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Old 22nd Jul 2016, 11:04 am   #1
SteveCG
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Default J. Beam aerials UHF Stacking Kit STK2

There is a brief description and a photo of the STK2 stacking kit in the 1972 J. Beam catalogue.

Its purpose was to allow two UHF MBM (Multibeam) or PBM (Parabeam) aerials to be connected together to increase the signal. It was intended for use in weak signal areas (presumably as distinct from areas where ghosting was a problem).

The kit seems to comprise a (horizontal) mounting pole to separate the aerials; and a cable phasing system.

Does anybody have further details on it?

I would expect there to be three versions - one for each TV aerial group.

Ideally I would like to make a reconstruction. But that requires knowledge of the cable lengths and also the aerial centre-to-centre separation.
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Old 22nd Jul 2016, 11:42 am   #2
G6Tanuki
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Default Re: J. Beam aerials UHF Stacking Kit STK2

I've seen several of these stackers in use - in houses near RAF Benson, where I'm assuming they were to minimise aircraft-reflection-induced flutter? The antennas are one above the other.
When I'm next in the area if the antennas are still there I'll try and get a photo or two.


There's a photo of horizontally bayed ones on the Wrights Aerials site:

http://www.wrightsaerials.tv/aerialp...ient/026.shtml


[p.s. for reference the company's name is 'Jaybeam' not J.Beam - neither of which should be confused with a popular brand of Bourbon!]
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Old 22nd Jul 2016, 11:51 am   #3
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Default Re: J. Beam aerials UHF Stacking Kit STK2

Thank you G6Tanuki for the offer of some pics.

By the way: J. Beam aerials was the name for many years which then indeed changed to Jaybeam. In the era I've been considering it was J. Beam Aerials (The MBM/PBM junction boxes have the initials J.B.A. upon them).
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Old 22nd Jul 2016, 11:55 am   #4
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Default Re: J. Beam aerials UHF Stacking Kit STK2

I've only ever seen them as Jaybeam: their corporate history - http://www.amphenol-antennas.com/Com...n-%281%29.aspx - only mentions Jaybeam too.

Though http://www.g4hfq.co.uk/g2hcg/jbeam.doc proves me wrong! They *did* start off as J.Beam after all!

Last edited by G6Tanuki; 22nd Jul 2016 at 12:11 pm.
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Old 22nd Jul 2016, 11:14 pm   #5
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Default Re: J. Beam aerials UHF Stacking Kit STK2

I am certain they were originally known as J.Beam Aerials. I can remember seeing the report of the name change in ERT must have been in the late 1970s; sadly I can't find the documentary proof as all but 1 or 2 of my ERTs eventually went to the tip; many of the older aerials and Gordon J. King's Practical Aerial Handbook (earlier prints) should corroborate my (and Steve's) story.
Steve, I'm not sure that the harnesses were grouped - I've a feeling the baluns in the 'slot' dipoles did most of the matching but I could be wrong. I can't remember where I put my old aerial catalogues because it would say in there on the end of the order code; i.e. if there was no suffix they wouldn't have been grouped.
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Old 23rd Jul 2016, 11:10 am   #6
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Default Re: J. Beam aerials UHF Stacking Kit STK2

Nicklyons2,

I suspect they were grouped because if you look at the Wright's Aerials picture which G6Tanuki gives the link to in post no. 2, you can see that the bottom part of the phasing harness is a 'T' which, I'm certain I remember, is a specific length of co-ax whose impedance allows it to act as a 1/4 wave transformer. This length will be different for the different TV groups. What the impedances are in this co-ax 'T' harness I don't know.

Certainly if there was a letter suffix to the order code that would be a starting clue !
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Old 23rd Jul 2016, 11:28 am   #7
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Default Re: J. Beam aerials UHF Stacking Kit STK2

Hello,
These two adverts from E&RT magazines from 1955 show them using both J Beam and J-Beam as does the 1967 advert in post 21 of this thread https://www.vintage-radio.net/forum/...d.php?t=127893
Yours, Richard
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Old 24th Jul 2016, 10:53 am   #8
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Default Re: J. Beam aerials UHF Stacking Kit STK2

I dont know or care whether it was J Beam or Jaybeam... what I do know is the MBM 48 was a "godsend" in my area... Ilkley and surrounding areas... the forward gain was magnificent.. compared with the usual "toast racks"..... even then we may have had to use a mast head amp... ahhh those were the days (1963 - 1971)...
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Old 25th Jul 2016, 9:42 am   #9
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Default Re: J. Beam aerials UHF Stacking Kit STK2

Phasing harnesses for 50Ohm system usually realised by 1/4 wave transformers of 75 Ohm cable (theoretically ideal 70.6 Ohms)

75x75/50= 112.5 2x112.5 in parallel =56.25 Ohms close enough
70.7x70.7/50=100 2x100 in parallel =50 Ohms perfect

For a 75 Ohm system it would seem to want 106Ohm cable. Never seen that.
Does it exist?
106x106/75 = 150 2x150 in parallel =75 Ohms

93 Ohm cable is available but that would give 58Ohm eventual impedance.
1.3:1 VSWR - could be worse I suppose.

Maybe these used a wilkinson combiner in the T and actually the cable was plain old 75Ohm after all.
Wilkinson could be made to have wider bandwidth too maybe.

Last edited by Jon_G4MDC; 25th Jul 2016 at 9:46 am. Reason: Further thoughts..
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Old 25th Jul 2016, 10:22 am   #10
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Default Re: J. Beam aerials UHF Stacking Kit STK2

Wendy,

Depending upon whose mast head amp you used then could it have been that it was the use of the amp that was making the dominant difference to the viewer's picture quality? (That is, the amp not only overcame the downlead losses but could well have had a better noise figure than the TV's tuner so determining the noise floor for the system - and for really old sets, have given enough overall gain for peak white to be achieved)
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Old 25th Jul 2016, 10:23 am   #11
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Default Re: J. Beam aerials UHF Stacking Kit STK2

Jon,

... indeed you put the 'questions' about this kit succinctly !
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Old 25th Jul 2016, 10:28 am   #12
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Default Re: J. Beam aerials UHF Stacking Kit STK2

Thanks Steve,

On further reflection using the quarter wave cables as a combiner is much worse than as a splitter.

The match at each aerial port is only about 3:1 VSWR and that is when the match at the common port is perfect!

Either they turned a blind eye to that or there was a Wilkinson in the T. That has a further benefit that the actual cable length to the aerials can be anything so long as it is identical for both.

Last edited by Jon_G4MDC; 25th Jul 2016 at 10:32 am. Reason: added a bit
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Old 25th Jul 2016, 1:24 pm   #13
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Default Re: J. Beam aerials UHF Stacking Kit STK2

Don't forget that you can't have a lossless matched passive 3-port. You have to decide what is most important and what you can do without. In this case I would guess that low loss is important, so less than perfect matching on at least some ports would have to be tolerated.
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