Misguided installation

Thought you might like a look at todays callout.
The local business has 4 plasma screens which were breaking up badly
and three radio tuners which were not performing at their best !


http://i874.photobucket.com/albums/a...didthis003.jpg
http://i874.photobucket.com/albums/a...didthis002.jpg
http://i874.photobucket.com/albums/a...didthis001.jpg

The interesting thing was that the mechanics of the installation were
quite good, with some attention to detail. The theory of it all was
not so good....to say nothing of the birdsnest !

We didnt need to resite the aerial in the end as the transmitter was
about 20 degrees left of where the aerial was facing, so the screening
of the building turned out not to be a problem. We did, however have
to spend some time with filters etc trying to get rid of very strong
tetra and other locally generated interference.

The radio tuners were DAB and FM so we took down the interestingly
positioned halo and fitted a vertical FM dipole which gave very good
results for DAB as well as it is almost exactly twice the length.

On Dec 17, 12:56*am, widgitt wrote:
Thought you might like a look at todays callout.
The local business has 4 plasma screens which were breaking up badly
and three radio tuners which were not performing at their best !

http://i874.photobucket.com/albums/a.../Whichprofessi...

The interesting thing was that the mechanics of the installation were
quite good, with some attention to detail. The theory of it all was
not so good....to say nothing of the birdsnest !

We didnt need to resite the aerial in the end as the transmitter was
about 20 degrees left of where the aerial was facing, so the screening
of the building turned out not to be a problem. We did, however have
to spend some time with filters etc trying to get rid of very strong
tetra and other locally generated interference.

The radio tuners were DAB and FM so we took down the interestingly
positioned halo and fitted a vertical FM dipole which gave very good
results for DAB as well as it is almost exactly twice the length.

It's odd isn't it? -- when you find one where the installer has
obviously been conscientious,
but is clearly ignorant of certain basic facts about the job. I went
to one the other week which
had a head end that was so neat and tidy it looked like the installer
had a mental problem.
But the whole system was total ******** when you looked at what he's
used and how he'd used it.

Bill

Twice the length? Well this surely would be a terrible mismatch impedance
wise.

Brian

--
Brian Gaff....Note, this account does not accept Bcc: email.
graphics are great, but the blind can't hear them
Email:
__________________________________________________ __________________________________________________ __________


"widgitt" wrote in message
...
Thought you might like a look at todays callout.
The local business has 4 plasma screens which were breaking up badly
and three radio tuners which were not performing at their best !


http://i874.photobucket.com/albums/a...didthis003.jpg
http://i874.photobucket.com/albums/a...didthis002.jpg
http://i874.photobucket.com/albums/a...didthis001.jpg

The interesting thing was that the mechanics of the installation were
quite good, with some attention to detail. The theory of it all was
not so good....to say nothing of the birdsnest !

We didnt need to resite the aerial in the end as the transmitter was
about 20 degrees left of where the aerial was facing, so the screening
of the building turned out not to be a problem. We did, however have
to spend some time with filters etc trying to get rid of very strong
tetra and other locally generated interference.

The radio tuners were DAB and FM so we took down the interestingly
positioned halo and fitted a vertical FM dipole which gave very good
results for DAB as well as it is almost exactly twice the length.

"widgitt" wrote in message
...
Thought you might like a look at todays callout.
The local business has 4 plasma screens which were breaking up badly
and three radio tuners which were not performing at their best !


http://i874.photobucket.com/albums/a...didthis003.jpg
http://i874.photobucket.com/albums/a...didthis002.jpg
http://i874.photobucket.com/albums/a...didthis001.jpg

The interesting thing was that the mechanics of the installation were
quite good, with some attention to detail. The theory of it all was
not so good....to say nothing of the birdsnest !

We didnt need to resite the aerial in the end as the transmitter was
about 20 degrees left of where the aerial was facing, so the screening
of the building turned out not to be a problem. We did, however have
to spend some time with filters etc trying to get rid of very strong
tetra and other locally generated interference.

The radio tuners were DAB and FM so we took down the interestingly
positioned halo and fitted a vertical FM dipole which gave very good
results for DAB as well as it is almost exactly twice the length.

Good photos. I'm always trying to learn new things & improve, but quite
often I see things like in your photos and it amazes me how many installers
are out there with poor knowledge. Even the basic rules of a good solid
installation seem to get ignored most of the time. Today I had to sort out a
high gain aerial, on a 16ft mast, mounted on a single 6" chimney bracket
(welded at least - not pressed). Just silly......

Brian Gaff wrote:
Twice the length? Well this surely would be a terrible mismatch impedance
wise.

