This is not to say that you cannot use N connectors on HF. Rather, the 
question becomes "What are the benefits derived versus the cost of 
changing to N connectors for HF, where constant impedance is 
unnecessary?". The answer

This is not a concern if you already have N connectors, though. You may 
use adapters or short coaxial jumper assemblies with differing 
connectors at each end, as offered by DX Engineering, with custom 
lengths available by telephone. These would cause negligible losses on 
HF, and would allow you to experience the superior port-to-port 
isolation of a DX Engineering RR-8 series Remote Antenna Switch immediately.

Let us know if you need any additional information. We look forward to 
hearing from you.

Thank you and 73,

Rod Ehrhart - WN8R
DX Engineering Customer Support

----------------------------------------------------------

Joel R. Hallas wrote:
> I agree with most of what has been said in support of UHF connectors for HF
> and low VHF -- with two reservations.
> 
> 1. Many SO-239s provided as part of radio equipment are not up to the
> quality of Amphenol units. In particular, the spring fingers of the socket
> for the male pin loosen over time to the point that the connection becomes
> less than solid. This is particularly a problem if the connector is used
> repeatedly. I usually replace with Amphenol units, if I can get to them.
> Another great option is the Oak Hills BNC kit, designed to fit the hole
> pattern of the SO-239.
> 
> 2. The other potential problem with UHF connectors, in my opinion, is the
> integrity of the shield connectivity through the connected pair. If the
> backshell loosens, there is no longer a solid connection either at the
> toothed parallel portion, nor through the backshell. This can be an issue in
> mobile or other high vibration environments and has caught me many times.
> Note that a type N, C or BNC pair makes it's shield connection through the
> inner spring tension sleeve, maintaining contact almost until the pair
> separates. The backshell is just used for mechanical, not electrical,
> purposes.
> 
> FWIW
> 
> Regards, Joel
> Joel R. Hallas, W1ZR
> 
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