Hi Group,

D-link has a vertical antenna for WiFi applications which is about a
foot long and operates at 2.4 GHz with a stated 7dbi gain.

Since the antenna is a vertical, I thought that the best low-angle
radiation is 5/8 wave for verticals, anything longer would put the
lobes higher and would result in a lower low angle gain.

If this is the case then the antenna should only be about 3.5 inches
long. ?

I am going to check, but I bet the true antenna is only about this
lenght and the rest of the lenght is the plastic, so the appeal is
bigger is better.

Any thoughts on this?

wrote:

Hi Group,

D-link has a vertical antenna for WiFi applications which is about a
foot long and operates at 2.4 GHz with a stated 7dbi gain.

Since the antenna is a vertical, I thought that the best low-angle
radiation is 5/8 wave for verticals, anything longer would put the
lobes higher and would result in a lower low angle gain.

If this is the case then the antenna should only be about 3.5 inches
long. ?

I am going to check, but I bet the true antenna is only about this
lenght and the rest of the lenght is the plastic, so the appeal is
bigger is better.

Any thoughts on this?


Collinear??

In article . com,
" wrote:

Since the antenna is a vertical, I thought that the best low-angle
radiation is 5/8 wave for verticals, anything longer would put the
lobes higher and would result in a lower low angle gain.

If this is the case then the antenna should only be about 3.5 inches
long. ?

PD-

The beauty of the 5/8 (0.625) wavelength groundplane antenna, is that its
feedpoint impedance has a resistive component of 50 Ohms, and can easily
be matched with a small inductor. Some such antennas have been
constructed by winding the coil using the base of the antenna rod. It is
my recollection from an antennas class 35 years ago, that the best signal
towards the horizon comes from a 0.58 wavelength groundplane antenna, not
0.625 wavelength. I can see how 5/8 and 0.58 might be confused, and many
may consider the difference trivial.

As far as the D-Link antenna, it is most likely a co-axially fed vertical
dipole with additional colinear elements separated by phasing networks.

Fred

Fred McKenzie wrote:

The beauty of the 5/8 (0.625) wavelength groundplane antenna, is that its
feedpoint impedance has a resistive component of 50 Ohms, and can easily
be matched with a small inductor. Some such antennas have been
constructed by winding the coil using the base of the antenna rod. It is
my recollection from an antennas class 35 years ago, that the best signal
towards the horizon comes from a 0.58 wavelength groundplane antenna, not
0.625 wavelength. I can see how 5/8 and 0.58 might be confused, and many
may consider the difference trivial.

Maximum gain at the horizon actually occurs at around 0.625 wavelength.
At lengths between 0.5 and 0.625, a higher angle lobe appears, but it
doesn't divert a significant amount of the total power away from the
main lobe until the antenna gets longer than 0.625 wavelength.

However, those gain figures are for a vertical antenna mounted on a
perfect, infinite, ground plane. Actual performance of 0.5 or 0.625
wavelength antennas compared to quarter wave ones varies a great deal in
most typical situations. And any kind of WLAN device I've seen deviates
in major ways from the ideal case. Gain of a longer antenna is as likely
to be due to placebo effect as to physics.

One big advantage of a 0.5 wavelength antenna is its high feedpoint
impedance. This makes it much more independent of the other half of the
antenna -- what amateurs like to call "ground", but is often the top of
a car, a person holding an HT, a router box, etc. The advantage isn't
shared by the 0.625 wavelength radiator.

Roy Lewallen, W7EL

wrote in message
ups.com...
Hi Group,

D-link has a vertical antenna for WiFi applications which is about a
foot long and operates at 2.4 GHz with a stated 7dbi gain.

Since the antenna is a vertical, I thought that the best low-angle
radiation is 5/8 wave for verticals, anything longer would put the
lobes higher and would result in a lower low angle gain.

If this is the case then the antenna should only be about 3.5 inches
long. ?

I am going to check, but I bet the true antenna is only about this
lenght and the rest of the lenght is the plastic, so the appeal is
bigger is better.

Any thoughts on this?

Unless you can see the wire, it may not just have a single radiator. We used
some Andrew 2GHz antennas at work that were about 3 feet long, and an inch
thick. I believe it was made up of multile 1/2 wave phased radiators.

Tam/WB2TT

On Tue, 02 May 2006 06:35:50 -0700, wrote:

Hi Group,

D-link has a vertical antenna for WiFi applications which is about a
foot long and operates at 2.4 GHz with a stated 7dbi gain.

Since the antenna is a vertical, I thought that the best low-angle
radiation is 5/8 wave for verticals, anything longer would put the
lobes higher and would result in a lower low angle gain.

If this is the case then the antenna should only be about 3.5 inches
long. ?

I am going to check, but I bet the true antenna is only about this
lenght and the rest of the lenght is the plastic, so the appeal is
bigger is better.

Any thoughts on this?

I suspect that antenna is a collinear antenna with elevated Feed.
Probably 1/2 of the of the antenna or more is simply transmission line.
They want the active elements be to in "Free Space" above any surrounding
equipment.
Its amazing what people will do with electronics and wonder why it does
not perform as advertised.

KD