[mythtv-users] OT: Indoor VHF gain antenna
Brian Wood
beww at beww.org
Wed Jan 27 02:31:05 UTC 2010
On Tuesday 26 January 2010 06:09:18 pm Tom wrote:
> At 04:06 PM 1/26/2010 -0800, you wrote:
> >Anyone have ideas for an indoor VHF gain antenna?
> >
> >I have one station on VHF I want to receive, on RF channel 9. It's fairly
> >weak at my location because I'm getting it via diffraction over two hills,
>
> You need to know the actual frequency of RF channel 9, and whether it's
> been moved to a UHF allocation in the digital transition.
Good point, a lot of stations still call themselves "Channel X", X being a VHF
channel, even though they have moved to UHF. "Channel 5" here is actually on
channel 30.
>
> >and I live in a first floor apartment. I've had some success with a
> >hand-cut half-wave wire dipole up near the ceiling, with a 4:1 balun to
>
> A dipole of course has no gain.
True, if the reference is to a dipole, then by definition it would be 0db.
gain. "dbi" is referenced to an isotropic source, not a dipole (hence the
"i"), so a dipole would have a positive gain "dbi", and zero "db.".
Generally dbi is used with transmitting antennas (hence isotropic "source"),
but it is sometimes used for receiving antennas.
An isotropic point is a mathematical abstraction, not attainable in the real
world, but it makes antenna calculations easy(er), and also makes products
look better.
A Yagi is a dipole, with reflector and director elements, so the impedance is
the same as the center of a dipole, unless the directional elements are close
enough to the driven element to effect the impedance.
Broadband TV antennas are usually log periodics, a modification of a yagi to
achieve a wide frequency response. True Yagis are generally for a single
channel, used by cable companies at headends, or in this case in 2nd floor
apartments.
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