[TowerTalk] Two 15M yagi stacking
jimlux at earthlink.net
Sat Feb 24 15:20:43 EST 2007
At 09:55 AM 2/24/2007, Stan Stockton wrote:
> >With 5 or 6 elements from here you can not
>cover all of Europe without rotating them.
> I strongly disagree. My 6 element KLM array
>has a 3 dB beamwidth of about 54 degrees, which
>is plenty for Europe.
>I guess when people talk about 3 dB beamwidth it is just a measure
>of reference, so I will use it as well. Let's say you could double
>the boomlength and realize about 3 dB more of gain than the original
>antenna. Now if you want to compare the two antennas look at what
>the 6 dB beamwidth is of the new antenna. However many degrees that
>is will be the place where the two antennas would be equal and
>everything less than that, the larger antenna will be better!
The only problem with this is that some of the time, the advantage of
a directive antenna is in the rejection of signals you don't want,
rather than the forward gain (that is, you might be willing to give
up efficiency to achieve a higher directivity). It might be that the
point at which the gain is down, say, 20 dB is the important thing.
(An example is antennas for receiving DBS satellite signals.. you
especially want to reject the signals from satellites that are
adjacent in the equatorial band... Or, maybe you're in southern
Californiai and want to suppress all those big signals from 5 land,
when you're shooting north to EU)
Especially when it comes to suppressing atmospheric noise (which is
very unevenly distributed) this might actually be more important than
raw gain (at least on receive). On transmit, of course, you want all
the gain you can get in the desired direction.
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