		***** AVANTI "ON-GLASS ANTENNA *****

			by Morris Lundberg, K4KEF


  You've, no doubt, seen them advertised and they sound pretty
good.  No holes to drill.  No clamps to mar the auto's paint. 
But how do they work, these "on-glass" antennas?

  The half-wave antenna, in contrast to the quarter-wave, is a
complete antenna.  The quarter-wave antenna requires a ground
plane.  Without a ground plane the quarter-wave antenna is a
sorry disappointment.  A half-wave antenna is twice the length of
a quarter-wave, but on the other hand, the quarter-wave radius of
the ground plane takes up space sideways which isn't needed with
the half-wave.

  In designing an antenna to mount on glass, a half-wave antenna
design is necessary.  Avanti's "on-glass" antenna consists of a
resonant half-wave radiator, a low-to-high impedance transformer
(tuned circuit mounted on the inside surface of the glass), and a
capacitance coupling (through the glass).  The antenna amounts to
an end-fed, half-wave antenna, mounted vertically.  Figure 1 is
the equivalent circuit, where the j100 ohms is the coupling
capacitor, consisting of the mounting plates on either side of
the glass and the glass is the dielectric, the load is the
radiating element and the source is the transmitter.  The input
impedance for an end-fed antenna of this type is very high
(several thousand ohms), allowing the application of the
capacitance coupling principal, since the capacitor's impedance
is comparatively low (little energy is dissipated in the
capacitive reactance; most of the energy is dissipated in the
load).  The capacitive coupling at the base of the antenna
results in reducing the antenna length by about 5%.







		FIGURE 1 LOCATION










  The optimum mounting position for this type of antenna appears
to be the upper portion of the center of the rear windshield. 
This places the radiating element above the roof of the auto and
minimizes radiation pattern distortion.  Some cars (Volkswagons)
have considerable roof above the rear windshield and radiation
pattern distortion may be unavoidable.  In this case it might be
better to mount the antenna on the front windshield.

  The presence of defogger wires located the the optimum mounting
position has been a point of some concern.  Avanti's testing has
indicated that running these wires through the capacitive
coupling area caused no harmful degradation to system operation
(in the Cellular Radio frequencies - information on 2 meters may
be available, but I haven't seen it).

  Some auto window tinting contains significant amounts of metal;
a point worth considering when you chose this type of antenna for
your car.  It may be detrimental to the efficient transfer to RF
through the glass.  Tinting, however, does not appear to be a
problem at Cellular Radio mobile frequencies (820 to 890 MHz).

  Some gain over a quarter-wave vertical (perhaps 3 dB) is
supposed to be achieved by antenna design which limits radiation
to very low angles above the horizontal.  In most cases, energy
radiated from this type of antenna at high angles is of little
use.  Concentrating the energy at low angles, in effect, provides
gain in the same sense that a yagi's concentration of energy in a
given direction provides gain.

  Our experience in Cellular Radio applications has indicated
that the Avanti "on-glass" antenna performance is every bit as
good as it's roof-mounted counterpart, which does require the
ground plane of the auto roof.

  As you can see, the "on-glass" antenna is not complicated and
it should be a good candidate for Amateur Radio experimentation,
especially at VHF and UHF frequencies.  

K4KEF.

Note to the Editor: Figure 1, along with a print of the article
will be provided separately.  Avanti antennas are from The
Antenna Specialists Co., 12435 Euclid Ave., Cleveland, Ohio
44106-4386.
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