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 aerial 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 aerial - how do i know which wire my little radio is currently using? I have about 6m of 75ohm coax wire also.

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:

 

[duke37]

The telescopic aerial is shorter than optimum so it is tuned with the extra coiled wire. There needs to be two connections to the aerial, the other is the ground connection of the radio which is usually a larger track on the printed circuit and connects to most parts of the radio.
Connect the outer of the coax to the ground track and the inner to where the existing aerial is connected.
Connect a wire 2ft 6in long to the center of the other end of the coax and another 2ft 6in long to the outer of the coax. The aerial can be suspended with string or can be tied to a bamboo to keep it straight. Position it for the best signal.
For information, the ARRL Handbook is very good but deals with a lot of topics you do not need yet.

 

[jogdish]

Thanks Duke, I had already opened up the radio and saw the solder point between that small coiled wire and the telescope. Then i stripped the coax down to the central copper wire, and touched it to the solder point that improved reception alot.
But If i understand you correctly i need to find the ground wire on the radio, and connect it to the fine outer metal sheath, and the central copper wire to that solder point - am i correct?

 

[KJ6EAD]

A folded dipole antenna made from 300Ω twin lead similar to the dipole that Duke described how to make can also be purchased cheaply or found with old stereo systems as they're often included with them. They look like this rolled up.

There's another method using just the coaxial cable with no connectors that involves stripping the cable sheath off 80cm, spreading the shield braid and extending the core out of the shield to separate the core and shield into a T-shape then trimming both ends to 75cm from the junction point. It's easy to do but difficult to describe and I can't find any good illustrations at the moment.

 

[duke37]

Yes, you are corect.

Connect the inner to the original aerial connection and the outer to the ground track.
The ground track will go to many places on the radio and often goes round the outside of the board. It will connect to the negative of the battery and the negative of several electrolytic capacitors. Chose a place to connect the coax screen near to the aerial input not at the other end of the radio.

This will be a good test of your soldering skills.

 

[jogdish]

Thanks KJ and duke, I think will have a go at what duke said, but i went to have a look inside the radio and can only one point of contact with the aerial and the circuit board. I have included a picture #1 is the solder point between aerial and small coil ( #2 ) and #3 is where the #2 is soldered to the circuit board. I lifted the aerial out and it only has #1 connecting to anything else - where is the ground in this case - do i need to make my own ground?




Also i had a go a second radio ( second image - what type of aerial is the small thin black wire? it only connected to one solder point, how would i go about hooking up an aerial to that.

Sorry about all the questions!

 

[duke37]

Don't be sorry about the questions, that is what we are here for!

I think there might have been a bit of a disaster, the coil seems to be connected to the board at each end and so is likely to be part of a tuned circuit. If it has been changed in shape, it will be off tune.
The rod aerial should have been connected to the board and this is where the external cable should have been connected.
The ground connection as I said before is the track which connects many parts of the circuit and to which the negative of the battery and electrolytic capacitors are connected. You will need to look at the track side of the board.

The second radio will be similar. The centre of the cable should be connected where the wire is connected and the screen to the ground track. There is little difference elecrically between a wire and a rod. The rod is more convenient for positioning to get the best signal.

 

[jogdish]

Thanks Duke, but that coiled wire is NOT connected to the board, it is only connected to the rod/aerial ( not so easy to tell from the pic sorry ).

Ya no idea why there is no ground for the aerial, basically my plan is: Frame of wood, wrap around a length of insulated copper wire, each end soldered to inner and outer parts of the co ax respec. ( ie one end to inner wire other end to outer frame of co ax ). Then run the co ax into the radio - inner wire in place of main aerial and outer part to the Ground wherever that is

 

[duke37]

The aerial should be in the shape of a T. This is called a dipole.
Tape the wires to a five foot bamboo and take the cable from the middle at right angles.

Your photos are better than average, if you show the track side of the board, we may be able to spot the ground track.

 

[jogdish]

Hi again Duke,
Here are some pics of the both sides of the radio - On the trackside the red and black wire from the bottom are the power lines from battery, and the blue wire that was attached to the coil and aerial on the top side meets the trackside at the solder point cirlced in red.





Hope thats of some use, or makes some sense to you.

