Re: usb adapter cantenna

Jeff,

Thanks for your reply. If I understand you correctly, that would mean
that a dish type reflector is more effective on a receiver than a
transmitter. Picturing the radio waves as rays of light, the rays
would be nearly parallel at a receiver very far from a transmitter.
Thus, nearly all the captured rays would go to the focus of the
reflector. For a reflector a few inches from the transmitter, however,
many rays would "spill out" and wouldn't contribute to the transmitted
beam. So it seems to make more sense to put a reflector on a client
antenna rather than an access point antenna. Is this verified by
experiment? (I know, why don't I do it - not fully set up yet.)

Another question:
I did some searching for IC RF amps starting from the page you gave in
an earlier post. I found this Maxim data ***:
http://pdfserv.maxim-ic.com/en/ds/MAX2240.pdf
which shows an application circuit at the top. In the circuit, the
signal is shown ac coupled to the antenna through a 10 pF cap. Is that
typical? Would I connect an external antenna at the RF out end of the
cap? Do I cut any traces to an existing antenna? Are compensation
circuits (not sure these apply to RF) sometimes used?

Also, at this page:
http://www.nodomainname.co.uk/ma401/ma401.htm
the author describes a way to add an external antenna to a particular
wifi card by removing an inductor and adding 2 150 pF caps. Why is he
doing that?

You gave some pictures of a dongle that you modified for an external
antenna. Where did you connect to the circuit?

I'm thinking now that the best approach might be a something like
this:
http://www.nodomainname.co.uk/cantenna3/cantenna3.htm
where a proper feed is made into the waveguide without using RF
cables.

Sorry for asking so many questions but I find this topic so
interesting. And thanks for the antenna theory references.

Cheers,
Bruce


On Sat, 24 Sep 2005 08:44:09 -0700, Jeff Liebermann
<jeffl@xxxxxxxxxxxxxxxxxxxxxx> wrote:

>On Fri, 23 Sep 2005 11:24:39 -0400, bjs555 <aaa@xxxxxxx> wrote:
>
>>I've been doing some more experimenting (nothing destroyed yet, but
>>soon no doubt).
>
>Actually, you already have but don't realize it. You need to match
>the antenna feed to the dish and that's rough with a USB dongle.
>
>>I mentioned earlier that I got about 5 dB improvement with a quick and
>>dirty corner reflector. I rummaged around the kitchen and found a
>>spaghetti strainer that's roughly a parabola.
>
>I have some not very nice comments to make about using a reflector
>with a USB dongle feed. The basic problem is that the original PIFA
>or circuit board antenna has roughly a hemispherical antenna pattern.
>That means that transmissions from the dongle will go in all
>directions and only some of which will hit the dish, reflect toward
>the receiver, and provide useful gain. That also means that you will
>observe different apparent gains in transmit and receiver. Receive
>will be largely unaffected by the dongle feed pattern and will yield
>reasonable results. Transmit gain will suck as much of the power goes
>in useless directions.
>
>See:
>| http://groups.google.com/group/alt.internet.wireless/browse_frm/thread/113ed746dc7446e7
>for details. I ran the numbers for an 18" pizza dish in:
>| http://groups.google.com/group/alt.internet.wireless/browse_frm/thread/e70d56f0dd9b9aa1/af7ddbbf5aa01dd7
>That's probably a worst case example as the deep dish rice dryer seems
>a be a better shaped reflector.
>
>Some not so light reading:
>| http://www.w1ghz.org/antbook/contents.htm
>See Part 4 on dish and feed basics.
>| http://www.w1ghz.org/antbook/chap4.pdf
>Just look at the figures and read the captions if you don't want to
>dive to deep.
>
>My suggestion is to replace the built in PIFA or whatever antenna in
>the USB dongle with a proper coax connector (SMA) or coax pigtail and
>use a dish that has a properly matched feed.
>
>Incidentally, the same problem with feed "overspray" or "spillover"
>happens with foil reflectors hung on vertical omnidirectional antennas
>as in www.FreeAntennas.com. However, the effect is much smaller than
>the 15dB loss in the pizza dish reflector because reflector captures
>most of the RF from the vertical and redirects it in a useful
>direction. For example, if the reflector covered a 120 degree azimuth
>wedge (looking down on the antenna), two thirds of the transmitted RF
>will go to places unknown, while one third will hit the reflector.
>That's about 5dB loss which isn't bad considering that even the best
>dish antennas and feeds have about a 3dB illumination loss.
>
>Also note that different parts of the dish provide different amounts
>of gain. This is best demonstrated by blocking parts of the dish and
>seeing how it affects the gain. Try this with an absorbent material
>(black foam or wet towel) and not with a reflector like aluminum foil.
>
>The flat part of the dish, near the middle contributes the most to the
>antenna gain. Unfortunately, that tends to be blocked by the feed (or
>USB dongle) so offset feeds are used (as in DBS dish antennas) to
>avoid blocking the center. Gain derived from the "sides" of the dish
>is much less, especially on a deep dish (f/D < 0.35). As the dish
>gets deeper, the edges of the dish approach being inline with the
>antenna direction, and therefore capture very little signal relative
>to the surface area. It's also more difficult to design a proper feed
>for a deep dish.
>
>This begs the question "Why are all these people building their own
>reflector antennas"? Easy. Even with all the problems mentioned, a
>suitable reflector type antenna is a major improvement over the stock
>antennas. Even the worst implementation will give about 6dB of gain
>which is the equivalent of 4 times the range. However, squeezing the
>maximum gain out of the system does require a proper design and
>testing.

.

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