70cms and 2m Dual Band Aerial
 My
prototype antenna was built using a single approx. 2.3 meter length of
4mm stainless steel wire. This resulted in a totally self supporting antenna.
I much prefer to see the least possible number of joins in an antenna
so as the thing ages, you do not get crackles and other noise problems
from your aerial.
In a similar manner the coax cable was soldered direct to the antenna
at the feed point which is some 86mm in from the antenna itself, as shown
to the right (the centre conductor goes to the top). I added some support
by way of a 75mm length of pvc tube 20mm diameter fitted between the wires
on the tower side of the feed point.
The coax runs through this tube. (the 20mm spacing is not critical but
should not be greater than 25mm for 70cm operation).
The mounting to the tower can be done in lots of ways.....my antenna
is attached to a piping mast and I used a old TV type boom to mast clamp
and drilled 4mm holes straight through it at one end in a manner so as
when the 2.3 meter wire was run through the holes to it's half way point
and bent at right angles the horizontal part of the antenna is in the
position as shown above.
A couple of bolts at the other end (close to the sides of the wire...
A total of 4 small bolts) resulted in a nice solid structure. Bend the
wire as shown above and trim to length as shown. The actual feed point
is found by mounting the antenna on some metal support and using a SWR
meter as close to the feed point as you can get. The length of the quarter
wave stub (510mm measurement) is not real critical so the 85mm dimension
can be changed a little for minimum SWR. (about 1.2 to 1 is easily found).
Do your tuning on 70cm as it is more critical on this band.
When the correct spot is found attach your feed line and seal it up.
I soldered the coax direct to the antenna. (you can't do this if you build
the thing out of aluminium!!) I tinned the 4mm wire at the feed point
with aid of some comweld 965 flux before soldering the cable on. I used
coax-seal to seal the cable. (if you must use silicon make sure that it
is a neutral cure type.) Oh and make sure you have a reasonable piece
of coax cable for the length you are going to use. (like if you are going
to use, say, 20 meters of cable then you
need to find a low loss cable...... RG 213 is not the best to use and
RG 58 is a good dummy load at 70cm) The most expensive part of this project
will be the coax cable.
73 de Mike VK4XT
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