Re: 405-Line VHF Aerials in 2013
 

Hello Gentlemen,

Here are 2 shots of a 405 "X" antenna taken in central London, Jermyn street from memory and not far from Burlington Arcade.

Yup, the Frenchman also chases 405 antennas!

Best Regards

jhalphen
Paris/France

Re: 405-Line VHF Aerials in 2013
 

Hi jhalphen,

We were in Paris last month, I looked for any surviving 819-line aerials but didn't see any where we stayed. Plenty of BIII types for Canal+ though.

Great photos. Please keep them coming!
Cheers,
Brian

Re: 405-Line VHF Aerials in 2013
 

re #35, contemporary trade leaflets indicate that the Antiference Antex was introduced in 1949. An Antiference leaflet refers to it having been in use for 2 years before the 1951 Festival of Britain, and a provisional patent application was also filed in 1949.

The Dublex may have been designed to get around the Antiference Patents. The (undated, probably circa 1952) Aerialite leaflet says that a patent was applied for the Dublex design. While Aerialite did have three patents granted around 1950, none relate to the Dublex configuration. At that time, only patents that were accepted were published, so possibly their application relating to the Dublex was abandoned or refused. I have another Aerialite leaflet, also undated, that mentions a "Unex X-shaped aerial", as well as the Dublex X-shaped : no illustration unfortunately, but the Unex was less sensitive (50 miles) that the Dublex (65 miles).

Re: 405-Line VHF Aerials in 2013
 

Can't see how an X can have much gain over a dipole?? Most of the current is near the feed point where there is not much spacing to the reflector. It is type of aerial only really ever used for Band I and not seen since. An H would be much better.

Re: 405-Line VHF Aerials in 2013
 

From memory my parents' aerial was an Aerialite although it was mounted on a wooden pole.

Peter

Re: 405-Line VHF Aerials in 2013
 

The Moxon antenna makes more sense where you have a decent dipole-reflector spacing where it counts where the current is high and then bend in the ends of the elements nearer the ends where the current is much lower. You end up with a smaller in area aerial with performance not far removed from a full sizes H.

Scroll down for schematic.

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=9322

Re: 405-Line VHF Aerials in 2013
 

Re #44, the polar chart in the Dublex leaflet purports to show a 6dB gain over a "H" aerial.

Re: 405-Line VHF Aerials in 2013
 

Re: post no 44.

I tried modelling an Antex X design using the computer modelling programme MININEC quite a few years ago. I found that the gain at the sound frequency was about 0 dBD relative to a vertical dipole, whilst the gain at the vision frequency was about 1-2 dBD. Bear in mind with 405 line System A, in its B channel allocations the sound was below vision in terms of the the carrier frequencies - eg CH B1 41.5 Mc/s sound, 45 Mc/s vision.

Note that all these yagi modelling programmes like MININEC produce results that need to be treated with a little caution.

The Antex X would have been successful in the UK in somewhat reducing the ghosting component spectral lines with the most energy, which are comparatively close in to the vision carrier. The use of an Antex X design on other TV systems where sound was above vision seems far more debateable.

P.S. I believe that an Antex X was a modified director-dipole design and not a dipole-reflector design.

Re: 405-Line VHF Aerials in 2013
 

Sounds like utter rubbish to claim 6dB over an H. If that was the case the design would be widely used today. By simple theory you would need 4 times the number of elements to gat a 6dB increase = an 8 element (at least).

Around 1 dB over a dipole sounds about right. More or less a waste of time.

What is the gain of a 2 element (H)? You get 3 dB by doubling the field power at a distance point and a bit more because you are firing mainly one way. 4 - 6 dB is claimed.

Re: 405-Line VHF Aerials in 2013
 

Leaving aside the diagram, I think the text is suggesting that the X gives 6dB of gain (presumably relative to a simple dipole). It does suggest that it gives more gain than an H but not 6dB relative to the H.

Peter

Re: 405-Line VHF Aerials in 2013
 

Oh well. The fact that AFAIK they were only ever used on Band I and by only by one or two manufacturers and is not AFAIK used today speaks for itself.

