A capacitor question.

[Dave Plowman (News)]

I've bought a DAB car radio. This has two aerial inputs - AM/FM as normal and a separate one for DAB.

I will be fitting a DAB aerial in the near future when I re-do the sagging headlining, but in the meantime I want to share the existing telescopic with both. So bought an adaptor off Ebay - all the way from Germany.

It sort of works, but the AM side is totally dead. FM ok as is DAB. Plug the aerial in direct and the AM works.

The adaptor is simply a Y cord with the appropriate plugs. Screens are commoned and each core is capacitor coupled. My capacitor measuring thingie isn't that accurate with tiny values, but gives the DAB one as

30pF and the AM/FM as 10pF which doesn't make sense to me. ;-)

It's obviously a home made device and not worth the bother of returning in the hope of getting a better one so I've opened it up - full of potting compound - and they've used surface mount caps with no markings I can find. There's room for ordinary axial lead polystyrene of which I've got a goodly selection, but what values to use?

RF is a total mystery to me...

[Steve]

The AM capacitor should be much higher in value than the DAB as the frequency is much lower.

I would try about 1nF to 10nF (1000pF to 10000pF) or doing away with the AM capacitor altogether.

Steve

[The Natural Philosopher]

Yes.

What you want is a small RF choke in series with the AM and a small capacitor series with the DAB..ah. that will screw FM tho..igf iots too big.

Hmm.

DAB appears to be around 227 Mhz or so,,wheres FM is 100Mhz or so.. what is neded is a high pass/low pass splitter at around 150Mhz and 50ohm impedance..

I think what I would probably do is as follows.

Replace the AM/FM capacitor with a coil of copper wire - something like

7 tsurns of T&E core round a 4mm drill shaft. Remove shaft and space turn to not touch. Coat with varnish maybe.

And leave the DAB connnection. Its about right.

This won't be as good as proper calculated values, but should get you going.

[Sylvain VAN DER WALDE]

I agree with Steve. I'd certainly try Steve's solution before trying the Natural Philosopher's one.

Sylvain.

[Dave Plowman (News)]

What I did before posting since I'd wrecked it in taking it apart was to replace the DAB cap with 36pF and the AM/FM one with 1000pF - the largest of the ones I've got that would fit easily. AM now works but not as well as without. I did look at some reactance tables but they might as well have been double Dutch. ;-)

I'll try getting rid of the FM/AM cap totally.

[meow2222]

X= 1/ (2 pi f c)

so 50 ohms at:

1MHz is 3nF 100MHz is: 30pF 220MHz is: 13pF

IRL you dont ideally want 50 ohm series impedances in there, but I'm sure you can decide what to try from there.

NT

[Dave Plowman (News)]

Something at the back of my mind says AM/FM aerials for cars ain't 50 ohms?

[The Natural Philosopher]

Anything but.

I am fairly sure the small choke will work best..it will block DAB, be a little bit of a loss at FM and not do much at all to AM bands. I didn't do a lot of time in RF design, but I do have a degree in electronics and about 15 years as a professional circuit designer..

[Dave Plowman (News)]

Rather the wrong way round for my requirements - the AM will only be used when all else fails. I don't think there's anything on that which isn't on DAB - or certainly that I'd want to listen to. AM will probably only ever get used on a trip north where R4 reception is poor in the border country.

You need a degree in witchcraft for RF design...

[tony sayer]

In article , Dave Plowman (News) writes

No not really, its just that RF is very poorly understood..

Hardly anyone I know, whilst very good with PC and programming and other electronics, would stand a chance of making a radio TX....

>

[meow2222]

Ones I've played with are, but its fairly much immaterial in this case, as your blocker C doesnt want to match the ae impedance.

NT

[The Natural Philosopher]

Not for something this simple.

Possibly the best solution would be a series tuned circuit in the DAB path peaked up for 220Mhz. And a parallel tuned on in the rest to block

220Mhz.

We are talking a few turns of copper wire and a few PF only. Its easy enough to 'play it by ear'

[The Natural Philosopher]

I have Tony. ;-)

[tony sayer]

About the only one who comes to mind;!....

[tony sayer]

A marconi quarter wave which is what most car aerials approximate to has a "typical" feed Z of 50 ohms.

Its the length Dave what matters;)....

300/f /4 x.90 will see you about right...

[Dave Plowman (News)]

I don't think my aerial is a quarter wave at 200 kHz...

[tony sayer]

In article , Dave Plowman (News) writes

Well it can't be .. but for most applications the VHF side takes preference ..

Surely your not saying that DAB sounds worse than AM;))

[Dave]

Is that Watford Gap? :-)

Dave

[Dave Plowman (News)]

To this Balham boy - Fulham. I need the sat nav the instant I cross Wandsworth Bridge. ;-)

[The Natural Philosopher]

No. Whips look like capacitors at this frequency - you match em with a tuning cap and a big coil on the front end.

Not sure they don't look more like a 1/4 wave at 100Mhz tho..thats about

2ft...IIRC.

which is nearer 75ohms anyway.