This antenna is an experimental combination of a 1/4 wave
monopole and yagi style reflector.
The problems I was looking to solve are three fold...
I have tried half wave, jpole, loop, quad and other
designs without much success. The biggest problem is that it's almost
impossible to tune up an antenna with so much metal in it's near field.
Second to that is the matter that all that metal is highly refelective,
tending to send any signal originating from inside the balcony back into
the building.
For some time I've solved the third problem by using a 1/4
wave monopole on my balcony railing. The first version was the Copper
Ducky that has proven to be a very reliable performer. More recently I
changed over to an aluminium rod design I call the
Balcony Buddy
which gives equivalent
performance but is far easier to build.
The trusty monopole grounded to the railing seems to be
the only one that will settle in and work decently. As a friend told me:
"If it's in the way of your antenna, make it part of your antenna".
So highly directional beams are out
because they would cut me off from local chatter. Omnis produce too much
RF in the building. What to do?
Answer: a fixed position, somewhat
directional antenna that will reduce RF in the building to safer levels
without being so directional as to cut me off from local repeaters and
hams.
So I started searching for antenna
designs to fit my needs and finally ended up at a 2 element
Yagi beam. The advantage of this design is that by changing
the spacing between the driven element and the reflector we can also
shape the radiation pattern somewhat. As the spacing increases the beam
widens, closer spacing gives a narrower beam.
This is the radiation pattern for a 2
element Yagi antenna with a somewhat longer than usual element
spacing...
If we take the blue line as the balcony railing with the
building being behind the beam (i.e. below it on the left image) we see
a radiation pattern that, while not strongly directional will
significantly reduce the RF levels in the building behind it. If we take
the antenna's gain over a dipole into account a reduction of about 6db
should be possible.
However; I'm not going to use a 2
meter yagi on this balcony. There just isn't enough clear space to tune
it up and there's no way the landlord is going to let me hang it outside
the balcony railing.
Still, it does suggest a rather
interesting compromise: add a reflector to my existing 1/4 wave antenna.
So I got busy and modelled my balcony and antenna.
This is the pattern for my monopole
on the balcony, confirmed more or less by Field strength readings. It's
not truly omnidirectional because of the influences of the balcony and
building...
Adding a reflector at .15 wavelength gave me this...
The difference in patterns is surprising and this does
seem to suggest that adding a reflector will accomplish my goals. Doing
a little math this seems to be saying that I will reduce RF in the
building by between 6 and 10 db over the omnidirectional antenna,
depending on direction. As a bonus I will have about 6db of additional
gain in my good direction.
This was encouraging, so, I decided it was worth
a try.
Building the Reflector
Antennas and plumbing supplies seem to go hand in hand.
The reflector is built using some common plumbing parts, a piece of
aluminium tubing and a bunch of hose clamps.
The parts are on the left. Included are 1/2" PVC "T"
fittings, hose clamps, 1/2" PVC pipe, a length of 1/2" tempered
aluminium tubing and plastic weather caps.
The first step is to cut the
aluminium reflector. For 2 meters I chose 1.08 meters, which the
modelling software indicated would give me the best front to back ratio
or, in my case, the best reduction in RF entering the building.
The boom for the reflector is made of
PVC plumbing pipe. This I cut long, because I wasn't sure of the spacing
I'd end up with. After experimenting and trimming so there's not a big
chunk sticking out over the balcony, it ended up 45cm long.
The second piece of PVC tubing was
cut to 30cm to give me a way to clamp it to the balcony's vertical bar
using hose clamps.
The "T" fittings have to be modified.
You need a slip through fit for the PVC boom and for the aluminium
reflector tube so your reflector assembly will be easily adjustable.
There are two steps... 
-
First use a rat tail file and cut out the internal
ridges in the long section that prevent putting pipes too close. Don't
remove the ridges from the short section. Test your work by trying to
slide them over the PVC and Aluminium tubes to make sure you get all
of the ridges out. You are looking for a snug fit that can be moved by
hand.
