[Fwd: Re: Debugging a helical antenna installation]

[S é bastien Wailliez]

Hello,

Though my helical antenna (http://users.bigpond.net.au/jhecker) may not
really be considered a finished product, I tried to test it as it is right
now and faced the same problem as was described by Alan. The helical
exhibits no gain at all compared to the integrated antenna in my PCMCIA
card, it even degrades my signal actually.

As I said, the antenna is not finished but I followed the design document
scrupulously so far. The antenna is still missing an N-connector at the
moment and my Orinoco pigtail cable was taken from the integrated Airport
wiring of a PowerMac G4 (i.e. it has an Orinoco proprietary connector on one
end and bare wiring on the other end). G4's have small range-extenders in
their handles and I chose to cut the pigtail cable just before the antenna
because I do not plan to use an airport card on that machine and even if I
changed my mind I would only need a tiny solder joint.

Since I do not have copper tape, I tried to make an impedance converter with
various strips of metal. I experimented a bit but never actually got the
antenna to work. Any good reference on impedance matching would be
appreciated because the explanation in the helical antenna design document
is a bit too weak and aimed at that specific project.

At the time, the antenna does not have an N-connector yet and I conducted
all my experiments by just putting the bare end of the pigtail cable in
contact with either the impedance converter or the antenna copper wire.

I am really wondering wether there is something wrong with the pigtail cable
(Apple Airport is just repackaged Lucent hardware, so the pigtail from a G4
should a priori work), the impedance converter or even the antenna itself.

Is there something obviously wrong in my description? Any idea for further
investigation? Thanks for helping!

Sébastien


> De : mick stuckings <mick at faceng.anu.edu.au>
> Date : Mon, 06 Aug 2001 10:12:15 +1000
> À : wireless <wireless at lists.samba.org>
> Objet : [Fwd: Re: Debugging a helical antenna installation]
> 
> Hi,
> If it's the cheap 50ohm ethernet, don't even bother trying, that's your
> problem. I tried this already :) You could easily be getting >50dB loss
> per 10m (I looked that up).
> If you order a pre-terminated cable of LMR or similar and cut it in
> half, it's cheaper than buying the bits. LMR-400 is a good trade off
> with loss vs flexibility for longer runs (that's what others suggested
> to me), but it's not so good for connecting straight to the card as it's
> a bit thick.
> For anyone interested here's what we did with webgears:
> http://hilbert.anu.edu.au/~mick/wireless/
> If anyone has any corrections to this I'd be happy to fix it :)
> Mick
> 
> Alan Wilkie wrote:
> 
>>> What kind of cable is it? When we were doing this we made sure there was a
>>> maximum of 2m of cable at each end and that was pretty expensive cable.
>>> We're dealing with 2.4GHz here and you'd be suprised at the kind of cable
>>> losses you can get.
>> 
>> 
>> I'm not sure exactly what grade the cable is, all I know is that it's 50
>> ohm coax.  I expected that there would be a reasonable amount of
>> attenuation in the cabling, but the antennae should be about 14dBi gain
>> and I thought there would be loads of signal to play with over the short
>> distance we're trying to span.
>> 
>>> With those full length ISA cards we had to chop the cable to get the
>>> connector off and we had minor difficulties with the joints, but other than
>>> that it seemed to work OK. From what I remember it's cable losses and
>>> impedance that are your major issues.
>> 
>> 
>> Thanks for that, we'll give it a try with shorter cables and try a
>> different method of connecting the cables to the wireless cards.
>> 
>> Alan
>> 
>> 
>> 
> 
>