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

CCW N4AOX wrote:
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


I've got one of those things. I use it for ATV. I consider it a 300 ohm
feed.
I've never tried to measure or model it, but I'm just trusting the
designers
considered that, being it's semi critical for UHF. Also, the antenna is

quite broadbanded, being a psuedo log periodic of sorts. I imagine the
feed impedance can change a bit with frequency. I doubt it's the exact
same at
600 mhz, as at 420 mhz.
BTW, using all 300 ohm line is less lossy than using the coax and
balun.
Well, unless maybe you use some super duper coax... At 420 mhz, I
always
had better results with the 300 ohm line. The only drawback is a good
bit of loss when it's raining hard.. :/
MK

"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

I should have mentioned that there are/were two versions of that antenna.
The one I was talking about is 40 inches long. The other one, which I think
is no longer available, is more than twice as long, and equal to the UHF
portion of the VU 120 or VU190 (count the elements). The picture in the
catalog of the short one is wrong.

Tam

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