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Old 2nd Dec 2022, 10:14 pm   #1
JR10891053
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Default BBC Band II (FM) Transmitter Powers

Just looking at the Ofcom TxParams datasheet and it is showing Sutton Coldfield has had its power reduced from 250kw ERP to 110 kw ERP. Divis in Northern Ireland is also showing the Tx power has been reduced by approx 3dB.

Probably on rolling program of power reductions on band II sites for the BBC with cuts to local radio and other services.
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Old 3rd Dec 2022, 9:12 am   #2
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Default Re: BBC Band II (FM) Transmitter Powers

I’ve had a look online but couldn’t find any information on why, Holme Moss is still on approx 250kw and Wrotham is approx 125kw, I don’t know if those have been altered.

It’s possibly cost cutting and/or an attempt to reduce the load on the National Grid in the Midlands. It would be interesting to know the effects on reception.
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Old 3rd Dec 2022, 9:56 am   #3
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Default Re: BBC Band II (FM) Transmitter Powers

Since about 1980 Transmitter power was doubled by having half horizontal and half
vertical polarisation. ERP reduction can take place with reduced aerial gain.
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Old 3rd Dec 2022, 10:57 am   #4
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Default Re: BBC Band II (FM) Transmitter Powers

Does any one know when SC reduced from 250kw to 125kw, approx split between VP/HP?
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Old 3rd Dec 2022, 1:19 pm   #5
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Default Re: BBC Band II (FM) Transmitter Powers

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Does any one know when SC reduced from 250kw to 125kw, approx split between VP/HP?
Don't know when Sutton reduced power. I do recall some sites were having new more efficient/reliable transmission equipment installed, so probably the power reduction took place at that stage.

The ERP figure is a figure which calculates power with the antenna gain, system losses and Tx power. VP/HP figure tends to be added. So 125kw HP and 125 kw VP = 250kw ERP.

The effect will be on reception on older receivers which are not as sensitive as newer receivers using dedicated receiver chipsets. So relevant in many ways to this forum. Probably notice a increase in the background noise and poor stereo quality.

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Old 3rd Dec 2022, 1:59 pm   #6
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Default Re: BBC Band II (FM) Transmitter Powers

Hmm - figures. I thought the hiss level on Radio 3 had gone up. Might have to think about more elements...
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Old 3rd Dec 2022, 2:13 pm   #7
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Default Re: BBC Band II (FM) Transmitter Powers

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Originally Posted by JR10891053 View Post
The effect will be on reception on older receivers which are not as sensitive as newer receivers using dedicated receiver chipsets. So relevant in many ways to this forum. Probably notice a increase in the background noise and poor stereo quality.
Modern receivers have "blend" when stereo reception get weak. With decreasing signal they gradually mix L and R together (cancels out stereo hiss) until it is eventually 'full mono'.

Ordinary people aren't going to notice less 'stereo effect' which a halving in power may well result in on modern receivers. It is of course going to be noticeable though on the limits of just about clear mono reception (be it in fringe rural areas or inside large buildings) where a few dB difference does make a difference between being able to listen or not.

Last edited by colourking; 3rd Dec 2022 at 2:21 pm.
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Old 3rd Dec 2022, 2:15 pm   #8
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Default Re: BBC Band II (FM) Transmitter Powers

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Originally Posted by JR10891053 View Post

The ERP figure is a figure which calculates power with the antenna gain, system losses and Tx power. VP/HP figure tends to be added. So 125kw HP and 125 kw VP = 250kw ERP.
SC BBC R3 approx 47kw HP, 62kw VP.

https://www.ofcom.org.uk/__data/asse...xParamsVHF.csv
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Old 3rd Dec 2022, 2:30 pm   #9
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Default Re: BBC Band II (FM) Transmitter Powers

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Hmm - figures. I thought the hiss level on Radio 3 had gone up. Might have to think about more elements...
Looking at the Associated Broadcast Consultant map based on 125Kw VP the Kington area is very patchy for R3 from SC, perhaps even more so at approx half the output. They only measured VP so HP could be better or worse.

http://a-bc.co.uk/MCA/BBC000003-SK11350032.html
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Old 3rd Dec 2022, 4:56 pm   #10
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Default Re: BBC Band II (FM) Transmitter Powers

Well, I'm on high ground 5 miles west of Kington, with reasonable LOS to SC, and it's worked well with a 3-element to date. The thought occurs, though - is it straight in-phase vert/lat or is it circular? If they're in phase could I squeeze a bit more by slanting the aerial, I wonder?
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Old 3rd Dec 2022, 6:11 pm   #11
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Default Re: BBC Band II (FM) Transmitter Powers

Hi

Back in the early 1970's, I worked for a couple of months at the BBC's Rowridge transmitter on the Isle of Wight. The 5 kW FM transmitters were all made by Marconi and dated to 1957. There were two banks of 3 transmitters (Light, Home, Third) with tuned coax combiners. The antenna was HP only, and consisted of a tall hollow cylinder with many vertical slots. These were covered with an insulating sheet to keep the weather out. I was never invited to climb the 500 foot mast to take a close look !

