Saturday, August 03, 2013

I guess it works - but not the best design!

I had a little more trouble when I went to the Jeep on Friday for my lunchtime QRP session.  I just wasn't hearing anything!  I know the bands can get that way, but the past few days had been relatively decent.  Like any other Ham Radio operator, my mind immediately went to "There's something wrong with the radio!"

As I disconnected the coax from the KX3, I noticed that the radio's BNC post turned a little bit as I removed the coax fitting.  I knew that by itself wasn't the problem, but I didn't like the fact that it was a tad loose.  I broke out the tools and did some field surgery on my KX3 right in the back of the Jeep!  I felt like Hawkeye Pierce, BJ Hunnicut or Trapper John from MASH.  I opened the KX3, removed the auto tuner board, and with a pair of needle nose pliers, I snugged up the nut that holds the BNC post against the inside housing of the radio. Then I put everything back together and turned the radio back on ..... still nothing.

I just recently replaced the PL259, so I twiddled that around, thinking that perhaps I didn't do as good an installation as I had thought.  Nope, no difference - that wasn't the problem either.  So even though I had taken the magmount assembly apart the previous night, I decided to look again there - a mistake on my part there? Once again, all was good there.  But I did notice that as I twirled the cable around, I was getting signals to appear, disappear and then re-appear. A break in the coax cable!

So I brought the magmount back in the house once again.  I decided that I was going to take it apart, cut back about 8 inches of coax and then put it all back together.  In the process of taking everything apart again, I just happened to pull on the center conductor of the cable and a six inch piece came out with my fingers.  It had indeed broken, back in the main part of the cable, and my plan to cut it back by a foot and reassemble turned out to be a good plan.

But I have to tell you, after working on this, I'm not really impressed with the design of this magmount. I hope you can figure out what I'm trying to describe.  The coax goes into a plastic housing. This plastic housing is roughly the diameter of a quarter and is maybe a 1/4 inch thick.  Each half has a hole.  The shield of the coax (which has been pigtailed) goes through the bottom hole, while the center conductor (which is kind of flimsy) goes through the upper hole. There are channels or depressions on each side for the wire to sit.

The is the order in which it all goes back together - from the bottom working towards the top.

1) Through bolt
2) Plastic insulator, so that the through bolt does not touch the magmount body (ground).
3) Magmount body
4) Coax housing with the braid sandwiched down, between the coax housing and the magmount body.
5) Metal washer.  The center conductor lays in a channel and is pointing up, to be sandwiched between the coax housing and the washer.
5) Threaded hex sleeve for the radiator.

Everything is held in place by the holding pressure created by screwing the through bolt into the threaded hex sleeve!  The center conductor is NOT soldered or otherwise electrically connected to the metal washer underneath the threaded hex sleeve, as I had previously thought.  And as I noted before, the braid is simply sandwiched between the plastic coax sleeve and the magmount body.  I made sure all metal surfaces were clean and shiny and used plenty of Deoxit to help ensure good electrical contact (there really are no true connections!) as best possible.

It seems to me that it would have been better for the coax to be soldered or otherwise connected to the metal sleeve and the magmount body other than just using physical contact and screw pressure to hold everything together.  But then I suppose that would have increased the cost of the product significantly.

In the near future,I think I am going to upgrade the quality of the coax, too.

72 de Larry W2LJ
QRP - When you care to send the very least!

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