FM signal too strong? - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

peter_sol · 14th Jul 2020, 6:40 am

I think the BBC transmission may be Horizontally polarised. the others vertical so try slanting the dipole.

Mr 1936 · 14th Jul 2020, 8:27 am

A photo of the tower shows a mixture of vertical and horizontal dipoles for FM. Crystal palace was chosen for some of the first BBC experiments in mixed polarization 40 years ago.

Derminator · 14th Jul 2020, 9:51 am

Thanks for the really interesting and helpful replies everyone. I did get up in the attic yesterday evening and managed to achieve some improvement in the radio 3 reception. Radio 2 is now excellent. This is with the dipole still aligned vertically but just moved by about 3 feet from where it was. This was done by trial and error, running up and down the stairs with the tuner on in the living room. I will get to work on a proper rotator so that i can align it horizontally and bring the tuner up to the attic with me for more thorough investigation.

paulsherwin · 14th Jul 2020, 11:35 am

London can be a difficult place to get a good FM signal, despite the big transmitters which are available. There are lots of substantial tall buildings, many of which are steel reinforced concrete. The really tall ones which have gone up in the last 20 years cast a huge reception shadow, and are also very effective at generating multipath. Add to that lots of interference sources and pirate broadcasters and clean reception becomes an absolute lottery.

Mach One · 15th Jul 2020, 3:58 pm

Crystal Palace is what is called "mixed" polarisation (realistically slant polarisation) and Croydon used to use Circular polarisation - I don't know if it still uses this. When I was living in London the commercial stations would come in beautifully with any "wet string" antenna (even a finger in the antenna socket...) whereas the similar strength signals from the BBC which were then horizontal were much more difficult in those circumstances.

Could the difference be down at least partly to the fact that Radio 3 has a much lower average modulation level than commercial radio which is generally as loud as possible? At any rate, Paul's suggestion of pointing the antenna away towards Wrotham might be a good solution.

crackle · 16th Jul 2020, 7:29 am

Quote:

Originally Posted by Mach One

Croydon used to use Circular polarisation

What is the advantage of circular polarisation, why use it?
Does the receiving antenna have to rotate as well?

Mike

Radio Wrangler · 16th Jul 2020, 9:05 am

Circular polarisation can be done with a vertical and horizontal antenna with a splitter and a bit of phase shifting line. No hardware has to spin !

It has several advantages/disadvantages.

Passage through some media interfaces can rotate polarisation. This doesn't affect circ pol. Also circ pol is receivable by horizontal or vertiacl polarised antennae, though a circ pol antenna will gather more signal power from a circ pol broadcast.

There are two circular polarisaton 'senses' - a right-hand corkscrew and a left hand corkscrew. The wrong one will null your signal.

Circ pol waves can have their sense reversed on reflection.

David

Derminator · 16th Jul 2020, 2:54 pm

Quote:

Originally Posted by paulsherwin

London can be a difficult place to get a good FM signal, despite the big transmitters which are available. There are lots of substantial tall buildings, many of which are steel reinforced concrete. The really tall ones which have gone up in the last 20 years cast a huge reception shadow, and are also very effective at generating multipath. Add to that lots of interference sources and pirate broadcasters and clean reception becomes an absolute lottery.

I should be largely immune to the effects of the tall buildings, Paul, as I am just over the ridge upon which the CP transmitter stands on the Kent side. Pirate radio interference? Now that's a different matter as Crystal Palace's elevated position over London has made it a mecca for pirate radio. The heyday was in the '90s but enough remain still. I can claim to have suffered pirate radio interference without even having switched my radio on! Enjoying the sunshine and a good book in my back garden a few weeks back my peace was suddenly interrupted by what at first sounded like the animated shouting of someone in the throes of a psychiatric episode, sadly a not unfamiliar sound on the streets of our great capital. When I listened more closely though the unmistakable patter of a pirate dj revealed itself. It was coming from the top floor window of a nearby building and continued sporadically for the next four hours. So there you have it, pirate radio interference with my radio turned off!

Dermot

paulsherwin · 16th Jul 2020, 3:46 pm

Croydon has lots of tall buildings which may be affecting you - only a possibility of course.

14th Jul 2020, 6:40 am	#21
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: FM signal too strong? I think the BBC transmission may be Horizontally polarised. the others vertical so try slanting the dipole.

14th Jul 2020, 8:27 am	#22
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 524	Re: FM signal too strong? A photo of the tower shows a mixture of vertical and horizontal dipoles for FM. Crystal palace was chosen for some of the first BBC experiments in mixed polarization 40 years ago.

14th Jul 2020, 9:51 am	#23
Derminator Pentode Join Date: Feb 2009 Location: Crystal Palace, London, UK. Posts: 118	Re: FM signal too strong? Thanks for the really interesting and helpful replies everyone. I did get up in the attic yesterday evening and managed to achieve some improvement in the radio 3 reception. Radio 2 is now excellent. This is with the dipole still aligned vertically but just moved by about 3 feet from where it was. This was done by trial and error, running up and down the stairs with the tuner on in the living room. I will get to work on a proper rotator so that i can align it horizontally and bring the tuner up to the attic with me for more thorough investigation.

14th Jul 2020, 11:35 am	#24
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: FM signal too strong? London can be a difficult place to get a good FM signal, despite the big transmitters which are available. There are lots of substantial tall buildings, many of which are steel reinforced concrete. The really tall ones which have gone up in the last 20 years cast a huge reception shadow, and are also very effective at generating multipath. Add to that lots of interference sources and pirate broadcasters and clean reception becomes an absolute lottery.

15th Jul 2020, 3:58 pm	#25
Mach One Heptode Join Date: Sep 2005 Location: Shoreham-by-Sea, West Sussex, UK. Posts: 611	Re: FM signal too strong? Crystal Palace is what is called "mixed" polarisation (realistically slant polarisation) and Croydon used to use Circular polarisation - I don't know if it still uses this. When I was living in London the commercial stations would come in beautifully with any "wet string" antenna (even a finger in the antenna socket...) whereas the similar strength signals from the BBC which were then horizontal were much more difficult in those circumstances. Could the difference be down at least partly to the fact that Radio 3 has a much lower average modulation level than commercial radio which is generally as loud as possible? At any rate, Paul's suggestion of pointing the antenna away towards Wrotham might be a good solution.

16th Jul 2020, 9:05 am	#27
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: FM signal too strong? Circular polarisation can be done with a vertical and horizontal antenna with a splitter and a bit of phase shifting line. No hardware has to spin ! It has several advantages/disadvantages. Passage through some media interfaces can rotate polarisation. This doesn't affect circ pol. Also circ pol is receivable by horizontal or vertiacl polarised antennae, though a circ pol antenna will gather more signal power from a circ pol broadcast. There are two circular polarisaton 'senses' - a right-hand corkscrew and a left hand corkscrew. The wrong one will null your signal. Circ pol waves can have their sense reversed on reflection. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

16th Jul 2020, 3:46 pm	#29
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: FM signal too strong? Croydon has lots of tall buildings which may be affecting you - only a possibility of course.