Text 34236, 55 rader
Skriven 2016-07-03 10:40:33 av Michiel van der Vlist (2:280/5555)
     Kommentar till en text av Björn Felten (2:203/2)
Ärende: A question to all the HAM radio experts here
====================================================
Hello Björn,

On Sunday July 03 2016 07:27, you wrote to All:

 BF> I know that this question would be more proper to post in the HAM
 BF> echo, but since I've never been into the HAM business, I never found a
 BF> reason to import the echo to my BBS.

Dont bo'ther. The HAM echo has been dead for year. No more critical mass.

 BF>    I've noticed that all my wifi equipments with a simple dipole
 BF> antenna is made for 3GHz and not the expected 2.45GHz, the center
 BF> frequency of the WiFi band. I.e. the supposed to be quarter wavelength
 BF> poles are 25mm long rather than the expected 30.6.

 BF>    Does anyone know it there's an electrical reason for this?

Yes, it is calle th e shorteningfactor. The quarter wave for an antenne in
resonance applies to an infinitly thin wire. If it is not an infinitly thin
wire, but a cylinder istead, there  is an end effect bevause the current at the
end of the cylinder is not zero. Current flows from the rim the center of the
end surface of the cylinder.

I do not know the formulas for the end effect o f th etop of my head. HAMs
usualy determine the factor by experiment. They make it a bit too long and then
cut off material until the antenne is in resonance. The usual tool to determine
resonance is the SWR meter. (SWR - Standing Wave Ratio)

 BF> Or can it be that the manufacturers took the cheap decision to make
 BF> the antennas compatible with the 5GHz band, where they could act as
 BF> half way designed antennas?

No. Making it too short is not a workable way to make a dual band antenna.

 BF>    I've searched the web for days now, and I can't find any
 BF> explanation.

https://en.wikipedia.org/wiki/Electrical_length

 BF> I'm almost inclined to adding five millimetres on either side of one
 BF> of my antennas just to see (with the help of my trusted Vistumbler) if
 BF> it makes any difference.

If you are prepared to scrap it, you can do the experiment. But I predict that
it will not make it perform any better and it is easier to cut material off,
then it is to put it back on again.

Good luck with the experiment.


Cheers, Michiel  (PA0MMV since 1964)


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