Unusually, the antenna was split into a separate upper and lower half, and each transmitter bank fed one half so two feeders were needed up the mast. An elaborate motor driven servo arrangment in the transmitter racks kept the RF phasing for each service within tight limits, so that the beam always pointed downwards by a degree or two. All this hardware occupied a long line of 6 foot racks.

The total antenna gain was (from memory) 8 dBi, which was rather better than usual because the coverage only had to be to the North, which avoided wasting power and upsetting the French.

The 10 kW plus 8 dBi gave a maximum ERP of 60 kW for each service. Occasionally a transmitter had to be turned off for maintenance. As the Chief engineer explained to me, when this was done not only did the transmitter power drop to 10 kW, but the antenna gain dropped to 5 dBi because it was effectively halved in size. The "cutback" ERP was therefore only 15 kW, and if maintenance dragged on too long it did result in a few phone complaints from listeners living at the edges of the service area

Last edited by Mr 1936; 3rd Dec 2022 at 6:20 pm.
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Old 3rd Dec 2022, 7:01 pm   #12
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Default Re: BBC Band II (FM) Transmitter Powers

Interesting thread, with some good information. My location is just barely LoS to SC and yes, there has been a noticeable effect as mentioned more noticeable on old (Roberts and Grundig) sets but much less so on more modern (Sony) receivers.

Wonder if the idea is to reduce the overall quality of the FM signal so DAB appears "better" ?
Then maybe they'll allow turning off the irksome FM - after this Winter at the earliest - as the Gov't have said to ensure you have an FM radio in the case of blackouts, according to the 'leaked BBC scripts' in the tabloids.
The story seems to be that an FM TX takes less power from any backup generator, I don't know whether this is true as the TX aerials I have seen seem to have much lower power-rated rigid copper CoAx feeders for DAB than FM. More likely seems that more folks have access to an FM than a DAB set even now.
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Old 3rd Dec 2022, 8:44 pm   #13
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Default Re: BBC Band II (FM) Transmitter Powers

Hi

As FM is a constant envelope system, modern transmitter efficiency can be roughly 70%. The DAB signal structure is noiselike, and needs highly linear RF power amplifiers, so transmitter efficiency is more like 40%.

However, there are numerous other factors at play such as the lower ERP of DAB for the same service area and the ability to handle multiple programmes. Overall, I suspect that DAB is the more efficient.

It's sobering to think that official AM broadcasting lasted 33 years (1922 to 1955) before FM came along as a rival. FM lasted 38 years (1955 to 1993) before DAB came along. And here we are 29 years later and AM is still around (just) and FM showing no signs of being axed just yet.
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Old 3rd Dec 2022, 9:21 pm   #14
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Default Re: BBC Band II (FM) Transmitter Powers

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It's sobering to think that official AM broadcasting lasted 33 years (1922 to 1955) before FM came along as a rival. FM lasted 38 years (1955 to 1993) before DAB came along. And here we are 29 years later and AM is still around (just) and FM showing no signs of being axed just yet.
...and the likes of RTE giving up DAB as a bad job. FM is a darned good system, and would be yet better without all the Optimods.
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Old 4th Dec 2022, 8:27 am   #15
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Default Re: BBC Band II (FM) Transmitter Powers

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... FM is a darned good system, and would be yet better without all the Optimods.
Ho Yuss!

Radio 3 in the car (FM) is horrible when the presenter crashes in after something orchestral one turned the volume up for.

I completely fail to understand why this has to be the case. Classic FM also does this, but oddly to a lesser extent (I think its music output is more compressed than R3's).

It wouldn't kill either of them to limit continuity speech (and adverts) to, say, 6dB below peak mod/deviation. Give full symphony orchestras a chance!

