[mythtv-users] OT: Indoor VHF gain antenna
beww at beww.org
Wed Jan 27 00:26:33 UTC 2010
On Tuesday 26 January 2010 05:06:12 pm David Brodbeck 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,
> 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
> match it to 75-ohm coax, but the signal is right on the edge of usable.
> 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.
> 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.
> 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.
I'm not putting much technical details into this post, such information is
available in several places on the web.
Whether an amplifier would help is a good question, it depends on whether your
problem is absolute level, or a signal to noise issue (actually S+N/N), but
you are correct that it's unlikely to help.
Your idea if using a yagi style reflector was a good one, but at that frequency
(approx 187 Mhz.) if would have to spaced a fair distance away (perhaps a foot
or 2), to get the best effect, likewise trying a shorter element as a director.
You might try moving your passive elements to different locations., or even try
two of them.
Could you perhaps put the antenna on the roof of your building? Getting higher
is the fastest way to increase level.
Knife-edge diffraction is a tricky thing, and is likely to be variable
according to many factors, as you have discovered.
I'm also curious, I thought a half-wave dipole had a theoretical impedance at
the center of 74 ohms, why do you need a balun to match it? You might also try
a "folded dipole", made from 300-ohm twinlead, based on the "more metal more
I'd try a direct feed into the coax, without the balun, and see what happens,
or a folded dipole.
In the real world it's results that count, get empirical and try some crazy
things, it's worked for others.
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