After having some luck with simply putting scrap metal into the air as a vertical antenna at 14MHz, we decided to try and do things properly. A trip to the local hardware store has proven quite useful for sourcing antenna elements. 3 metre lengths of 10mm aluminium tubing came out to only AU$8 or so. Of course, it aluminium tubing can be sourced more cheaply (especially as scrap), but this was more convenient for simple experiments.
It seemed like the biggest drawback to our previous verticals was the height above ground. Our copper gas pipe experiment had the base of the antenna only 3 metres above the ground, with the radials lazily laying on nearby objects.
This time, we decided to utilise our EMDRC 9 metre collapsable mast. We had decided to use PVC water piping as the mount system. Unfortunately we never bothered to collapse the mast and measure the outer diameter of the upper section, and ended up buying pipe which was too wide, and possibly too thin-walled. With a bit of persitance and trying again to get the holes to match up with the screws (how do you properly measure straight lines on a cylinder?), we had about 5 metres of aluminium in three telescoping segments on PVC. For the radial system we used self tapping screws to attach a 5cm wide alu strap around the PVC below the vertical element, with one larger hole for attaching an SO-239 socket. The centre conductor of the SO-239 is attached to a short jump lead with a terminal plug around the mount screw at the base of the vertial element.
20m vertical test on ground
Detail of vertical element and radial mount
Mounted vertical ready to raise. Dipole mount for 80/40 below.
Raising 9metre mast with 20m vertical
It began to get dark by the time everything was constructed and roughly tuned. Turns out we had mixed up some of the radials and guy wires, which wrapped around each other at the top. Not expecting good results the first on the first try, I was surprised to find it gave little to no reactive loading at resonance, which was around 13.9MHz. The resistive load of the antenna was much higher than expected, at nearly 200 Ohms. I had read the angle of radials to vertical element can change the Zo of the system, but had no time to try at the time.
Coming back to it, and getting the radials as close to 45 degrees from vertical as possible gave good results. We were able to get the Zo down to a good-enough match of 60 Ohms across the lower section of the 20 metre amateur band. the 2:1 bandwidth was across any usable frequency for amateur use.
The 1964 ARRL Antenna Handbook gives ideas for multi-band verticals containing multiple resonant vertical elements with 4 resonant radials for each band. This PVC mounted system may lend itself well to this setup, just in time for the growing 10 metre openings.
de VK3GNU. 73
Tags: antennas, musings, tech by Hannah
No Comments »
