When does an antenna amplifier do any good?

Archived from groups: alt.tv.tech.hdtv (More info?)

A friend of mine is having difficulty getting all the local HDTV channels
over-the-air. The question is, Would an amplifier such as this one help?

http://www.buy.com/retail/electron [...] u=90125305

I would figure that putting an amplifier right before the tuner wouldn't do
much good, because the tuner itself is quite capable of amplifying the
signal if necessary. Therefore, the only use of an amplifier would be right
after the antenna but before the cabling. The logic of this is that the
signal amplification is useful in traversing a long cable from antenna to
tuner.

Is this logic correct? Or would an amplifier right before the tuner
actually do some good also/instead?

Archived from groups: alt.tv.tech.hdtv (More info?)

"Lawrence G. Mayka" <lgmayka000@ameritech.net> wrote in
news:ktvhc.189$3r4.114@newssvr15.news.prodigy.com:

> A friend of mine is having difficulty getting all the local HDTV
> channels over-the-air. The question is, Would an amplifier such as
> this one help?
>
> http://www.buy.com/retail/electron [...] u=90125305
>
> I would figure that putting an amplifier right before the tuner
> wouldn't do much good, because the tuner itself is quite capable of
> amplifying the signal if necessary. Therefore, the only use of an
> amplifier would be right after the antenna but before the cabling.
> The logic of this is that the signal amplification is useful in
> traversing a long cable from antenna to tuner.
>
> Is this logic correct? Or would an amplifier right before the tuner
> actually do some good also/instead?

You are more or less correct. It depends on the tuner somewhat, though.
If the tuner's "front-end" is a bit noisy, then you might want to
consider an external low-noise pre-amp. A lot of newer TV's have the
feature built in for their OTA reception on VHF and UHF, but I have no
experience of digital TV receivers. You would need a preamp that could
handle the frequencies used by the digital channels you want to receive,
though and the general rule of thumb is to put the amplifier as close to
the antenna as you can practically get it. Digital signals require a
much better signal to noise ratio than what will give tolerable results
on an analogue device, but it is a kind of threshold effect. Once you're
over the threshold, data recovery becomes acceptable and you can watch
TV. Below it, you see nothing but pixelation.

A lot of HDTV reception problems stem, not from weak signals but from
various types of interference, the worst of which is ghosting. On an
analog set, this appears as a secondary image to the right of the primary
signal image. If you're seeing a lot of this on analog signals from the
direction you're trying to receive the digital signal from, then you may
well be better off to invest in an antenna with a sharper pattern than in
an amplifier. And this type of interference is not really susceptible to
treatment by an amplifier because it is the station interfering with
itself via different signal paths.

Another source of problems is noise from other services. A good trap to
remove the FM broadcast band from your feedline before it goes into the
receiver (or amplifier) can help. And, if you MUST use 300 ohm line from
the antenna to the receiver, make sure you dress it away from other metal
objects and bring it away from the antenna at right angles to the
elements themselves. 300 ohm line IS better at transmitting signals, but
it is also better at picking up extraneous noise from the environment
unless you dress it very carefully. And good sattelite-quality RG6 is
cheap and well-shielded. A balun and a preamp at the antenna, a good,
high-gain antenna with a sharp pattern, and RG6 feedline will do wonders
sometimes, even at a fair distance from the transmitter.

I've engineered a fair bit of VHF and UHF packet radio stuff in my time
and the problems are quite similar. You will need a solid signal (but
not so strong as to overload, especially if there are multiple channels--
in that case consider an attenuator, not an amplifier) and it will need
to be relatively ghost-free.

So you need to check out a lot of things before you decide exactly what
to do to improve your reception. If you tell us more about where you are
and what the signals are like, then maybe we can make suggestions.

--
Dave Oldridge
ICQ 1800667

Paradoxically, most real events are highly improbable.