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Old 1st Jul 2019, 12:52 pm   #641
SteveCG
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Default Re: 405-Line VHF Aerials 2013 to the present day.

When looking at these few surviving aerials it is always useful to bear in mind that although a strong signal might be obtained by a 'dry piece of string plugged into the back' it might be badly ghosted. So you have the incongruity of directional external aerials being needed - but with a line of attenuators round the back of the set.

Contrast that situation with one where you are chasing every microvolt 'beyond the fringe' so again a multi-element aerial is called for.
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Old 1st Jul 2019, 1:12 pm   #642
G6Tanuki
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Default Re: 405-Line VHF Aerials 2013 to the present day.

And equally, in some parts of the country there was the issue of co-channel interference where having a decent bit of directivity was helpful to keep the Continentals from interrupting your viewing.

Some of the Band-I "H" or "X" antennas were horrible in this respect - they used a dipole-and-director, with no reflector!

Who needs a front:back ratio?
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Old 6th Jul 2019, 7:22 pm   #643
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Default Re: 405-Line VHF Aerials 2013 to the present day.

I had a day trip to York by train today and on both parts I kept an eye on the rooftops as I was passing through built up areas of older buildings.

There was the odd band III aerial though most looked too modern to have been for 405 line TV.

I did see 1 or 2 H band 1 ones visible, including one in York not far from the station.
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Old 7th Jul 2019, 10:23 am   #644
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Charmouth receives quite a strong signal from Stockland hill but in the analogue days it was unusable in most places due to ghosting. Its at the bottom of a long hill to both East and West and most of the signal was reflected from the hill to the east.

There were a few places where it would work, just isolated small spots, in one I can remember the signal was useless from a roof aerial but worked fine on a set top!

They put in a receiving station on Stonebarrow hill which was quite well located to receive the Mendip transmitter, that was relayed by cable down into the town and was known as the 'Veiwline' system.

That was in use until a relay station for Stockland hill was built on Stonebarrow. Now we have digi it doesn't seem to mind ghosting and works in many places, although we still have the relay.

At my location I had a slightly ghosty and weak picture from Stockland Hill but since it went digi the upstairs set will work with no aerial at all!

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Old 7th Jul 2019, 11:18 am   #645
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Ref post 642, the Antiference X aerials were very successful, installed many in the early 60,s. Of course if it was a challenging location then other types may work better. Sometimes it was trial and error, what worked next door may not work well in the new install.

At least being in NW England we didnít have co channel from across the channel and when SpE was causing problems it didnít matter which way the aerial was pointed.
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Old 9th Jul 2019, 12:48 am   #646
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Default Re: 405-Line VHF Aerials 2013 to the present day.

A directional band 1 tv aerial might help sometimes depending on the direction of the Sporadic E opening. Continental interference could come in any direction from Scandinavia to the Northeast from Mainland Europe to the east and southeast and Iberia to the south.
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Old 9th Jul 2019, 1:13 pm   #647
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Re posts 645,646:

The BBC opened a Band III, channel B12 transmitter on the ITA Winter Hill site in the mid 60s because of the recurrent SpE interference to reception from its B2 transmitter at Holme Moss in parts of the NW of England.
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Old 9th Jul 2019, 3:21 pm   #648
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Arranged many channel 9/12/UHF GroupC aerials to receive the Winter Hill transmitter from about 1966 to 1969 when went to fitting UHF only.
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Old 12th Jul 2019, 9:01 pm   #649
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Here's a nice condition X aerial near to Taunton town centre. There's still quite a few 405 aerials to be seen on older properties and shops.
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Old 13th Jul 2019, 7:12 am   #650
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Wenvoe Ch5 country!
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Old 15th Jul 2019, 2:44 pm   #651
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Default Re: 405-Line VHF Aerials 2013 to the present day.

This one is visible from the A350 on the western outskirts of Westbury.

It points North-West, so is almost certainly for Wenvoe.