Brian


Why, surely it would be 1/2 wave if it started at 1/4 wave, or 1 wave if
started as 1/2 wave? Twice is good?

--
Tony

Tony wrote:

Brian Gaff wrote:
Twice the length? Well this surely would be a terrible mismatch impedance
wise.

Why, surely it would be 1/2 wave if it started at 1/4 wave, or 1 wave if
started as 1/2 wave? Twice is good?

The typical dipole to feed 75 ohms is a half-wave dipole made up from two
quarter-wave elements. A quick Google search tells me that full-wave
dipoles can work but shouldn't be fed directly to the coax because of the
mismatch -- but this can be fixed with an air choke:

http://www.hamuniverse.com/kl7jrverticaldipole.html

However, this option wasn't available to the OP because the dipole's
primary use was for FM, and in any case, theory is trumped by:

"...and fitted a vertical FM dipole which gave very good
results for DAB as well..."

--
Dave Farrance

On Thu, 17 Dec 2009 18:09:49 GMT, Dave Farrance
wrote:

Tony wrote:

Brian Gaff wrote:
Twice the length? Well this surely would be a terrible mismatch impedance
wise.

Why, surely it would be 1/2 wave if it started at 1/4 wave, or 1 wave if
started as 1/2 wave? Twice is good?

The typical dipole to feed 75 ohms is a half-wave dipole made up from two
quarter-wave elements. A quick Google search tells me that full-wave
dipoles can work but shouldn't be fed directly to the coax because of the
mismatch -- but this can be fixed with an air choke:

http://www.hamuniverse.com/kl7jrverticaldipole.html

However, this option wasn't available to the OP because the dipole's
primary use was for FM, and in any case, theory is trumped by:

"...and fitted a vertical FM dipole which gave very good
results for DAB as well..."

In a large proportion of cases where an inappropriate or just plain
wrong aerial is fitted, it gives perfectly good service simply because
of a sufficiently high field strength.
In such an environment the proverbial length of wet string would do
just as well, with or without a balun or correct impedance match.

On Dec 17, 6:09*pm, Dave Farrance
wrote:
Tony wrote:
Brian Gaff wrote:
Twice the length? Well this surely would be a terrible mismatch impedance
wise.

Why, surely it would be 1/2 wave if it started at 1/4 wave, or 1 wave if
started as 1/2 wave? *Twice is good?

The typical dipole to feed 75 ohms is a half-wave dipole made up from two
quarter-wave elements. *A quick Google search tells me that full-wave
dipoles can work but shouldn't be fed directly to the coax because of the
mismatch -- but this can be fixed with an air choke:

http://www.hamuniverse.com/kl7jrverticaldipole.html

However, this option wasn't available to the OP because the dipole's
primary use was for FM, and in any case, theory is trumped by:

"...and fitted a vertical FM dipole which gave very good
results for DAB as well..."

--
Dave Farrance

Vertical FM dipoles without a balun work so well on DAB it's not
always obvious which feed is which is you've fitted a DAB dipole as
well. But FM dipoles with baluns are crap for DAB. However, since DAB
coverage is based on indoor aerials the field strength on the roof is
often enough for a very inefficient aerial to work well. In fact, most
DAB dipoles feeding head ends do so via a 6dB or 12dB attenuator. Or
they should. Often they don't and the result is that the DAB signals
are launched at the same level as the analogue TV.

Bill

Bill

Bill

Dave Farrance wrote:

"...and fitted a vertical FM dipole which gave very good
results for DAB as well..."

I use my vertical FM dipole for DAB reception. I was surprised to receive
distant muxes with it. I lashed up a dipole cut for Band III Ch 11, and
compared the results. There were no differences, only that the Band III was
crap at Band II (which you would expect).



--
Mark
Please replace invalid and invalid with gmx and net to reply.

www.paras.org.uk

"Mike Henry" wrote in message
news
In , "-GB-Carpy"
wrote:

Good photos. I'm always trying to learn new things & improve, but quite
often I see things like in your photos and it amazes me how many
installers
are out there with poor knowledge. Even the basic rules of a good solid
installation seem to get ignored most of the time. Today I had to sort out
a
high gain aerial, on a 16ft mast, mounted on a single 6" chimney bracket
(welded at least - not pressed). Just silly......

For my untrained eye, could I tentatively ask what is wrong with the TV
aerial part? I can see that the radio aerial is wrong, but all I can think
that's wrong with the TV aerial is that it's too close to the radio
aerial.

TIA :-)

Pity there's isn't a number 4, "after", picture!

TV aerial looks ok - certainly better than most!