RadioBanter - View Single Post

September 20th 05, 12:44 AM

wrote:
Hello,

Can someone tell me what type of electical length does 802.11 antennas
(lambda/2, lambda/4, etc) have? The 2.4GHz has 12.5cm wavelength but
none of the rubber duck antennas that come with the wirless router have
a length of 12.5cm, 6.25cm, or 3.125cm, etc.

I recently purchased a 9dBi antenna to replace the 2.2dBi stock rubber
ducks. It also does not appear to measure in any n*lambda/4 lengths.
What puzzles me is that an outdoor $200 antenna of 3 foot long and a
$60 antenna of slightly less than 1 ft both have about the same gain.

And before the purchase I have tried to make a few higher gain antennas
out of a wire coat hanger in n*lambda/4 lengths but they don't have
much of an impact in signal strength. Why?

I did an antenna course years ago and remember the gain of a monopole
is related to its radiation resistance, which is proportional to
n*lambda/4 from what I can recall.

Thanks!

t
You are correct, the radiation resistance partly depends on, or partly
governs, the dimensions (depending on how you look at it). The size
(length) of the antenna depends not only on the diameter of the wire,
but on the surrounding material too. An antenna can be made shorter
than what you would expect by simply increasing the permittivity (or
dielectric constant) of the material around it. My guess would be that
the antenna is coated with a high permittivity material which (in the
material) makes the wavelength of any frequency appear shorter than it
actually is. So the antenna only actually has to be as long as the
quarter wavelength in the dielectric.

Another possibility would be to introduce a reactance (inductance or
capacitance [I can't remember which at the moment]) to shorten the
electrical length of the antnna, thereby, once again, only requiring a
shorter than expected antenna length.

As far as your tests go, I can't really comment on why you don't see a
change. But... If you'll recall from your antenna course, a lambda/2
dipole has a figure 8 pattern when looked at from the side (E plane).
If you increase the length of the dipole (or monopole with a ground)
you are effectively creating an antenna array and you get interference
from each element (each section of lambda/2). Your figure 8 with a low
gain broad main beam begins to look like a flower with thin petals.
Each petal (lobe) has higher gain but a narrower beamwidth. You may be
getting multipath (a type of interference) due to the extra lobes or
you simply may be looking from an undesireable angle. Perhaps you
could try moving your non homebrew antenna station in the E plane of
your antenna and see if the signal strength changes.

I hope this sheds some light!

Dario