Another 2M Mobile Antenna
Another 2M Mobile Antenna - April 1995
This antenna idea was shown to me the other day, it comes from a two part article in "Practical Wireless" o f September and October 1992. I f you need further details, I suggest you refer to those articles.
Construction o f the antenna is very simple as shown in the accompanying sketches. The antenna appears to be a five-eighth wave radiator with a ground plane consisting o f a quarter wave length o f rod formed into a ring just below the resonating coil. The dimensions shown should give a good match to 50 ohm coax; if anything is critical, it appears to be keeping the gap in the ground plane ring at about the lOmm.
The antenna can be constructed from brass rod or copper coated welding rod. The coil in the example I was shown was self supporting although depending on the material used, it may be prudent to mount the coil on a rigid former with its based solidly glued into the end of the PL259 plug.
Affixing the ground plane ring to the outside of the PL259 without damaging the plug or the coil former (if used) may require a bit of thought and quick soldering come brazing. Again, the example shown to me used small brass spacers soldered to the plug with the ring rod inserted into the spacer hole and also soldered. The example also used a shield over the coil to give it weather protection and help prevent water finding its way down through the PL259 into the feedline.
To tune either carefully snip off the top of the whip, stretch or close-up the coil or use a combination o f both to get the lowest VSWR.
The good feature of this antenna is that it has gain (approx 3db) and the ground ring in neatly aranged so that it doesn't present a health and safety hazard and is ideal for mobile use.