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Doug McDonald
Guest
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 Posted:
Tue Jan 18, 2005 7:54 pm Post subject:
Re: Is this an FM aerial? Where did it come from? |
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davidrobinson@postmaster.co.uk wrote:
| Quote: | It's a difficult choice: simple outdoor
aerial (at loft height) or better aerial in the loft itself. Obviously
it's easier to put something in the loft
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That's not a real choice.
A small antenna may work well indoors, while a larger one
likely will not. In other words, your reral choices are,
in order of worst to best, a small antenna indoors,
the small antenna outdoors, or a large antenna outdoors.
A large antenna indoors will likely .... not certainly but
likely ... not work better than a small antenna indoors,
unless you have a window to point it through.
Doug McDonald
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DAB sounds worse than FM
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davidrobinson@postmaster.
Guest
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| Posted:
Thu Jan 20, 2005 3:09 pm Post subject:
Re: Is this an FM aerial? Where did it come from? |
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DAB sounds worse than FM wrote:
| Quote: | I don't think David has a rooftop dipole, though. Assuming he's just
got
an internal wire aerial for FM, if he installed an external dipole or
Yagi he could vastly improve the SNR of the stations he wants to
receive.
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If I had a simple roof-top vertical dipole, I think I'd have to filter
out or reduce the two local stations before distributing the signal -
they're spectacularly strong. Still, that may be the case with whatever
I buy.
Cheers,
David. |
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Alex Bird
Guest
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| Posted:
Thu Jan 20, 2005 4:18 pm Post subject:
Re: Is this an FM aerial? Where did it come from? |
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Mike Gilmour wrote:
b) it feeds a good tuner Magnum Dynalab 'Etude'.
Whooo. Nice tuner. |
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DAB sounds worse than FM
Guest
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| Posted:
Thu Jan 20, 2005 6:53 pm Post subject:
Re: Is this an FM aerial? Where did it come from? |
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davidrobinson@postmaster.co.uk wrote:
| Quote: | DAB sounds worse than FM wrote:
I don't think David has a rooftop dipole, though. Assuming he's just
got an internal wire aerial for FM, if he installed an external
dipole or
Yagi he could vastly improve the SNR of the stations he wants to
receive.
If I had a simple roof-top vertical dipole, I think I'd have to filter
out or reduce the two local stations before distributing the signal -
they're spectacularly strong. Still, that may be the case with
whatever I buy.
|
Depending on the angles of the transmitter you want to receive signals
from and the transmitter with the local stations that you don't want to
receive, then if you got a Yagi then it may do the filtering for you due
to the directional radiation pattern of the aerial.
If you look at typical antenna radiation patterns:
http://www.gigaant.com/?id=246&show=434
these patterns are in the horizontal plane, so the omni-directional
vertical dipole has a circular radiation pattern, because it has the
same antenna gain in all directions. The radiation pattern superimposed
on the dipole's pattern is for a directional aerial such as a Yagi
(although the main lobe is quite narrow, so it's likely to have quite a
few elements). But the important thing as far as filtering-out signals
go is that if the unwanted signal is coming from an angle, say, 60
degrees relative to the desired signal then it's in one of the nulls of
the antenna pattern, so it's pretty effectively filtered out, because
the relative attenuation is the gain of the main lobe (which will be
pointing at the desired transmitter) minus the gain at the undesired
angle which, as you can see from that diagram, can be quite significant.
Here's some more info on radiation patterns:
http://www.astronantennas.com/radiation_patterns.html
Antiference say the acceptance angle of their FM1083 3-element Yagi is
32 degrees, and they define acceptance angle here:
http://www.antiference.co.uk/def.htm
as:
"the angle from the centre line to the point where the signal falls to
half power (-3dB)"
The front to back ratio is specified as being 15dB, so if the unwanted
signals were coming from the opposite direction to the desired signals
then you've got 15dB of attenuation from the front-to-back ratio, and
looking at a radiation pattern for a Yagi in a book I've got, the back
lobe has the highest gain.
Sack this, they say a picture tells a thousand words, so I've done a
graph:
http://www.digitalradiotech.co.uk/antenna_pattern.htm
(There's a null (theoretically infinite loss) at +90 and -90 degrees, so
to make the graph look right I've arbitrarily allocated a gain of -40
dB.)
