Steve- you are compairing apples to onions! Why?? Simply, there is no
absolute relationship between forward gain, in dB, and the front/Back ratio
(also in dB)! The forward GAIN is relative to either a DIPOLE , or an
Isotropic point source! If you were to place a dipole at the same location,
as the beam
(for arguements sake), the signal would be 5 dB louder on the beam, then on
a dipole. And an isotropic
point source (really, an immaginary "antenna", is aprox. 1.8 db dB below a
dipole (beam would be aprox. 6.8 dBi rated) The front to back ratio, on
the other hand, simply means that the signal off the front of the
beam will be 20 dB LOUDER (for want of a better word) than it will be
recieved, if the antenna is turned 180 degrees! Hope this explains it!
that is how any gain antenna works, by takeing power from an
unwanted direction, and concentrateing it in a desired direction! Jim NN7K

--
No trees were killed in the sending of this message. However, a large number
of electrons were terribly inconvenienced !

" People who never get carried away, should be! " --- Malcom Forbes

"N4LQ" wrote in message ...
Then how come it doesn't give us 20db of forward gain? I mean, we blocked
20db off the back and applied it to the front and we still only have 5db
of
forward gain. Where did the extra 15db go?
I think those are typical figures for most triband beams I've seen
advertised. 5db forward and 20db front to back. I would expect the guy on
the other end to hear me by 5db stronger than a dipole and my beam to
reject
signals by 20db off the back. Well let's say the guy on the back side of
my
beam hears me at S9 then I rotate the beam around to him. My signal then
goes up 20db. Right? Is that 20db of gain? Well in comparison to the other
way, yes but in comparison to a dipole, no since the dipole would have
been
only 5db less than the beam in the favored direction. Now the dipole would
be 15 db stronger than the signal off the beam's back end since it has no
front to back ratio but it's only 5db down from the beam in the forward
direction. Thus we conclude the dipole produces 15db more signal than the
beam. Impossible.
Something just doesn't add up.

--
Steve N4LQ
"Fractenna" wrote in message
...

Say a beam has 5db forward gain and a front to back difference of 20db.
Where does the extra 15db go? I mean, if you loose 20db off the back on
transmit, I assume you also loose that much on receive. What happened
to
the
20db? Did it burn up as heat? What am I missing here?

--
Steve


The analogy to a balloon is pretty apt.

Squeeze and pinch the back so only about 1/50 th of the air that used to
be
there is left. The rest gets distributed towards the front, and makes
the
front
side more than a factor of two bigger. But the ratio of the amount of
air
in
the front to that of the back is very big--say, 100.

That's a 20dB F/B.

Hope that helps.

73,
Chip N1IR