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Traffic FM Signal and Antenna
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PostPosted: Sun Oct 01, 2006 9:29 pm    Post subject: Reply with quote


I think I've sort of kind of like almost got something like a solution. But it's a bit bizarre, and probably not replicable. But when I've a moment I'll describe it, if only for a laugh.

However, it does seem to give me 'two bars' for Classic FM where previously I got none.

Sorry - this is not meant to be a teaser, just to say I've not forgotten.
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PostPosted: Thu Oct 05, 2006 12:28 am    Post subject: Reply with quote

Well, I think I’ve gone as far as I’m going to. And – success, of a kind. What a saga. Are you sitting comfortably?

After finding that the draped mains cable worked better than anything else so far, I first dumped the various amplified antennae I’d tried. Then I went back to the web to search around some more for inspiration and guidance. (Although I’m into electronics, r.f. has always been too much for my little brain.)

Among various sites I found two really helpful ones – both practical while apparently theoretically sound as well. KALX Radio did a survey of antenna types: http://kalx.berkeley.edu/recept/antsurvy.htm
And L B Cebik discusses antenna types from a slightly more theoretical standpoint:
These sites inspired me to make some more experiments,

First, although the supplied ‘external antenna’ is a single wire connected to just the end pin of the ‘stereo’ jack, I reckon that the other signal pin (the middle section of the jack) is also ‘live’ in some way. Best guess is that it is the other side of a balanced input that is not used by Michelin, but I really don’t know. However, when I talk of ‘dipoles’ below, I was connecting them to both signal connectors of a stereo jack.

I tried various forms of half-wave and quarter-wave set-ups, with approximations to monopoles and dipoles in various configurations, and some were better than my mains cable, some much the same. But then KALX persuaded me to try out a folded dipole. This is the floppy bit of twinfeed ribbon cable that comes free with most fm receivers and that you usually chuck, presuming it to be rubbish. To my surprise (but less so after reading KALX’s report) the folded dipole was significantly better than anything else thus far.

So I refined it a bit for my car. Now, the top of my windscreen is only 108cm wide, so whatever the theory, I’m never going to get a proper dipole in there (which would be getting on for 150 cm for Classic FM). Even taking into account KALX’s ‘velocity factor’ of 0.82, (and I’ve no idea if he’s right or not) for 100.9 Mhz we want 122 cm. So what to do? Shorten it to fit? Symmetrically or not? Or let the ends hang down a la Cebik (see ‘Making a dipole fit the space available’) – again symmetrically or not? At first I followed Cebik’s implied advice not to shorten, and I let the ends droop down. It worked fine at first.

But whenever I tidied this arrangement up the performance dropped. I think this is because, against all the rules, I am mounting the antenna very close to the metalwork, because I wanted it right out of the way around the border of my windscreen and hidden in the trim. So I suspect I was getting ‘detuning’ effects each time I tidied up the wires.

So fiddling about with the droopy ends, and trying shortening instead, I eventually decided to cut the folded dipole short to fit the top of the screen. (Not forgetting to reconnect the two arms at each end.) Performance was down on what I had seen, but better than anything else thus far.

Given that the impedance of the folded dipole is around 300 ohm, and not knowing the impedance expected by the X-950, I experimented with a balun (Maplin, <1 GBP) between the antenna and a 75 ohm coax feed to the X-950. It didn’t help, so I’ve left it out. Instead I’ve run the ribbon twin-lead to within a few inches of the X-950, where I soldered it to a short length of coax terminated with the jack for the X-950 – and blow the rules.

And then, by a route that is too complicated to explain, I discovered quite by accident that a quarter-wave monopole (more 2.5 mm mains cable) connected to one end of the folded dipole, trailing down the side of the screen, made a significant improvement:

I said I added a quarter-wave monopole. But actually its length turned out to be quite critical. According to most text-books, the ‘velocity factor’ for a monopole would be around 0.95, implying an ideal length of about 70.5cm. That did work, but with a LOT of trial and error I found that around 64 cm works best in this particular situation (which is inside the trim around the windscreen). I suspect that this is because of the proximity of the metal of the windscreen pillar, but as I said, I’m no r.f. engineer. Also, the position of the wire makes a big difference. As I said earlier, each time I seemed to have something working, when I put the car back together again (pushing cables around in the process) the antenna performance had gone. So eventually I fixed the position more reliably by taping it to trim clips and by sticking a foam rubber spacer on the end (see pics).

And then, just when I thought I’d got it all working, I put the trim back on and was brought back down with a bump: signal strength had dropped dramatically again. Eventually I worked out that I was forcing the feed line too close to the metalwork. So, I re-routed it, with foam rubber spacers:

(No – I don’t have wooden car. That was on the kitchen table before installation.)

And it sort of works. Here is the rather subjective evidence ...

