[mythtv-users] OT: Indoor VHF gain antenna

[Tom]

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.

>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.

>match it to 75-ohm coax, but the signal is right on the edge of usable.

I think you can build a yagi with 75-ohm impedance.

>Some weeks it comes in, some it doesn't.  Clearly I need a bit more
>antenna gain than the 2.1 dBi the dipole (theoretically) has.

Aiming it could be tricky, not to mention picky.

>The antenna will go into a back room, so it doesn't have to be beautiful,
>but it can't be completely unwieldy, either.
>
>I've experimented with adding a second length of wire on the ceiling,
>behind the dipole, to act as a Yagi reflector, but didn't see any
>difference in signal strength.

A Yagi could be built with multiple directors and reflectors. The directors 
go in front of the driven element and are shortened with respect to the DE. 
The reflectors are slightly longer than the DE. In many designs the 
directors increase in size as they get farther from the DE.

But the dimensions of the reflectors, directors, and driven element are 
critical. Contruction is not so critical. It's better to use larger size 
conductors, like maybe 1/4" aluminum rod, but heck, even 12 gauge copper 
wire will do. You could tape it to the ceiling, if there isn't tons of 
metal in the walls facing the station.

I'm not sure what the Frequency of channel 9 is. Say it's 186.31 MHz.
Take that to any online antenna calculator, like 
http://bfn.org/~bn589/antenna.html
Plug your numbers in and a basic 3-element yagi design pops up:
Director: 2 feet 4 and 11/16 inches
Driven Elements: 1' 3-1/4'' each, or 2' 6-15/32 total
Reflector: 2' 8-9/32"
Spacing: 1' plus 1/16" between director and DE, and DE and reflector.

Adding directors and reflectors increases directionality and maybe gain 
too. Here's a calculator that lets you change the number of elements. It's 
much more complicated than the first one: 
http://fermi.la.asu.edu/w9cf/yagipub/ and click on the "Click Here for the 
Yagi Modeler"

It's very slick- takes conductor diameter into account, and plots a polar 
pattern graph of how it will perform. One of the examples is a 13-element 
yagi, with one reflector behind the DE, and 11 directors out front. The 
reflector is at zero inches from the back of the antenna and it 41.5 inches 
long. The DE is at 20 inches and is 39.5'' long. Etc etc etc. This critter 
is 282 inches long-  23.5 feet! So clearly, it's possible to get nuts with it.

Here's a list of antenna and line-of-sight calculators for all purposes:
http://www.educypedia.be/electronics/electroniccalculatorsant.htm

Hope that's of some help.




>Am I correct in thinking that a preamp is unlikely to help in this
>situation?  My understanding is that preamps are mainly useful for
>compensating for cable losses, and in this case the feedline is only six
>feet long.
>
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