Kits and Modules
Not a kit as such, but Geoff Theasby G8BMI repurposes a camera tripod.
Not a kit as such, but Geoff Theasby G8BMI repurposes a camera tripod.
Ireceived a camera tripod for my birthday, and it was while I was contemplating this that I realised that with a 2m whip antenna or a small beam attached, it could be useful for going portable, for example in contests such as the PW Low Power and the RSGB Backpackers as well as in SOTA events. Collapsible poles are good, but need guying, and the PW Tenna-Tourer is very good if you can drive to the transmitting point, but I was looking for something that would pack down very small, and would be quick and easy to erect. I then thought that the legs of the tripod, if connected up as the ground plane of a vertical, would complete the ensemble. The reason being that a horizontal ground plane exhibits a low impedance at the base of the antenna, of about 25Ω. Drooping the radials at 45° increases this impedance to about 50Ω.
Idly thinking about tripods in the all-toobrief period between waking and setting off to work, I conceived the idea of using a magnetic mount antenna, such as I have for my car. If a small sheet of steel had a central hole, tapped to match the screw on the tripod, then radials could be attached to this plate in the manner of a ground plane, and the mag-mount would sit on this. Of course, if the steel sheet were tapped in the middle, then protrusion of the tripod screw would interfere with the mag-mount. Therefore, if a matching nut were to be brazed or glued to the steel sheet, the mag-mount would be free to attach. Furthermore, to stop the waving of the whip antenna stressing the brazed or glued joint, if a ring, approximating to the dimensions of the top plate of the tripod, could be fitted, or perhaps four more nuts were similarly attached, the strain on the mounting nut would be reduced, Fig. 1.
A mag-mount antenna would also be pre-matched to a horizontal ground plane, as when attached to a car, so the tripod legs need not be used as radials, nor the radials drooped. What is more, the concept of attaching a nut to the steel sheet could be carried further. A small beam, HB9CV or a little larger, could be similarly prepared, with three nuts in a line, placed at the centre of gravity of the beam. The pan and tilt head of the tripod would allow for rotation and, if need be, polarisation change. (Although the metal tripod legs would interfere with the radiation pattern of the antenna when set for vertical polarisation) Rather than solder the radials direct to the steel sheet, if small sockets were to be attached to the corners, or every 90° around a circular plate, then the radials would be easily removable for transport. The result is more or less the finished article, Fig. 2.
Construction
I cut a 150mm square sheet of steel 0.5mm thick, from a piece I found in a scrap yard. Wrappers from cookers, fridges or office equipment, or even car body panels can be used for this, the only requirement being that it is flat, stiff enough to resist the bending effect of the antenna, yet thin enough to cut easily, and magnetic. I obtained a piece for nothing from the scrapyard, after I explained my requirements. I was given a hard hat and told to look “over there”. Please be aware that, although aluminium alloy sheet, when cut, is reasonably easy to deal with, steel sheet when cut with tinsnips or a nibbler, is razor sharp, and will draw blood easily. Run down the edges with a file or grinding wheel, and keep the Elastoplast handy!
Five nuts were Araldited to the centre of the plate in a diamond formation. To prepare the plate, remove all paint and rust from the steel, and degrease the nuts, especially if they are new. I washed the nuts in white spirit. They need to be ¼in BSW thread, which is fairly standard for tripods. If you do not remove the paint, you are dependent on the strength of the bond between the paint and the steel. With the paint removed, the bond is much better, Fig. 3.
The precise spacing depends on the size of your tripod’s camera mounting surface. Many tripods have a quick release at the tip, Fig. 4.
Then, solder four 4mm wander sockets to the corners. Use a large bit in the soldering iron, because there is quite a lot of metal to heat up, but clean steel solders well.
To make the radials (Do this while the glue on the nuts is setting) cut four pieces of stiff copper wire. I used 16gauge harddrawn enamelled copper wire offcuts from a 40m antenna project. Cut them to 550mm long, assuming they are for the 2m band. This is to allow for trimming to frequency later. Radials should be one quarter wavelength at the frequency in use. Solder each one to a 4mm wander plug. That is more or less it!
In Use
The square plate screws to the tripod, the whip antenna (I used a 5/8-wave whip) mag-mount sticks to that and the radials plug in to the corners. If you are using a beam, attach that to the tripod instead of the steel sheet, and you don’t need radials. Extend the tripod legs to their fullest extent, wind up the centre column if there is one, and bingo! A vertical or small beam ready for action. What is more, the antenna is more than a half wave above ground on 2m or above, which is recommended. If you use a camping chair, or one which is quite low, you will not distort the radiation pattern of the antenna too much, as you will be ‘below’ the ground plane and the antenna will not ‘see’ you. My 5/8-wave whip used in this configuration returned an SWR of 1.2:1 when mounted on the car roof. (Which is a perfect ground plane, being a continuous steel sheet) On the tripod, which extends to a height of 5ft, it gave an SWR of 2:1.
Trimming the radials down by a couple of centimetres reduced this to 1.7:1, but trimming more off this length had no noticeable effect. During construction I found that the radials I used were too short and I had to lengthen them. This may be seen in the photographs. I tried a smaller tripod, which extends to 3ft high. This also resulted in an SWR of 2:1. I tried connecting the legs electrically as drooping radials, using short lengths of flexible wire and crocodile clips and no radials. This made the SWR 4:1, and with the radials reconnected 2:1. This suggests that there is no advantage to connecting up the legs as radials. The HB9CV gave an SWR of 2.2:1 when mounted on the large tripod, which is acceptable, and suggests that the whips are reasonably matched. Tripods of the type described can be found from about £20 in photography shops, or the CPC Catalogue:
Going equipped for remote portable operations requires some thought, and taking directional antennas more so. This option provides an opportunity to take lightweight, directional or omnidirectional antennas quite some way off the beaten track for very little effort.