I don't know what exact antenna that is for, because it doesn't say what
antenna it's for in the book other than it's a Yagi. But that'll be the
typical shape of the graph of gain against direction. So, try and find
out what the relative angles are of the transmitter that you want to
receive from and the transmitter that the local stations are on which
you don't want, and if you're lucky the unwanted signals may be coming
from angles where they'd be filtered by the antenna pattern of a Yagi.
--
Steve - www.digitalradiotech.co.uk - Digital Radio News & Info
Find the cheapest Freeview, DAB & MP3 Player Prices:
http://www.digitalradiotech.co.uk/freeview_receivers.htm
http://www.digitalradiotech.co.uk/dab_digital_radios.htm
http://www.digitalradiotech.co.uk/mp3_players_1GB-5GB.htm
http://www.digitalradiotech.co.uk/mp3_players_large_capacity.htm |
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Arthur
Guest
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| Posted:
Thu Jan 20, 2005 7:27 pm Post subject:
Re: Is this an FM aerial? Where did it come from? |
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On Thu, 20 Jan 2005 13:53:12 GMT, DAB sounds worse than FM
<dab_is@low.quality> wrote:
(sound advice snipped)
| Quote: | if you're lucky the unwanted signals may be coming
from angles where they'd be filtered by the antenna pattern of a Yagi.
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If you aim the aerial to drop one of the "spectacularly strong" local
transmitters into a notch, this will certainly help in listening to weaker
stations. But when listening to the local station itself you may be
beaming at a reflected signal with comparable strength to the direct
signal. This can cause appalling distortion to FM audio reception.
Arthur |
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Richard Fry
Guest
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| Posted:
Thu Jan 20, 2005 7:44 pm Post subject:
Re: Is this an FM aerial? Where did it come from? |
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"DAB sounds worse than FM" <dab_is@low.quality> wrote
| Quote: | Depending on the angles of the transmitter you want to receive signals
from and the transmitter with the local stations that you don't want to
receive, then if you got a Yagi then it may do the filtering for you due
to the directional radiation pattern of the aerial. (etc)
_________________ |
Other considerations are the frequency separation between the desired and
undesired station, and the field strength of the undesired station.
A very strong local station on a nearby freq can de-sensitise a receiver,
making it difficult to receive adjacent channels. The carrier ratio between
local and distant stations easily can be 50dB or more.
Even the best FM rx antenna will have difficulty reducing the field strength
of the local station below that of the distant one. Reflections of the
local station can arrive via multipath along with the distant station, on
the pointing axis of the rx antenna. The reflection itself may only be
10-15 dB reduced from the direct wave -- still far above that of the distant
station -- and there is no way to get rid of it using the antenna pattern
alone.
There are methods of introducing a sample of the interfering signal into the
output of the rx antenna line, and adjusting its amplitude and phase so as
to null the interfering RF wave. Usually this is not practical for other
than commercial applications.
RF
Visit http://rfry.org for FM transmission system papers. |
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DAB sounds worse than FM
Guest
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DAB sounds worse than FM
Guest
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| Posted:
Thu Jan 20, 2005 8:20 pm Post subject:
Re: Is this an FM aerial? Where did it come from? |
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Richard Fry wrote:
| Quote: | "DAB sounds worse than FM" <dab_is@low.quality> wrote
Depending on the angles of the transmitter you want to receive
signals from and the transmitter with the local stations that you
don't want to receive, then if you got a Yagi then it may do the
filtering for you due to the directional radiation pattern of the
aerial. (etc) _________________
Other considerations are the frequency separation between the desired
and undesired station, and the field strength of the undesired
station.
A very strong local station on a nearby freq can de-sensitise a
receiver, making it difficult to receive adjacent channels. The
carrier ratio between local and distant stations easily can be 50dB
or more.
Even the best FM rx antenna will have difficulty reducing the field
strength of the local station below that of the distant one.
Reflections of the local station can arrive via multipath along with
the distant station, on the pointing axis of the rx antenna. The
reflection itself may only be 10-15 dB reduced from the direct wave
-- still far above that of the distant station -- and there is no way
to get rid of it using the antenna pattern alone.
There are methods of introducing a sample of the interfering signal
into the output of the rx antenna line, and adjusting its amplitude
and phase so as to null the interfering RF wave. Usually this is not
practical for other than commercial applications.
|
Very interesting, thanks.
