[mythtv-users] OT Question About OTA Antennas

Brian Wood beww at beww.org
Tue Sep 25 02:19:30 UTC 2007


James Oltman wrote:
> So here's the deal.  I took the amp (with adjustable gain) out of the
> picture.  I connected  Tuner0 of the HDHR directly to the antenna.  Of
> course it used RG-59.  I ran the channel scan on the backend with the
> mythbacked off.  It didn't detect any new stations.  It actually lost
> one.  So it looks like I'm SOL.  I connected the amp back up and I'm
> getting the channel that I lost.  However, I'm having a guide problem
> now.  I have created a new email regarding that issue.  Antennaweb.org
> <http://Antennaweb.org> shows that I can only get FOX in digital.  It
> just figures that this station is pushing out 1 MegaWatt and I can't get
> anything else.  Antennaweb doesn't say anything about the "church
> channel" that I get.  I was hoping that they didn't know "everything." 
> Oh well.  Should I try a different amp?  ChannelMaster i hear is good. 

Don't know about particular brand names, but:

Be aware that there are basically two types of amplifiers: Antenna
pre-amps and distribution amps.

Taking the latter first, distribution amps are basically designed to
make up for losses in cable, splitters, couplers and the like. They are
not particularly low-noise, and are designed for high output levels (on
the order of +45dbmv. or more per channel) and many-channel capability.

Pre-amps, OTOH, are designed to be as low-noise as possible (for the
price) and operate at much lower output levels (ca. +15 or +20 dbmv.).
They are not designed for a lot of channels (such as a cable system
might have) as you do not usually have many channels coming off the
antenna. These amps are usually single-ended (dist. amps are generally
push-pull) and usually get their power feed through the coax.

Often people mistakenly use a distribution amp for an antenna amplifier,
with predictably bad results. (it doesn't work well the other way around
either).

So whatever you might use, make sure it was designed for the purpose you
intend to use it for.

If you have strong local transmitters (TV or even FM) they can push the
output of your preamp over the spec, thus such signals should be trapped
down to a reasonable level before they hit the amplifier input (what's
"reasonable"? As close as you can get to the level of the other
channels). Amplifiers often have internal adjustable FM traps for this
purpose.

In case anybody's interested, +0 dbmv. equals one millivolt across a
75-ohm load, thus +60 dbmv. would be one volt across 75 (a really
smokin' level). The dbmv was originally called a dbj (the "j" for
Jerrold, the company that pioneered CATV systems (named for Milton
Jerrold Schapp, former Pennsylvania Governor, who founded the company,
long since sold to General Instrument).

beww


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