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

"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?

Well... you won't handle it like that. dB aren't preserved in this
fashion. Power is...

The balloon analogy it good. Poke it "in" at one point so there is NO
radiation in that direction. The signal in the "poked-in" direction will be
really low or zero or MANY MANY MANY dB down...hundreds.


However, try this as another way to see it:

Imagine an antenna that radiates equally well in all directions AROUND the
antenna like an ideal vertical. It will have SOME signal strength in all
directions, which is the same.

Now, lets imagine another antenna (more complex and let's ignore how) which
only radiated HALF way around, but equally distributed, just like before.
That is, it cuts off all radiation to one side as though it was cut with a
knife -- and re-distributes the "cut-off power to the "good" half.
Nothing to the "bad" half -- everything to the "good" half.

The good half just went up 3dB, right ... twice the power.

The bad half went to zero. or infinity dB.

You don't conserve dB.

Help ???

--
Steve N, K,9;d, c. i My email has no u's.

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

N4LQ has been a licensed ham since 1964, I would dare say he already knew
the answer to this goofy question and was just looking for some funny
responses.....

Mark W4UDX


"N4LQ" 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

Forgot to mention.... it kinda reminded me of when that one guy who was
looking for a 160M HT...

Mark W4UDX


"W4UDX" wrote in message
...
N4LQ has been a licensed ham since 1964, I would dare say he already knew
the answer to this goofy question and was just looking for some funny
responses.....

Mark W4UDX


"N4LQ" 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

On Wed, 16 Jun 2004 19:06:30 -0400, "W4UDX"
wrote:

Forgot to mention.... it kinda reminded me of when that one guy who was
looking for a 160M HT...

Mark W4UDX

The transceiver is portable, except for the antenna. ;-)

Happy trails,
Gary (net.yogi.bear)
------------------------------------------------
at the 51st percentile of ursine intelligence

Gary D. Schwartz, Needham, MA, USA
Please reply to: garyDOTschwartzATpoboxDOTcom

Why NOT ??? Afterall, the "Gibson Girl" worked on 500 kHz !! Now THAT is
REAL QRP DX!
;) Jim NN7K


Mark wrote:
Forgot to mention.... it kinda reminded me of when that one guy who was
looking for a 160M HT...

Mark W4UDX

W4UDX wrote:
Forgot to mention.... it kinda reminded me of when that one guy who was
looking for a 160M HT...

We had quite a few 160M HT's right after WWII, thanks to MARS
and military surplus equipment.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

I bet they didn't have a rubber antenna with a BNC connector....


"Cecil Moore" wrote in message
...
W4UDX wrote:
Forgot to mention.... it kinda reminded me of when that one guy who was
looking for a 160M HT...

We had quite a few 160M HT's right after WWII, thanks to MARS
and military surplus equipment.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----

W4UDX wrote,
Message-id:

I bet they didn't have a rubber antenna with a BNC connector....


"Cecil Moore" wrote in message
...
W4UDX wrote:
Forgot to mention.... it kinda reminded me of when that one guy who was
looking for a 160M HT...

We had quite a few 160M HT's right after WWII, thanks to MARS
and military surplus equipment.
--
73, Cecil http://www.qsl.net/w5dxp



-----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
-----== Over 100,000 Newsgroups - 19 Different Servers! =-----


They were probably called "walkie-talkies," too. Who thinks up the names
for things like this?
73,
Tom Donaly, KA6RUH