Although we live only 30m north of Charing Cross, we are at the bottom of an E-W valley. FM (and terrestial tv) reception here is terrible. I can barely get Classic FM on the bedside radio. For FM on my hi-fi I have a huge, loft-filling array. And where I am working on the car is at the very bottom of that valley with all sorts of multipath problems, so that moving the car backwards or forwards a foot can make a significant difference to signal strength. I don’t have any means to measure signal strength properly – at least, not when you take coupling to the radio into account. But this is what I get looking at TMC Status on the X-950.

Without any external aerial it generally sees one or two bars of Chiltern (our very local station 10 miles away) and nothing else. Zilch:

With the supplied bit of wire it sees Chiltern and a few hints of other stations, but Classic FM either doesn’t show at all or we get maybe ‘zero bars’.

But now, with the 'folded dipole plus monopole' I generally get 2 green bars of Classic FM and it locks on to the TMC channel:

I’ll try to summarise what I have arrived at. Here is a very rough sketch:

First, one of those floppy ribbon aerials along the top of my screen. (I’m lucky, it’s outside my visual field and not really noticeable from outside.) In my case I shortened it to fit the width of my screen (remembering to reconnect the top and bottom arms at each end).

The feed ribbon runs around the headlining, down the driver’s side pillar trim, and into the dash. The last few inches are made with a piece of coax connected to the end and centre sections of a Maplin’s 3.5mm stereo jack.

Then – no laughter, please – where the passenger’s side arms of the ribbon aerial have been re-soldered together, a piece of wire rather less than 70cm is also soldered, to hang down inside the passenger’s side pillar trim. It is positioned and cut carefully to get the best signal, and held in place with clips, tape, rubber spacers, etc etc.

I don’t know what you would call such a mish-mash of an antenna. ‘Stupid’, maybe. I have no idea how or why it works, but it does seem to, so far. I know the horizontal (folded dipole) section will have nulls at each side, but I’m prepared to live with that for the better signal in other directions. Maybe all the supposed folded dipole does is provide a rather complicated feed to the ‘monopole’. Dunno. However, one good thing about the X-950 (at last) is that although it takes for ever to seek and lock on to the TMC channel, once it is locked on it seems to remember for quite some time during signal drop-outs.

So, I am happy to report success of a kind, and I am sorry: doubtless this is not likely to be replicated easily.

Now, I’ll try to find something less nerdy to be obsessed about.

BTW, Paul Groves maintains an excellent information site. I recommend downloading his ‘Whole list’ (PDF, 286KB) of FM transmitters in the UK from:
As I travel around, I find his list of Classic FM transmitters really helpful to understand what the X950 is seeing when it is searching for a traffic channel.
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PostPosted: Thu Oct 05, 2006 9:54 am    Post subject: Reply with quote

Wow that was certainly a read and a half Smile I am glad you have it all working now, but it must have taken you ages to figure all that out?

I will print this off and have a more detailed look at the weekend and see if I can come up with anything.

Thanks for all your help and pictures.
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PostPosted: Fri Oct 06, 2006 11:01 am    Post subject: Reply with quote


Sorry about the length, but I think I can now distill a fairly short message.

Knowing now what works (for me, at least) and what doesn’t, if I were to do it again I’d go straight to the twinfeed ribbon aerial, connected to the two signal connectors on a stereo jack. That costs about a fiver in total (maybe 6GBP). Check that out. If it works, tidy it away as best you can, avoiding – as far as possible - losing signal by routing the feed too close to the metalwork.

I certainly would not recommend faffing about with my funny tacked on monopole – though don’t let me stop you having fun. And now realising that some of the variability I experienced was probably due to multipath reception, I might even try the simpler version again. Later.. maybe ...

Meanwhile – the good news: how to make the best of a bad job (TMC on the X-950) ...

I reckon that in order to lock on, it needs about 5 continuous seconds of ‘2-bar + RDS’ strength signal. Driving around where I am that is a tall order – the signal zings up and down between none and very occasionally 2 bars, but never (or very rarely) long enough to lock on.

OTOH it seems to stay locked on unless it gets about 5 minutes without any signal at all (on the TMC frequency).

So I’ve taken to stopping where I can get a strong enough signal to lock on, and then it stays on all day. Since with the latest ‘daft’ aerial (nicer than ‘stupid’, I think) I can find a spot on the drive where it will lock on, and I haven’t found anywhere yet where (on the move) I lose signal altogether for 5 minutes at time, I’m sorted.

Can I get off the couch now, doctor?
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PostPosted: Fri Oct 13, 2006 6:22 pm    Post subject: Reply with quote

The saga staggers on ...

I have now had a little time to explore a little more - both theoretically and empirically.