Shades of the Jay-Beam Multibeam (think it was was) for UHF. J-Beam claimed high gain figures but tests showed them to be very poor.

Re: 405-Line VHF Aerials in 2013
 

Ours was certainly a poor performer. I think J-Beam's claim to fame was the 'skeleton slot' (as they called it) or 'trifold' (as others call it) active element and reflector? This necessitated the use of a helical balun transformer to match the very high impedance.

The only advantage of a Band-I X-array over an "H" that I can see is mechanical strength at the attachment point and reduced wind resistance.

It has to be said that a skinflnt uncle used a Band-I X-array for many years to receive not only Band-I but also Band-III transmissions from Sutton Coldfield about twenty miles away. There was no noticeable degradation in picture compared to our H+5-element yagi aerial mounted on a three-strorey chimney!

Re: 405-Line VHF Aerials in 2013
 

Hi to All,

@Focus Diode, Hi Brian,

You would never have seen a Band I VHF antenna in the Paris region for the good reason that the Eiffel Tower's transmitter was on Channel 8A in the E system frequency plan and this was in the 180 MHz region, i.e. VHF band III.

The Pre-War and up to 1951 441 line Paris transmitter used nearly the same frequencies as Alexandra Palace (from memory, 42 and 46 MHz) but this really dates back a lot.

The 441 line TX "conveniently" caught fire in 1951 (sabotaged?) to make way for 819 which was officially adopted in 1949.

The government had promised Parisians 441 service until 1958. When the TX was destroyed, it was never repaired and Paris viewers all had to go out and buy a new TV 7 years before the planned retirement date.

So much for politician's promises...

Best Regards

jhalphen
Paris/France

Re: 405-Line VHF Aerials in 2013
 

Re posts 51 & 52.

I concur with those who say J-Beam's UHF MBM range was not all it was cracked up to be. In later years J-Beam's/ Jaybeam's advertising started to reflect the more real figures. I think this was because of the adoption of a measuring standard.

However when it comes to the UHF PBM range - I'm much more of a fan, a PBM 18 element, Group A, especially the early, more rugged, 'squareback' reflector type is a sight to behold - and a repectable performance.

J-Beam used what they called an inverse balun to effect a 300 Ohm Balanced to 75 Ohm unbalanced transformation - for use with 75 Ohm coaxial cable downleads.

The main problem with all these aerials is the contact corrosion in the junction boxes.

Re: 405-Line VHF Aerials in 2013
 

Visiting my Mum in Wiltshire and saw this on top of the local Indian. Is it a TV array or something newer. ??

Re: 405-Line VHF Aerials in 2013
 

I remember that Band III aerial, just can't remember its name. Quite common in its day where Band III signals were weak. Now where is that 1973 ITV year book? Or the BBC one ? In the loft, I expect.:-[

Re: 405-Line VHF Aerials in 2013
 

X aerials were a dipole and director. Their main claim to fame was that they had nulls at the sides rather than the back.

Re: 405-Line VHF Aerials in 2013
 

Nigel re post #55 - that installation is indeed a VHF/405 installation, most probably around mid 1960s. I can't quite make out whether the Band I "H" is a dipole-director or dipole-reflector version but, in any case it would have been for BBC1. The ITV aerial is an horizontally polarised Band III "double" with a slot feed. It may well be a J-Beam "double six" which has moulted a couple of bits. Horizontal polarisation was not so common in the VHF TV era being reserved mainly for relays and just the odd main station.

The UHF aerial at the bottom is more recent and looks like some no-brand cheap rigger's special.

Re: 405-Line VHF Aerials in 2013
 

The horizontal band 3 aerials on Membury perhaps?that was horizontal polaristaion to prevent CCI to Caradon Hill.

There are still a load of 405 line aerials on the rooftops in my area,i will take some pics and put them up here shortly......

Band 3 came from Lichfield,Band 1 from Sutton.

Re: 405-Line VHF Aerials in 2013
 

Band 1 on channel 4 approx 60Mhz Band 3 on channel 8 approx 186Mhz. The X aerial was probably working on the 3rd harmonic, the impedance match would have been close.