To receive all the frequencies
you hear through this website, I built two identical antennas from plans
I found on the internet. Both antennas are mounted in the attic of my
house, and are attached to a horizontal section of roofing support. The
antennas I built are known as quarter wave antennas. The quarter wave
antenna is a simple design and can be constructed (or tuned) to receive
whatever frequency range you are after.
There is a relationship
between frequency and the wavelength, so the physical size of
the antenna is a function of the frequency range you are aiming for.
To get the wavelength of any frequency there
is a simple formula. 300 divided by the frequency in MHz .
A vertical quarter wave length
of wire, or metal rod by itself will
not perform that well, and is really only half of the antenna. We need
to add the other half of the antenna, which is known as 'the ground
plane'. The easiest way to do this is to add similar length radials to
the vertical quarter wave. They are not electrically
connected to the quarter wave, and if we bend them down at a 45 degree
angle, the impedance of the antenna ends up around 50 ohms which is what
the majority of communication devices perform best with, and is
certainly what is required for radio scanners.
Construction of a quarter
wave antenna is quite simple, and in the design that follows, the cost
is very low. Some wire, coaxial cable and a couple of plugs. Any
electronics or hobby store should have all the parts you need. In the
Kitchener Waterloo area Orion
Electronics is as good, and as reasonably priced as anyone.
Solid wire - I
use straightened out coat hangers for one antenna, and some similar
thickness metal rods my wife said she bought at a hobby store for some
reason. Any metal rod that holds it's shape will suffice.
One SO239 plug
Two PL259 plugs
(or whatever plug you want) to suit the coaxial cable you will be using
4 small bolts
(small enough to fit in the holes on the corners) and matching nuts (depends
on the method – see below)
For the Waterloo
Control Tower frequency of 126.0 MHz you apply the formula of 300 divided by
126.0, and come up with 2.38 meters. Since we are constructing a quarter wave
antenna, we divide the calculated wavelength by 4. In this case 2.38 meters
divided by 4. Therefore the length required for each metal radial on a
quarter wave antenna tuned for 126.0 MHz is 2.38 divided by 4, which equals .60
meters, or 60 centimeters (23.6 inches)
Before you cut your lengths of rods for the four radials, you need to decide
how you will fasten them to the SO239 socket. There are two ways to do this –
one is to solder them directly to the corners of the socket (where the holes
are), and the second is to use a small bolt to fasten it to the socket. I used a
combination of the two (bolts and solder). If you construct this antenna
for a lower frequency, the radial will be longer of course, and the bolt method
may help with portability, but either method works well.
Note small loop at end for bolting radial to socket
Five lengths of
wire (one quarter wave in length) need to be cut. In this example they are each
60 cm long. If using the bolt method to secure the radials, a small loop needs
to be bent at the end of four of these rods to allow the bolt go through and
this is part of the 60 cm length. The fifth piece of rod does not have a loop in
it for either method. If soldering the radials directly to the socket, all 5
lengths are the same length and do not have a loop in them. The length without
the loop is soldered onto the socket of the SO239 plug (on the opposite side to
the socket where the PL259 is screwed in. Metal coat hangers from the
drycleaners are a good fit and allow a sufficient gap for solder to flow into
the hole, creating a good electrical connection on the SO239 plug.
Socket with the straight wire soldered in.
The other 4 lengths are bolted onto the
socket using the holes on the corners, and then bent DOWN to an angle of 45
degrees (which gives you the 50 ohm match so that your radio is happy).
First of the radials bolted onto the
The completed project
Your antenna is now completed, and you
need to feed it and mount it. Attach your coaxial cable to the PL259 plug, and
screw it into the SO239 platform and you have a new antenna to use.
A simple mounting method is a length of
25mm PVC pipe which allows the coax to be fed inside it, and also has the
benefit of offering a snug fit for the PL259 plug, and holding the antenna up
well. I simply attached the PVC pipe to a roof member with some copper plumbing
A note about coaxial connecting cable.
Both my antennas have about a 50 foot run of cable to the scanners. One antenna
uses your basic cable TV 75 ohm cable, and the other antenna uses 50 ohm cabling.
The reception with the 50 ohm cable may be slightly better, but in my particular
be hard pressed to really notice the difference. Now I am only a few kilometers from
the airport and the police transmitters, and distance is a huge factor in
determining the quality of reception. The farther away you are from the source transmitters, the more
noticeable the effect will be between the 50 and 75 ohm cabling. The difference
between the 50 and 75 ohm cabling will also become more noticeable the greater
the distance between your antenna and the radio.
Because I'm using the antenna for
reception only and not transmitting, the measurements are not as critical. I'm
also using this antenna to pick up the Police and Fire transmission with
excellent results. Their frequency range is in the 868 MHz area. Again,
reception quality will vary depending on several factors, not limited to type of
cabling, distance from transmitter source, antenna size, nearby sources of electrical interference , line of
If you have any questions,
Many thanks to Matt of ScanACT in
Australia for publishing this design originally