A capacitor question.

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

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

And leave the DAB connnection. Its about right.

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

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

Sylvain.

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.

X= 1/ (2 pi f c)

so 50 ohms at:

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

NT

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

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..

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...

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....

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

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

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

I have Tony. ;-)

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

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;)....

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

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;))

Is that Watford Gap? :-)

Dave

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

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

which is nearer 75ohms anyway.