Archived from groups: alt.tv.tech.hdtv (
More info?)
In article <2pd841lc0kcesj3ajeavurnn4eticin7n0@4ax.com>,
Richard <rstaples312@yahoo.com> writes:
> I am particularly interested in the messages here about current
> receivers for digital reception. Why? At my location in central KY
> reception of digital has been no easy task and, in fact, it has been
> an extremely frustrating experience.
>
> One of the problems is that two of the available digital stations
> broadcast on VHF channels 4 and 13, while the other digital and analog
> stations all broadcast on UHF. To make matters worse, one the two VHF
> channels is located at the BOTTOM of the spectrum and the other is
> located at the TOP. Try to find an antenna with good gain at both
> ends of the spectrum that is not a mass of metal!
>
>
> The closest DTV station (UHF) is only 2.2 miles away and its tower
> will also eventually become the home for a competing second UHF
> station which is VERY good news.
>
One comment -- 2.2miles away from a VHF/UHF transmitter can create
very significant signal levels, and with an antenna with gain, you'd
have to have a front end that can withstand very high signals. This
high signal can even interfere with other channels.
Sometimes an attenuator can be just as helpful (or moreso) than a
preamp. I use splitters (like a 2 way 3.5dB loss unit) to help
create some loss, yet it doesn't lose so much as to ruin all but
the weakest signals.
In overly high signal level areas, some of the distortion products
do reduce rapidly with small signal level decreases. For example,
the IM3 product levels decrease several times faster than the decrease
in an overly strong signal.
An attenuator can help matching and force nice matches to mitgate some
various notches and other damage to the frequency response.
Given a moderate gain antenna and transmitters that are 2-10miles away,
I'd strongly suggest the use of some 6dB attenuators. I wouldn't use
a preamp except after the attenuators, and use the preamp only for
distribution applications. If you are going to split the signal
for multiple HDTV tuners, then it might be wiser to use a 3.5dB splitter
instead of a 6dB attenuator -- totally avoiding the use of a preamp.
In normal circumstances, I wouldn't use a preamp in the vicinity of high
power transmitters. We could specify a good preamp that would work well
in those circumstances, but the output might be too strong for the HDTV
tuners further down in the chain. After such a preamp (e.g. one that
can do 10-20dB gain, P1dB 16dBm, OIM3 approx 30-40dBm, NF 1.0dB or
less with any impedance between 30-100ohms, input/output return loss
of at least 10db), then you'd probably need at least one 6dB attenuator
so that your TV front end doesn't overload with the possibility of
100-500mv signal levels!!! Such an attenuator can undo most of the
purpose of a near-perfect preamp, and the best choice is to avoid
the preamp all together.
MORE CONCRETE INFO:
The distortion products decrease MUCH MUCH more quickly than the
signal levels. So, if you drop the signal level by 3dB, you can
loose between 6dB and 9dB (or more) of the distortion products.
So, using a 6dB attenuator can make a HUGE difference in the ability
of the system to work well. One can loose perhaps 10-20dB of interference
by adding a 6dB attenuator.
John
/staff.washington.edu/mrc
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