Aerial madness (long)
 

On Wed, 4 Aug 2004 01:58:59 +0100, Neil Jones
wrote:


The oddest thing to me is that the aerial is pointing more-or-less at
the T.Wells transmitter which is on a bearing of 252 degrees from here,
whereas Bluebell Hill is 35 degrees. Not only that, but Tunbridge Wells
and Bluebell Hill have opposite polarisation, and the Wolfbane thingy
gives a predicted field strength of 61dBuV/m from T.Wells vs 48dBuV/m
from Bluebell Hill.
are you sure it's not just reflection..
in my area digital is only transmitted on main transmitters not relays
i'd assumed this was the case everywhere
i'm having a day off at mo but have a transmitter book with all main
and relay tx in it will look for you
try looking at www.dtg.org.uk
Taz

also worth a look
http://www.cai.org.uk/asp/home.asp
http://www.freeview.co.uk/index.html
Taz

"Bill" wrote in message
...
Your new aerial is a higher gain and hence more directional. Thus its
gain off axis (i.e. toward Bluebell Hill) will be lower.

Hmm. . .

All other things being equal it is true that the ratio between the
aerial's
gain on-axis and it's gain off-axis will be greater if the aerial has
'more
gain'. However, this increased ratio comes from the greater gain to
the front,
not an absolute reduction of gain from other directions. As a matter
of fact,
the more complicated the aerial (odd shaped elements, etc) the more
chance
there is of large spurious rear lobes appearing. That's a
generalisation, of
course, and the manufacturers go to great lengths to eliminate such
effects.
Bill

I think I'll either have to a) work out how to get the aerial installed
outside or pay a professional to come and play.

Cheers

Neil

Bill wrote:

Possibly the truth is that a longer yagi has a narrower and longer front lobe.
In other words, the extra gain on axis is achieved by a slight reduction of
gain a few degrees off axis, the rearward lobes being unaffected. That's just a
theory. It might be wrong.

That sounds quite likely. To some extent the sensitivity from the rear
will also be governed by the quality and effectiveness of the reflector
not only in focusing wanted signal from the front, but also in screening
/ deflecting unwanted signal from the back I suppose.

--
Cheers,

John.

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"John Porcella" wrote in message
...
Your roof is likely to provide masses of attenuation, perhaps of the
order
of 17 dB from previous correspondence here.

Might I suggest that you have your aerial stuck up at roof level if at
all
possible. Indeed you could have two aerials pointing to one
transmitter
each and then having them combined together, if that is the correct
word.


Well, I have swapped back to my old aerial for the time being and
perhaps my roof isn't attenuating the signal as much as one would think.

I appear to be picking up mux 1 & 2 on channels 31 & 21 respectively
which would make the transmitter Reigate - 24 miles away, on a bearing
of 280 degrees for which Wolfbane predicts a field strength of 23 dBuV/m
and describes as 'extreme outer fringe' - and this is channels out of
the designed band for the aerial.

Neil