SOME IDEAS ABOUT GROUND-PLANE ANTENNAS
by Don Irving
People seem to like to spend lots on scanner antennas as if there were some
magic about them. This article explains how the simple 18.5 inch rod may
be the best all around scanner antenna for general emergency services use.
Ground plane antennas including whip antennas on car-tops function as
quarter-wave antennas. "Quarter-wave" means that the antenna length is
1/4 of the wave length of the frequency at which it is resonant.
Resonance is not nearly as important for receiving as it is for
transmitting. In most cases it is not feasible to have resonance for
scanner listening because you listen to many different frequencies. A
given antenna is resonant at only a single frequency and certain harmonics
of it. By and large, you should not be concerned about resonance for
scanner listening.
A neat trick, however, is to cut a whip antenna (a simple metal rod) to be
resonant at a particular frequency that you have special use for, so that
you get a bit better performance at and around that frequency. For
example, maybe your local fire department operates simplex in the 154 Mhz.
range, and you would like to pull in the weak mobiles a bit better. You
could cut a whip to be resonant at that frequency and get a stronger signal
without going to a special high-gain antenna. The whip will still work
fine for other frequencies.
This article gives a formula for cutting a whip antenna to a resonant
frequency. The formula provides much more precision than necessary for
receiving, but a little precision never hurt anyone.
The general formula for relating wave length to frequency is:
wavelength = speed of light / frequency
Using 300 million meters/second for the speed of light we have:
wavelength in meters = 300 million / frequency in Hz
If we change Hz to MHz we would have:
wavelength in meters = 300 / frequency in MHz
For a quarter wavelength we would have:
quarter wave in meters = 75 / frequency in MHz
Changing meters to inches would give us:
quarter wave in inches = 2953 / freqency in MHz
The speed of light used for this calculation was the speed of light (and
radio waves) in free space. Radio waves travel slightly slower in metal,
so the formula must be tweaked slightly to be accurate for a metal antenna.
Instead of using 2953 in the formula, use a value from the following table
to make the calculation more accurate for a metal rod.
Diameter of rod: 1/16 1/8" 3/16" 1/4"
-----------------------------------------------------------
Band desired
------------
VHF LO (30-50MHz) 2878 2873 2870 2867
VHF HI (150-174 MHz) 2867 2861 2855 2852
UHF LO (450-512MHz) 2855 2843 2828 2825
Here is the formula now in summary:
length (constant from table above)
desired for = -----------------------------------
your whip desired resonant frequency in MHz
Example: To make a 1/8" rod resonant at 155 MHz:
desired length in inches = 2861 / 155 = 18.5 inches
The desired length is the total length including the metal base to which
the rod attaches, so you have to consider that before you cut. Cut your
rod so that you can make the total length the desired length.
HARMONIC OPERATION FOR A WHIP ANTENNA
------------------------------------
A ground plane antenna works well at 1/4 wavelength and at odd multiples of
1/4 wavelength. It does not work at even multiples, and performance is
mediocre at less than 1/4 wave. The technical reason is that at odd
multiples there is a current maximum at the end of the antenna (which is
good), and at even multiples there is a current minimum.
Here is how an 18.5 inch whip antenna behaves. (Performance does not change
abruptly. It changes gradually as you move through the table.)
1/4 wave at approximately 155 MHz. (good performance)
1/2 wave at approximately 310 MHz. (bad performance)
3/4 wave at approximately 465 MHz. (good performance)
4/4 wave at approximately 620 MHz. (bad performance)
5/4 wave at approximately 775 MHz. (good performance)
6/4 wave at approximately 930 MHz. (bad performance)
Well below 155 MHz it gives mediocre performance.
At lengths beyond a few wavelengths, this progression no longer works. The
antenna starts acting as a random long-wire antenna.
Notice that the 18.5" whip is great for all the emergency services that are
clustered around 155 MHz. It is also great for the emergency services
around 460 MHz. It's not so hot for most of the 800 MHz band. It is just
so-so for the 42 MHz Highway Patrols and State Police. Nevertheless,
because of its good performance at 155 MHz and 460 Mhz, the 18.5 inch whip
is a great, standard scanner antenna for general emergency services use.
I have tried lots of scanner antennas over the years, and I keep coming
back to the basic 18.5 inch whip.
A good way to make a scanner whip for a car is to buy a cheap VHF mobile
whip with base and cut it to 18.5 inches. For an outdoor house antenna,
Radio Shack makes an inexpensive ground plane scanner antenna that is
already about the right length.