I was bitten by the “scanner bug” in 1978,
and started out with a four-channel, crystal-controlled unit from Radio Shack. Since then,
I’ve probably owned a dozen scanners, and currently have a myriad of radios around
the house, car and in the “shack”.
Through the years, I’ve picked up a lot of great tips for increasing the
functionality of my radios. The purpose of this article is to share some of these pointers
and ideas with you. Whether you’ve just unwrapped your new radio, or are a seasoned
enthusiast, hopefully, there’ll by something for everyone.
When it comes to antennas, there are as many different designs and variations as there are
scanners. And like the correct speakers for your stereo, the right antenna for you scanner
can make all the difference in the world.
Antennas For The Handheld Scanner
For handheld applications, your selection is a bit limited. First, there is the standard
flexible, rubber coated whip, affectionately referred to as a “rubber duck”.
Unfortunately, most of these whips are nothing more than an untuned coil of wire, and
their performance usually leaves a lot to be desired. If you plan to monitor only one band
(i.e., UHF, VHF, 800 MHz), you may consider an antenna tuned particularly for that band.
Optoelectronics offers a full line of antennas tuned for specific parts of the spectrum.
Other antennas, such as the Austin Condor, are tuned for several bands, and will allow you
to obtain much better general coverage from your hand-held scanner.
Mobile Scanning
When you take to the car with your scanner, there are countless factors that come into
play. Since you’re in a metal bodied vehicle, the effectiveness of anything except an
external antenna will be substantially diminished. Fortunately, there are numerous
antennas available for mobile use. Again, a multi-band antenna may be your best all-around
choice. You should probably consider a magnetic-mount antenna if you plan to move the
scanner from vehicle to vehicle. For my mobile, I use a mag-mount MFJ dual-band (UHF/VHF)
ham antenna. It’s less than eighteen inches high, looks just like a cellular antenna
(which won’t tip off thieves that there’s a scanner in the car), and cost less
than fifteen bucks. It doesn’t work particularly well on the low band, but does a
pretty good job on UHF and VHF.
If you don't want another antenna poking out of your car (my wife drew the limit at
three), there’s a great way to use your existing car antenna with your scanner. The
Para Dynamics PDC 63 Mobile Multicoupler is an antenna splitter which will allow your
scanner and car radio to simultaneously share the existing AM/FM auto antenna. And like
the mag-mount MFJ, it has the added benefit of not alerting the whole world that you have
a scanner in the car.
And while we’re on the subject of mobile scanners, intermod is an annoying problem
that often manifests itself as you drive past powerful transmitters (pagers, cellular
phone, etc.). I have found that the Radio Shack variable inline attenuator (#15-578) is
very effective in reducing or eliminating this problem. Now keep in mind that the higher
you crank up the attenuator, the less signal that will get through. However, by adjusting
the level of attenuation, you should be able to find the point where the intermod stops
and the desired transmissions can still be heard. I have one of the attenuators hooked to
my mobile scanner and use it quite frequently. It may be also be used on a base station,
particularly when near a large urban area where intermod is a constant headache. And by
the way, don’t ever try to transmit through an attenuator unless you want to find out
what burning components smell like!
Base Station Antennas
For base stations, one of my favorite antennas is the discone. A discone has no gain, but
is almost equally effective over a wide range of the spectrum, typically up to 1300 MHz.
It is especially suited for air band and satellite monitoring because it works well from
the horizon all the way to straight overhead. I’ve even used my discone to receive
packet data from the SAREX amateur transmitter aboard the space shuttle.
A major problem with many discones which lack a vertical top element is their
ineffectiveness below 50 MHz. However, there is a simple fix for the problem. On the top
of most discones where the horizontal radiators are attached, there is a short section of
bolt (which may be covered by a rubber cap) that can be used to attach a vertical whip. I
added a six foot whip to mine, which improved reception considerably down into the mid 40
MHz range.
One of the nice features about a discone is that it can also be used for transmitting. My
only advice is to check the SWR for your particular radio before extended use.
