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Old 26th Jul 2016, 12:44 pm   #21
brian_mk
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

You are right G8HQP Dave - it is Bow Brickhill. I just checked my list of BBC frequencies I can receive against that given in the earlier link.

I think my dipole probably needs to point further south for Bow Brickhill.
The TV antenna may obstruct the signal path if I simply rotate the dipole, so I may need to move it to another part of the loft.
I will try and realign it with the help of a compass and report back.
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Old 26th Jul 2016, 12:59 pm   #22
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

Probably not relevant to the OP's problem but a lot of new build house roofs use warm roof construction techniques, if so the the whole roof system might incorporate aluminium foil...two layers of it..

Lawrence.

Last edited by ms660; 26th Jul 2016 at 1:01 pm. Reason: changed words
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Old 26th Jul 2016, 1:05 pm   #23
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Arrow Re: Homemade VHF 88-108MHz FM dipole antenna

Quote:
Originally Posted by brian_mk View Post
By re-positioning the dipole lower down in the loft below the rafters, I managed to get about the same signal level as I had downstairs.
With VHF aerials, that is not an uncommon experience. From that observation, it is clear that the proximity of the roof (and possibly other things in the loft: e.g. a water tank, plumbing, wiring, etc.) are having a deleterious effect upon the aerial: i.e. the aerial is being de-tuned. There may be other effects coming into play to account for not obtaining the expected increase in signal level between the aerial in the downstairs room and the loft, but I suspect that the position of the aerial in the loft, very close to the roof, is the predominant factor.
If the finally chosen position of the aerial in the loft provides adequate signal strength, I'd settle for that. You can spend a lot of time experimenting with different positions of a VHF aerial in a loft (or attic), only to find that the improvement is signal strength from the 'best' position to another position is actually quite small. Compared to an aerial mounted outside and 'in the clear', a VHF aerial, when mounted 'inside' a building, will always be a compromise.

Al.
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Old 26th Jul 2016, 1:06 pm   #24
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

Success!

I can only think that when I initially tested the dipole downstairs, the tuner must have been set to a station that was not coming from Bow Brickhill. I must have been receiving a signal in the direction of Sandy or Oxford. It meant that Bow Brickhill stations were in the dipole null.

All I need to do now is fine tune the position and angle to maximise the signal strength.

Thanks for your help folks.
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Old 26th Jul 2016, 1:40 pm   #25
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

This seems to echo my experience. RF is a Dark Art and when you get up to mega-cycles you're well into the realms of witchcraft.

Glad to hear you've got it working.

— Joe
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Old 26th Jul 2016, 4:32 pm   #26
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Arrow Re: Homemade VHF 88-108MHz FM dipole antenna

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Originally Posted by ThePillenwerfer View Post
RF is a Dark Art and when you get up to mega-cycles, you're well into the realms of witchcraft.
Joe: I fully appreciate your remark (as above) and I know where you're coming from. Your remark has inspired me to write what follows below . . .

The mysteries of R.F. at high-frequencies is something that has bedeviled analogue (and digital) design for ages. For myself, it is the tackling of the problems thus arising that gives me one of the most absorbing reasons and motives why I have pursued that topic during all my working life and as a hobby - and continue to do so now that I am in retirement.

The problems one encounters with HF are usually caused by a perceptive mis-match between what we physically observe (by way of layout and components) and what is 'really' there: 'hidden' resistance, inductance and capacitance. And that's just passive components. With active devices, not only are those hidden components there, but the electrical values of those components which are intrinsic to that device vary in magnitude and phase as the behaviour of that active device changes. So, how do we deal with those situations? An accurate simulation of all the components, visible and invisible is required and a subsequent prediction of the subsequent behaviour is required. And yes - that often not only involves a lot of hard work, but will also include some guesswork as to the values of those invisible components. As you, others here and I well know, the usual alternative and practical approach to R.F. design is to follow a few 'golden rules': keep leads short; add copious screening, etc. And even then there is the matter of subsequent testing: the effect of the connected measurements on the circuit under test, etc.

So, in conclusion, witchcraft it all may appear to be, but the appropriate physics covers all the relevant variables. It's just a question of accounting for everything in a given design, either by a deep analysis, or the 'golden rules' approach - and even then, keeping fingers crossed! An attitude of patience, persistence and perseverance usually helps too! Besides, if it was too easy, it wouldn't be fun, would it?

