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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

Off topic: VHF FM radio aerials



 
 
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  #11  
Old January 25th 07, 10:12 AM posted to uk.tech.digital-tv
tony sayer
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Posts: 5,001
Default Off topic: VHF FM radio aerials

In article , charles
writes
In article ,
Bill Wright wrote:
For many years, since the introduction of mixed polarisation FM
broadcasts, I have favoured a simple vertical dipole for general purpose
applications. The days of co-sited FM transmitters are long gone.
Although I would prefer a genuine omni pattern, I usually have to accept
that there will be a mast a quarter wave away from the dipole, so the
pattern will be cardiod. It doesn't matter. There's usually one
direction where there are no feasible transmitters.


No-one sells dipoles


[Snip]

Does anyone know of a good straightforward FM dipole, folded or
otherwise, with a decent length of mounting arm and ideally some sort of
balun or matcher?



The Triax website lists an 'FM1' which they say is 75 ohms. Their aerials
certainly used to have Baluns. Unlike Antiference who once told me that
baluns weren't necessary at Band II frequencies.

And the same thing for DAB?


Triax list a 'DAB Omni' - again single element.


Triax, at least the one I have, do have Baluns and their very good
performers too. However they might just be a tad expensive for most
users.

I'm sure Bill you could get some manufacturer to make one somewhere?.
As pointed out by others that Halo thing is useless for FM reception due
to its loss, though oddly enough a similar aerial has been used for
Transmission, but they sometimes stack them to make up the losses.

I reckon that FM on a dipole for general purpose usage is fine but
someone more interested in serious listening would want a directional
jobbie aimed at the main BBC national TX.

As Mark C says its only the smaller low powered stations that use
vertical only, but these if they had any sense would add in a Horizontal
component for mobile reception especially in Urban areas where multipath
fading is problematical.


--
Tony Sayer

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  #12  
Old January 25th 07, 10:53 AM posted to uk.tech.digital-tv
charles
external usenet poster
 
Posts: 4,016
Default Off topic: VHF FM radio aerials

In article ,
tony sayer wrote:

[Snip]

As Mark C says its only the smaller low powered stations that use
vertical only, but these if they had any sense would add in a Horizontal
component for mobile reception especially in Urban areas where multipath
fading is problematical.



Interesting. The addition of the vertical component to the BBC's existing
horizontally polarized transmissions was for the benefit of mobile
reception. How does HP help the mobile listener. The usually reason for VP
only is cost. Some existing masts could not cope with the load of omni MP
antennae.

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

  #13  
Old January 25th 07, 11:11 AM posted to uk.tech.digital-tv
tony sayer
external usenet poster
 
Posts: 5,001
Default Off topic: VHF FM radio aerials

In article , charles
writes
In article ,
tony sayer wrote:

[Snip]

As Mark C says its only the smaller low powered stations that use
vertical only, but these if they had any sense would add in a Horizontal
component for mobile reception especially in Urban areas where multipath
fading is problematical.



Interesting. The addition of the vertical component to the BBC's existing
horizontally polarized transmissions was for the benefit of mobile
reception. How does HP help the mobile listener. The usually reason for VP
only is cost. Some existing masts could not cope with the load of omni MP
antennae.


Well as you drive round there are constant polarisation shifts due to
reflections i.e. the vertical is getting skewed and goes more Horizontal
in nature, apart form that there are of course phase cancellations etc.
Course if theres a horiz component thats there, this will then get
skewed towards the vertical.

Its very interesting to drive through a build up area like a city centre
and look at Mixed and Vertical only transmissions the Vert only is up
and down like the proverbial!!, whereas the mixed is far more constant
in signal level.....
--
Tony Sayer

  #14  
Old January 25th 07, 07:29 PM posted to uk.tech.digital-tv
Bill Wright
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Posts: 8,408
Default Off topic: VHF FM radio aerials


"tony sayer" wrote in message
...
In article , charles
writes
In article ,

Triax, at least the one I have, do have Baluns and their very good
performers too. However they might just be a tad expensive for most
users.

I'll look into it.


I'm sure Bill you could get some manufacturer to make one somewhere?.

It wouldn't be hard because it's all standard off-the-shelf parts. If all
else fails . . .

