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