J. Beam Parabeam UHF aerials - start date

[SteveCG]

I've been given an early 'Squareback' style 18 element J. Beam Group B aerial. What I'm interested in is knowing when J. Beam started this range of aerials? My estimate is about 1967.

[Aub]

Hi,

Not sure if it's any indication, but a friend of mine had a J-Beam 14 element Parabeam for 2 metres in 1974.

Cheers

Aub

[Nuvistor]

Oct 1966 according to this article.
http://www.americanradiohistory.com/...%22parabeam%22

Frank

Edit, we were using the J-Beam Multibeam aerial in 1967, similar Skelton slot reflector and dipole but with X directors instead of the normal Yagi directors used in the Parabeam.

[SteveCG]

Thank you Frank (Nuvistor) for the link.

The particular model I have (which I've taken apart for cleaning, so can't do a photo) comprises a 6 element section (Slot Reflector, slot dipole and 4 directors) which is coupled to a 12 element director array. The junction box cover is such that the coax cable enters from the Reflector side.

Interestingly, the booms are held together with two special brackets that combine the boom coupler part and a straight section for mounting to the mast via a clamp. In other words this model does not use the trombone type mount that later UHF J. Beam aerials do. I don't know whether the aerial originally had a special clamp (some surface marks imply so) as I was told some years ago that the aerial had been moved somewhat - and the clamp it has now is not the type I believe J. Beam would have used.

By the way, the downlead Coax read open-circuit from inner to outer. I expect to find everything inside the junction box open-circuit if the aerial is like other Parabeams/Multibeams I've come across in the past.

[Nuvistor]

I have Multibeam MBM46 group C/D in the loft, been there working since 1970 with the same coax. When all the shuffling around has been completed in the UHF band I will have to see how well it fairs if the channels change to below 49.
Frank

[SteveCG]

Frank,

People living in service area of the Malvern TV relay which used C/D channels in its analogue days (including Channels 66 and 68 - ie the top of the band) found that J. Beam designs did not work well for those top two channels. The local aerial riggers confirmed this.

Now I cannot speak for the later J. Beam products but I'm pretty sure that earlier J. Beam aerials for this frequency band were just group C and not C/D.

Moreover later Parabeam and MBM designs that used the inclined slot reflector design eventually used a larger slot reflector. This was so that the reflector actually worked as a reflector at the bottom of the band.

To check this I did an experiment with a Group A 10 element Parabeam tuned to channel 22; it was the same size reflector/ slot-dipole design. Watching a signal strength meter I brought up the larger reflector design from behind. What I saw was the signal strength reading varying cyclically as I brought the extra reflector closer to the original reflector.

I then looked more closely at Parabeam aerials on the skyline and after a lot of observation concluded that later Group B aerials used the original Group A size as a reflector and later Group C aerials used the original Group B reflector size.

To sum up Frank, it's possible you might find your MBM46 has not got quite the front to back ratio that it had on the higher analogue channels. But being digital would you notice??

[Nuvistor]

It could be a C it is one of the first but I thought it was C/D, the highest channel we had was 65 and it worked very well for that. It does help that I can see Winter Hill line of sight about 10km away, the aerial was s/h and given to me, when first installed gave about 20mv into the front end of a valve UHF tuner, never had overload or noise problems. No doubt the coax is not what it was and that has reduced now. Channel 5 I think was on ch49 but it worked ok.

One unusual occurrence when both digital and anolgue were transmitted was that in a very small area of about 4 sq miles, aerials had to be turned vertical to get a decent digital signal without being pixelated, I was in that area. When switch over occurred and the ERP increased that was cured, there are still a few aerials around that are vertical, presume working ok.

My biggest problem is not aerial performance but program performance, the TV not been switched on all week, but off topic.

Frank

[G6Tanuki]

There are diagrams and discussion of these in my 4th edition of "RSGB Radio Communication Handbook" printed Sept 1968.

[SteveCG]

Frank,

My first MBM46 Group C was bought new in 1968/69 (can't quite remember !) It had a square reflector. It was bought on the claims of superior forward gain compared to the 18 element yagis of the time. The experiment was to see if Winter Hill on Channel 62 could be received in a southern part of the Lake District. The experiment used a friend's dual-standard Phllips portable and lugging it up the side of some hills. Yes, we could get a weak signal...

