Frequency of 405-Line VHF Aerial.

[Restoration73]

Split from this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=95890

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Can anyone confirm the driven element was cut for the vision carrier frequency ? I have just been given a new Band I H (Antiference) and as channel was not known I measured it to confirm it as B1 45 MHz. Assuming a strong video signal would guarantee adequate sound reception.

[Nuvistor]

I cannot answer your question but I understood that the presence of other elements on the aerial would affect the centre frequency of the aerial, they did affect the impedance, the more elements the less the impedance. Hence the use of a folded dipole to increase the impedance by 4x.
A bit of antenna theory on this web page.
http://www.antenna-theory.com/antenn...lling/yagi.php
Note comment about length of dipole with parasitic elements.

[G6Tanuki]

Originally Posted by Restoration73

Can anyone confirm the driven element was cut for the vision carrier frequency ? I have just been given a new Band I H (Antiference) and as channel was not known I measured it to confirm it as B1 45 MHz. Assuming a strong video signal would guarantee adequate sound reception.

Given that after the front-end, the sound and vision strips are essentially separate in a band-I/III TV it was never really a problem if the audio carrier presented from the aerial was a couple of dB down on the video. Indeed, it was often the case that with a weak 'fringe reception' signal you would get still listenable-to audio but the video was trembly/snowy and unwatchable even if your receiver had flywheel-sync in the video stages.

Seeing some of the BI/III aerial setups - where the mast passed through the centre of the vertically-polarised elements in an essentially-random position so acting as an additional director (or sometimes reflector) I suspect that caused more detuning than anything!

[SteveCG]

Restoration 73,

Good fortune on you for being given a CH B1 Antiference H !

Was it out from a loft?

My experience with Yagi aerials is that the different makers used different exact dimensions and as Nuvistor says, the precise performance of any aerial is affected by the interactions between the different parts. To be honest, the question you ask has no simple answer - text books are written on the subject.

However given that B1 sound carrier frequency was 41.5 Mc/s, you could measure the length of the reflector element (I assume it is a dipole - reflector H design) to see if it would resonate near that freq - that is a clue as to the bandwidth of the overall aerial.

As G6Tanuki says, aerial mounting also affects performance for V Pol use, depending on the design.

[Restoration73]

I do not know if it was in a loft but certainly has been stored well - there is no evidence of any connection made to the terminal box. CP is line of sight in this area. It is not a Trumatch type, and is too long to be for B2. The reflector is of course longer.

A colleague has an antenna analyser so we'll see what the resonant frequency is. I intend to mount and use it in the event of the resumption of the 405 line service

[bobsterkent]

I'd not mount it yet as they don't yet know what bandwidth OFCOM will allow them to use.

[SteveCG]

Restoration 73,

If CP is line of sight for you, then, had you had that aerial at your present location but back in time, you would have needed a line of attenuators on the TV's aerial input!

Two construction questions for you: Is the dipole junction box fixed to the boom using two 'U' bolts? and is the junction box cap just a rectangular, black plastic, push fit?

If it is in very good condition then I'd be loath to mount such a rarity outside.

And finally: is it possible for you to measure the dimensions of the aerial please? (That is: element tip-to-tip length, element separation and finally boom diameter) Imperial units or Metric - preferably Imperial as these were ones used to make the aerial.

Try modeling it in antenna analysis programme, something like MMANA (free download). Warning, these programmes get adictive, you can loose days!

[Restoration73]

Thanks for your interest . I do have the modelling program. SteveCG, I think this is a
late aerial as the junction box is circular held on with a 0BA nut bolt. In addition, the
elements are held on by the usual 2BA bolt/wingnut arrangement with washers and springs. There are no marks on the boom caused by a clamp.
Sadly I have no contemporary Antiference catalogue for identification, I assume VHF
TV aerials were available right up to the late 70's. I remember seeing an edition of
"Nationwide" broadcast in around 1976 which showed TV being brought to a remote
Scottish island (vhf), and the Thorn 1400s were still being made around then.

[SteveCG]

Restoration73,

Thanks for the info. Your description of the junction box intrigues me - I have never had my hands on a late Antiference Band I H, so the exact type of junction box interests me as a possible dating clue. Is the cap on the junction box a screw-on type?

On the aerial modelling prog. I have not heard of the MMANA, but years ago I spent a lot of time with MININEC. It was using this prog that I had confirmation of the serious discrepancy between claimed aerial gains and what I measured - the gains were much closer to what MININEC said. That said, MININEC could not model some important features properly - such as the effect of (metallic) boom diameter on frequency.

I had an example of a J-Beam channel B2 dipole and a J-Beam channel B2 (reflector type) H. They both had exactly the same dipole length - yet the presence of the reflector altered the (MININEC computed) performance parameters. I guess that for TV reception these changes were not significant - Analogue TV was rather forgiving.

[emeritus]

I have just found my late father's "Wireless World" diary for 1966 while clearing my late mother's house. It gives design data for VHF and UHF aerials, but only tabulates values for three-element arrangements using folded dipoles. However it has this to say about reflectors.

" Reflector to dipole spacing alters the impedance of the aerial and can provide a useful means of matching to the downlead. It does not, to any extent, affect the forward gain characteristics within the specified spacing. "

The "specified spacing" between dipole and reflector is between 0.125 and 0.25 lambda.

[Nuvistor]

A really interesting read is in the ARRL Antenna handbook chapter 4, max forward gain is not normally achieved at max front to back ratio, the length of the parasitic elements also requires changes with element spacing.
It is a very complex subject, as Steve says, text books have been written on the subject.
I can just about scratch the surface of the theory.

[Electrone]

Restoration 73- who would want a resumption of the 405 line service apart from people like yourself who restore old 405 line sets? In any case Bands I and III are now used for other services

[Amraduk]

Originally Posted by Electrone

Restoration 73- who would want a resumption of the 405 line service apart from people like yourself who restore old 405 line sets? In any case Bands I and III are now used for other services

British Heritage Television

Dave.

[Restoration73]

I think that Band I is somewhat user light. Amateur and PMSE activity is almost nil, with
the odd wind profiler radar. Ofcom may release some low vhf spectrum in thefuture.
I will post some pictures soon.
Tony

[ntscuser]

The test transmissions were closer in frequency to channel B5 than channel B1:

http://405-line.tv/?page_id=762

[Restoration73]

Model no. is 120 (Band I Outdoor)
http://www.wrightsaerials.tv/alberts...ce-1974-75.pdf

[SteveCG]

Thanks Resoration 73 for the info and the link - that 1974/75 Antiference catalogue is a useful source.

[colourking]

Originally Posted by ntscuser

The test transmissions were closer in frequency to channel B5 than channel B1]

Interesting reading of the 2011 405-line test transmission (which I hadn't known of), what audio went with the transmission of that test card or was the audio carrier left blank? (the AM sound frequency, if modulated, could of course have been received by hams and other radio enthusiasts with scanners/other receivers that could cover the frequency).

Could ofcom have allowed a regular TV broadcast such as BBC1 (with their permission) to have been relayed on such a test transmission ? (Obviously with 625->405 conversion)

[Martin Allen]

Although we had originally intended to transmit both vision and sound in the 2011 tests, in the event we decided to radiate vision only - and focus on delivering the best quality pictures with the kit available.

For the limited occasions of those tests, it was felt that a static test card would allow for easier comparison of Reception Reports that mostly related to differing periods of a transmission lasting several hours in total.

A feed of (say) BBC1, if allowed, would have been 'widescreen' - and jarringly 'modern' in style and content!