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...
- First; using an omnidirectional antenna on an
apartment building balcony results in a considerable amount of RF
entering the building. This is a waste of signal as the building
simply absorbs most of it. It also poses a health risk as RF exposure
has been shown to cause problems for some people.
- Second; I am surrounded on all sides by other hams
and repeaters. If I used a strongly directional beam antenna I would
surely lose contact with a couple on the back side of the building.
Turning the beam their way is not an option since it would flood the
building with even higher levels of RF than are present now.
- Third; the balconies on this building aren't exactly
antenna friendly. They are solid sheets of steel a meter tall with an
angle iron hand rail about 20cms above that. The clearance between the
handrail and the balcony above is a rather tight 140cms. Just barely
enough room for a 2 meter half wave.
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.