Bill


  #15  
Old January 25th 07, 07:34 PM posted to uk.tech.digital-tv
Bill Wright
external usenet poster
 
Posts: 8,408
Default Off topic: VHF FM radio aerials


"tony sayer" wrote in message
...
In article , charles
writes
Its very interesting to drive through a build up area like a city centre
and look at Mixed and Vertical only transmissions the Vert only is up
and down like the proverbial!!, whereas the mixed is far more constant
in signal level.....


Interesting. And presumably this is on a vertical aerial.

Bill


  #16  
Old January 25th 07, 08:49 PM posted to uk.tech.digital-tv
tony sayer
external usenet poster
 
Posts: 5,001
Default Off topic: VHF FM radio aerials

In article , Bill Wright
writes

"tony sayer" wrote in message
...
In article , charles
writes
Its very interesting to drive through a build up area like a city centre
and look at Mixed and Vertical only transmissions the Vert only is up
and down like the proverbial!!, whereas the mixed is far more constant
in signal level.....


Interesting. And presumably this is on a vertical aerial.

Bill



Yes a vertical whip on a motah in the test case a magbase aerial plugged
into an analyser..
--
Tony Sayer

  #17  
Old January 25th 07, 11:45 PM posted to uk.tech.digital-tv
William
external usenet poster
 
Posts: 7
Default Off topic: VHF FM radio aerials

Bill Wright wrote:
I'm sure Bill you could get some manufacturer to make one somewhere?.

It wouldn't be hard because it's all standard off-the-shelf parts.


These guys have been making aerials for the ham radio market
for many years.

http://www.sandpiperaerials.co.uk/

I'm sure they'd be more than capable of doing a Band II dipole.



  #18  
Old January 26th 07, 01:27 AM posted to uk.tech.digital-tv
Mallory
external usenet poster
 
Posts: 46
Default Off topic: VHF FM radio aerials

I have come across some plans for building a Slim Jim aerial for both the FM
and the DAB band

I wonder what its performance is like, as I've not built one yet

It is omnidirectional and has an integrated Balun

anyone care to comment?

http://www.irational.org/sic/radio/omni-aerial.html gives the details for a
SLim Jim for FM 88MHz to 108MHz

and for DAB,

http://www.hack247.co.uk/2006/06/06/...-the-slim-jim/ gives
the dimensions to use with the instructions for building given earlier for
FM use.


I wonder if a dual slim-jim could be built on the same plastic pipe so it
could recieve both ranges simulatneously and output them on two downleads,
for combining later on? This would certainly reduce the wind loading on the
pole.

on the webpage, it states "Why is the Slim Jim so much more efficient than
the popular 5/8l or other ground plane aerials, despite the latter's claimed
3dB gain over a dipole? The Slim Jim vertical angle of radiation is almost
parallel to ground so maximum radiation is where it is needed, straight out
and all around. With all ground planes, including those with radials even
one wavelength long, the vertical angle radiation is tilted upwards at an
angle of 30 or more."

Turning that round into recieving implies a very narrow vertical acceptance
angle despite having a horizontal acceptance angle of 360degrees.....

Regards

Stephen


"William" wrote in message
...
Bill Wright wrote:
I'm sure Bill you could get some manufacturer to make one somewhere?.

It wouldn't be hard because it's all standard off-the-shelf parts.


These guys have been making aerials for the ham radio market
for many years.

http://www.sandpiperaerials.co.uk/

I'm sure they'd be more than capable of doing a Band II dipole.





  #19  
Old January 26th 07, 02:36 AM posted to uk.tech.digital-tv
Bill Wright
external usenet poster
 
Posts: 8,408
Default Off topic: VHF FM radio aerials


"William" wrote in message
...
Bill Wright wrote:
I'm sure Bill you could get some manufacturer to make one somewhere?.

It wouldn't be hard because it's all standard off-the-shelf parts.


These guys have been making aerials for the ham radio market
for many years.

http://www.sandpiperaerials.co.uk/

I'm sure they'd be more than capable of doing a Band II dipole.


That looks like a good firm. Thanks for that.

Bill


  #20  
Old January 26th 07, 02:49 AM posted to uk.tech.digital-tv
Bill Wright
external usenet poster
 
Posts: 8,408
Default Off topic: VHF FM radio aerials


"Mallory" wrote in message
...
I have come across some plans for building a Slim Jim aerial for both the
FM and the DAB band


I've built a few slim jims over the years and always found the design to be
efficient and easy to do. I wonder why some aerial manufacturer doesn't come
up with a slim jim for FM and one for DAB.

Bill


 




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