Note that the Band-I setup is a dipole-and-director; the BIII setup uses the BI dipole as its dipole (that 'third harmonic' trick) and again only has directors, so any signals off the back of the beam would still have come romping in.
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Old 15th Jul 2019, 2:59 pm   #652
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Claimed polar diagram of the Antex X, WW 1949. I canít corroborate the figures but on the whole they worked very well. We didnít have any fringe areas but certainly some with problem reflections, in which case they worked at least as good if not better than a standard H. Of course every installation was unique in those areas.
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Old 16th Jul 2019, 10:02 am   #653
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Quote:
Originally Posted by G6Tanuki View Post
This one is visible from the A350 on the western outskirts of Westbury.

It points North-West, so is almost certainly for Wenvoe.

Note that the Band-I setup is a dipole-and-director; the BIII setup uses the BI dipole as its dipole (that 'third harmonic' trick) and again only has directors, so any signals off the back of the beam would still have come romping in.
Nice to see such a tidy complete B1/B3 TV aerial, Its stood up to the test of time very well.
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Old 16th Jul 2019, 12:53 pm   #654
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Re post no. 653;

... it may be the shorter elements of channel B5 mean that the mechanical stresses are sufficiently less for the elements not to bend/fracture. Vertical polarization operation helps as well as you don't have the constant torque on the dipole and element holders due to gravity. Also you don't have well fed crows landing on the elements !

But I agree with Simon, the Band III sections look in good nick as well. It make you wonder whether it is still a functioning aerial - or will junction box corrosion have got it?
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Old 16th Jul 2019, 3:13 pm   #655
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Quote:
Originally Posted by G6Tanuki View Post

Note that the Band-I setup is a dipole-and-director; the BIII setup uses the BI dipole as its dipole (that 'third harmonic' trick) and again only has directors, so any signals off the back of the beam would still have come romping in.
I'm wondering how many combined band 1/3 aerials used the "3'rd harmonic trick" to receive band 3? I presume this would only work in certain parts of the country with favourable BBC/ITV channel combinations...
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Old 16th Jul 2019, 3:50 pm   #656
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Is the idea that a quarter-wavelength at some frequency f is also three-quarters of a wavelength at 3*f, which means there is still a null at the tip and a maximum nearest the feed point?
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Old 16th Jul 2019, 4:14 pm   #657
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Quote:
Originally Posted by julie_m View Post
Is the idea that a quarter-wavelength at some frequency f is also three-quarters of a wavelength at 3*f, which means there is still a null at the tip and a maximum nearest the feed point?
Yes - the idea is that a half-wave on BI frequencies works as a 3-half-waves on BIII, and the impedance at the centre will be relatively low on both bands so presenting a reasonable match to 75-Ohm coax.

How well it works - that depends! If the BI element is optimised for the old London channel 1 its dipole will be cut for something around 43MHz. So the 'third harmonic' resonance will fall at around 130MHz - well outside Band-III - indeed at lower Band-III frequencies the BI dipole will 'look' more like a centre-fed pair of half-waves meaning high impedance at the feedpoint and distinctly sub-optimal matching. At the top end of BIII a "channel 1" BI dipole will start to look more like five half-waves and again the feed impedance at the centre will be low(ish).

But a Band-I Channel-5 dipole as shown here works better - cut for 66MHz its 'third harmonic' coming out pretty much bang in the centre of Band-III.

I guess how well this all worked depended on the channels in use in the area.
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Old 17th Jul 2019, 11:05 am   #658
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Re "3rd harmonic trick": Belling and Lee sold an 'H' design with sloping elements - like an arrow - for the Midlands area that was receiving BBC on CH B4 and ITA on CH B8, and this relied upon the 3rd Harmonic. Otherwise I agree with G6Tanuki that some Band I and Band III channel combinations were better suited than others for the style of combined aerial featured in post no. 651.
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Old 21st Jul 2019, 7:02 pm   #659
Peter.N.
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Tatworth Road Chard. Mendip or Stockland Hill. Looks to short for Ch 1.
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Old 22nd Jul 2019, 6:24 am   #660
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Default Re: 405-Line VHF Aerials 2013 to the present day.

Looking at Wenvoe Ch 5 probably.
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