| When winter comes,
it's time for antenna works. Some people believe winter is for skiing and skating. No way,
winter is antenna time! I made my elements out of 6 meter long aluminium tubing, 50 mm in
diameter and a 12 meter spiderbeam fiberglass mast at the top. The bottom element has a 52
mm inner diameter, so it fits on top of the aluminium tube. I wound 4 turns of
electrical tape round the top of the alu tubes in two places one meter apart, to get exact
fit. The fiberglass element overlaps the aluminium tube with one meter, giving a total
height of 17 meters. The antenna wire is a 2,5 sq mm electrical wire, taped at every
section of the fiberglass pole. This also serves as stops for the sections so that they
don't contract.
The wire is 14,5 meters long, wound 14 turns round the lowest fiberglass element,
making up a coil with 55 mm diameter and about 100 mm length. The end of the wire was
connected to the alu tube with a tube clamp. The antenna is perfectly resonant at 3.8 MHz,
if you want it to resonate lower, you must add 4 extra turns to the coil for every 100 KHz
down.
The feedline was connected at the bottom of the alu tube with a tube clamp. 4 elevated
radials was added to each element, made of 18,5 m electrical wire.
I mounted the alu tube on a 100 X 34 mm impregnated wooden pole 3 meters long. The bottom
of the alu tube ends a good meter up on the pole to get height for the elevated radials.
The pole was screwed into the roof of my garage (and other buildings for the other 2
elements) with a 140 mm galvanized French screw (we call them French in Sweden). I can
easily erect the element alone round the mounting point. I am going to add 3 guy lines to
each element to keep them up in strong winds.
Well that's about all, the system is working as expected, with a front to back attenuation
of about 15-20 dB, and a forward gain of 3 dB compared to a single vertical. Many times
you experience much better receiving gain, but then again: Phasing is believing!
73 and cuagn
Johan
SM3HXD |
