If you are using the Balcony Buddy or a
similar monopole on your balcony you may benefit from adding a set of
ground radials to the antenna.
Ground plane antennas are nothing new. Hams have been
using them for a very long time. But there are some problems with
putting them on balconies. By the time you arrange a stand for them, get
them set up and positioned on the balcony, they take over half of the
space. Additionally the perfomance can be seriously hindered if parts of
the antenna are hidden behind the balcony's structure.
But what isn't common knowledge is that you don't need 4
ground rods to make a groundplane antenna, it can be done with 2. This
gives us a more or less 2 dimensional antenna that can easily be stood
up on a short piece of pipe from a balcony rail without losing any space
on the balcony itself.
There are numerous advantages to a groundplane antenna
over a monopole. First you get approximately twice the gain (about
9DBi). But more importantly the ground radials isolate the antenna from
the balcony itself. This reduces the risk of RFI in the building and
greatly reduces the levels of induced electronic noise you will hear
from other electronics in the building.
This project is a bit more advanced than the
Balcony Buddy in that it
requires fabrication of parts. However there's nothing here that can't
be done at home using basic tools.
The first step is to get all the bits and pieces you
will need to build a 2 meter version of the Balcony
Buddy . In addition you will need need a piece of
1/2 or 3/4 inch pipe about 60 cm long for the mast pipe, a piece of 3/16
inch tempered aluminium rod at least 120 cm in length for the radials, a
second piece about 53cm for the top whip, and a bit of 1/8 inch
aluminium stock the width of your bracket and about 4 cm tall. Depending
on your balcony you may need a couple of U bolts or hose clamps to mount
the mast.
Making The Radials
First you want to bend the ground radials from the 3/16"
aluminium rod. The bending plan is below, along with a picture of the
finished groundplane.
Caution: You can re-bend the 3/16 tempered aluminium a
little bit to adjust your angles but if you try to straighten it and
start over it is going to break so be very careful to get this right the
first time.
The dimensions are not particularly critical but you
should try to get them as close as possible. The short 10cm "wing span"
is a slight adjustment for more uniform SWR. The downward angle of the
radials determines the antenna's impedance. Their length affects the
resonant frequency. You can be a couple of millimetres off without too
much detrimental effect, more than that will begin to show in the
antenna's performance.
I
started in the center of the rod, with a 90 degree bend. Then moved out
2cms each side to make the two 45 degree bends that bring the horizontal
parts out level. When done the rod should lay flat, the two ends should
be exactly in line with each other. This is the portion of the radials
that have to fit against the bracket, so you can make some adjustments
to suit your bracket here if you need to.
Next I trial fitted the center section to my bracket
just to make sure everything was going to fit properly before
proceeding.
On
the right you can see how the center section sits on the bracket. This
portion will be clamped into place by the aluminium bar we are going to
fabricate once the ground radials are finished.
The next step is to get the 10cm horizontal part right.
While I still had the rod on the bracket for trial fitting I measured
the bracket, subtracted it's width from 10cm, divided by 2, then
measured out and marked my next 2 bends. If you are using the RV-1
brackets I use, this is easy... the bracket is 5 cm wide so you need 2.5
cm on either side.
Next make the 45 degree angle bends that bring the
ground radials downward. This angle sets the impedance of the antenna,
be extra careful to get these bends as close to 45 degrees as you can.
Also, some care is needed here as you want the whole thing to lay flat
when it's done, so be careful to get the radials parallel to the the
center section and to each other when you make these two bends.
The final step in fabricating the radials is to measure
down the 45 degree sloping rods, starting from the center of your bend
radius and cut them to 44.5cm. This measurement isn't all that critical,
you can be off by a millimetre or two but the antenna is going to work
best if both are the same.
The Clamp Bar
The next step is to make a clamping bar to hold the
radials against the bracket. This has to be some pretty sturdy stuff if
you don't want them to move, so use at least 1/8" aluminium, 3/16" is
better.
On
the left is a shot of the bar itself. The actual dimensions will depend
on the bracket you are using. The hole spacing must match the hole
spacing of your bracket as you will be using it's mounting bolts to trap
the ground radials into place. Note that the lower section in the
picture is longer than the upper. The long side is cut to exactly fit
between the upper bolts of the bracket and the bottom of the top plate.
The closer this fit the better. The shorter distance is less important
so long as it is sufficient to support the bolt heads.
