Kaasplank(je): a selection of cheeses, usually served on a cutting board.
Based on the design for a 4m antenna by John, PA0ETE, I decided to try for myself. Initially I wanted to stay as close as possible to the original design, but eventually selected different materials.
- one dirt cheap plastic cutting board ( €0,69 at the budget household store )
- two 1m lengths of 8mm round aluminium tube ( € 2,39 each at the DIY store )
- one 1m length of 15mm square aluminium tube ( € 4,00 or so )
- two m3*16mm screws with nuts ( several cents )
- two 6mm bolts with wingnuts ( several cents )
- two faston connectors ( several cents )
- length of Aircell7 with a N-female connector ( € 10,00 or so )
- tie wraps (several cents )
I started out by drawing a template on paper. Drew the outline of the cutting board, and two lines at a 120 degree angle from each other (and the center line). Then one centimeter from those lines is drew a few 5mm holes… 2 for each tie wrap one set about 15mm from the center line, the other set at about 1cm from the board edge. Then taped the template to the cutting board and drill the holes for the tie wraps. Turns out that the holes are a bit far from each other… next time I’d prefer to make more of a slit and a bit closer to the radials.
Then I drilled a 3mm hole at about 7mm from the end in of each the round tubes. Through these holes the 3mm screws will be used to connect the coax cable. I attached some simple faston connectors to the coax and then wrapped everything in self-fusing rubber tape. In the picture below you see the details of the connections.
Below you se the entire cutting board. The coil of coax is needed to cancel out the RF signal on the outer shield, I made it 6 turns. The advantage of the cutting board is already obvious: the coil can be positioned very close to the connection. Again a few holes through the cutting board for a few tie wraps to keep the coil in place.
The cutting board is mounted on the square aluminum rod using m6 bolts and wingnuts for easy disassembly. I clamped the aluminium rod to the fencing of my balcony. Below a picture of the whole thing.
The Aircell7 and N connector as well as the 15mm square tube were leftovers from a previous attent at an antenna that failed. Non the less, I prefer N connectors for outdoor use over PL. Yes that doubles the price of the antenna, so be it. RG58 will probably do just as good. Anyway, this is what I had, so why bother with something less…
The only purpose of the square tube is to be able to mount the antenna at some distance from the mast (or balcony).
Without any tuning the antenna’s SWR is 1:1 for the entire 4m band. The first QSO (thanks Thijs, PA5TYS) yielded good results given the positioning of the antenna: clamped to the balcony and several high apartment buildings blocking line of sight, including my own.
About the dipole
The antenna is a normal dipole with a twist. The 120º angle helps reduce the impedance of the dipole from 75Ω to 50Ω, Exactly what we want. Normally a dipole requires a symmetric feed, the coil will cancel out any current on the coax outer mantle. Five or six turns will do the job.