Re: Re: will satellite dish enhance the R2?

Hi Brian, all,
----- Original Message -----
From: "Brian" <rocketmannm-/[email protected]>
To: <[email protected]>
Sent: Tuesday, May 13, 2003 3:52 PM
Subject: Re: [icomr2] Re: will satellite dish enhance the R2?


> Aside from the whether or not this rather silly question was asked
seriously or just to ping the group (I'm beginning to wonder if some
realized that), basic radio theory says Ubbe is correct - to a point.
>
> Any parabolic reflector will focus recieved radio waves onto an antenna -
that's why they are used. The "feedhorn" that is normally used in this setup
is essentially nothing but an antenna itself that is specially shaped to
take advantage of the circular reflection of the dish, and the parabolic
dish focuses the radiant energy onto it (the dish itself is not really the
active portion of the antenna, as most people assume, just a passive
reflector, like the last element of a beam antenna).
>
> The problem comes in when you take frequency into account. The location
and efficiency of a parabola's focal point is frequency dependent. There is
also a range where focus is possible dependent that is dictated by the arc
of the parabolic dish. When you combine these two basic laws of phyics you
find that outside of this range the parabolic reflector has no more
efficiency as an antenna 'helper' than a flat metal plate - or worse. Since
you would have to calculate that focal point for a wide range of frequencies
for normal scanner use (some, most, or all of which would be outside the
design limitations of the dish), and constantly move your radio to get the
focus for those signals unles they were very close on the spectrum to each
other.
>
> To summarize, using a parabolic dish for your scanner as a passive
'antenna helper' would (technically) give some small reflected gain. Not
enough to be worth the trouble - because the dish was optimally designed for
frequency reflection at a far frequency higher range than the scanner picks
up - that's why you only see dish antennas being used to microwave
frequencies. Yes, they will work at lower frequencies (I once saw a huge
parabolic dish designed for 10 meters, of all things!) but they are
ridiculously cumbersome compared to the high gain antennas commonly used for
VHF/UHF, and horribly inefficient if not designed perfectly for the
frequency band they are used for.  The reflective element of a good yagi or
LPDA will do magnitudes better in gain increases. So would a good low-noise
signal amp.
>
> The best suggestion for using a dish with the R2 I have heard so far is
catching some sun on it while listening to your scanner. You may as well
hold a sheet of tinfoil behind the antenna for the same reflective
efficiency level. This holds true even as one person suggested using a
feedhorn and cable, because unless the horn is constructed for the correct
frequency pickup range, it is  basically a random chuck of metal located in
a random focal pattern -and about that efficient, too.
> For those of you with electronics degrees, I realize this is an
oversimplified explanation, but it is done to get the basic idea across,
while avoiding the math for capture areas, reflective incident angles, etc.
It all boils down to trying to use an antenna outside of it's design range -
if you clip a coat-hanger to the R2's antenna jack it will recieve - but any
'improvement' is a different story (and dependent on the frequency
characteristics of the hanger, obviously).
>
> -Brian
>
> Ubbe <[email protected]> wrote:
> But if you where to hold the R2 in the dishes focus, wouldn't that
increase the reception regardless of frequency? My guess is it would very
much do so, and by moving the R2 around the focal point it would be possible
to find the maximum. At the maximum the dish would be very directional with
a small beam but with the R2 out of focus the direction and beam would open
up. The question is what happens to vertical polarized signals in a dish
that normally use circular polarization?
>
See my last reply to this question. It says what Brian as said but in a much
shorter form......Ken.


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