On Sat, 24 Sep 2005 13:44:17 -0400, bjs555 <aaa@bbb.com> wrote:
>If I understand you correctly, that would mean
>that a dish type reflector is more effective on a receiver than a
>transmitter.
No. A properly designed dish and feed have the same gain in both xmit
and receive. In improperly designed system, with large amounts of
overspray from the feed, will cause the dish to less than the
theoretical maximum gain in transmit and about the same as the
theoretical gain in receive.
>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.)
This is rather muddled. A client radio, access point, repeater, and
just about any type of wireless device works best when the transmit
and receive gains are about the same. More specifically, they work
best in a symmetrical system, where the fade margin on both directions
is about the same. If you radically increase the gain in one
direction, you obtain no additional range because the signal in the
other direction hasn't improved at all. Whether you create such a
situation by adding a tx power amplifier or by throwing together a
poorly designed antenna system is not important.
>Another question:
Groan...
>I did some searching for IC RF amps starting from the page you gave in
>an earlier post. I found this Maxim data sheet:
>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?
Chuckle. I just love the polarity marking in the 10pf capacitor,
especially since ceramic caps are not polarized.
The reactance of 10pf at 2.4Ghz is:
Xc = 1 / (2 * PI * freq * C)
Xc = 1 / (2 * 3.14 * 2.4*10^9 * 10*10^-12)
Xc = 6.6 ohms
Compared to the 50 ohm load, that will result in about a 10% loss or
about 1dB. Tolerable. Looks like the output matching network is
trying to compensate for some lead inductance in the flip chip. The
10pf is therefore probably part of this matching network, in which
case the loss is probably much less. I'm too lazy to model it to be
sure.
If you're thinking of making your own amplifier, there are better
chips that put out more power and in easier packages. Flip chips were
made for automatic vapor reflow soldering and are not really suitable
for do it thyself breadboards.
You might wanna dig through the collection of 802.11a/b/g app notes
at:
|
http://www.maxim-ic.com/appnotes10.cfm/ac_pk/38#12
for anything interesting. Maxim also sells prototype boards with the
chip and all supporting circuitry ready to test and clone.
>Would I connect an external antenna at the RF out end of the
>cap?
Reading between the lines of your question, I detect that what you
really want to know is where to cut the trace on your USB adapter and
attach either a pigtail, connector, or antenna. Yes, it's on the
antenna side of the capacitor to avoid having DC appear on the
antenna. No, I don't have a clue where you should cut because I don't
have the board layout, photo, schematic, or the time.
>Do I cut any traces to an existing antenna?
See above.
>Are compensation
>circuits (not sure these apply to RF) sometimes used?
Always used in the better radios. It's almost impossible to build a
perfect 50 ohm match. The designers take their best shots and
compensate for any mismatches with a matching network.
>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?
Probably because he had some 150pf caps handy or couldn't handle the
tiny 10pf or so capacitors. Self resonance is a big problem and needs
to be avoided. 150pf is way too big. He added them because Netgear
apparently has some DC on the ungrounded antenna elements and
diversity PIN diode switch. External antennas should be AC coupled to
prevent accidentally shorting parts of the radio to ground.
>You gave some pictures of a dongle that you modified for an external
>antenna. Where did you connect to the circuit?
I also mentioned that it's on a neighbors roof and I can't find the
photos. The PIFA antenna has 3 soldered connections. The middle
connection is the 50 ohm point and is where you connect the coax
center conductor. The nearest solder blob is ground. That's the
shield point. The other solder blob is just for mounting. I used a
PCB mount SMA connector and as much ground as I could solder.
>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.
Note that the Compaq WL215 is an Orinoco Gold classic card with an
external coax connector. He could have done it without cramming the
whole mess into the tube. I personally don't find anything wonderful
about the arrangement.
>Sorry for asking so many questions but I find this topic so
>interesting. And thanks for the antenna theory references.
No problem.
--
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
Re: usb adapter cantenna 
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