Al.
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Old 26th Jul 2016, 8:11 pm   #27
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

That is very true in principle, but optimising the performance of a loft FM aerial with all the reflections, refractions, resonance distortions and screenings involved can certainly be described as a black art. I'd like to see even the most skilled RF engineer model reception under those conditions. The same is true of any indoor aerial of course, but loft aerials are no easier, despite being a bit higher and usually having a bit less screening.
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Old 27th Jul 2016, 9:23 am   #28
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

Intrigued to hear that the VHF FM input is on an F type. I have never seen that on any piece of equipment I have used or worked on. The F type is generally for the DAB aerial and the VHF FM has a male Belling Lee.
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Old 27th Jul 2016, 9:34 am   #29
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

Tuners for the American market would use 'F' connectors for FM coax cable input (and they use 'F' for terrestrial TV too)
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Old 27th Jul 2016, 10:25 am   #30
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

Yes I was aware that the Americans use them and it is probably the only American invention I approve of. ;-)
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Old 27th Jul 2016, 11:43 am   #31
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Arrow Re: Homemade VHF 88-108MHz FM dipole antenna

Re: post #27 by Paul Sherwin.
Yes, indeed Paul: point taken.

Al.
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Old 27th Jul 2016, 6:22 pm   #32
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

Slightly OT but I hope the Mods understand as this TRUE story is worth telling. I was called out to a house reporting loss of TV signal, just a blank raster. The renter used an indoor aerial and upon entering the living room I could see the fault. Just for a change, the renter had decided to wall paper the room with Aluminum foil! I recommended the installation of an outside aerial which naturally cleared the fault, I tell this tale quite often but hardly anyone believes it. Trust me it is a TRUE story, as the saying goes, I was there
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Old 27th Jul 2016, 8:56 pm   #33
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Arrow Re: Homemade VHF 88-108MHz FM dipole antenna

I believe that anecdote, Les.

Al.
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Old 27th Jul 2016, 10:41 pm   #34
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

RF engineers don't predict reception/coverage anymore.
Software blokes do. They model it on equations and fine data then promise the earth if you can wait long enough.
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Old 28th Jul 2016, 10:45 am   #35
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Question Re: Homemade VHF 88-108MHz FM dipole antenna

Is there really such a sharp divide between "RF engineers" and "software blokes" these days? Today's RF engineers use computers in their design work and surely these "software blokes" will need a some level of understanding of how RF 'behaves'?
I would have thought that instead of there being two discrete 'camps' here that there would have been a gradual blend from one to the other.

However, if I am seriously mistaken here (quite likely), then please educate me! Thank you.

Al.
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Old 29th Jul 2016, 8:34 pm   #36
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

In the 1970's I made a simple loft-mounted dipole for FM reception (still in use) from two offcuts of 15mm copper pipe, rammed into a piece oval plastic conduit. I seem to remember getting the dimensions from data in an old "Wireless World" diary, now mislaid, and cut for 100MHz. I just soldered TV coax to the inner ends and it works very well without a balun. I used to pick up French stations in Stereo on my budget Alba UA 700/800 setup in the days before the FM band filled up. The sharp end-on null was very useful for neutralising strong local BBC transmissions. The figure-of-eight polar diagram of a horizontal dipole means that aiming at a wanted transmitter is not critical.
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Old 29th Jul 2016, 8:44 pm   #37
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

As I said earlier, I use a folded dipole built from wire and a stick. I chose this design over a simple dipole because it is a good match for a 300:75 balun, which are cheaply available and which I had to hand anyway. A simple dipole can be directly connected to a 75 ohm downlead but a 75:75 balun will provide better results because the dipole is balanced and the downlead is unbalanced.

Often this is all pretty academic given the vagaries of loft aerials. Nobody is going to use a loft dipole if they have poor reception (not if they have any sense, anyway).
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Old 29th Jul 2016, 11:57 pm   #38
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

RE #32- I believe it as well, from my uncles tales.In his younger years he was the top dog in his electrical firm, with the coming of TV. One customer had bought a very expensive set ,on the priviso that decent signal would be received. Other blokes had failed, and uncle was sent out to get set working, on this bungalow, in a poor 405 area. Uncle walked all over the roof, with apprentice watching for a picture. Eventually uncle gave up and dropped aerial to ground, where upon there was a shout from inside "that's it" .Aerial ended up tied to clothes pole.
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Old 30th Jul 2016, 6:56 pm   #39
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

As others have mentioned, to comply with energy-saving rules and regulations the "sarking" [felt used between the rafters/battens and the roof-tiles] is often these days a thermal-sandwich which includes either aluminium foil or metallised-plastic film; I've also come across loft-insulation called "Space blanket" which is the usual plastic/fibreglass-wool but sandwiched between two aluminised-plastic films.

All this can make loft-antennas problematic. Still, it seems you've found a solution that works for you. It will be interesting to see what it behaves like when it rains: I've found that wet roof-tiles can provide an additional 10dB of attenuation !!
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Old 6th Aug 2016, 8:00 am   #40
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Default Re: Homemade VHF 88-108MHz FM dipole antenna

Musing aloud: I wonder if a hole could be cut in the above-mentioned metallised insulation to form a slot antenna
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