On
the right is a shot showing how the bar fits over the bracket and how it
clamps the ground radials into place. You need to measure and arrange a
similar fit for the bracket you are using.
I do feel duty bound to tell you that I cheated just a
bit making this bar. I had an old bracket with the back piece missing,
so I just cut the bar from the bottom of the bracket with my hacksaw and
cleaned it up with a file. Sometimes it pays to hang on to bits and
pieces of stuff.
Assembly
Now that we have all our bits and pieces together it's
time to make this into an antenna.
The first step is to assemble the bracket and radials using
the clamping bar and two of the bolts supplied with the bracket. (Note
on some mast pipes you may need to replace these two top bolts with
longer ones to get enough reach). On the left we see a back shot of the
bracket showing two extra nuts added to clamp the ground radials into
place. This is a better plan than relying upon the standard nuts as
these can be tightened a lot more firmly to minimize the risk of the
radials moving after installation.
On
the right is a shot of the completed assembly, with the RF parts added
and all ready for a top whip.
If you are using the Balcony Buddy
You will need to make a new vertical whip for it as
this antenna uses a slightly longer whip than a monopole. The vertical
whip should start at about 56cm from the tip to the top of the bracket,
for this configuration.
Now,
on the left we see the complete antenna assembled on a mastpipe and
ready to go out onto my balcony for tuning and use.
Installation and Tune Up
The
only special step when installing this antenna is to keep the tips of
the ground radials above the balcony rail; not across it. Also they
should be about 15 or 20cm above the railing.
Tuning this antenna is a matter of getting the top whip
the right length. You should not have to trim or adjust the ground
radials.
After mounting the antenna on my balcony with a couple
of U bolts, the task became one of tuning for the lowest SWR. This
turned out to be a lot easier than I expected. I was able to get the SWR
under 1.1 all the way across 2 metres in a matter of minutes. A bit of
time with a file, trimming slivers off the top whip got it down to
1.05:1 on each end and 1.0:1 in the middle.
At a beginning height of 54cm the antenna came in long,
as expected, and I followed the same tuning process as for the Balcony
Buddy...
- Take an SWR measurement at 144mhz and write it down.
- Take another measurement at 148 and write it down
too.
- If the low end reads better than the high end, the
antenna is too long. You should trim about 5mm off the aluminium top
whip and go back to step 1.
- If the high end reads better than the low, the
antenna is too short, go on to the next step.
- Once you get your first "too short" reading stop
trimming the whip and move it up a little in the AS-1 adaptor and
gently snug the setscrews.
- If the ends of the band now read nearly equal, you
are done.
- If the antenna still reads too short, go to step 5.
When done the top whip on mine measured 518mm tip to
bracket. Of course the conditions on your balcony may change this length
somewhat, so don't be surprised if you end up a bit longer or shorter.
During my early testing I ran into a problem with "common
mode" signals on the coax (aka "feedline radiation"). I eventually
solved the problem by making a coil of 5 turns of coax wound around the
mast pipe and taped into place about 15 cm below the bottom of the
mounting bracket. This improved and stabilized the SWR of the antenna
with only minimal retuning. As a bonus it also got rid of a problem I
had with my signals interfering with my computer monitor. It's probably
best to add this choke when installing the antenna, before you start
tuning.
One final tip: to keep the unplated aluminum parts
looking bright and new for a long time, polish them gently with fine
steel wool and give them a triple coat of car wax.
Results
I will say I am quite pleased with this antenna so far.
As expected there has been only a little change in
received signals from local area repeaters. Signal reports on my
transmissions have, so far been about the same as for the Balcony
Boomer, a little better than for the basic Balcony Buddy.
However, there has been a huge improvement in the
quality of received signals. Noise levels are way down. Since the
antenna's radiator is now isolated from the balcony structure I no
longer have to listen through all the electronic hash in the building to
hear a weak signal.
Update: August 2007
After using this antenna for a
few months I find it's performance to be excellent. I've been able to
turn down power for several repeaters and still have good copy with
others. The simplex range extends to the horizon and beyond, often with
only 10 watts of power. The match is holding beautifully at under 1.1:1
across the entire 2 meter band.
My little "2 dimensional groundplane" seems to be living
up to all expectations and, so, I will call this design a
success.
Editor's note: This
project uses mostly metrics.
Use this calculator to
convert lengths, sizes,
etc
