Another Bad N-connector

After a lengthy bout of frigid weather, the sun came out and the temperature soared to 3° C. The snow was melting into puddles and it felt tropical outside. I was briefly tempted to wear a t-shirt. Of course my thoughts soon turned to tower work. Several issues have been languishing over the winter and this was an opportunity to deal with some of them.

One of those was the malfunctioning TH6. When I last mentioned it, the balun enclosure was full of water. Removing the water didn't help. From the ground the coax measured as an open circuit. My intent was to pull the coax from the balun and test the antenna, balun and the long run of Heliax. I pulled the coax jumper from the 2×8 switch to attach a 50 Ω load. That way the Heliax could be tested at the same time as the antenna with an analyzer. Preparations made and analyzer in hand (kept warm inside my parka), I stomped through the deep snow out to the tower and up I went.

To my surprise the antenna and balun passed the test. However, when the analyzer was connected to the coax I again measured an open circuit. I really didn't expect that outcome due the trouble I've had in the past with either the antenna and balun. The antenna in particular is an antique at well over 50 years old!

I reconnected and sealed the coax connection to the balun and descended. There is an RG213 jumper from the balun to the Heliax but I decided not to fuss with it just yet. Until now I'd only done an SWR test and I wanted to do a TDR (time domain reflectometry) test from the ground of the full system.

I visually inspected the jumper between the switch and the Heliax and manipulated them to check for looseness. So far so good. I connected the analyzer to the coax and set the analyzer to plot R and X from 1 to 51 MHz.

Notice the peak R and X on the right of the plot. The approximate frequency is 47 MHz. For the 0.66 velocity factor of RG213,  ½λ is approximately 2.1 meters. That raised my suspicions. I disconnected the RG213 jumper and took it indoors. I measured its length: 2.1 meters. What a coincidence! No, not a coincidence.

Modern analyzers have a TDR feature, but I don't have that on my relatively old RigExpert AA54. However it is pretty easy to achieve the same by plotting R and X and doing a little calculator math. I could have zoomed in (narrow frequency plot) for better accuracy but that was hardly necessary in this instance. 

At right is a picture looking into the N-connector on the end of the jumper that connects to the female N-connector on the Heliax transmission line. The problem is obvious: the male pin has pulled inward. It does not make reliable contact with the female receptacle on the Heliax connector. It had been intermittent for some time before failing completely a few months ago.

This is a common fault with N-connectors on coax similar to RG213. I didn't expect it to happen in a short jumper that wasn't subject to stress or weight. This is a connector where the pin is soldered to the coax centre conductor. I prefer N-connectors with a captivated centre pin but those are not available for coax with a stranded centre conductor.

I tried to pull and wiggle the coax while holding the connector and did not notice any appreciable movement of the centre pin. I have yet to open it up for a closer inspection. My priority was to bring the TH6 back online.

I happened to have a short RG213 jumper already prepared and tested. It has UHF male connectors on both ends. I keep an ample stock of N-male to UHF-female adaptors --whenever I see one at a flea market it comes home with me. 

Since I have to deal with N-connectors on most of my Heliax runs the N-to-UHF adaptors come in handy. The pins are mechanically stable so the pins cannot creep. That is critical for N connectors. They have become my preferred choice for transitioning between N and UHF terminations. They let me exclusively use the more positionally stable UHF connectors on RG213.

The new/old jumper and the adaptor were attached to the Heliax and the antenna system swept with the analyzer. The antenna was back in service. I cut off the old bits of tape and sealant and then weatherproofed the connection. It took more material that most coax connections due to the added length of the adaptor. 

Have I told you (again) that I hate N-connectors? Hate and N-connectors show up several times in the blog, as a search will demonstrate. Had I less restraint the subject would appear 10× as often in the blog! 

My thoughts on the matter are well summarized in the linked article. It accrues well above the average number of hits for articles in this blog. N-connectors should be avoided whenever there is an alternative, and there almost always is, especially for HF systems.

You might think that I am relieved to have the TH6 back in service since the ARRL DX CW contest is only days away. You'd be wrong. Yes, I'm relieved but not for that reason since I'll be joining a multi-op team elsewhere for the contest. However it's one less worry for forthcoming contests.

Although I plan to retire the TH6 this year the Heliax run will be needed for whatever antenna, or one of the antennas, slated to replace it. The balun itself requires service to prevent water ingress whether or not it is needed for the replacement. With the immediate problem dealt with that job can wait for warmer weather. Although tower work in winter isn't always difficult or uncomfortable I prefer to avoid it if I can.