Replacing FM short wire aerial

[Adrian C]

I've got a FM tuner (ok, it's a Sony clock radio ... ) that relies on a permanently attached stringy bit of single core insulated wire as an aerial. This doesn't work very well...

I intend chopping this wire off and replacing it with a coaxial socket for a decent distributed feed from the aerial on the roof.

It's about 63cm long, so it's a base fed quarter wave aerial of sorts, and from known theory on that, a 36 ohm match to the front end of the receiver. So I need signal to go 75ohms into 36ohms. 2:1 ish.

Anyone know the whereabouts of a cheap and easily obtainable 2:1 RF matching transformer (I failed looking at eBay, CPC and Maplin), or a suitable minimun loss pad design with L/C/R passive components, or can recommend funny stuff chopping measured bits of coax about?

Thanks

[Woody]

It's difficult not to say a little knowledge is a bad thing. There will be no matching to the aerial as it will be connected to a (relatively) high input impedance of some sort. The length of the wire is immaterial, but you can try attaching another wire to make the cable longer.

I use a Sony clock radio and it used to suffer signal disturbance as anyone walked around the room. My fitted bedside table is attached to a side panel of the wardrobe, so I drilled a small hole through, pushed the cable through, and attached a longer piece of wire which I hung vertically inside the wardrobe. Problem solved.

[Adrian C]

Ah, yes I see that. The input of the front end could be as you say a high impedance. I'll sling a 75ohm resistor across it, and connect the coax. Job done :)

[Paul - xxx]

Heh, I had a similar predicament, wanting better radio reception from the basic wire .. I happened to drape it over and into a metal cup I'd won many years previously, and it worked better! Coiled it inside with the bare end touching the inside of the cup and almost perfect (certainly good enough for me), reception!

I'd suggest just you try just 'playing' with it and try extending it by adding other wire or simply arranging it differently before 'doing it properly' .. ;)

[Adrian C]

Whoops, I'll better rethink that. Probably kill it doing that, not knowing the DC conditions of the input.

[NT]

Its 1.4:1.

[Bill Wright]

To be pragmatic rather than theoretical, if you feed it from a decent outdoor aerial the problem is likely to be too much signal for the poor little thing rather than not enough.

One option would be to leave the wire aerial in place -- pin it up so it's straight, then make a 1/4wave dipole at the end of your outdoor aerial feeder and see how things work out when the two are in proximity. Anything from almost touching to a foot apart might give the best results. The signal from the outdoor aerial will most likely be so much stronger than anything the wire picks up direct that phasing won't be an issue. The advantage of this is that you haven't hacked the radio about.

Alternatively, simply connect the inner of the feeder to the stub of the wire aerial, and experiment to see what happens if you ground the coaxial outer to the radio chassis (if it is accessible).

If you have one of those old 75/300 balun/matchers around it could be worth trying that.

As you mention a distributed feed I wonder if there's amplification involved. Years ago in the days when shops used to demo radios it was common to install [outdoor aerial]> [30dB amp]> tx dipole above the suspended ceiling near the radios (and well away from the rx aerial of course!).

Bill

[Ian Jackson]

On 100MHz, a wavelength is appx 3m. As the existing aerial is 63cm long, that makes it just a bit shorter than a quarterwave, and that suggests that receiver input impedance is low. However, there's no saying what the impedance really is, so I doubt if you'll see any benefit at all from trying to fit an impedance matching device. Matching a 75 ohm aerial to a 37.5 ohm receiver is essentially a total waste of time.

[The Natural Philosopher]

I dont think so. 1/4 wave is 75 0hms.

So I need signal to go 75ohms into 36ohms. 2:1 ish.

[The Natural Philosopher]

be careful of that too. May detune the RF stage - or broaden it. Jolly difficult to say what A piece of wire is mapped into

Typically on 35MHz/72MHz RC stuff its tapped in at either v low impedance on a bit of coil or V hi via a capacitor.

But you SHOULD be OK in either case on a short bit of coax

[Woody]

Is the clock not two-core double insulated? DC conditions therefore do not matter.

[thirty-six]

It's too thin and too short.

I wouldn't bother, these are usually very sensitive receivers and just need a decent bit of wire.

Don't worry about it, it's not a transmitter and building shielding negates much advantage in choosing specific lengths.

You don't.

Pull out a 0.75mm^2 wire from a mains flex and use that as a replacement. Start with at least enough length to get to the ceiling plus 4 foot. You could solder a dog clip on the end and clip it to a heating pipe or radiator.

[Terry Casey]

Be very careful the clock radios often use live chassis techniques.

In which case there will be an isolating capacitor in series with the throw-out aerial.

Make sure that you fit an adequately rated capacitor in the 'earth' feed to your socket!

[Ian Field]

With those type of radios I've never had any problem just adding a random length of wire to the 'tail'. Just join it on the end with a single segment of "choccolate-block" connector.

If you really must use a correctly terminated 75 Ohm co-ax, just isolate both core an braid with a pair of 1nF capacitors and you won't upset any DC conditions.

[NT]

Please tell me you typed that wrong

NT

[Brian Gaff]

Well, from experience, it won't really matter if you match it or not, but obviously you will need to be very careful about isolation etc, so some capacitors are going to be needed, screwing up any matching attempts. All I can say is that I did this on a Goodmans unit, well someone did it for me, and thereafter all stations had gurgling in the background, and aicraft could clearly be heard breaking through on the image. I suspect the sensitivity of the front end is huge, but the overload and image rejection are crap. Brian

[Brian Gaff]

Good grief...

Brian

[Woody]

Clock radios using live chassis techniques? Eh??!!!!!

Perhaps CRT valve TV's used to do that decades ago but I would be surprised, nay shocked, to see that in a small domestic device. A clock radio will, at a push, probably draw about 5-10W or so, so a mains transformer will be very small and cheap to make - it wouldn't even justify the cost of a minuscule switched mode unit.

I suspect what you meant to say is that the mains lead will be connected direct to the PCB so there is a fair chance of a few bits of track and maybe a fuse of some sort being live, but as for a 'live chasis' no way.

[Frank Erskine]

Tha last few bedside clock radios I've had (currently DAB) have operated from "wall warts", so there've been no problem with mains connections :-)

[Scott M]

Friend of mine had a DAB radio where they'd broken off the telescopic aerial. Reception in the area is poor anyway so listening was a ropey experience. One day I was stood there with a 2m length of coax in my hand and contemplated the relative diameters of both. I stripped an inch off the outer insulation, folded the braid back and shoved it down inside the broken aerial (ie just using the braid as the antenna.)

Reception now better than it ever was.

Radios really aren't fussy!

Scott