2Meter 5/8-Wavlength Verticl Whip
Mag-mount Antenna
Having won the Fox-hunt RDF trophy last year, together with Peter, a fellow member of the Trowbridge and District Amateur Radio Club, it fell to both of us becoming the Fox (hidden radio station) for this year - July 2020. Because of Covid-19 we would of corse, need to observe social distancing. This is easily done by arriving at the location in separate cars and siting in folding chairs at the appropriate distance.
As neither of us had a suitable omnidirectional antenna to use for the Fox station, making one I thought, would be the best corse of action. Why spend hard cash for something you can make yourself? However, was it possible to have it ready in time? Absolutely!
The basis for construction of this antenna was taken from an article in the 1987 ARRL Handbook, page 33-28 of Chapter 33. Images taken from the article are shown here, however, only the element lengths and coil dimensions were used.
Changes to some of the materials and methods were made to take advantage of the materials I had on hand. Also my requirement for the antenna to be placed on the car with a magnetic mount was a guiding factor in my chosen method of construction.
In the ARRL article, the coil former was a piece of perspex, however, I had a 9" length of 1" diameter SRBF Tufnol, so I used that. After cutting 3.5" off its length, I turned down 3" of it to 3/4" diameter ready for the coil as dimensioned in the ARRL article.
Finished coil former for the antenna matching coil.
Hole drilled in the top of the former to take the thicker section of brass rod. A solder tab has been soldered in place ready for the top end of the coil.
Here we have a 4" diameter disk of PCB with two neodymium magnets salvaged from computer hard drives glued in place with epoxy resin.
At the base of the coil former two holes were drilled to make anchor points for the bottom end of the 10.5 turn coil and the taping point wire. A hole has been drilled at the base of the former and PCB disk.
The centre of the disk has also been tined in preparation for soldering the bottom of the coil and the coax screen.
The coil and former are fixed in place onto the PCB disk with epoxy resin and a screw into a treaded hole at the base of the former. Top and bottom ends of the coil are soldered to there respective places at the base and thicker brass section of the whip antenna.
Centre of the coax is soldered to the anchor point for the coil tap wire which is soldered 4 turns from the top of the coil.
After an initial tune up with a VNA, the cable was better secured with a liberal amount of hot glue.
To tune up the antenna, it was placed in the centre of the car roof and the VNA connected to it.
The VNA was set to 145.375MHz that being the clubs usual frequency for the fox. With the final top section of the brass whip in place readings continually taken as small snips of rod were cut of.
As can be seen here in the picture, a good match was made.
