FM Radio/Coax/Antenna

[Dougal]

I know this subject came up a little while ago, but i'm after more info...
If I'm using a 12volt FM receiver designed for a car, as I suspect are many/most.

Should I be using 50 or 75 Ohm coax?

When I simply joined a long (approx 2.5mtrs) piece of electrical wire (unshielded) to the end of the coax run, and dumped it in a 'snotty heap' over my davits, the reception was good to excellent, only getting interference when mains battery charger (or neighbours wind gen) running.

I then followed instructions from here and other places; cutting the unshielded wire to the specified length, and running vertically. The reception is now shite;-) so I'm trying a few (cheap) alternatives.

Would I be correct to assume that physically connecting the coax to rigging has no value (or worse) ?

Would a car radio antenna need grounding at source, like the car bodywork?

What about 'stubb antennas' - any good?

All input most welcome.

EDIT: In France and would LOVE to get BBC Radio4 but its only available on LW. Is there any way to increase LW reception, or is that an internal winding?

 

[nigelmercier]

I just used a cheap as chips car wing antenna, no bonding. It came with cable.

If you have enough internet, BBC iPlayer Radio http://www.bbc.co.uk/radio4

 

[earlybird]

75 ohm co-ax is correct. My FM aerial is a ~1 m. length of single core insulated wire soldered to the end of ~3 m. of co-ax and stuck vertically in an inconspicuous corner of the cabin, fairly near the mast as it happens. I'd expect the stripped co-ax inner to be just as good.
I get good reception, with only occasional slight interference.

 

[rogerthebodger]

I have used an internal car aerial on my car radio and reception is great until I turn my computer on and that inside a steel hull boat.

 

[Dougal]

That apears to be an 'active' powered antenna is it? Presumeably for amplifying the signal?

 

[rogerthebodger]

That apears to be an 'active' powered antenna is it? Presumeably for amplifying the signal?

Yep that correct.

 

[PaulRainbow]

I've used normal car antennas, no grounding. I used this style (not the exact aerial, but the style) :

http://www.ebay.co.uk/itm/like/2220...rlsatarget=pla-146652016866&adtype=pla&crdt=0

Currently using a VHF antenna on the pushpit, which acts an an emergency VHF antenna.

 

[William_H]

For the OP one thing to try is the orientation of the antenna. Try horizontal compared to vertical. It does make quite a difference that the polarisation of the antenna should be the same as the transmitter.
Yes a 1 metre wire connected to the centre of the 75 ohm coax should be fine however another 1 metre piece connected to the outer of the coax at the end and run in the opposite direction may also make a useful difference. This makes a ground plane.
Now for LW reception things get more tricky. Car radios usually do a compromise for LW/MWv VHF FM. Old car radios fro MW/LW used a very low capacitance antenna cable looks like coax but very fine centre wire. This became part of the capacitance of the input circuit of the radio. They often had an adjustable capacitor to tweak the tuning to the antenna.
I don't know what they do for a compromise with VHF FM and MW/LW on same antenna.
However I would suggest 75 ohm coax would be bad for LW. I suggest you try a long wire or even rigging connected directly into the antenna cable socket of the radio for LW. good luck olewill