-
Once you have a slip over fit,
take a hacksaw and put a cut along the long side of the fitting. This
will allow you to use hose clamps to tighten up against the tube
inside the T fitting locking it into place, as shown on the right.
Now it's time to assemble the reflector...
First use some PVC solvent cement to put the modified T
fittings on the ends of the plastic tubes, one on each. This stuff dries
in seconds and doesn't let go, so make sure you get them installed all
the way down the first time. My technique was to put the glue on the end
of the tube and inside the T fitting's short side and then quickly pound
the T fitting into place with my hand.
Now
insert the aluminium tube into the T fitting on the longer PVC piece and
slip it about half way in. Add a couple of small hose clamps, tightened
only snug as you will want to be able to move this for adjustment.
Next insert the boom section into the
T fitting on the other PVC tube and position it just in from the end.
Add a couple of small hose clamps, again tightened firmly but not
finally, so you can adjust it.
Place the plastic weather caps on the
top of the reflector and the open end of the boom and you're all done.
The finished product should look like the picture on the
left. The active element is the aluminium tube reflector, the rest is
just support structure. You should note that depending on the structure
of your balcony, you may have to modify the mounting technique used
here. The important thing is to be able to adjust it to find the best
SWR and RF reductions.
It is now time to install everything
and see what happens.
Installation and Tuning
The first step is to install the
Balcony
Buddy or a similar monopole onto your balcony,
if you don't have it there already. You should make up a second vertical
whip for your antenna. When you adjust it for the reflector it will have
to be shortened a fair bit, so keep the original in case you want to go
back to an omnidirectional antenna.
Now comes the reflector. Position the
upright white PVC tube against the side of the railing, with the boom at
the top, immediately under the antenna. Using hose clamps lock it into
place making sure the back end of the boom piece can slide in and out
for adjustment.
The
result should look like the picture on the right.
The first step in adjustment is to center the reflector on
the monopole. Measure and mark the center of the reflector. Now measure
from the top of the monopole's bracket to the floor of the balcony.
Finally slide the reflector up or down until it's center is at the same
height as the top of the monopole's bracket. (Note: this is a fairly
critical adjustment.)
Now measure between the reflector and
the antenna. They should be 30 centimetres apart, center to center.
Slide the boom section in or out to get this distance, making sure the
reflector and antenna are exactly parallel.
Now that you have your preliminary
settings, it's time to run coax to your radio, fire it up and start
adjusting the monopole's SWR. The goal is to have equal SWR readings on
144mhz and 148mhz so you want to take readings at opposite ends of the
band and write them down.
If the antenna is too long you will
have a better reading on 144 than 148 and you should trim a few
millimetres off the top of the monopole. Do not cut the reflector!
Stay in the cycle of test-adjust,
test-adjust until you get the lowest SWR you can, with both ends of the
band being equal. I managed to get mine to read under 1.1:1 all the way
across the 2 meter band.
The antenna is now ready for use!
If you are inclined to experiment more, you can try
changing the spacing between the reflector and monopole and see what
effect that has. You can also try moving the reflector up and down to
gauge it's effects.
My reflector is currently at 30.3
centimetres and the monopole turned out to be 468 millimetres tall for
best SWR. The final adjustments on your balcony may be quite different.
The Results
I haven't noticed any big differences in S-Meter readings
from repeaters in my area. One repeater got stronger, another weaker,
but most have stayed about the same. A couple of hams have commented
they can hear me better and one even told me he could hear me now, after
not hearing me at all. So I am guessing my transmitted signal is a bit
better, perhaps with a more even pattern perpendicular to the balcony.
The big news came from Field Strength
readings taken 1 meter inside my balcony window, just before installing
the reflector and right after finishing the SWR adjustments. The before
and after readings, both taken at 10 watts, are shown below...
Is this a good
antenna?
Signal reports have been good so far, the match is
excellent and the neighbor's cat seems to have survived... So I'm
calling it a success.