Theoretically I discovered that the daft antenna (DAFT = Daft Antenna For TMC; nicely recursive, don’t you think?) may not be so daft after all. For a start, there are some that think that the folded dipole can be improved by putting shorts across the ‘free space’ length dipole (vf = 0.95) at the ‘transmisson line’ length (vf = 0.82). And this is equivalent to having the shorter dipole with ‘pigtails’ on each end. OK – so mine is lop-sided, but that is some kind of support for having an overall length greater than the twin-feed length.

And then there is the effect of all that metalwork around our aerial. Adding inductive and or capacitive loading is a standard way (I think) of getting a shorter antenna for a given frequency. (Not sure I’ve got that quite right but, in essence, weird things can happen when you’re not in free space.) So it is not so surprising that it seems to resonate at a frequency different from what you might expect of a pure folded dipole.

Empirically, I have been trying all sorts of other arrangements. BUT I just discovered that the connectors on some of my Maplin stereo jacks are not good. (The solder tags are not crimped on very effectively.) So a whole load of my tests were invalid. All I have done so far is a quick check with a ‘stripped coax’ (75 ohm) dipole off the car. That seemed to be slightly better than the daft antenna, but I’ve yet to summon up the strength to re-do everything in the car again.

What I have not explained is why I’ve been doing all this. It would be simple enough to stick a truly external whip aerial on the car, but I really didn’t want to be drilling holes in my fairly new toy. So I want to conceal any ‘external’ aerial, and while the windscreen and headlining trim are all plastic, any wires inside the trim are going to be pretty close to the bodywork. Hence the weird effects. And hence the necessity to suck it and see. And hence why what might work for me might well not work for you.

If there is any good news here it is that, with patience, it *is* possible to improve dramatically on the supplied antenna. TMC is now staying locked on for whole journeys, where previously it would not lock on at all, or for only a short stretch as I passed Milton Keynes (the Bow Brickhill transmitter) at best.
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PostPosted: Fri Oct 13, 2006 6:45 pm    Post subject: Reply with quote

With 10,000W of power the Bow Brickhill transmitter should be easily picked up when passing by on the M1, or other close by roads.

The simple structure of the car being steel will give you problems getting a decent signal, although so far you seem to be doing quite well, I took the easy opt out and fitted an external aerial, but I like the idea of the non damaging fit you are pursuing.

My car resembles a hedgehog with various antenna fitted, one more made very little difference, please keep us up to speed with your "experiments" as I am sure quite a few others will benefit in the longer term - Mike
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PostPosted: Thu Nov 02, 2006 9:18 pm    Post subject: Reply with quote

Well, Mike, you did encourage me ...

I previously mentioned that I’d discovered that the Maplin jacks had faulty connections, so I was thinking of trying again with a simpler (than the DAFT antenna) arrangement. I eventually got round to it. Sorry – no pics of it in situ.

I’ve got room for not much more than 50 cms. inside the windscreen pillar trim. There is already some capacitive loading simply from the proximity of the antenna to the bodywork (just a bit more than a centimetre away along the whole length). This should cause it to resonate at a lower frequency than in free air, but I thought I’d try some inductive bottom loading to bring it down still further.

Here is something very similar to what I ended up with:

And here is a close-up of the coil at the bottom end:

I did NOT ‘design’ this. It was really trial and error – plenty of both.

To connect to the X-950 I made yet another lead from some 75 ohm coax, with the core to the centre pin of the stereo jack and the braid to the ‘middle’ connector. (Checking this time that the connections were really made!) At the other end of the coax, at the bottom of the windscreen pillar I drilled a small hole and connected the braid to the metal bodywork with a self-tapping screw. I connected the core of the coax to the bit sticking off the bottom of the coil. The rest of the antenna is held in place with foam spacers and sticky tape, much like the earlier ‘added-on monopole’ shown in a previous post, ending 1 or 2 cms. away from the bodywork at the top of the windscreen.

For the bottom coil I started with about 12 turns around the body of a ball-point pen. (No calculation – just a stab in the dark.)

I then monitored the signal strength for Classic FM as I cut off, from the bottom, one turn at a time.

Eventually, around 8 turns, I got some signal. Gradually the signal strength increased. Then I tried stretching the coil slightly as another less destructive (i.e. reversible) way to reduce its inductance. And after a couple of overshoots and much stretching and squashing, I got performance which is at least similar to that of the daft antenna if not slightly better.

Sooo – not sure I’ve gained much, but at least this is a much simpler and even less obtrusive arrangement.

All I have to do now is to find away to get the very sticky goo of the double-sided tape (that I used to fix the folded dipole) off the windscreen.

Incidentally, Maplin were very good when I pointed out the problem with their jacks. Instead of the usual “Can’t be us, must be the way you’ve mistreated them” I got a genuine apology and an offer to replace those that were faulty.

Pity viaMichelin seems to fail to make use of TMC information, after all that!

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