Tam,

Thanks for the information and the URL on Lewallen'w work. This
configuration appears the same as the front end of the channel master 3018
VHF/UHF combo. As such it appears I could expect 10 dBi to 8dBd from it. I
will try to run down the analysis source and see if a dipole feed impedance
was computed. Thanks again.


"Tam/WB2TT" wrote in message
...

"CCW N4AOX" wrote in message
...
I thought about using one of these for DTV reception. I am curious about
the feed point impedance. The antenna (see it at radioshack.com) looks
like a corner reflector of sorts. But, it has an extended boom ahead of
the feed point dipole with yagi like elements (cut much shorter than the
dipole). The feed point is an approx. 1/2 wave split dipole, insulated and
center fed.

The feed point 1/2 wave dipole (center fed) would have a 75 ohm
impedance free space. Corner reflector design data shows an approx. 70
ohm impedance for a similar configuration, but without the extended boom
ahead of the feed dipole.

The RadioShack accompaning literature implies a 200-300 ohm feed point
impedance, ie. it states that a 300 / 75 ohm balun and RG-6 feed line
should be attached. Has anyone measured, or modeled the feed point
impedance of this antenna or have an explanation of how they arrive at
200-300 ohms?

N4AOX




Sounds like my antenna, if the current price is 24.99. It is 300 Ohm
output. It's called a Corner Reflector Yagy. I would'n get too exited
about the impedance, but you can get some idea about the gain by going to
http://www.hdtvprimer.com/ANTENNAS/comparing.htmlI
Scroll down to the Radio Shck VU75, and look at the UHF gain. Keep in mind
this is an old design consumer product, and probably predates any computer
simulations. More than likely (hopefully) the 300 Ohms means it works
better feeding a 300 Ohm load than a 75 Ohm load. In general, when you add
directors to a dipole the impedance goes down from 75 Ohms, not up. They
may have gotten the impedance up because the feed point is several inches
from the center of the driven element.

I am using it with a 300:75 Ohm transformer at the feed point, feeding a
Channel Master UHF only ptreamp, mounted just below the rotator. Feed line
is about 50 feet of RG6. On good days I get HD stations from 65 miles
away. Antenna is at about 25 feet.

Tam