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6th Jun 2014, 8:20 am | #1 |
Nonode
Join Date: Jun 2013
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Frequency of 405-Line VHF Aerial.
Split from this thread:-
https://www.vintage-radio.net/forum/...ad.php?t=95890 ------------------------------------------------------ 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. |
6th Jun 2014, 9:01 am | #2 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
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Re: Frequency of 405-Line VHF Aerial.
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. |
6th Jun 2014, 9:29 am | #3 | |
Dekatron
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Re: Frequency of 405-Line VHF Aerial.
Quote:
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! |
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6th Jun 2014, 2:38 pm | #4 |
Nonode
Join Date: Jun 2009
Location: Herefordshire, UK.
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Re: Frequency of 405-Line VHF Aerial.
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. |
6th Jun 2014, 3:28 pm | #5 |
Nonode
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
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Re: Frequency of 405-Line VHF Aerial.
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 |
6th Jun 2014, 6:56 pm | #6 |
Hexode
Join Date: Apr 2006
Location: Medway towns, Kent, UK.
Posts: 271
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Re: Frequency of 405-Line VHF Aerial.
I'd not mount it yet as they don't yet know what bandwidth OFCOM will allow them to use.
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7th Jun 2014, 11:51 am | #7 |
Nonode
Join Date: Jun 2009
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Re: Frequency of 405-Line VHF Aerial.
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. |
7th Jun 2014, 11:54 am | #8 |
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Re: Frequency of 405-Line VHF Aerial.
Try modeling it in antenna analysis programme, something like MMANA (free download). Warning, these programmes get adictive, you can loose days!
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7th Jun 2014, 2:51 pm | #9 |
Nonode
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
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Re: Frequency of 405-Line VHF Aerial.
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. |
7th Jun 2014, 3:56 pm | #10 |
Nonode
Join Date: Jun 2009
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Re: Frequency of 405-Line VHF Aerial.
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. Last edited by SteveCG; 7th Jun 2014 at 3:57 pm. Reason: added a word |
7th Jun 2014, 8:08 pm | #11 |
Dekatron
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
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Re: Frequency of 405-Line VHF Aerial.
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. |
7th Jun 2014, 8:50 pm | #12 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
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Re: Frequency of 405-Line VHF Aerial.
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. |
7th Jun 2014, 10:18 pm | #13 |
Retired Dormant Member
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Re: Frequency of 405-Line VHF Aerial.
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
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7th Jun 2014, 11:27 pm | #14 | |
Hexode
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Re: Frequency of 405-Line VHF Aerial.
Quote:
Dave. |
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8th Jun 2014, 12:28 am | #15 |
Nonode
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
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Re: Frequency of 405-Line VHF Aerial.
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 |
8th Jun 2014, 10:50 pm | #16 |
Hexode
Join Date: Mar 2013
Location: Coventry, West Midlands, UK.
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Re: Frequency of 405-Line VHF Aerial.
The test transmissions were closer in frequency to channel B5 than channel B1:
http://405-line.tv/?page_id=762
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10th Jun 2014, 11:12 pm | #17 |
Nonode
Join Date: Jun 2013
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Re: Frequency of 405-Line VHF Aerial.
Model no. is 120 (Band I Outdoor)
http://www.wrightsaerials.tv/alberts...ce-1974-75.pdf |
11th Jun 2014, 11:21 am | #18 |
Nonode
Join Date: Jun 2009
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Re: Frequency of 405-Line VHF Aerial.
Thanks Resoration 73 for the info and the link - that 1974/75 Antiference catalogue is a useful source.
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16th Jun 2014, 3:27 pm | #19 | |
Pentode
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Location: Cork, Ireland
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Re: Frequency of 405-Line VHF Aerial.
Quote:
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) Last edited by colourking; 16th Jun 2014 at 3:33 pm. |
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16th Jun 2014, 4:37 pm | #20 |
Triode
Join Date: Oct 2007
Location: Hatfield, Herts.
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Re: Frequency of 405-Line VHF Aerial.
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! |