"aiming" antennas

"aiming" antennas

Post by Warren5 » Sun, 22 Mar 1998 04:00:00

how do SW stations "aim" their antennas?

is it possible to aim a dipole antenna?
if so, how?

thanks
warren
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"Yes, I'd like a McChicken Sandwich and medium fry."
"Sorry, sir. We don't have a medium fry. We only have Large,
Extra-Large, & Super-Size fries."

 
 
 

"aiming" antennas

Post by rsm.. » Mon, 23 Mar 1998 04:00:00




Quote:>how do SW stations "aim" their antennas?
>is it possible to aim a dipole antenna?
>if so, how?

Shortwave broadcasters often have elaborate antenna arrays that are
designed to radiate in a particular pattern.

A dipole at a reasonable height (about as high as the antenna is long,
assuming we're talking about half-wave dipoles) has a directional pattern,
too, although it's not very sharp, and will receive signals best at right
angles to the antenna.

In other words, a dipole running from east to west will receive stations
best that are coming from the north or south.

The closer to the ground the dipole is, the more ground reflection will
decrease the directionality.

For more information on basic antenna systems, Radio Shack has a helpful
booklet for SWLs, and the ARRL sells the well known ARRL Antenna Book,
which you may also find in your local library.

73, Bob, VE7HS

Quote:>thanks
>warren
>--------------------------------------------------------------------------
>-------------------------------
>"Yes, I'd like a McChicken Sandwich and medium fry."
>"Sorry, sir. We don't have a medium fry. We only have Large, Extra-Large,
>& Super-Size fries."

--
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"aiming" antennas

Post by Tom » Mon, 23 Mar 1998 04:00:00


> how do SW stations "aim" their antennas?

> is it possible to aim a dipole antenna?
> if so, how?

> thanks
> warren
> --------------------------------------------------------------------------
> -------------------------------
> "Yes, I'd like a McChicken Sandwich and medium fry."
> "Sorry, sir. We don't have a medium fry. We only have Large,
> Extra-Large, & Super-Size fries."Hi, Warren. The answer is complicated but, to make it short, antennas and

their elements are "phased" - ie. carefully positioned - so that the
radio waves emiting from them combine "in phase" in the direction of
interest.  Thus, multiple radio signals, each from it's own antenna (or
element) combine in space (or the atmosphere)...similar to waves ove
water building one upon the next to become larger.  Phasing can also be
done to reduce a signal in an unwanted direction.

Dipole antennas at less than 1/2 wavelength above ground are
multi-directional. However, if you take one and suspend it from a metal
structure (like a tower) - one end to the top and the antenna*** on
a 45 degree angle toward the ground with the lead-in wire coming off the
antenna at a 90 degree angle back to the tower, it is said that the
antenna will exhibit slight directionality in the direction the antenna
is*** from the tower - I've tried it and didn't see much.

73/tom.

 
 
 

"aiming" antennas

Post by CW » Tue, 24 Mar 1998 04:00:00

Shortwave stations use directional antennas such as log piriodic dipole
arrays, Sterba curtains, ect. that have to be pointed in the direction that
they want the signal to go. They are either turned with a rotator or are
built facing the desired direction. On the lower frequencies they are so
huge that they are generally not rotated but it can be done. If you are
really interested I can direct you to a manufacturer that sells multi-band
shortwave log periodic antennas ( 3-30 Mhz ) and rotators for them but be
forwarned the monsters weigh 16000 pounds and the entire tower must rotated
to aim them. A little much for my  backyard.
On a more serious note, a dipole will show some directive caractoristics.
On it's resonant frequency and below it's highest gain will be off the
sides of the wire. On higher frequencies it will have highest gain in the
direction of the wire. As you can see, to achieve directionality across the
entire band would involve multiple rotatable antennas, such the log
periodic dipole array mentioned above.
--
CW
KC7NOD



> > how do SW stations "aim" their antennas?

> > is it possible to aim a dipole antenna?
> > if so, how?

> > thanks
> > warren

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