FM signal too strong?

[Derminator]

Hi All, I live between the Crystal Palace transmitter and it's near neighbour, the Croydon transmitter about one quarter of a mile and one mile respectively and have a clear line of sight to both from my house. I have a Leak Troughline stereo in my living room connected via coaxial cable to a simple dipole aerial in my attic that I made from two pieces of half inch copper pipe cut to length so as to tune it for radio 3. The copper elements are connected to the coax via a balun that I bought online and the dipole is hanging vertically.
Problem is I can't get a clean reception on radio 3. Radio 2 and radio 4 are a little better but not great. These are all broadcast from Crystal Palace just across the park from my house. The stations broadcast from Croydon a mile away, Heart 106.2, Capital and Smooth Radio, come over beautifully which proves to me that the Troughline is working well and sounding fantastic even with the much derided original stereo decoder. Why then is there so much hiss when I tune to the beeb at Crystal Palace? Is the radio being overloaded? How can I test for this?

[paulsherwin]

It's much more likely that you are getting shading or multipath problems. Getting a clean signal with a loft aerial is often a challenge, even in a strong signal area. Try moving the aerial around or modifying its polarisation.

There is often a signal shadow very close to a large FM transmitter.

[Hartley118]

It's worth trying an attenuator in series with the aerial coax. If that improves matters, you'll know that you had an overload. Here's an example of a simple cheap variable attenuator:

https://www.amazon.co.uk/Electronics...VZV7NQ6T88V6C0

Martin

[Station X]

Unlikely to be down to too high a signal strength, but you could test for this by using an inline attenuator, Commonly supplied with male and female Belling Lee connections.

[mickm3for]

Hi C.P. is a relay of Wrotham. Crystal Palace is on 91mhz at 4kw Whrotham is on 91.3mhz at 125kw Mick

[Derminator]

Thanks all, I think firstly I'll bring the tuner up to the attic and try repositioning the aerial. If I get no improvement then I'll try the attenuator and report back. That's interesting about Wotham transmitting a much stronger signal Mick. I have a clear line of sight to that transmitter too, well, to my attic at least. Could they be interfering with each other?

[paulsherwin]

They are on different frequencies so shouldn't interfere with each other.

Wrotham was the original transmitter for the London area (it covers most of SE England). The relays at CP were installed in the 80s because tall buildings were causing reception problems from Wrotham. You may well find you get a cleaner signal from Wrotham.

It's normal to need to set up a loft aerial by trial and error.

[Mr 1936]

I suspect that at your location the Crystal Palace BBC signal will be stronger than Wrotham, as the much shorter path length will more than compensate for the 18 dB lower transmit ERP (4 kW versus 250 kW). Even so, a frequency spacing of 300 kHz doesn't allow for much adjacent channel rejection, especially for an older tuner design with L/C IF filters. You could therefore be getting interference effects due to receiving both signals together.

A loft location will always tend to have multipath reflections from the various surrounding objects, which will give nulls at certain places. These nulls will be frequency conscious, so for example Radio 3 may be weakened but Radio 2 is OK. You may have just been unfortunate enough to have positioned your aerial in such a null for your favourite station, in which case changing the aerial's polarization angle and/or moving it sideways a few feet one way or the other may improve reception.


In the very early days of VHF television from Alexandra Palace, the engineers were initially puzzled by complaints of poor pictures from viewers living close by. Loads of signal, but in some cases the picture went "negative" with blacks rendered as white and vice versa. The cause was eventually traced to the the transmitting aerial array, which was designed to send power out to the horizon. At short distances, where the angles were well below horizontal, there were deep frequency selective nulls in the elevation pattern. Modern arrays are more scientifically designed to largely fill in these nulls, so its unlikely but not impossible you could be suffering from a similar effect. If you are a quarter mile away from Crystal Palace and their FM transmit aerials are say 160m high, your receive aerial viewed from the transmitter will be about 22 degrees below the horizontal.

[paulsherwin]

As I suggested back in #2, most of the CP signal may be radiating out some distance above the OP's roof line.

Also try the arial horizontally, this way you can at least null out one transmitter.

[Boulevardier]

It's always worth experimenting in all sorts of ways with VHF aerials in high signal strength areas. I've lived in a few places that were "line-of-sight" to the transmitter, and just a few feet of wire (or even a screwdriver blade) on the aerial socket gave perfect reception that couldn't be improved on with any amount of aerials! Also, set-top aerials used to be very common, and often gave excellent signal strength. Wrotham is a pretty "meaty" transmitter!

Mike

[stevehertz]

Originally Posted by merlinmaxwell

Also try the aerial horizontally, this way you can at least null out one transmitter.

Yes, a vertically polarised aerial is not 'pointing' at anything in particular, so it's not directional. I would hang it horizontally and swing it around until you get the results you want with the tuner by your side in the attic. You may have to use a long stick to move it as your presence right next to it will cause reflections etc.

[Hartley118]

It occurs to me that if you're in a really high signal strength area, then you may be picking up sufficient signal on the feeder to overload the tuner. So Mike's suggestion of just a length of wire is worth trying.

To my own embarrassment, I found some while ago that, owing to an accidental short in the loft, my own pretty satisfactory FM reception had been coming just from the feeder coax pickup and not the 4-element aerial on the roof!

Martin

[G6Tanuki]

On the other hand... if you have a massive local signal *and* a load of local reflections, sometimes a highly-directive multi-element antenna is needed!

[A 5-element beam pointed at the desired line-of-sight transmitter, coupled with 30dB of attenuation at the receiver's input, may seem counterintuitive - but you're using the beam to get directivity to exclude the odd reflections/multipath that cause distortion. Then you attenuate to stop the beam's incidental 'gain' overwhelming the receiver].

[PJL]

FM is not the only transmitter at Crystal Palace...

[Restoration73]

Where I am, CP is the stronger signal and I haven't noticed any degradation, it also
carries Classic FM. Back in the analogue TV days, I saw a receiver fed by a VCR very close to the CP mast. To reduce ghosting an 18-element aerial was used with attenuators
totalling 200dB, so that might be worth considering.

[Ambientnoise]

200dB ! That is some attenuator. I think it amounts to one ten-billionth (10^-10) if my arithmetic is correct?

[emeritus]

I use a home made, loft-mounted, horizontal dipole made from 15mm copper pipe for my FM reception. I didn't bother with a balun. Before the band filled up with local and commercial stations, it was mounted on a home-made rotator. The very sharp end-on null could be used to attenuate UK stations to allow listening to French stations in stereo. A consideration of its polar diagram shows that a horizontal dipole does not need to be accurately aimed at a wanted station, so accurately aligning the null with an unwanted station shouldn't significantly affect the signal strength of the wanted station.

[Hartley118]

Originally Posted by Restoration73

Where I am, CP is the stronger signal and I haven't noticed any degradation, it also
carries Classic FM. Back in the analogue TV days, I saw a receiver fed by a VCR very close to the CP mast. To reduce ghosting an 18-element aerial was used with attenuators
totalling 200dB, so that might be worth considering.

I guess that 200dB attenuation would only leave you with a spot of thermal noise.

Martin

[crackle]

As you are so close to the transmitter could you actually be in a shadow and the signals are going straight over you.

Mike