importance of swr with wifi

I've lately picked up an interest in wifi, mainly, as an option to
untangle myself from cables. As an amateur radio hobbyist, I understand
the principles of the radio hardware involved but not the
firmware/software aspects of wifi. I can save that for later. The
question I have is about the antenna system. I've seen several DIY
parabolic contraptions you can make from cardboard and aluminum foil to
place on the 'antenna' to increase the range and directional
characteristics. With the transmitters I'm familiar with, you just
can't start modifying the antenna system, with no regard for SWR. Could
someone explain if SWR is critical to a wifi transmitter? I'm assuming
since they appear to be a relatively low power device, they can either
tolerate a high SWR or have built-in protection. I'm not so concerned
with SWR as it relates to the efficiency of the antenna system but more
with the potential to cause damage to the transmitter.


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I suggest you might find more satisfying answers if you pose your question
on the "rec.radio.amateur.antenna" newsgroup..

For my own education, I bought a Wi-Spy USB spectrum analyzer module, set it
in a shield enclosure, and made measurements of various antennas as
receiving devices. This approach is easier than trying to make SWR
measurements on antennas as transmitting devices. You can find SWR bridge
construction for 2.4 GHz on the web, but in the end only results count.
Leave the SWR problems and protection to the chip designer with a laboratory
of expensive test equipment to guide him. Certainly standard configurations
of simple antennas will behave fairly predictably - - for instance, use a
folded dipole as the radiator in a 23-element Yagi design, because you know
the parasitic elements will knock down the feed impedance to something you
can use - - that sort of thing. There's a lot of information on Wi-Fi
antennas on the web, and I built most of the designs myself. The worst
result was from the "helical" type, and the best result was from the
"quarter-wave radiator in a tin can waveguide" and from the "Biquad"
configuration.

Good luck

Chuck W6PKP

"awizardalso" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>
> I've lately picked up an interest in wifi, mainly, as an option to
> untangle myself from cables. As an amateur radio hobbyist, I understand
> the principles of the radio hardware involved but not the
> firmware/software aspects of wifi. I can save that for later. The
> question I have is about the antenna system. I've seen several DIY
> parabolic contraptions you can make from cardboard and aluminum foil to
> place on the 'antenna' to increase the range and directional
> characteristics. With the transmitters I'm familiar with, you just
> can't start modifying the antenna system, with no regard for SWR. Could
> someone explain if SWR is critical to a wifi transmitter? I'm assuming
> since they appear to be a relatively low power device, they can either
> tolerate a high SWR or have built-in protection. I'm not so concerned
> with SWR as it relates to the efficiency of the antenna system but more
> with the potential to cause damage to the transmitter.
>
>
> ------------------------------------------------------------------------
> View this thread: http://www.wirelessforums.org/showthread.php?t=30298
> http://www.wirelessforums.org
>

awizardalso <(E-Mail Removed)> hath wroth:

>I've lately picked up an interest in wifi, mainly, as an option to
>untangle myself from cables. As an amateur radio hobbyist, I understand
>the principles of the radio hardware involved but not the
>firmware/software aspects of wifi. I can save that for later. The
>question I have is about the antenna system. I've seen several DIY
>parabolic contraptions you can make from cardboard and aluminum foil to
>place on the 'antenna' to increase the range and directional
>characteristics. With the transmitters I'm familiar with, you just
>can't start modifying the antenna system, with no regard for SWR. Could
>someone explain if SWR is critical to a wifi transmitter? I'm assuming
>since they appear to be a relatively low power device, they can either
>tolerate a high SWR or have built-in protection. I'm not so concerned
>with SWR as it relates to the efficiency of the antenna system but more
>with the potential to cause damage to the transmitter.

Good observation. Most antenna builders don't bother with VSWR,
running simulations, field testing, and all the niceties of
calculations and measurement. There are several reasons for this (in
order of decreasing importance).

1. Any antennna is better than the stock antennas that come with the
typical router. Even if the VSWR absolutely sucked, it would
constitute a substantial improvement gain. For example, compare these
two biquad antennas:
<http://802.11junk.com/jeffl/antennas/Biquad/index.html> (good)
<http://802.11junk.com/jeffl/antennas/biquad-junk/index.html> (bad)
Numerous web piles show both methods of construction.
The 1st has a gain of 11dBi with a VSWR of appprox 1.5:1.
The 2nd has a gain of 11dBi with a VSWR of 6:1. Yech.
The first is properly designed and constructed. The 2nd is a feel
line problem between coax and the quad elements. However, the gain is
the same, so nobody notices or cares.

2. Crude is effective. You can build a wi-fi antenna fairly crudely,
with miserable tolerances and crude construction techniques, and still
end up with something that's better than the stock antennas. I'm used
to squeezing the last dB of gain out of the antenna, so my first
impression was very unfavorable. I'm used to precision machining, but
have learned to tolerate cardboard and aluminum foil. After building
and testing some antennas, I find that the average user doesn't need
that last dB.

3. Test equipment is expensive and difficult to build and calibrate.
<http://pe2er.nl/>
It can be done, but requires precision beyond what the average user is
willing to pay for. (Incidentally, look at the AMOS sector antenna on
this page for a really good and easy wi-fi antenna)

4. VSWR is only a major issue with high gain ( >15dBi ) antennas.
That's because the VSWR bandwidth tends to get narrow as the gain
increases. Above some gain ( 24dBi???) the bandwidth gets
sufficiently narrow that the antenna has to be tuned to a specific
group of channels and will not cover the entire band.

5. Transmitters are very tolerant of high VSWR. All power amp MMIC's
have built in VSWR protection so there is not going to be any blown
amps. Most are also fairly conservative, running at 50mw out on a
250mw spec power amplifier. If I were running 1 watt, I might be
concerned, but even those have VSWR protection.

Gotta run....

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558