In later years, during the '80s / early 90's, being older and wiser and armed with a Labgear Signal strength meter, I tried comparing different designs because I had this feeling that these 'fancy director' aerials did not seem all they were claimed to be.

Now measuring aerial gain using a Terrestrial source is difficult (Radio-Astronomers have it easier) but I concluded that the length of the aerial boom was possibly a better indicator of gain than director shape (ie the 'X's etc).

Later still I saw in the aerial makers literature that the aerial-gain-claims came down in value once they had adopted a more rigorous approach.

Using the aerial modelling programme MININEC for simple Yagi designs had fuelled my suspicions.

As for TV content - having gone to all these Herculean efforts to receive them - well, I know what you mean !

G6Tanuki - I haven't read your work - I shall endeavour to find a copy.

[Nuvistor]

Steve,
One installation I do remember was at Billinge near Wigan. Billinge is on a high point, indeed at the top there are commercial relay stations. However this customer was the wrong side of the hill, not too far from the top in deep shadow.
We tried various combinations to get a good signal for the S/S colour TV the customer wanted, money was not a problem. The only way we got a signal that was noise/fade free was with 4 MBM46 stacked on the garage about 20 yards from the house. Other various designs just did not work in that location. We may have got away with slightly less and used a mast head preamp but that perhaps would have had other problems with the transmitters on the hill a few hundred yards away.

Appreciate the X directors may not have been as good has what was originally claimed though, just had to go off what worked for the location.

No doubt in another place a different design could have worked better but for that one we had a happy customer who paid in full without a quibble.

Luckily for us there were few really bad locations we had to deal with, Parbold was one but they got their own repeater.

I left the trade in around 1980 so any changes to specification would have passed me by.

I saw a note in one of the mags, of course cannot find it now, which stated that J-Beam had altered the reflectors and dipole to strengthen them from folding back due the being used as bird perches, they could also have changed the size as well but not commented on it.

Frank

[radioredcat]

Hi Frank was it this one as I live round the corner from it and is still there.



I had a Multibeam 48 Group B for Sutton Coldfield that would be 1971.

Andy

[Nuvistor]

Originally Posted by radioredcat

Hi Frank was it this one as I live round the corner from it and is still there.

It could be, if my memory is correct it was Beacon Road, if it is the one it has done extremely well, cannot say the exact date it was installed but I should think no later than 1974. Presume the photo was taken over a wall, the mast was on the front of a garage, I think it was a double garage.

Well fancy that.

Frank

[radioredcat]

Yes Frank your spot on it is Beacon road and the photo was taken over the wall and as you said it is on a double garage.
I was inside the property some years ago and was in good nick then well it's still standing.
The present owner has Sky so doesn't use Terrestrial TV but if they did then Storeton would serve them well there now.

Andy

[Nuvistor]

Thanks for that Andy, it's over 40 years old so could well be called vintage I suppose. I was reading an article on line and one comment from the manager of J Beam, document at http://www.g4hfq.co.uk/g2hcg/jbeam.doc

quote "We made those aerials too well, they are still on the chimneys to this day some 30 years later." so some truth in that.

Sorry Steve, seemed to have high jacked your thread.

Frank

[Nuvistor]

Andy, you have a PM.

[ntscuser]

I thought the whole point of the Multibeam was to avoid the need for stacking and matching harnesses?

[Nuvistor]

They are basically a yagi design so you can stack them like other Yagi aerials, the matching harness in that photo if original was supplied by J-Beam including all the hardware.

[SteveCG]

Frank - its been an informative thread for me. I think I only ever saw one 4 x MBM46 in the flesh, and that was years ago, so to have a photo from Andy of a 'beast still at large' is quite a treat!

Judging from the amount of bird droppings on the Parabeam I have - yes our feathered friends loved them...

G6Tanuki - Is the section you mention still in the 5th edition of the RSGB publication ?

[SteveCG]

Just found this image of the exact design of aerial I have, courtesy of Wright's Aerials

http://www.wrightsaerials.co.uk/aeri...ient/021.shtml

I note the comment about specific channels. Well, I'll have to get my ruler out to see if that is so.

[Nuvistor]

I have not seen that mounting bracket before now.