TV Reception
Threads - Renewable Energy, TV Reception
Paul Williams posted:
I've been asked why at between 5.30 - 6pm television programmes appear less
clear than at other times.
Under duress, I watched a little, and this did appear to be the case. My
question: if this is so - why is it so?
My very shaky hypothesis is that the (dinner making; shower having) power
drain is greater at this time - resulting in the transmission strength being
weaker?
[someone else suggested that all the television sets turned on dilute the
signal strength :-)]
David Martin
wrote:
The suggestion that "all the sets turned on dilute the signal strength" may
not be quite so silly as it seems.
When the TV wave washes over your neighbour's aerial some of it will be reflected,
unless some very unlikely conditions are met.
If the reflection arrives at your aerial out of phase with "your" signal
then the two signals summed together will have a *smaller* amplitude than
yours by itself. This is a common phenomenon whenever waves are involved
and is called destructive interference.
The question now arises as to why the opposite effect, constructive interference
giving an increase in picture quality, isn't observed in about half the TV
sets.
On the other hand, maybe your hypothesis is the better one.
Paul responded:
> When the TV wave
washes over your neighbour's aerial some of it will be
> reflected, unless some very unlikely conditions are met.
Would not these reflections occur
whether the TV is turned on or off? I understand that some people have
boosters installed for UHF signals. I imagine that a booster would
require power. Could this power corrupt/boost the reflected signal? Sorry
to be so full of questions, but I'm learning more in a few days than I've
learnt in 20 years of wondering.
David
Martin replied:
Hi Paul, and any others following this thread. Also my apologies to anyone
whose desire to wear clothing more appropriate to the opposite sex may have
been misled by the title :-)
Please don't apologise for being "full of questions"; that's the purpose
of the list after all. Many of the questions stimulate me (and I'm sure many
others) to think more carefully and your recent questions are a particularly
good example. I just wish I was full of answers.
This whole subject becomes more interesting the more I think about it. You
are of course correct to point out that the aerial is there whether the TV
is turned on or not, but I'd be surprised if it reflects in quite the same
way.
Although metals (such as aluminium TV aerials) have almost 100% reflectivity
at UHF frequencies, a combination of aerial plus TV input electronics should
be designed to have a much lower value.
My reasoning would be that the TV signal will go down the transmission line
to the set (or mast head amplifier) which will also set up a reflection if
the TV is on. 100% efficient reception occurs then these two reflections
are out of phase, so that the total reflection of the aerial is zero (no
reflection means that all the power is absorbed by the receiving system ).
On the other hand, maybe the input electronics is set up so that the TV is
effectively on all the time. I don't know enough about this, but will try
and consult an expert (we have one in Physics here) ASAP.
At least I can make a suggestion which is open to experimental test and is
therefore good physics. If you have a friendly neighbour, you can ask her
to switch her set on and off and check to see if it has any effect on your
reception (this might lead to other experimental benefits :-)
You also asked:
"Would not the number of possible destructive waves outnumber the possible
constructive waves by a large margin? UHF signals to aerial (in my poor understanding)
need to be in direct line of sight. One would (perhaps erroneously) suppose
that a constructive interference would be therefore rare?"
This is another difficult one. The wavelength at UHF frequencies is about
30 cm, very much less than the distance between you and your neighbour's
aerials. This is the reason why I would expect an increase as often as a
decrease in signal.
On the other hand, maybe it's not just the nearest aerial which is responsible;
the effect of all surrounding aerials might need to be included. I
still can't think of a logical reason why this combined effect should always
reduce the signal however.
To summarise: if different sets show an increase or decrease on a more or
less random basis then it seems to me like an interference effect from the
nearest aerial. If reception always decreases during some period then I haven't
any idea what the cause might be and the (in any case valid) points made
by the electrician about weather conditions and sunset time might have to
be looked at more closely.
David Allen responded:
<snip>
> I understand that some people have boosters installed for UHF signals.
Indeed, some fringe dwellers require
masthead amplifiers for VHF signals.
<snip>
> I just passed this by an electrician who told me that I must factor
out
> weather conditions and sunset times. Perhaps the dew point may be
> involved?
On the fringe it is abundantly
clear that weather & time of day have a significant effect on VHF reception.
Quality may vary from perfect picture with full stereo to an unwatchable
mess. The commonest manifestation of depreciated reception is stereo flicking
in and out. Bad reception is commonest around sunset.
