Another Design

There is another new design available at QSL.NET. Unlike other designs this one looks logical and the maths seem to work. I suspect that there will be some type of missmatch since I suspect that this antenna might not be 50 ohms, but for all intents and purposes I believe it will be close enough
I am certainly going to build this one. I will post some pictures here when I do.
Two Main Designs

There are two main designs for vertial 802.11 antennas available. The first is
based on a design from
Guerilla.net. A few other groups have nicked this design ( tux.org and
MelbWireless ). The second is based on a design by JA7YTB in the
The Radio Amateurs Magazine from Japan
here and here.
The design of the antenna from guerrilla.net has a non-obvious derivation.
There are errors in the design as presented. In fact attempts at
construction have led us to believe that this design should not be
used until the design itself has been verified. Measurements for the
design mostly do not make sense.
One of the important elements of the design is winding the
coils. According to the documentation coil is to be wound on a screw
of 6mm diameter and pitch of 6 threads per 10mm. There are no such
screws available anywhere. We did build this antenna making some
assumptions, with poor results. The design is also tedious
to put together...
The second design is much better in terms of design calculations. It
is simply 3/4 wavelength sections with 1/4 wavelength sections
bent for phasing.
FYI.....
----- Original Message -----
From: Richard Talbot
To: melbwireless@melbwireless.dyndns.org
Sent: Monday, January 21, 2002 3:44 PM
Subject: [MLB-WIRELESS] Client side access
Ok, some more testing was done on the weekend, this time with hardware
suitable for user (leaf) nodes.
We have noted several issues with the present method of, buying a Lucent
card, pigtail and 10-15m LMR-400 including:
Most users have desktops, necessitating a $120 investment on a PCI ->
PCMCIA adaptor.
LMR-400 is not cheap and there is an appreciable investment involved
to buy it (around $120-$140 for a 10m cable)
Pigtail cables are very lossy and expensive to boot. The Lucent connector
alone is rated with a SWR of 1.56:1 at 2.4 Ghz, to say nothing of feedline
losses and poor construction techniques.
In short, Losses between the card and the antenna are high,
as are the costs involved in setting the whole thing up.
In our particular application there are bigger problems. We are planning
to mount the Galaxy antenna on the roof, about 6-8 m up a TV antenna mast.
Common sense would kind of dictate that we are not going to run a
15-18m length of LMR-400 to the office downstairs. The smartest thing to
do would be to put an AP or a computer in the roof and run ethernet the
rest of the way. In order to do this we are also going to need 10 m of
feed line. The AP is also not to good as it is expensive and requires our
neighbours to have a card in infrastructure mode.
We wanted to try another method. The plan was to use a USB based card (in
this case the Dlink DWL-120). These are cheap, easily modified and allow
us to run a simple USB cable up the mast. The unit is smaller than a
AP and cheaper.
Our first job was to test the performance of the card as both of
us had heard they didn't go as well as a PCMCIA unit. Testing
on a celeron 400 running win 2k and 64 mb ram, showed us downloading a
20Mb file from a local ftp server at 470k/sec. Not too shabby at all,
so this was obviously worth the trouble to test. There may be an issue
with USB being a bit slower than PCMCIA, but will everyone really
be running at 11Mb?
So far so good, the next question is, can we get into it? This turned out
to be fairly simple. The board is setup for easy access
to the antenna terminals. We also found that the second antenna is easily
disabled and connected to an external antenna. This has
some serious possibilities. We could run two antennas in diversity mode.
Due to the negligible amount of additional coax, this is quite inexpensive.
The whole lot can be mounted in a waterproof box very close
to the antenna feed point.
Having some surplus Galaxy's about we had an interesting idea. What if we
put the card in front of a grid in about the same place as the dipole sits
in the feedhorn? Some experimentation came up with a fairly predictable
result. Approximately 13db gain on the original signal levels. With some
more accurate positioning and a few mods, I am sure this could be better.
So with some work we might not even need to open the box and take to it
with soldering iron.
Now the cabling.... It seems you can only run 5m
of USB cable between two devices. That's a real shame because we were
so close. However, we found a 5m active USB cable advertised at
www.ht.com.au . Seems you can use this with a simple 5m cable to do 10m.
You can use 4-5 of these things if you really want to get 20-25m of USB.
We have seen prices elsewhere as low as $30 / 5m cable. So it works out
about $6 per meter in cable. Less than LMR-400 and definitely less
than the LMR-400 and the connectors for each end. The USB
cables are definitely easier for the amateur to setup too.
The only remaining problem was that of our PC. We have some old 486's and
some low end Pa's that we want to use in the roof at each end.
Unfortunately no USB ports. Nevermind www.ht.com.au has some USB dual
port cards for $29 and they work with Linux. Much better than the
$120 PCI -> PCMCIA converters.
The testing continues and I'll keep you all posted.
Cheers
Richard
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