Interesting UHF aerials.

[Hybrid tellies]

Ref Post 33
We had ran into a few problems caused by trees and it always effected BBC2 which from Mendip was on ch64.
One customer would have wildly fading reception on BBC2 with perfect reception on BBC1 and ITV, another customer had poor and fading sound on BBC2 but the picture was perfect and another customer had poor and varying colour on BBC2 with a perfect black and white picture and good sound. All the problems were solved by relocating the aerial to another part of the roof.
All three good examples at how frequency selective trees can be.

[SteveCG]

Re Post no. 33,

I suppose there are two factors at play with the 'viewing through the trees problem'.

The First is that the slightly different paths of the "direct signal" through the foliage will give a wavelength dependent (vectorial) addition at the dipole. This will fluctuate statistically as the leaves/branches move in the wind. At the top end of UHF (eg Mendip when it was on ch 64) the path difference does not have to be much to have cancellation (compared to VHF/FM Band II say).

The Second is that the direct signal is likely to be attenuated compared to any rear/side reflected signal. This will give a varying ghosting that is more prominent than would have been seen in a "normal" situation. Here a well-designed aerial with good low sidelobes/backlobe characteristics can help the viewer. Hence my comment about the indifferent performance aerials that had been used in the pictures of post no. 33.

Mind you now we have all gone digital all these problems should have gone away - or so the Broadcasters claim... I wonder what those in the aerial erection trade now a days have to say about that?

[ntscuser]

My bedroom aerial faces a tree and whereas in analogue days all five channels were just about watchable, digital reception fluctuates between perfect reception of some channels and none at all.

[terrykc]

I'm fascinated by all the references to Bow Tie aerials in this thread when the pictures clearly show the double halo type!

A bow tie element is a plate the same shape as the tie you wear and seems to have been used a lot in the US where it was the active element of a corner reflector aerial.

[SteveCG]

Terry,

A bow-tie aerial is just a bi-cone aerial with filled-in space. A discone aerial is half a bi-cone aerial. They can cover a 5 to 1 frequency range.

As you correctly say, two circular elements forming the dipole is the beastie that you call a double halo type.

[valvekits]

I started my TV apprenticeship in the summer of 1970. I didn’t want to be late and one day I arrived a bit too early with the only people around being the aerial riggers. I was tasked with loading huge boxes of all the UHF aerial components, such as elements, reflectors, etc onto their van. The boxes came straight from the aluminium pressing factory. It never dawned on me then that aerials had to be assembled and I guess that this was the most cost effective and profitable way of providing an aerial service. I assume the bits were assembled by pop riveting but I don’t remember exactly. At that time the riggers were out from dawn 'til dusk, only making a brief appearance to replenish their stocks. I’m not sure if this was the peak period for UHF aerial installations, because colour TV was still in it’s infancy at that point.

Eddie

[Nuvistor]

Depended were you located, in late 1969 some of the main sites started transmission of the 3 stations on UHF, this with colour perhaps was the reason for the surge in aerial installations.
Interesting that the aerial were supplied in pieces, the normal way was to have them partially preassemled for quick and easy installation. The company perhaps did its sums and thought letting the riggers be part of the assembly line was good business, it would certainly slow installations down.

[terrykc]

Yes - as Frank says, they were usually supplied with the directors in two sections complete with the fixings to join them together. The dipole would already be fitted but the reflector would also need to be bolted on.

Before BBC2 started, I remember the day that the BBC announced that the new channel would cater for 'minority' interests. The following morning I had trouble getting through the shop to the workshop because it was jammed with customers queuing up to cancel their 625-line conversions!

3-channel colour from November 1969 heralded cheaper single standard set and increased demand, so that was probably the cause - in many cases being the customer's first UHF TV.

Of course, the BBC had really solved much of their BBC2 viewing number problem to a large extent by putting Match of the Day on the channel!

[valvekits]

The areas were covered by Waltham and Sutton and I can visualise the different coloured plastic end caps but I don’t know how the aerials were physically different. Maybe in some cases of overlap it could be either/or?

Eddie

[Refugee]

This one has got the birds on it

[1955APREN]

Hope they enjoy the grain Ho my spelling again(gain)

[terrykc]

Ah, that must be the Twitter feed ...

[SteveCG]

Re post no. 50

Those aerials look impressive - being a variation on JayBeam (the later J-Beam name) MBM style - I tried a few out from the JayBeam era and found that the results were more mediocre. If you look at the later data sheets from all the aerial manufacturers who had signed up to a specific measurement standard then you will see about 5-6 dB being knocked of their previous forward gain claims. It is one of the reasons that I like the early J-Beam PBM designs - their good performance was real, shall we say...

[G6Tanuki]

One thing I don't get about the antenna design shown in post #50: the ones I've seen, rather than having the 'forked' directors working as an 'X-tilted-on-its-side' - whose operation I could understand as the legs of the 'X' would essentially be quarter-waves, the X-ness giving broader bandwidth - well, on the antenna shown they are configured as two 'V's on their sides - that is, with no electrical connection between the pairs of "V"s or the Vs and the boom.

So you've essentially got two half-wave 'Vees' (each with a very small apex-angle) for the directors. How does that work? And the 'loop' dipole looks to be something similar: I always thought you normally needed the elements to be spaced at least 1/2-wave apart and with some kind of phasing network between them.

Most odd.

[SteveCG]

I agree G6Tanuki - a most curious design. I remember getting a very early MBM46, Group C, when they first came out in 1967/68 time hoping that it would help TV reception of Winter Hill on Ch 62 in the Lake District - a fringe challenge if there ever was one. Looking back I should have saved half of my money and have got an 18 element PBM instead...

[Refugee]

Those birds are more intelligent than normal because all the ones that were waiting for there turns to go square and appear on the screen were spaced accurately along the roof at twice the spacing of the elements in the aerial.
They even perched on alternate elements while thy were square on the screen.

[Heatercathodeshort]

Clever birds! I think they use their feet as a matching 'Balun' to pick up the vibes direct.
J.

[Refugee]

They were certainly well organized in the way they were taking shifts to keep the screen all blocky.
I got the photo at the moment of a shift change with the left one flying off and the relief one flying in from the right.

They were certainly trying to add a harmonic to the response of the aerial by perching on alternate reflectors.

[Heatercathodeshort]

Caught this mess on the roof of a shop premises adjacent to a car park in Cranleigh High Street. Surrey/Sussex borders. John.

[SteveCG]

Re post no. 59

Hmm "an aerial for all seasons " eh?

Squinting at the picture it looks like one of the UHF aerials (missing a plate-type reflector) on the right hand most pole has actually fallen off its perch.