Tunable, Water-Bottle Loaded compact dipole antenna for 40m
I’ve been in on sabbatical in Israel at the Weizmann Institute of Science since August. I’ve been lightly operating as 4Z1ML my new Israeli call sign. We live in an apartment on campus, so space for antenna is a problem. In the beginning I was using my buddy stick antenna on 15m and 20m, and sometimes the alexloop on 40m. Since propagation on 20m has not been great at night, I decided to put a fixed 40m dipole in our porch.
The porch is about 8m long, so a full dipole is not possible. One evening, I was bored enough and took some copper wire, two water bottles. I coiled the wire around the bottles to make a loading coil. Connected the wires to the base of the buddy stick plate and vuala! got a nice dipole!
To my surprise, the first time I measured it, I saw that I the resonance was at ~7.400 MHz. Only 330KHz above my desired JT65/9 frequency. Instead of playing with wire lengths, I found a nice trick to get it down to the right frequency. I added some water to the water bottles, and found that the additional dielectric constant within the coil was enough to get it right down to the right frequency!
Here’s a miniVNA trace of the dipole on 7.050MHz
and here’s another one after poring out a bit of water to move it to 7.076MHz
Here are spots received over 2 hours:
and here are spots other received:
