"Adrian" wrote in message
...
HI Brian / Andy

On Tue, 17 Oct 2006 08:38:09 GMT, "Brian Sharrock"
wrote:


"Christian McArdle" wrote in message
...
The existing aerial's a sort of '4 dipoles and a single mesh
reflector' arrangement - not a Yagi

Can you identify it? 95% of all TV aerials are Yagis. (And 95% of
statistics are made up on the spot).

Christian.


Chipping in, again, I can't quite parse what you're saying; is the aerial
array; -

query
Case A:

-- -- -- -- four off dipoles
--------------------------- mesh reflector

Yes - that's the one !

or;-

Case B:
snip

Let that be a lesson to me; I assumed that it must be a Yagi. After all,
that's what _everybody_ uses. I re-read your ' ... not a Yagi' and
thought; - "No! they couldn't use that type ... could they?"


Case B would be normal and what RTE intend when they call for a UHF Band A
aerial; CASE A is highly unusual - what RF Engineers describes as a phased
array with 'intriguing' possibilities of gain, bandwidth and beamwidth as
a
function of frequency!

Even better if the mesh is electrically connected to the gurt big
I-beam (by virtue of being physically in contact with it !

The groundng effect of a lump of steel shouldn't hurt it too much.
'Theoreticaaly' the reflector is the minimum area that one can 'get away
with' . Additional material, provided it's behind the dipoles, shouldn't
matter.

RF energy is what 'we' call 'funny stuff' (another technical term ) bits of
cable, such as co-ax, that seem to be normally resitsive become 'open
circuit' and/or 'dead short circuit' at different points along the cable
once one starts trying to stuff rf energy along it: technically it's all to
do with lambda and halve and quarters but as lambda changes with frequency
it all get too hard for humnas to suss out. { partly becaue it's all tto
hard to imagine 'receiving' things off an aerial - we utilise the theory of
reciprocity and pretend that the aerial is a transmitter }
Your Irish installers seem to have attempted to install a phased array,
wherein one dipole will 'interfere' with it's adjacent neighbour cauisng
nulls and maxima to overlap creating a narrowing of the beam without
significantly increasing the received signal .
Adding these four signals together in such a manner that they sum
accumulatively and not destructively is a non-trivial task. Attempting to
transmit such a signal down a non-homogenous cable to a receiver is another
minefield. Reflections at junctions and terminations at cable end will
almost invariabley caue a 'Standing Wave' to be generated, and Sod's Law
[may be known as Murphy's Law in the Republic] will almost guarantee that
you may have an minima at the end.

Had a quick discussion with the installers today and suggested that I
might be interested in paying their bill if they might be interested
in coming back and doing a proper installation...

..we'll see....g

Thanks
Adrian

Now to find out if there's any 'good euro'

--

Brian