Jeff Liebermann <(E-Mail Removed)> wrote in
news:(E-Mail Removed):
> On Fri, 8 Oct 2010 03:56:59 +0000 (UTC), Hobbyist
> <(E-Mail Removed)> wrote:
>
>>a) As far as reception power goes, will it improve or worsen reception
>>to make the reflector larger beyond the dimensions of the the whip
>>antenna?
>
> Not really. If you double the aperture size of the antenna, you'll
> get a theoretical increase in gain of 3dB. However, that doesn't
> include reflector efficiency, cancellation due to multiple paths (more
> on this later), feed overspray, etc. Also, antennas of this type tend
> to have different gains in TX as in RX (due to overspray). To get
> them equal, matching the feed pattern to the dish f/D (focal length to
> diameter) ratio is important. With an uncontrolled feed (i.e. omni
> rubber ducky antenna), much of the RF radiated by the feed does NOT
> hit the dish, and therefore is not sent out the window in the desired
> direction. If I arbitrarily assume that half the radiation from the
> omni does not hit the dish, the TX gain is automatically 3dB less in
> TX than in RX, where all the RF that hits the dish ends up somewhere
> on the omni antenna.
>
> Start here:
> <http://www.w1ghz.org/antbook/contents.htm>
> and especially read the section on feed design and overspray
> (spillover) and matching the feed to the dish.
> <http://www.w1ghz.org/antbook/chap6-0.pdf>
>
>>I was thinking of doubling the size in both dimensions which
>>will make it significantly larger than the antenna. Instead of 1' by
1'
>>approx 2' by 2'. Directions call for it to be equally tall as it is
>>wide, do they have that right?
>
> There's no technical requirement that the antenna be symmetrical. It's
> just easier to calculate that way. The gain is mostly determined by
> the frontal area (aperture size), which can be rather asymmetrical.
> However, make an effort to make the antenna longer than the omni
> antenna feed or RF off the end of the omni will miss the reflector.
>
>>b) what else can be done simply to improve the receive portion of the
>>signal in construction of this antenna.
>
> Simplify it. There's really no need for a parabola. A flat plate
> reflector or two flat plates forming a corner reflector will work
> almost as well. See:
> <http://802.11junk.com/jeffl/antennas/FullWavePlateReflector/>
> <http://802.11junk.com/jeffl/antennas/HawkDish08/>
> This is a comparison of a parabolic dish, as found on a Hawking
> something client radio, and an equivalent sized flat plate reflector.
> 7.7 for the dish, and 8.3 for the flat plate. A corner reflector will
> do better, but I'm too lazy to model one.
>
> What you must realize is that as antennas get bigger, and gains
> increase, the dimensions and limitations become more critical. The
> reason the aforementioned antennas, and most of the salad bowl and
> kitchen accessories reflectors work is that all of these offer an
> improvement over a simple rubber ducky antenna at low gains. However,
> to use the same techniques for higher gain antennas just isn't going
> to work. A slight construction or dimension error, and it just won't
> work as well as expected.
>
>>c) I will use a 10 foot usb extension to my radio, does that cut the
>>power of the tranceiver at all?
>
> Nope. The xmit power at the end of a USB cable will be exactly the
> same as the xmit power when the USB dongle is plugged directly into
> the computah. The only loss is in the +5V DC power to the device,
> which is adequate for most such dongles. On the other foot, 10ft of
> coaxial cable has a rather high loss at 2.4GHz and should be avoided
> if possible.
>
>>Does the placement of the usb extension
>>wire factor into receive sensitivity?
>
> Nothing you do to the antenna affects the "receive sensitivity".
> That's measured at the receiver input, without an antenna.
>
> The gain of the antenna is affected by the placement of the USB cable.
> It's best to run it through the center of the dish, where it doesn't
> block the signal in any way.
>
>>d) the antenna screws into the top of the little usb radio. The usb
>>radio is about 2-3 inches in height itself. Does the radio body take
>>part in any signal pickup or interference thereof?
>
> A little. The radio itself is not very well shielded and can radiate
> quite a bit of RF without any antenna. Most of the RF comes out of
> your unspecified model USB radio, but there is a little leakage from
> the case. If you mount the USB radio in the aperture of the dish or
> reflector, it's going to block some of the signal. Same with the
> cable. Look at it from the optical point of view and pretend your
> working with light instead of RF.
>
>>e) where exactly is the pickup inside most of these antennas as shown
in
>>the pictures?
>
> I can't tell because you haven't bothered to supply what device you're
> using. Usually, it's a tiny ceramic antenna at the very end of the
> USB dongle, which will form a rough hemispherical pattern.
>
> Now, I have a question. A few months ago, you posted this photo:
> <http://www.flickr.com/photos/4975853...n/photostream/
#/>
> and engaged in an irritating and pointless dialog over its purpose,
> despite several people having successfully guessed the manufacturer
> and its function. Wanna explain what that was all about?
>
You are a thorough bloke aren't ya? Hehe, I like you Jeff, so as one
sometime irritator to another I will answer.
I was getting open wifi connections from several of those pictured
devices that could not seemingly be found to be emanating from any other
AP. While I knew ahead of the post that these devices are *supposedly*
fiber optic hardware of some kind, after alot of exploring I could find
no other probable nearby source of the APs I was connecting to. Neither
antennas, nor any other conceivable source of the signal. The signals
were also strongest when pointed directy at those devices, and dropped
off when pointed away. I concluded, rightly or wrongly that the signals
were coming from the pictured devices, located in areas that suggested
some type of mesh network. I was attempting, with some success, to draw
out replies from a group that is usually either ignorant or anal-
retentive in trying to answer questions from anyone who is not
(stupidly) paying $40 a month for a simple wifi connection. In fact this
group would be rather worthless if it was not for the occasional good
answers coming from you. Since I am no electronic guru like you, it was
impossible for me to determine if the wires leading away from those
devices were power cables or not and cannot answer your question about
that. In fact RF wifi communication is so bizarre I frequently wonder if
etheral spirits are guiding it's weird permutations. I have no desire to
enroll back in college to attempt to gather the math necessary to get to
your level of expertise in these matters, which I what I would have to
do to fully understand WHAT THE HELL IS HAPPENING??!! :-) Have a nice
day, Jeff.
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