Very Basic FM radio help

jogdish

Hi,
If this is the wrong place to post sorry ( any ideas where i should )?

I am trying to get a decent signal on for my FM radio. As a test i went to the €2 shop and picked up an FM radio and opened it up. Attached to the telescopic ariel is a coiled copper wire ( looks like enamelled wire ) and then thats hooked into the circuit board.
From looking up online guides i will need either 300ohm or 75ohm wire to solder on to make my own ariel - how do i know which wire my little radio is currently using?

If anyone knows of any online guides/books for basic radio electronics that would be cool - i understand the physics just no idea how to understand what each piece of the circuit board is about.

Thanks:

watty

Has your radio got an Aerial socket?

jogdish

No, I can find plenty of help for radios with a socket.
My radio is the following, it has a telescopic antena - I also have a second radio and all it has is one of those tiny thin wires that acts as the aerial.

watty

you can fit a switch disconnecting the wire to telescopic rod and wire other contact to inner pin of a 75 Ohm coax socket. The outer of socket with short wire to 0V rail on PCB near where wire fro aerial comes from. The switch, socket and point on PCB where aerial wire comes from all need to be close.

Then you can use 75 Ohm TV or Satellite coax to an attic or Roof aerial. Do not use a halo aerial as they are very low gain and horizontal polarised. All FM radio here is Vertical, Slant or Circular.

Three kinds of attic / roof aerial possible
Discone
Dipole (or Band II Yagi)
or 1/4 wave whip. A rod about 75cm long connected to inner of coax and four radials about 75cm long connected to outer of coax. You can make it out of 5 lengths of coax with inner shorted to outer at both ends. Lay the 4 radials across the rafters in attic and suspend the single 0.75m vertical using nylon cord from peak of roof. Or other insulating material (even Briquette binding tape). You can use speaker wire or bell wire or CAT5 for the 5 lengths of wire, as long as if it has more than one wire, all the wires shorted at both ends. A screw terminal block will work to connect the 4 + 1 wires to the end of the coax cable.

Unless your aerial is on roof, or at least in attic, or at worst an upstairs window, there is no point in modifying the radio for an external aerial.

Maplin and many online sellers have a simple 1 pole change over toggle switch and 75 Ohm aerial socket. You could also fit an F-Connector instead of TV style socket. Professional Radio use 50 Ohm BNC, but without experience it's difficult to fit a BNC plug on coax, and most are for 50 Ohm coax which is not in the local shops.