 

[duke37]

Better eyes than mine might make out the tracks.
In the third picture, there is a large track area in the top rifgt corner, this could be ground.
You appear to be holding a headphone socket, this will have a ground connection to the body of the plug.
The tuning capacitor will also have a ground control and is adjacent to the aerial input.
I do not understand where the battery wires go, surely one should go to the switch which is bottom left with the volume control.

Nice nail varnish!

 

[jogdish]

Hi Duke,
So i had go at making a very simple aerial for the small radio - just something to test out the idea:






The aerial is a T shape, 83cm long on each side, enamelled copper wire from each side comes down at the middle and is joined to inner and outer coax respectively. I then touched the other end of the inner coax to the aerial connection on the small radio - made a huge difference to quality - i then tried the same but also touched the outer coax, first to ground on battery then some other points - it made a very small difference for the better in quality. I then moved the aerial around - up heigh at 2m, ground level, inside, on the ground - made no difference.
I live in a city and the radio reception is pretty good most of the time so im guessing its hard to tell if its really working at all.
Finally I tried touching the inner coax to the small thin black wire aerial from another radio - it makes zero difference and actually seems to make it a bit worse.
Just wanted to let you know how i got on, perhaps you have some tips?
Thanks:

 

[duke37]

Glad to know you have had some success.
FM radios need a certain level of signal input and the quality of the output does not improve much above this.
The aerial you have is called a dipole and is broadly tuned to the frequency you want. I suggested that you made each side of the dipole 2ft 6in and KJ6EAD suggested 75cm. Yours is a bit long.
Happy experimenting.

 

[jackorocko]

There's another method using just the coaxial cable with no connectors that involves stripping the cable sheath off 80cm, spreading the shield braid and extending the core out of the shield to separate the core and shield into a T-shape then trimming both ends to 75cm from the junction point. It's easy to do but difficult to describe and I can't find any good illustrations at the moment.

Spot on this. I got what you meant and so I hacked up a coax cable last night. I had been struggling for 2 weeks to get a signal, I have a garage that is just stuffed with metal. I tried everything and was about to buy a 300 Ohm split cable. Then I read this thread which was posted at the perfect time. I pulled out the inner cable through a hole I made in the copper mesh. Pounded in a couple nails to the rafters in the ceiling, attached my copper wire.

I went from 2 channels to 12 and I live in a rather rural area. So that is extremely impressive. I cut mine at 2ft 5in to tune in a slightly higher freq better.

My only concern is that one of the two original channels I got real well is now it is the worst of the twelve. Almost like there is too strong of a signal. It is louder then the rest and very noisy now. What happen?

 

[(*steve*)]

That is possible. A local station could quite easily now be overloading the input stage.

You may be lucky enough to be able to fix this by orienting the antenna in a way that has its minimum sensitivity toward the strong station, but this may also compromise the weaker stations.

If you connect the antenna via a plug, you can simply disconnect it for the local station.

 

[KJ6EAD]

You can add a simple resistive T or pi attenuator for local stations.

 

[davenn]

You can add a simple resistive T or pi attenuator for local stations.

and that will also attenuate the weaker stations accordingly.
Better selectivity is what is really needed

Dave

 

[(*steve*)]

Better selectivity is what is really needed

That is certainly the case if a stronger station causes interference on other stations.

This does not sound like the problem to me (but I could be wrong)

And I also note that davenn is the expert in this area

 

[jackorocko]

You can add a simple resistive T or pi attenuator for local stations.

Want to expand upon this a little more. I am not sure what you mean exactly.

That is possible. A local station could quite easily now be overloading the input stage

This is almost the perfect explanation for what is happening. The station that has a lot of noisy now was certainly the strongest signal before I made the antenna.

 

[davenn]

Hi jackorocko

these are Pi and T attenuators .....

Pi ....

T ...

often just refered to as pads As I said in the previous post... "IF" you have 1 or 2 very strong stations overloading the receiver and drowning out the weaker stations, then adding attenuator pads will only attenuate all signals weak and strong by the same amount of dB. So you dont gain anything, the weaker stations are even weaker then they were before. In a reasonable radio they use a mix of good selectivity of the tuned circuit. This is achieved by making the tuning reasonably sharp ( a Hi Q).

They also use AGC, Automatic Gain Control, this controls the level of strong signals using feedback within the receiver to drop their level when they hit a predetermined setting. The main purpose of the AGC is so that weak and strong signals have a relatively similar audio output level from the speaker.

cheers
Dave