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