--
Steve - www.digitalradiotech.co.uk - Digital Radio News & Info
Find the cheapest Freeview, DAB & MP3 Player Prices:
http://www.digitalradiotech.co.uk/freeview_receivers.htm
http://www.digitalradiotech.co.uk/dab_digital_radios.htm
http://www.digitalradiotech.co.uk/mp3_players_1GB-5GB.htm
http://www.digitalradiotech.co.uk/mp3_players_large_capacity.htm |
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Mark Carver
Guest
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| Posted:
Thu Jan 20, 2005 8:40 pm Post subject:
Re: Is this an FM aerial? Where did it come from? |
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DAB sounds worse than FM wrote:
| Quote: | Arthur wrote:
If you aim the aerial to drop one of the "spectacularly strong" local
transmitters into a notch, this will certainly help in listening to
weaker stations. But when listening to the local station itself you
may be beaming at a reflected signal with comparable strength to the
direct signal. This can cause appalling distortion to FM audio
reception.
Fair enough. Mark Carver said it might be these: Chiltern 96.9 and BBC
3CR 95.5, so I assumed he wouldn't want to ever listen to them. ;)
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I did, I only posted that reply in alt.radio.digital, so to avoid
confusion I suggested making a couple of quarter wave stub filters, and
place them in the downlead. |
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wrightsaerials@aol.com
Guest
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Richard Fry
Guest
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| Posted:
Fri Jan 21, 2005 1:26 am Post subject:
Re: Is this an FM aerial? Where did it come from? |
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<wrightsaerials@aol.com> wrote
| Quote: | I suggested making a couple of quarter wave stub filters
___________ |
However the reject bandwidth of such filters is rather broad, and they can
introduce significant losses on many FM channels beside the one(s) targeted.
Also their sloping RF amplitude response and differential RF group delay
across the nearby channels will add stereo crosstalk and distortion to
stations received on those channels.
RF
Visit http://rfry.org for FM transmission system papers. |
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Arthur
Guest
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| Posted:
Fri Jan 21, 2005 2:43 am Post subject:
Re: Is this an FM aerial? Where did it come from? |
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On 20 Jan 2005 12:07:09 -0800, wrightsaerials@aol.com
<wrightsaerials@aol.com> wrote:
Are you sure it would have a sufficiently high 'Q'? I think you'd need a
cavity resonator.
Arthur |
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Marky P
Guest
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| Posted:
Fri Jan 21, 2005 3:55 am Post subject:
Re: Is this an FM aerial? Where did it come from? |
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On Tue, 18 Jan 2005 09:42:25 -0000, "Mike Gilmour"
<mike@tfjazz.freeserve.co.uk> wrote:
| Quote: |
"Neil" <nobody@nospam.net> wrote in message
news:cshhuj$ni5$1$8300dec7@news.demon.co.uk...
I used to own a Galaxie 17, later upgraded to a 23. Unless Ron Smith has
re-designed these since 1997, I must say their performance above 104 MHz is
for one of a better word 'sh*te'. Granted, when he designed them the FM
band only extended to 105 MHz but these days the 105-108 sub-band is
heavily populated with stations and the performance is pretty poor with a
4-6dB roll off between 104 and 106 MHz.
The centre frequency of most of the models is 93.0 MHz but if you wanted
good performance above 104 MHz then maybe a GTE model may have suited you
better whose centre frequency is 102 MHz, available on the 17, 20 & 23
models.
Mike
My G23 had a centre freq of 95 MHz for some reason. Apparently it was |
a custom buit effort that someone didn't want. At that time, I use to
listen to Capital FM on 95.8, so it was perfect, & also had better
gain up the top end of the band.
Marky P. |
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Mark Carver
Guest
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| Posted:
Fri Jan 21, 2005 8:28 am Post subject:
Re: Is this an FM aerial? Where did it come from? |
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Arthur wrote:
I used one with success to reduce a strong TV carrier (UHF E31) that was splashing
across on a much weaker one (E34). It didn't impair the level of the E34 carrier noticably,
and the E31 carrier (also wanted) was still usable. That's a 24 MHz range in 550ish MHz.
I suppose scaling that down to the OP's problem in Band II might be expecting too much ?
One advantage is that it's very cheap (just off cuts of co-ax and a bit of soldering) to try :-) |
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