"Mark Atanovich" wrote in message
news:k7eKb.26608$i55.6640@fed1read06...
Big Yagi:


http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=41 298

Is this BS or is this kind of gain at least theoretically possible?


Most likely big BS . I did not see what the db was related to. The
biggest Yagi I could find in a quick search of my material at home that I
have any faith in is 40 elements and 20.8 dbi is all that it has. That is
almost 3 times as many elements as the ebay antenna. Assuming almost a
perfect antenna it would hav to have a boom of over 30 wavelengths to get to
24 dbi.

The antenna may well have 36 db of gain over the internal antenna of the
cell phone. If you cup your hand around a cell phone you may loose the 20
or more db of gain. That and about 15 dbi of gain that a 15 element beam
will have may be close to the 36 db of antenna gain.

Any antenna that gives much over 10 to 15 db of gain is not to be trusted.
Some will do that but not many that are on the market and you have to know
the physical size (boom lenght) to get a good estiment.

Ralph Mowery wrote:
"Most likely big BS."

Also:
"---40 elements and 20.8 dbi is all that has. That is almost 3 times as
many elements as the ebay antenna."

Ralph is on the mark.

John Kraus in "Antennas", page 705, demonstrates how the gain of a
curtain antenna adds up:

"The gain of a single 1/2-wave dipole is 2.15 dBi and of 2 collinear
in-phase 1/2-wave dipoles is 3.8 dBi. The array of 8 such collinear
dipoles adds 3+3+3 =9dB. The reflectoer screen adds 3 dB more and the
ground bounce another 6 dB for a grand total of 3.8+9+3+6=21.8 dBi or a
directivity of 151 approx."

36 db is the sort of gain that one can get from a dish that is very
large in terms of wavelength and that has extremely small imperfections
in its surface. As Kraus shows, you only hope to gain 3 db more when you
double the number of elements in an array.

Best regards, Richard Harrison, KB5WZI