[Scanner] "Waterfowl" antennas

Sat, 15 Jun 2002 23:39:31 -0700

Just a few comments.  Read "between the lines".

73,  DW, KE6WYB, Shasta County California, CM80 <><

----Original Message Follows----
From: Edmund F Leavitt <[email protected]>
Reply-To: [email protected]
To: [email protected]
Subject: [Scanner] "Waterfowl" antennas
Date: Sat, 15 Jun 2002 17:46:52 -0700

On Sat, 15 Jun 2002 17:46:18 -0500 "Mike Urich" <[email protected]> writes:
 > From: "anthony costa" <[email protected]>
 >
 > I have a radio shack pro 91.I listen mainly around the 800-900 mhz
 > freq,I
 > now use a telescopic antenna in the retracted position.6" in
 > height.the
 > signal's are in and out.Now on the vhf freq's it has great
 > sensitivity,But i
 > listen mainly on 800 mhz.any good ht antenna's for this band??
 >
 >
****************************************************************

Regarding "rubber duck" antennas:

Based on those I've seen, the antenna supplied with most scanners are
resonant at approximately 153 Megahertz.  Considering how devoted the
owners who show up here are to using their devices predominately for UHF,
I am forced to wonder what the decision process was to market them with a
VHF antenna.

Two reasons.
1.  Cost.  Why pay to re-engineer something that's "always worked" if it's 
not costing sales??

2.  In my experience, there is still a lot of public service on VHF-Hi.  
Also NOAA weather and Aircraft.  It's an acceptable performer for most 
people who listen here.  Personally, I use a homebrew wire ant 39.5" long.  
Yes, it's a bit unwieldy at times.

Regarding selecting a better flexible antenna, the following are some
generalities about "rubber duckie antennas" and a simple performance test
worth trying.

1.  The lower the desired frequency, the harder it is to make a  "rubber
duck antenna" appear resonant, and the bigger the compromise such an
antenna constitutes.  Conversely, the higher you go in frequency (within
reason) the easier it is to manufacture a credible antenna and then cover
it with rubber.   A mentor of mine told me decades ago  that  "A rubber
duck antenna is 3 dB better than a dead short."  Another used to say "A
rubber duck is a radiating dummy load."   I certainly would not quibble
with either until you get up to around 800 megahertz.

The point is that for VHF the difference in performance between a rubber
duck and a real antenna can be quite substantial.  However, in the lower
and mid-UHF part of the band, a rubber duck may actually be a real
antenna disguised by a coat or rubber.

2.  Several generalizations  worth considering when choosing an antenna:
         a.  Performance of any quarter wave antenna is dependant upon the
groundplane to which it is referenced or attached.  The chassis of a hand
held radio, is generally too small to make a good ground plane.

I  have had some success improving the duck's performance by squeezing a 
thin wire (28 ga.) 40" long wrapped once or twice around the connector 
between the duck and the case, making a jury-rigged dipole.

         b.  A vertical 5/8 wavelength antenna exhibits gain (relative to
a quarter wave antenna, and  TOWARD THE HORIZON)  when used with a proper
ground plane.  Without a good ground plane, a 5/8 wave antenna should be
suspect.
         c.  A half wave antenna is generally able to perform well,
independent of a ground plane, making it desirable for use on hand held
devices.  (Another place this comes in handy is for mounting on
fiberglass roofs of travel trailers and the like.)  Typical designs
include end fed coil matched "whips", and at higher frequencies, center
fed "coaxial" or "sleeve" antennas.

[As an aside:  If you've ever wonder how long is a half wave antenna, the
math is simple...divide 150 by the frequency in MHz and you have the
approximate length of a half wavelength IN METERS.   If you are more
comfortable with inches, you can approximate  by multiplying meters times
39.   eg.  150/900 MHz = 0.16 meters or about 6 1/2 inches.

That says that if you are looking for a half wave antenna, expect it to
be about one meter long for 150 MHz,  0.3 meters or 30 centimeters long
at 450 MHz,  around 18 centimeters for Cellular/Trunked freqs, and around
16 centimeters for 900 MHz.]

3.  "Coaxial" design antennas are quite common for UHF 800 and 900
systems, and perform well.   They are generally easy to recognize because
they reduce diameter above the midpoint of the antenna body.  (Some VHF
flex antennas combine a helical wound base with a slimmer top element,
and look similar.  Usually the transition from thick to thin occurs in
the bottom third of the antenna.  If you find an 800 MHz antenna with the
diameter change, it is almost certainly a coaxial or "sleeve" design, and
should be a decent performer.)

Recommendations:
         a.   If you are receiving the stations in your area well with
whatever you presently use, be happy...invest your money in the stock
market.  If you need better, buy better.
         b.  If you use the factory rubber duck a lot, recognize that it
is designed to work fairly well in the 153 MHz range.    Consider buying
one(s) designed specifically for the frequency ranges you like.   Same
comment applies for mobile antennas.  With either, it's probably
reasonable to ignore the difference between a cellular (824-896 MHz)
antenna, an "800" (806-866 MHz) antenna, and a "900" (865-965 MHz)
antenna.  The profusion of cellphones probably makes mobile cell antennas
the easiest to find and the cheapest to afford.
         c.    Want to play around to see just how foul that fowl antenna
is?  Find a weak station in the 860 part of the band using your factory
rubber duck antenna.  Listen with the squelch open or just barely closed.
   Cut a paper clip to 3 3/4 inches in length (approximating a 1/4
wavelength antenna at 860 MHz).  Make sure there are no jagged ends on
the paper clip that could damage the radio's BNC contacts.  While
monitoring a weak conversation, carefully replace the duck with the paper
clip, by inserting the end of the clip into the center contacts of the
BNC connector.  Let go of the paper clip.  Also, be certain not to change
the radio's position when you switch back and forth.  If the duck works
consistently as well or better, keep it
If the 1/4 wavelength paper clip works consistently better, I would
consider shopping for a UHF antenna.  Splurge on a half wave 800 MHz
rubber duck.  It will probably be worth the bucks.   Does this procedure
sound too simple and too crude to be meaningful?  It is not.  I'd be
interested to hear from a few users how their factory ducks stand up
against the 1/4 clip.

True fact.  And usually the paper clip wins

***************************************************************
Edmund Leavitt                        Ph:    253 582-5034

Ham/MARS/SHARES:               KA7UKN / AFA5AH / KPS654
Lakewood, WA                         N 47� 09'   W122� 31'
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