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

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

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

Then how come it doesn't give us 20db of forward gain?

It does, if you (incorrectly ) define forward gain as being relative to the
backside.

As I said, the signal (that) now no longer goes out the back gets its
power---redistributed--and that redistribution is like pushing the air forward.
It doesn't -- all-- get concentrated in one spot up front, thus the forward
gain isn't 20 dB.

73,
Chip N1IR

Look at a polar antenna plot (either in the vertical (elevation) or
horizontal plane, though horiz would be best for this exercise). The greater
the gain *straight* forward the less the angle that the gain is within
say -3dB of the peak. F/B ratio is a measure of how much more sensitive it
is dead ahead compared to dead behind, and ignores the sensitivity at any
other angle, generally (but not always) there will be one large lobe
pointed forward and only little bumps in sensitivity to the sides.

Another way to think of it (a thought experiment if you will) is as a garden
hose w/ a fine stream out the front (very high gain and very high F/B ratio
(very little water shoots straight back at you, and only a small spot that
you aim at gets wet, vs. a hose w/ the end cut off and flattened to form a
flat fanlike spray -- still good F/B (you stay dry) but the water goes less
far and covers a bigger horiz angle. Last, imagine you unflatten the open
hose end, point it straight up and hold a plate over it so water sprays in
every direction horizontally -- this is an omnidirectional pattern (like a
dipole, where RF leaves perpendicular to the wire or element) and goes even
less distance (gain) but goes in every horizontal direction. In all cases
the same amount of water comes out, but as you change the nozzle you send
more in one direction at the expense of all the others. The antenna is the
same when transmitting or receiving (collecting if you will) RF energy.

If your hose has some small pinholes in it you may get "side lobes" but they
will usually be narrow angled or weak (low gain) -- differant cause than the
antenna, but same effect for the purposed of the thought experiment.

Last, if you had a hollow spherical ball much bigger than the hose diameter
that fastened on the hose's end and this ball had little holes evenly spaced
all over it (and a good imagination) water would go in EVERY direction
EQUALLY -- the equvalent of the *theoretical* isotropic antenna.

The analogy is not exact, but it should give a feel of the thing.

Note too some water or RF goes out at an angle from horiz (this is what you
see in the vertical plane polar charts) and if you hold th ehose or antenna
too close to the ground none can go down (it either soaks into the ground or
bounces off and goes upward a bit) giving a slight "takeoff angle" to the
beam (or stream).

That help, or only confuse?

"Fractenna" wrote in message
...
Then how come it doesn't give us 20db of forward gain?

It does, if you (incorrectly ) define forward gain as being relative to
the
backside.

As I said, the signal (that) now no longer goes out the back gets its
power---redistributed--and that redistribution is like pushing the air
forward.
It doesn't -- all-- get concentrated in one spot up front, thus the
forward
gain isn't 20 dB.

73,
Chip N1IR

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

Oh, and one thing NOT mentioned, about the Isotropic-- It is defined to
radiate equally well in ALL directions-- It has NO Front to Back Ratio (if
it ever could exist!) This is why it has a loss in comparison to a dipole ,
which is directional parallel to it, and has great loss perpendicular to it
! -- Jim

N4LQ wrote:

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?
. . .
Something just doesn't add up.

My car goes about 100 MPH (I assume) in the forward direction, but only
about 15 MPH in reverse. When I'm in reverse, where did the other 85 MPH go?

The answer to that question is about the same as the answer to yours, so
when you figure out the answer to my car question, you'll have the
answer to your antenna question.

Roy Lewallen, W7EL

The beam width is not 1 degree -- more like 35 degrees + so the energy is
spread over that area
Narrow the beamwidth and the gain goes up

--
Lamont Cranston

The Shadow Knows
"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

And then, there are all those side lobes!

Phil

"Da Shadow" wrote in message
news:bguzc.4044$0z6.3664@fed1read07...
The beam width is not 1 degree -- more like 35 degrees + so the energy is
spread over that area
Narrow the beamwidth and the gain goes up

--
Lamont Cranston

The Shadow Knows
"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