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TUCoPS :: Radio :: scantips.txt

Using a TV antenna with a scanner





 > Some time ago I read here about someone using a TV antenna for their
 > scanner.  Would anyone still have the specifics to get the maximum
 > signal to the scanner.

Here's what Bill Cheek posted awhile back.

==============================================================================
Mon 14 Aug 95 12:05
By: Bill Cheek
Re: TV Antenna Notes
------------------------------------------------------------
This message is copyrighted (c) 1995 by Bill Cheek.  All rights reserved.

Questions and comments of late suggest that we better rehash the fundamentals
of using TV Beam Antennas for scanners.  This stuff is elementary, and well
known by those who use them, but since the scanning crowd turns over
periodically, the old knowledge can get lost or misplaced.  Hence, here is a TV
Antenna Primer for those who want SUPERIOR listening capability at a modest
cost and who wish to minimize the mechanical nightmares that can be associated
with antenna installations.

1.  The nominal or typical scanner antenna is the discone and/or rubber duckie,
    neither of which are high performers.  Discones have a 0-db gain over their
    bandwidth with respect to an isotropic radiator that radiates equally well
    in all directions of a sphere.  Rubber duckies probably have a gain that
    varies across the spectrum, perhaps slightly higher than a discone on some
    frequencies, and defnitely less (- dBi ) on others. The height of a discone
    makes for MUCH better performance than a rubber duckie under most,
    conditions.  There is a nominal 3 dB gain improvement for each 10-ft of
    height increase over the VHF-UHF spectrum.  Finally, there are other
    antennas made for scanning that may be better performers than discones
    and rubberducks.  The "ScanTenna" is one, but omnidirectional antennas of
    this type have "gain" at the expense of loss on other frequencies.  Caution
    is advised before purchasing this type of antenna.  Finally, there are
    directional beam antennas..........

2.  Discones, rubberducks, and "ScanTenna" types of antennas are said to be
    omnidirectional....more or less equal performance from all directions in
    a 360-degree circle around the antenna.  Typically, gain is low; bandwidth
    is wide.  Good antennas for all-round scanner use, with the discone
    probably THE best, since it covers the greatest spectrum with a fairly
    flat, predictable performance spec.

    Then there are directional beam antennas.  CB'ers, hams, and professionals
    are familiar with these, whereas scannists may not be.  Directional beams
    transmit and/or receive better in one direction than in others.  This is
    the factor that gives substantial GAIN to beam antennas, exactly like the
    reflector behind a flashlight bulb makes the light much stronger in one
    direction and much weaker in other directions.  Beam antennas work exactly
    the same way; just not as dramatically.

3.  There is almost nothing by way of a "scanner beam" antenna on the market
    for scannists.  The most widely known would be the Grove Scanner Beam.
    Less well known, would be what is called a "log-periodic", examples of
    which are sold by a company called CREATE.

    Log-Periodic beam antennas are excellent for scanner work, but
    unfortunately they are expensive, cumbersome, heavy, and not readily
    available to the casual scannist.

    The Grove Scanner Beam is little more than a small TV antenna, perhaps
    optimized for scanners; perhaps not.  But it is a definite value for the
    advancing scannist because it does work, and it comes with complete
    instructions for assembly and installation.  The price is consistent with
    that of a TV antenna of about the same size.

4.  TV Beam Antennas can be used to great advantage with scanners!  Yes, they
    are designed for TV signals, but most also include the FM Broadcast band,
    so the coverage and gaps are as follows:

    Designed Coverage                Scanner Frequency Gaps
    =============================    =======================
     54 -  88 MHz TV VHF Ch 2-6        25 -   54 MHz
     88 - 108 MHz FM Broadcast        108 -  174 MHz
    174 - 216 MHz TV VHF Ch 7-13      216 -  470 MHz
    470 - 890 MHz TV UHF Ch 14-83     890 - 1300 MHz

Gaps?  Well, they're mentioned solely because those bands are not TV
frequencies and some people think the antenna won't work in those gaps.  Well,
it will work just fine because it would be more costly to make the antenna NOT
work in those gaps than to just optimize coverage for the TV bands and let it
go at that.  Also there are the principles of "overlap" and "harmonic
relationships" which combine to fill in those "gaps" with very good
performance.  The bottom line is that a TV antenna offers excellent performance
across the scanner spectrum of 25 MHz - 1300 MHz, especially with respect to a
discone and other omnidirectional scanner antennas.

Now the CAVEAT.  The GAIN of a TV antenna varies with size and design, but a
mid-grade model will have 10-12 dBi GAIN or more within its designed bandwidth
and 4-8 dBi gain at the band edges and within some of the gaps.  This GAIN
comes at the expense of reduced performance in other directions.  Therefore,
most installations will need a rotator by which to point or aim the TV antenna
in the desired direction.  This means that you will hear much farther and more
signals in the pointed direction and worse with fewer signals in the off-
directions.  Therefore, most scannists who use a TV antenna or other
directional beam will also use an omnidirectional antenna for casual and local
monitoring.

As a beam antenna is rotated around the 360-degree azimuth, the scanner will
detect signals never before heard, and from "impossible" distances.  On the
other hand, when the beam is not pointed correctly, some nearby, strong signals
may not be heard.  This is both a blessing and a curse.  The curse is that you
can't depend on a beam antenna to hear everything you want to hear at any given
time.  The blessing is that you can use a beam antenna to REJECT undesired
signals while emphasizing desired ones.  Another blessing is that you can hear
much farther in desired (pointed) directions than with an omni antenna.

A TV antenna to be used for scanning MUST be mounted in the vertical plane,
with the ends of the longer elements pointing straight up and down (90-degrees
shifted from the normal position of a TV antenna.  This is easily done with a
short "mast" about 2-ft long coming out of the antenna mount and mechanically
fixed to a longer vertical mast that comes up out of the rotator.  Imagine an
upside-down "L", or:

              |
              | B           B = short 2-ft horizontal boom
    Antenna   o====|
              |    | M      M = longer mast out of rotator
              |    |
                   |        R = rotator
                   R

The installation is completed with RG-6 satellite coaxial cable with gold-
plated Type F connectors at each end.  At the antenna, a TV matching
transformer is used to mate the coax to the antenna.  Down in the shack, use a
gold-plated F-to-BNC Adapter to mate the coax to the scanner.

These are only highlights of the subject, so feel free to ask questions.
==============================================================================



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