[mythtv-users] OT: Indoor VHF gain antenna

[Brian Wood]

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.