FAQ's (Frequently Asked Questions)
I moved my on-glass antenna from one car to
another. It worked great on the old car, but now
I have a very high VSWR? Why?
Your new car probably has passivated glass ---
glass impregnated with tiny metal particles that
render the glass incapable of coupling RF between
the inside and outside couplers.
How can I tell if the glass on my vehicle is
suitable for on-glass antennas?
First, look near a corner of a window for words
that mean "sun", relate to sun, or
refer to ultraviolet or other forms of radiation.
If you find terms like "Soft-Ray",
"Solar-Cool" or similar terms, your
glass is probably passivated and wont work
with on-glass antennas. If you want to be 100%
certain, and you dont find helpful words on
the glass, ask a 2-way or cellular shop
technician to test your glass with a capacitance
Why is a wavelength (or fraction of) in coax
cable physically shorter than what I calculate
from the formula for wavelength?
The formula you used to calculate the wavelength
was probably meant for calculating a "free
space" (air) wavelength. In fact, RF energy
moves more slowly in a transmission line than it
does in air because the materials used in cable
slow it down. Therefore, a wavelength in cable
takes up less length.
With my low-band antenna mounted on the roof of
my car I had great VSWR. I moved it to a trunk
bracket and now it doesnt tune properly.
The antenna is responding to the ground plane
size and shape in the immediate field of the
whip. Ground plane effects are significant at low
frequencies, because a quarter-wave is fairly
Im using one of your rubber duck antennas
on our electronics box. Our system hasnt
been working right, and when I tested the antenna
VSWR I found it was very high. Whats wrong
with your antenna?
We need to understand what kind of rubber duck
youre using. Most of them require a good
ground plane for good performance. If your box is
made of plastic, fiberglass, or other
non-metallic materials, there may not be enough
ground plane. If your box is metal, but is small
in wavelengths, there may still be inadequate
ground plane. One solution may be to use a
half-wave antenna if your application will allow
the additional height. Half-waves work
independent of a ground plane.
I put an Ohmmeter between the center contact and
the outer ring at the mount (bottom) end of a
coil and measure a dead short. On another of your
coils I measure an open between the center
conductor on the bottom of the coil and the whip.
Dont these Ohmmeter results mean the coils
Not necessarily. DC performance of a matching
coil seldom relates at all to how well it works
at radio frequencies. Some coils employ a tap a
few turns from one end that yields excellent RF
performance even though its a dead short
for DC. Other coils use capacitors that block DC
current, and although theyre open to DC
they conduct RF just fine.
I just installed a ________ antenna on my car,
and I measure reflected power of 2 Watts with 55
Watts forward. Is this is o.k? Should I have 2
Ideally we want no reflected power at all, but
thats seldom realizable in practice. Your
antenna is performing to specification, because
the percentage reflected power youre
measuring is only 3.6% of forward power. That
means 96.4% of the power is transmitted. Most
antennas have a VSWR specification of 1.5 or 2.0.
Your example is equivalent to a VSWR of 1.47.
How do wireless repeater antennas work?
The average vehicle is a reasonably good barrier
to radio waves getting in and out. A wireless
repeater creates a leak so to speak in the
barrier. A signal from a handheld phone induces a
tiny current in the inside antenna mounted on the
glass. That current is directly coupled to an
outside gain antenna and is radiated. Of course
it works in reverse on received signals. For most
installations, signals coupled through the
two-antenna-glass coupler media are stronger than
signals which must penetrate the vehicle
Some Manufacturers do not specify gain for its
portable antennas, whereas some manufacturers do.
Why is this?
Virtually all portable antennas compromise gain
for small size and the convenience that size
affords the user. True antenna gain results from
directivity and the use of efficient (low loss)
materials. Directivity requires a certain amount
of "aperture" or physical size.
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