Back on topic, the thick walls and urban location of our property make DAB a non-starter (without a loft aerial), but VHF FM is mostly fine (from Wenvoe, across the Bristol Channel). Whatever the reason that the BBC FM TXs are being turned down, I hope it's temporary.
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Old 4th Dec 2022, 3:57 pm   #16
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Default Re: BBC Band II (FM) Transmitter Powers

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Originally Posted by PaulDarzi View Post
Interesting thread, with some good information. My location is just barely LoS to SC and yes, there has been a noticeable effect as mentioned more noticeable on old (Roberts and Grundig) sets but much less so on more modern (Sony) receivers.

Wonder if the idea is to reduce the overall quality of the FM signal so DAB appears "better" ?
Then maybe they'll allow turning off the irksome FM - after this Winter at the earliest - as the Gov't have said to ensure you have an FM radio in the case of blackouts, according to the 'leaked BBC scripts' in the tabloids.
The story seems to be that an FM TX takes less power from any backup generator, I don't know whether this is true as the TX aerials I have seen seem to have much lower power-rated rigid copper CoAx feeders for DAB than FM. More likely seems that more folks have access to an FM than a DAB set even now.

I recall reduced power operation on FM was 6dB reduction. So it would be half power on the amplifiers and half antenna = 6dB. So for a site like Sutton when running 250kw, the power would reduce to 62kw. Was noticable in some areas depending on receiver.

Today if the same still applies the output is 31kw ERP. So reduced power operation would produce very marginal reception at some locations.

The band II RF amplifiers at SC were Pye when I visited the site some 20 years ago, whilst the modulator was in another room with curved windows and was a BBC design. There was 2 x Pye amplfiers per service when I visited.
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Old 4th Dec 2022, 7:05 pm   #17
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Default Re: BBC Band II (FM) Transmitter Powers

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Originally Posted by Mr 1936 View Post
Hi

Back in the early 1970's, I worked for a couple of months at the BBC's Rowridge transmitter on the Isle of Wight. The 5 kW FM transmitters were all made by Marconi and dated to 1957. There were two banks of 3 transmitters (Light, Home, Third) with tuned coax combiners. The antenna was HP only, and consisted of a tall hollow cylinder with many vertical slots. These were covered with an insulating sheet to keep the weather out. I was never invited to climb the 500 foot mast to take a close look !

Unusually, the antenna was split into a separate upper and lower half, and each transmitter bank fed one half so two feeders were needed up the mast. An elaborate motor driven servo arrangment in the transmitter racks kept the RF phasing for each service within tight limits, so that the beam always pointed downwards by a degree or two. All this hardware occupied a long line of 6 foot racks.

The total antenna gain was (from memory) 8 dBi, which was rather better than usual because the coverage only had to be to the North, which avoided wasting power and upsetting the French.

The 10 kW plus 8 dBi gave a maximum ERP of 60 kW for each service. Occasionally a transmitter had to be turned off for maintenance. As the Chief engineer explained to me, when this was done not only did the transmitter power drop to 10 kW, but the antenna gain dropped to 5 dBi because it was effectively halved in size. The "cutback" ERP was therefore only 15 kW, and if maintenance dragged on too long it did result in a few phone complaints from listeners living at the edges of the service area
I have happy memories of listening & watching from Rowridge at Swanage. Used to get a very good signal. I was also impressed when UHF came about, measuring around 5 millivolts at the aerial! A few inches of random wire was quite good enough for FM.
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Old 5th Dec 2022, 12:33 am   #18
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Default Re: BBC Band II (FM) Transmitter Powers

I only listen to FM radio in the car. I'm not sure if it's related to power levels at the transmitter, but Radio 1 always sounds louder than Radio 2 in this area.
Perhaps it's a compression thing, or is Radio 2 quieter to suit the old folks who are supposed to listen to it?

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Old 5th Dec 2022, 10:46 am   #19
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Default Re: BBC Band II (FM) Transmitter Powers

Have you ever known such fiddling about with broadcasting as a whole? It's getting ridiculous.

Oh Aub, Radio 2 still has a reasonable middle aged audience but I think maybe R4 is for those that have sadly passed. J.
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Old 5th Dec 2022, 7:25 pm   #20
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Default Re: BBC Band II (FM) Transmitter Powers

Where we live on the Eastern side of Leicestershire, Sutton Coldfield is the best source for BBC R2.

Just tried it on my modern Roberts Stream with telescopic antenna & no problem on FM. However my older radios struggle indoors unless connected to an external dipole.

Don't tend to listen to the BBC R2 of recent though because sadly he station does seem to be morphing for the younger listener's benefit.

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