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Old 22nd Nov 2020, 12:17 pm   #1
SteveCG
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Default Jaybeam SBM2 FM aerial

When this aerial came out in the 1970s it was sold as covering the 87.5-100 Mc/s FM radio band as was then. My question is this: When the decision to extend the FM band in the UK to cover 87.5 to 108 Mc/s came into force did Jaybeam alter the dimensions of the aerial? Or for that matter had Jaybeam been taken over by then and the product disappeared off the market...
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Old 22nd Nov 2020, 12:49 pm   #2
Edward Huggins
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Default Re: Jaybeam SBM2 FM aerial

The extension to 108M/cs came before the 1970s. Your Jaybeam aerial should work fine just as it is.
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Old 22nd Nov 2020, 7:22 pm   #3
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Default Re: Jaybeam SBM2 FM aerial

The band itself was always 88-108 but there was nothing above 100MHz in the UK until the 'mid band' VHF PMR was cleared. I have had examples of both an early 70s Jaybeam 3 element FM1083 and a later one and the early model has slightly larger elements and spacings.
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Old 23rd Nov 2020, 11:41 am   #4
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Default Re: Jaybeam SBM2 FM aerial

Did the FM1083 have just a plain dipole? (I assume the FM1083 was the earlier aerial.)

Last edited by SteveCG; 23rd Nov 2020 at 11:46 am. Reason: Added sentence
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Old 23rd Nov 2020, 12:09 pm   #5
Ted Kendall
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Default Re: Jaybeam SBM2 FM aerial

Quote:
Originally Posted by Jonster View Post
The band itself was always 88-108 but there was nothing above 100MHz in the UK until the 'mid band' VHF PMR was cleared. I have had examples of both an early 70s Jaybeam 3 element FM1083 and a later one and the early model has slightly larger elements and spacings.
...which would imply that the sensitivity peak was shifted nearer the centre of the band in later production? If memory serves, the first arrays for stereo reception were arranged to peak around 91-93MHz, where the Radio 3 carriers were.
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Old 23rd Nov 2020, 4:05 pm   #6
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Default Re: Jaybeam SBM2 FM aerial

Quote:
Originally Posted by Ted Kendall View Post
If memory serves, the first arrays for stereo reception were arranged to peak around 91-93MHz, where the Radio 3 carriers were.
That’s my memory as well.
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Old 24th Nov 2020, 11:10 am   #7
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Default Re: Jaybeam SBM2 FM aerial

Well folks here is some further information that you might find of interest:

1) an international meeting held in Atlantic City (USA) in 1947 reserved 87.5-100 Mc/s for FM broadcasting in Europe. I've no doubt that 88-108 Mc/s was reserved then for the Americas. I think it was only in the late 70's that up to 104 Mc/s and then finally 108 was allocated for Europe. Hence my question about the change in aerial dimensions.

2) In a Feb 1972 J-Beam catalogue the FM4S aerial was being advertised as being optimised for stereo reception. It is notable for having a folded dipole as a reflector.

3) In a May 1983 Jaybeam catalogue the SBM2 was advertised as covering 88-102 Mc/s. Indeed all the FM aerials SBM2, SBM3, SBM4, SBM6 and FM9S covered the same 88-102 band.

4) In a July 1986 Jaybeam catalogue the SBM2 is no-longer listed, but the remaining aerials are listed as covering 88-108 Mc/s, except for the FM9S which is listed as still covering 88-102. The dimensions and gains etc, though are identical to those quoted in the 1983 catalogue.
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Old 24th Nov 2020, 12:09 pm   #8
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Default Re: Jaybeam SBM2 FM aerial

I wonder how an aerial can be "optimised for stereo reception"? Surely both mono and stereo signals benefit from any attributes of an aerial that isn't extremely narrowband?

I suppose higher gain could be the issue since a stereo signal has to be stronger than a mono one for equivalent S/N, though higher gain won't do the mono results any harm.
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Old 24th Nov 2020, 12:53 pm   #9
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Default Re: Jaybeam SBM2 FM aerial

Quote:
Originally Posted by Herald1360 View Post
I wonder how an aerial can be "optimised for stereo reception"? Surely both mono and stereo signals benefit from any attributes of an aerial that isn't extremely narrowband?

