The Diamond RHM8B is a nice little antenna for backpacking. The antenna covers from 40m to 6m continuously, although on 40m it is not too efficient being only 1.7 metres long. On 20m and above it has made some pretty reasonable contacts. The big weakness of the antenna is the way it mounts onto the FT817 via a BNC connector. There can be a lot of force on the connector and it could break very easily.
This little project gets the stress off the BNC connector and mounts the antenna on a small tripod. This setup would be useful for situations where there is not much room on a summit and you don’t want to put up something long and high that may attract negative attention.
The tripod was purchased from Ebay for $14 delivered, it is very lightweight at about 400 grams. The tripod has a socket for a 16mm tube that supports the camera pan and tilt mount. The tube with pan and tilt mount were removed and replaced with a 20cm length of aluminium tube that is used for supporting the antenna.
An adaptor was made that clamps around the lower 1cm of the antenna. This is above the BNC connector and the clamped part is electrically the end of the antenna. The BNC connector for the antenna is inside the adaptor, but unconnected. The adaptor is made from a short length of 20mm electrical tubing (Bunnings). A vertical split was hacksawed into the tube and a finger adjust worm drive clamp (Bunnings) holds this end of the adaptor over the antenna end. A BNC connector and ground screw with wingnut are about half way along the adaptor tube. The other end of the adaptor tube fits over the tripod tube.
The photo is pretty much self explanatory. On the opposite side to the BNC connector is a small hole around 5mm diameter. This hole provides access to the BNC solder centre spigot. Along the inside of the tube from the top, is a length of adhesive copper foil (Bunnings, sold under Slugga product name). The foil provides the connection between the end of the antenna and the BNC. A soldering iron through the hole solders the foil to the BNC centre spigot.
The BNC ground lug inside the tube connects to a solder lug on the ground screw. The 4mm ground screw has a nut on the outside and a wingnut to clamp the antenna ground radials. A 4mm nylock nut upside down stops the wingnut from coming off in transit and getting lost.
Here is a photo back a bit showing a bit more of the tripod and antenna. The tripod is pretty light weight and light duty (what did I expect for $14 delivered!), but it will probably be OK providing it is not too windy.
Here is the antenna on the tripod on test on Mt. Beenak. It went very well on 6m, 10m and 20m. I didn’t bother trying it on 40m as I know it performs poorly, although presenting a nice match to the radio. On 6m, the radials had to be raised up a little for a perfect match. Also a loop of about 10 turns in the coax about 10cm in diameter helped decouple the coax outer. Several contacts were made on 6m and 10m with good reports.
The photo on the left shows it all disassembled. The total length is of the tripod collapsed is less than then length of the antenna loading coil.
There was only a light breeze on Mt. Beenak when testing, and there was no sign of instability, although it would not be good in high winds if the legs weren’t anchored down.