25 June 2007

Restoring a 15 years old CB antenna

I still own my first (and only) mobile antenna: a CB antenna by CTE named "Dallas". It has been outside for many years and used seldomly until I dedicated it to my 10m beacon (pictures).

One day I measured it with the MFJ antenna analyzer and discovered that it showed an horrible SWR, with a best 2.5:1 at 27.5 MHz. On 28.322 it was above 3:1 and unusable anywhere else!

So I opened it up. It's very easy: unscrew the whip part and slide off the plastic cover of the coil. Surprise surprise: it used a tapped coil of 15+3 turns (also called autotransformer). There is no visual damage to the coil and continuity tests showed zero ohms: no obvious damage to be fixed.

Since I am also looking for a 6m mobile antenna, I thought I might reuse the support for the base loading coil. Let's first try a quick fix for 10/11m: interrupt the tapped coil so that the antenna becomes a base loaded vertical:

A second run on the antenna analyzer made me smile: the resonance now moved to 28.8 MHz with 1.1:1 SWR after the 3m of RG58 coax. The antenna on the balcony fence is usable on the whole 10m band! :-)

What's next: measuring it on the car. Bandwidth might broaden a bit.