I suppose higher gain could be the issue since a stereo signal has to be stronger than a mono one for equivalent S/N, though higher gain won't do the mono results any harm.
Between 1966 and 1972, this basically meant that the array was peaked for around 91 MHz, which was the Radio 3 frequency. Other stereo services had to wait until the introduction of PCM distribution in 1972 - and of course re-equipping studios for stereo working took time and money. All other things being equal,you need 20dB more signal at the aerial to get the same signal to noise ratio on a stereo transmission as you had in mono.
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Old 24th Nov 2020, 1:49 pm   #10
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Default Re: Jaybeam SBM2 FM aerial

Wonder why 20dB more was required, it’s a factor of 100 in power terms. Simplistically, 3dB more sounds right, half power in each channel. Although nothing is perfect, I am missing something major !
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Old 24th Nov 2020, 2:59 pm   #11
Ted Kendall
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Default Re: Jaybeam SBM2 FM aerial

Quote:
Originally Posted by Ambientnoise View Post
Wonder why 20dB more was required, it’s a factor of 100 in power terms. Simplistically, 3dB more sounds right, half power in each channel. Although nothing is perfect, I am missing something major !
Yes, in a word. The stereo subcarrier is AM, takes up only 10% of the radiated power, and the noise bandwidth increases from 15kHz to 53kHz. Number crunching from there gets you to the 20dB figure, which is borne out in practice.
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Old 24th Nov 2020, 4:34 pm   #12
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Default Re: Jaybeam SBM2 FM aerial

WW Sept 1966 page 445 gives an outline of the pilot tone stereo system.
https://worldradiohistory.com/UK/Wir...ld-1966-09.pdf
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Old 24th Nov 2020, 4:41 pm   #13
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Default Re: Jaybeam SBM2 FM aerial

Further info:

5) The FM4S aerial claimed a gain of 7.0 dBd, F/B ratio of 16 dB and had a boom of 74" (1.88m). It was sold in versions covering "Any 5Mc/s band between 88 and 100 Mc/s". The boom length of its successor, the SBM4, was 1.32m, which had a claimed gain of 7.0 dBd covering the 88-102 Mc/s band.

6) In the 1980s I obtained the dimensions of an 'unknown' SBM6 and modelled its gain using the Mininec software programme. This cut off above 102 Mc/s - which was consistent with the element lengths, especially compared to known Antiference 88-108 Mc/s aerials. So I'm confident that the SBM6 as sold in the later 1980s must have had its director element lengths reduced.

7) Many years ago I saw an FM4S to the West of the Malvern Hills, pointing to Wenvoe. Now its Radio 3 Frequency would have been 96.8 Mc/s back then - quite how that fitted into J-Beam's version choice I'm not sure! Sadly the aerial has long gone.
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Old 24th Nov 2020, 6:47 pm   #14
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Default Re: Jaybeam SBM2 FM aerial

I don't think an inch or so extra length on a folded dipole will make much if any noticeable difference to the sound coming out of the speakers. Though I can appreciate that you want to get it 'right'.

I dual use my FM dipole for DAB use and it works fine on the latter despite the size being quite 'wrong' according to DAB spec. Ok, it's DAB so as long as the signal level is high enough (greater than the knee in the receiver), there will be no difference. They connect into a Labgear distribution unit.
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Old 25th Nov 2020, 11:45 am   #15
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Default Re: Jaybeam SBM2 FM aerial

stevehertz,

I agree with you about general usage - however I'm using an SBM2 because it has a balun and I'm trying an experiment in a location where a good polar diagram shape is helpful.
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Old 26th Nov 2020, 10:18 am   #16
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Default Re: Jaybeam SBM2 FM aerial

I've attached a couple of 'photos of an SBM2. The location is west of the Malvern Hills and the aerial is pointed towards Ridge Hill / Wenvoe. Clearly the owner is interested in radio reception of all sorts, but by the looks of the state of the SBM2 I reckon that it is the oldest one there.
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