Paul Butzi |||

Improving NMO antenna magmount

I use a mag mount for the antenna on my car. I’ve run through a number of mounts, though, because I just route the coax through the rear hatch and let the hatch close over the coax - the weatherstripping has enough compliance that it doesn’t mash the coax. But the fit of the hatch into the hatch opening is close enough that it pinches the RG58 coax and puts strain on the coax, and also tugs on the coax as the hatch closes. Inevitably this causes either the coax to fail where it gets pinched, or the mount to fail from the pulling on the coax.

I’ve replaced the coax on mounts before, using RG8x or LMR240. But the sheath diameter is basically the same and the problem persists.

In a fit of pique I ordered some RG174 (which is about half the diameter of RG58), and replaced the RG58 on a failed magmount.

image of magmount

As you can see, no more problem with pinching the coax at the close fit hatch.

The downside of RG174 is that it’s fairly lossy at UHF frequencies. The trick here is that I carefully measured the length of coax needed and made it as short as possible. This is a contrast to the origial RG58, which was preposterously long (something like 18 feet).

At 18 feet the loss in RG58 is 1.6dB. If I made the RG174 coax the same length I’d get about 3.6dB of loss. But I only needed about 6 feet, and the loss on that shorter length is 1.2dB, which is better than what I was getting with the needlessly long RG58.

Sure, I could have improved performance by shortening the RG58 to the minimum length, but everything was working fine and I was just ignoring the coil of 12 feet of coax tucked into the corner. Once acceptable performance is obtained I tend to just let things go. It was only the annoyance of the repeated failure that provoked me into making the obvious improvement of replacing the coax with something thinner and only the threat of increased signal loss in the coax that provoked me into making the new coax as short as possible.

Up next Get Home Bag/EDC Bag I got a good start on an EDC backpack/bag. This has prompted some thinking about what belongs in an EDC bag, and how much such a bag can weigh. So Antenna Suspension Maintenance
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