[mythtv-users] OT: Indoor VHF gain antenna

[Ben Kamen]

On 1/26/2010 8:31 PM, Brian Wood wrote:
>
> 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.

You can check www.antennaweb.org with your zip to see what stations are in the area and
what "real" channel they're on. From there, you can look at the ATSC frequency map here:

http://en.wikipedia.org/wiki/North_American_broadcast_television_frequencies

to get an idea if they're VHF or UHF (which now dictates which part of the antenna they might use)

Sometimes I wonder if they should just dump the VHF band altogether and stick 'em all in UHF.

>>
>>> 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.

Not necessarily. http://en.wikipedia.org/wiki/Dipole_antenna

> 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.

And their elements are about the same length. (note, "about" and not exactly.)

Typically, the directors are close if not same length, the radiator is another length
(slightly longer than the director) and the reflector is a third length (slightly longer than the radiator).

http://en.wikipedia.org/wiki/Yagi_antenna

Cheers,

  --Ben