On Mon, 10 Oct 2005 20:39:36 +0000 (UTC),
dold@XReXXdipol.usenet.us.com wrote:

>Jeff Liebermann <jeffl@comix.santa-cruz.ca.us> wrote:
>> On Mon, 10 Oct 2005 08:04:47 -0400, bjs555 <aaa@bbb.com> wrote:
>
>>>My understanding is that a dipole antenna is in the shape of a T where
>>>the length of each horizontal branch is equal to a quarter wavelength.
>
>> Correct. Actually, it's more like 0.95 * 1/2 wavelength because of
>> "end dispersion" effects.
>
>Isn't he speaking quarter wave and you half wave?
>He said quarter wave, and you counter with .95 * half.

Yah, sorta. I tend to refer to a dipole antenna as a "half wave
dipole" because of the overall length. The 0.95 is the calculated
fudge factor based on the total length of the dipole. It is NOT a
fixed number but an estimate. I need to know the wire diameter to
calculate the exact value.

>Maybe I'm too fussy, measuring with a micrometer, marking with a piece of
>chalk, and cutting by biting the wire with my teeth.

Ummm... I've seen worse.

>My dilema: I see 30.5, 31, and 32 suggested, all of which seem to long,
>given a center frequency of 2437MHz and the .95 factor.

It's difficult to define the length because I don't know the aspect
radio. Tell me the wire diameter and I'll give you the free space
numbers to about 4 decimal points. I usually cheat and just pound the
numbers into an NEC2 modelling program such as EZNEC or 4NEC2.

A nifty trick to inscreasing the bandwidth of a driven element is to
use conical shaped radiators or rounded ends on the rod. Most of my
900MHz rod antenna have rounded ends on the elements to improve the
bandwidth. Fat elements also dramatically increase the antenna
bandwidth.

>It would seem that the optimum would be 29.2mm for Channel 6.
>299792.458 / 2437 / 4 * .95
>
>Why, then, do I see suggestions of 30.5 (which would be 100% of 2400MHz,
>outside of the band), or 31, or 32mm? Where is the .95 applied?

The 0.95 is my guess as to the fudge factor for the overall 1/2 wave
dipole. I don't know the magic correct number without knowing the
construction details.

>> Watch your accuracy. At 2400Mhz a wavelength is about 125mm. However,
>> each MHZ is equal to:
>> 125mm / 2400 = 0.052 mm/MHz
>> The band is 83.5 MHz wide, so your overall tolerance on cutting the
>> elements is:
>> 83.5 * 0.052mm/MHz = 4.35 mm.
>> it doesn't take much cutting error to end up with a non-functional
>> antenna.

>A margin of error of 4.35mm on an element that is only 31mm long is
>obvious enough that I would hope people can get it right.

Try again. The above calcs are for a full wavelength. The necessary
accuracy for a 1/4 wavelength would be 1/4th of that or about 1.1mm.
That's 1.1mm difference to mistune the antenna over the entire 83.5MHz
wide band. If you model the antenna and get exact numbers for
building the beast, then to keep the antenna inside the 2400-2483.5MHz
band, you need +/- 0.55mm accuracy on the 1/4 wave elements.

Another way of demonstrating the accuracy is:
1/4 wavelength at 2400.0 MHz is 31.25mm
1/4 wavelength at 2483.5 MHz is 30.20mm
Therefore, the total allowed cut range accuracy is:
(31.25 - 30.25) /2 = +/- 0.5mm
If you want it to operate on a specific channel (1, 6, or 11), then
the accuracy required is one third of the 0.5mm.

Can you say critical?

>But that's a full wave, not the margin on each 1/4 wave element. It also
>suggests that such an antenna would be perfect for some frequency with the
>range of 14 channels, but doesn't say anything about the tolerance for an
>acceptable antenna that will be used specifically on channel 6.

Bandwidth is directly affected by the antenna gain. Very roughly, for
a given antenna type, a 3dB increase in gain (by doubling the size)
will also double the Q (quality factor of the antenna) which
effectively cuts the bandwidth in half. That means that fairly low
gain resonant antennas (<12dBi) will have no problem operating over
all the channels without retuning, while higher gain resonant antennas
may only cover a few channels.

>If that were applied to each 1/4 wave element, are you saying that the
>margin of error on cutting the element is 1mm?

Worse. +/- 0.5mm. See above calcs.

--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558