Backplanes and
Mounting Hardware
The backplane is a metal sheet that lies behind the BiQuad and controls
the shape of the antenna emissions. It is important to size the
backplane accurately, that it be flat and the side walls have tight 90
dg bends. The BiQuad itself should lie about 15 mm above the
backplane.
I'll describe a couple of methods I've used to hang the BiQuad itself
15mm in front of the backplane. Depending on your choice of cable
(thick LMR400 vs thin LMR195) and your personal preferences, you may
like one more than the other. The details about cable choice and
mounting the BiQuad follow.
Note the small hole drilled in the upper rightmost pipe below.
See how ground wire is fit and soldered into the same hole in the upper
left piece, below. This makes a really good mount. Solder
the wire to the pipe, then add the backplane. Once that is done,
the antenna soldering isn't likely to melt the ground wire free from
the pipe and backplane.
The thick LMR400 cable will make
a
press fit in the pipe I use with the copper tube. The
slimmer LMR195 won't and may need some caulk or crimps in the pipe to
stabilize the wire in the mount. If you want to use a heavy
copper wire as an antenna mount,
drill a small hole in the copper pipe the size of your support
wire. Drill it just behind the backplane between the backplane
and the Tee, as shown above. Bend your support wire to an "L"
shape, stuff the
short leg of the wire thru the drilled hole and solder it to the mount
pipe. You can bend/solder/re-solder the support and BiQuad to
adjust the SWR. It's easier to solder to than the slim copper
tube. You can improvise any support as long as you get a good
ground and a stable position for the BiQuad.
Most situations need a Tee and post (above right). To mount
the copper pipe to the dish, use a piece of PVC pipe. Find some
PVC pipe that just fits over the feed arm of the dish, as shown
below. In the US, a 2" heavy wall PVC has worked well for me.
The
length of the pipe depends on the make of the dish, but typically they
are about 0.3 m long-this is the length of the long side of the
Construction Template.
In the photo below, I cut off the last 10-20 cm of the feed arm to make
up for the antenna
and backplane being too far from the dish. That allows me more
range
to tune the BiQuad to the parabolic dish focus point. It's important to
get the
antenna at the right height, distance and angle to the dish.
It's easy to set a drill press to the right angle if you have a
reference.