Yagi Antennas
For those who may be a fairly long distance from a large metro area and want to tap into
the scanning action, your best bet may be a Yagi (also know as a “beam”)
antenna. You can usually obtain good results by simply aiming the antenna in the middle of
the area to be monitored. A rotator may not even by necessary, though it may come in handy
if you’re close enough for the directional characteristics of the antenna to be a
problem. By its very design, a Yagi will be usable for only a relatively limited frequency
range. However, a variation of the Yagi, know as a log periodic, will cover a very wide
range of frequencies. I have an associate who lives on a hill about eighty miles from San
Francisco, and by using a Create brand log periodic, is able to copy most of the action
very satisfactorily.
An excellent cross between the omni-directional antenna and the log periodic is the Grove
Scanner Beam. It is tuned to cover a wide range of frequencies and provides up to nine dB
of gain. However, it isn’t so directional as to exclude those signals coming in from
the side. And when mounted on a rotator, you can quickly aim the antenna for best
reception of weak signals.
One final note about Yagis: since almost everything you will be scanning above 30 MHz will
be vertically polarized signals (originating from a vertical antenna), be sure to mount
your antenna vertically as well. Otherwise, you will be cross-polarized and will reduce
the effectiveness of the antenna significantly.
Ground Plane Antennas
The ground plane is probably the most common type of outdoor scanning antenna in use
today. Though it doesn’t exhibit much gain, if any, it is quite efficient when used
on its resonant band. The Radio Shack ground plane antenna works quite well for UHF/VHF,
and sells for under twenty bucks. However, if you want to build you own ground plane,
optimized for best performance on a particular band, you can do so for around five
dollars.
To determine the optimum frequency for the antenna, ascertain the middle frequency of the
band you wish to scan. For instance, if you want to scan the domestic aviation band, which
covers from 118 MHz to 136 MHz, subtract the lower frequency from the upper. In this case,
the result is 18 MHz. Divide this number in half (nine), then add it to the lower
frequency, which was 118 MHz. The result is middle of the band, 127 MHz.
The formula for determining the length of the radials is as follows: 2952 / Frequency (in
MHz) = Antenna Length (in inches)
Now don’t get ahead of me and cut the radials just yet, otherwise the ground radials
will be too short when you make the loops for the screws. First, start with a Radio Shack
chassis-mount SO-239 (#278-201). For mounting the ground radials, Radio Shack also sells
4-40 screws (#64-3011) and nuts (#64-3018). At your local hobby shop, you should be able
to obtain the required number of 3/32” brass rods. These rods typically come in
36” lengths. Five would be the most you need, but you may be able to get away with
fewer if you are using a frequency high enough to cut some of them in half.
Using the above formula, cut the radiator to the exact length required. It doesn’t
have to be perfect, but try to get it as close as possible. In the case of the air band
example above, that would be 23.24409448819 inches. We can probably safely round up to
23.25 inches. It may take a bit of force to fit the brass rod into the solder lug on the
SO-239. Once firmly in place, carefully solder the rod to the SO-239. I have found that a
minimum of a 100 watt soldering iron is usually required to do the job right.
The final step is to attach the ground radials. Using a pair of needle-nose pliers, bend a
loop on each of the ground radials just large enough to accommodate the screw. Attach the
ground radials to the SO-239 and cut to the same length as the radiator. Finally, for
impedance matching, bend the radials down at a forty-five degree angle.
If you would like to purchase a ground plane antenna, customized for a specific frequency,
MAX System Antennas provides this service at a reasonable cost. The antennas are
constructed of high quality materials in a weather-proof PVC housing that makes mast
mounting much easier.
Antenna Related Tips
And while on the subject of antennas, just a few words about coax. The best antenna in the
world will never reach its full potential if the signal doesn’t make it to the radio.
