DIYbanter - View Single Post - One more antenna question: Antenna pitch?

Arfa Daily

"hr(bob) " wrote in message
...
On Dec 1, 3:42 am, "Arfa Daily" wrote:
"mm" wrote in message

...

One more antenna question: Antenna pitch?

That is, my attic has a pitched roof and the antenna for channels 7 to
60 that I'm thinking of now is almost 13 feet long.

It would fit more easily in the attic if I pointed it down a little
bit just like the roof pitches down from the center. Does the angle
of the axis matter that much if the individual elements are all
horizontal?

After all, if the tranmitter is higher than the antenna, it's as if
even a horizontal antenna is pointed down, from the pov of the
transmitter.

I've been reading but nothing has addressed this. All outdoor
antennas are of course horizontal, and I don't get to see people's
indoor antennas.

That's an interesting question. I guess to some extent, it depends on the
frequency involved, and how close you are to the transmitter. I feel that
the potential losses are likely to be far worse at UHF than VHF. An
example
that comes to mind is my local ATV repeater. It is sited on a hill, and I
clearly remember some of us helping out a new licensee, who was located
within line of sight of the transmitter mast, about 3/4 mile away, and at
the bottom of the hill. He could barely pick up a signal from it, and
could
not access it with his own 2W transmitter at all. This was at 23cms
(1.2GHz)
so a little above the top of the UHF band. We did some field strength
checks, and found that there was quite a distinct 'shadow' around the
transmitter site, out to a distance of around a mile. The repeater's
antenna
is a slotted waveguide.

The first thing that we did was to re-site his antenna pair onto a taller
mast, which produced a significant improvement to his situation, but was
still not good, considering how close he was to the repeater. The final
thing that we did was to tilt his antennas up at a similar angle to the
hill
itself, and this brought about the improvement to solid P5 copy and
repeater
access, that we had been expecting. I believe I have read that it is
common
to find this shadowing effect around high power TV transmitter masts, and
that it can extend out to several miles in some instances. Michael T could
probably comment better on this as he was involved in the industry.

So, I suppose that on paper, the answer has to be that there will be an
effect on received signal strength with the antenna tilted down, but it
will
probably not be enough to notice on a modern TV set, if the signal is not
marginal and hovering on the set's AGC threshold in the first place. If
you
take it to its logical conclusion, if you stood the antenna on its 'nose',
it would receive virtually nothing. At some point, horizontal or pointing
slightly *up*, you would be receiving a maximum signal. Anywhere inbetween
must, in theory at least, be less than the potential maximum.

Of course, now we're talking digital TV transmissions, all bets are off on
this ... :-)

Arfa- Hide quoted text -

- Show quoted text -

Digital signals travel through space the same as analog signals at the
same frequency. The best reception is for the antenna to point
directly at the transmitter. If there are reflections, then pointing
either upward or downward from the transmitter maight improve
reception. Digital formatting of the signal may make reflections more
or less of a problem than it was with older analog signal format.

I see that you're well into all the media and government hype about how good
digital terrestrial TV is then, Bob ...

The (tongue in cheek) point I was making had nothing to do with the format
of the modulating signal, which of course has absolutely nothing to do with
how well or otherwise, the carrier propagates through space. I was actually
referring to less-than-ideal reception circumstances such as those the OP
suggests that he may have, which with an analogue signal, may well give
perfectly acceptable results, but with a digital signal might result in
digital cliff pixellation and freezing. Digital terrestrial TV signals are,
in my experience here in the UK, nothing like as robust, or easily received
in many transmission regions, as the government - whose primary interest
lies in how much money they can make by selling off large chunks of the UHF
TV band to cell phone operators - would have Joe public believe through
their hyped-up and often misleading media campaign directed at the subject
....

Arfa