For example, if you achieve a 3 dB improvement by using RG-8 instead of RG-8/M for a 100
foot run, the perceived difference will be the same as if the transmitting station doubled
its power. I strongly suggest you check the coax specifications for the loss
characteristics for the frequency range you intend to monitor, then choose the right coax
for the job. For those who find themselves frequently switching between antennas and want
to avoid the hassle of having to manually disconnect and reconnect each time, I would
recommend an antenna switch. Unlike the expensive type used for switching transmitting
antennas, the Radio Shack “high isolation” A/B/C switch (#15-1248) works very
well with scanners. There is no apparent loss of signal, and you can choose the right
antenna for the job at the touch of a button. However, since the switch is designed for
television use, you will have to pick up a few “BNC to ‘F’ jack”
(278-256) adapters if you plan to use the BNC connectors found on most coax.
RF Amplifiers
If you wish to increase the strength of the signal coming down from the antenna, you may
choose to use an RF amplifier. There are a number of good amplifiers on the market, and to
get into the attributes of each is beyond the scope of this article. However, as a general
observation, I would recommend a GaAs FET model which generates much less internal noise.
Also, some amplifiers offer the option of a variable gain control. This is a very helpful
option that can be used to optimize the gain for both weak signal reception, and to avoid
overloading the scanner when a strong signal is received.
By its design, an RF amplifier will boosts the signal level of everything in its operating
frequency range. Unfortunately, that may present more problems than it solves. Not only
will the amplifier boost the signal you are trying to monitor, but all the noise and
undesired garbage as well. One way to minimize interference when using RF amplifiers is to
use a narrow band pass filter. As its name implies, the band pass filter allows only the
frequencies for a specific band to pass through, and rejects all others. To eliminate a
specific type of interference, such as a local CB, ham, FM, or TV broadcast station, you
can use a filter. The Grove Adjustable Scanner Filter or the Grove Interference
Eliminators are both options you may wish to explore to solve your interference problems.
As far as getting the most bang for the buck with the least aggravation, I prefer to
invest my money in good antennas with low-loss coax, and mount them as high as possible.
However, if you’re after weak signals such as satellites, you may wish to explore the
RF amplifiers a bit further.
Tape Recorders
One of my favorite scanning techniques is to hook up the scanner to a good tape recorder
and compress a whole day’s listening onto forty five minutes of tape. This is
especially useful for logging seldom used or hard-to-find stations. If your radio
doesn’t have a tape recorder controller, you will have to rely on a voice activated
(VOX) recorder. The following list of features should be considered when purchasing a
recorder for use with a scanner:
After evaluating a total of five recorders from various
manufacturers, I finally settled on the Radio Shack CTR-69. I’ve used mine for over a
year now and it has served the purpose very well.
As an added feature, you may wish to put a timestamp on your tape to help establish a
pattern of activity. This project will take a bit of soldering, and some understanding of
basic circuitry.
For this project, you will need a Radio Shack “talking” alarm clock with an
“hourly announce” feature. I used the VoxClock 2 from Radio Shack, but this unit
has been discontinued. However, there are a number of these clocks on the market
(including several from Radio Shack) which should be suitable for the purpose.
The first step is to open up the clock and remove the two wires from the speaker. Then
attach the same two wires to either a speaker jack or to a wiring harness which will be
used to connect the clock to the scanner and recorder. I opted to install a 1/8”
female jack on my clock which bypasses the clock speaker when it is connected to the
recorder.
Next, add the proper speaker (or tape output) jack for your scanner to the harness. And
finally, add a jack to interface the output from the clock and scanner to your recorder.
Remember, all connections are parallel.
I must admit that I had reservations when I assembled this contraption. I was concerned
about the audio levels and impedance matching. But believe it or not, it works quite well.
These are just a few of the ways you can get the most out of your scanner. Publications
like the “The ARRL Antenna Book” and Bill Cheek’s “Scanner
Modification Handbook” are both excellent sources of additional information. However,
making friends with other scanning enthusiasts is probably the best (and most rewarding)
way to learn more about this exciting hobby.
The following products are available from Grove Enterprises (800-438-8155 or
704-837-9200):
All articles are property of Haskell L.
Moore, and may not be reproduced
in whole or in part without written permission from the author.
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