audioguru's FM transmitter, antenna design.

[rdljpl]

RadioRon

Yes, I still believe the problem lies in the transmitter operation, I have found that P3 circuit is too hard to tune (very sensitive) and just touching it seems to cause the transmitter to drift off frequency plus there is a hum transmitted also. I think that I will need a stronger magnifirer to see what may be wrong. I am also thinking that there are some voltage points that I can check with a VOM that will give me some clues.

How would I measure the power output?

 

[RadioRon]

P3 is, by design, a very sensitive control and there may not be anything malfunctioning just because it seems that way. If you want to make it easy to manage, you could use a multi-turn trimmer pot, or you could break P3 up into two fixed resistors and one variable, to reduce the range of voltage adjusted by P3.

Measuring the output power is most easily done by checking its range as you are doing. However, I think there is more going on here than you are telling us. The schematic shows a direct connection from the antenna to a capacitor, yet you said that you have coax between the two. This changes things a fair bit. Can you explain or show by picture or diagram, exactly what lies between the output transistor and the air. Closeup photos of your connections, your circuit board, your cables and your antenna would be useful.

 

[rdljpl]

Here are some pictures.

Antenna pictured is a straight insulated peice of #12 copper wire 29.5" long connected directly to an RCA plug. no ground.

The second antenna is exactly the same except that there is a 30' piece of coax (RG6) between the transmitter and the antenna. The coax shield is connected the ground point at the transmitter end only

Performance between the two antennas seems to ba about the same except when I use the one without the coax I become part of the antenna when I try to tune the transmitter.

 

[RadioRon]

OK, good photos, thanks. I think the next step is two-fold. First, I want to know what receiver you are using to check your range. Can you post a photo of it?

Then, perhaps its time to make some voltmeter checks for DC levels on your circuit board to see if all is in order. Do you have a voltmeter?

 

[rdljpl]

I am using a Yamaha AV-1 receiver (ca 1999) and the FM radios in 2004 and 2005 Ford vehicles. 91.1 is a vacant frequency in my area and that is where I am transmitting.

Yes, I have a very good but old digital vtvm and a newer digital vom.

rdljpl

 

[audioguru]

I also have a Yamaha receiver about 20 years old. Its FM is very sensitive.
When I tested the range of my FM transmitter my car radio was also very sensitive.
My Sony Walkman radio got overloaded by my FM transmitter plus all the local stations. My cheap "radio" from The Dollar Store received my FM transmitter no farther than across the street.

 

[RadioRon]

I am using a Yamaha AV-1 receiver (ca 1999) and the FM radios in 2004 and 2005 Ford vehicles. 91.1 is a vacant frequency in my area and that is where I am transmitting.

Yes, I have a very good but old digital vtvm and a newer digital vom.

rdljpl

I don't know if you've checked the circuit operation, so let's do that now. Can you measure, with your DC voltmeter, the following points and tell us what you see?
R19 (either side)
output of 78L05
top of C20
top of C19
top of R15

These should be measured with the simple wire antenna attached to the RCA connector.

 

[rdljpl]

I don't know if you've checked the circuit operation, so let's do that now. Can you measure, with your DC voltmeter, the following points and tell us what you see?
R19 (either side)
output of 78L05
top of C20
top of C19
top of R15

These should be measured with the simple wire antenna attached to the RCA connector.

R19 (either side) = 11.9v
output of 78L05 = 4.8v
top of C20 = 6.9v
top of C19 = 11.0v
top of R15 = 1.9v

rdljpl

 

[audioguru]

R19 is powered from the output of the 5V regulator through the tuning pot P3, so how can its voltage be almost +12V? Its voltage should be variable from +0.45V to +5V adjusted with P3. There must be a short circuit near R19 to +12V.

 

[rdljpl]

I think I found the problem, tomorrow when I have good light I will try and fix it. Then I will check the voltaqges and try it again.

I will let you know.

rdljpl

 

[rdljpl]

Sorry, my eyes got to me again, I looked at my notes again and the voltage at R19 was actually 4.9 or 4.7. I somehow mistook a 4 for 11 when I transcribed.

rdljpl

 

[rdljpl]

The other day I took my transmitter out for photographs and I managed to drop it. It landed right smack on the tuning pot and it quit working after that except at the high end of the FM dial. Naturally I took it apart and found a crack that caused the problem.

I replaced it with a 10K, .75 watt, 15 turn pot and that is what I am using now. It does make it a bit easier to tune.

This morning I got out the pencil soldering iron again and went through and cleaned up all of the ugly soldering I had done before. Then I cleaned the back of the board with some denatured alcohol and a Q tip. After that I brushed it with my wife's toothbrush. (the old one she keeps under the kitchen sink, I 'm not totally stupid.)

When I plugged it in again it was no better, so I suspected the CD player line outputs, so I got out my portable CD player and put a CD and some new batteries in and tried it with both the line output and the headphone output with the volume as low as it would go. No change.

I played with the antennas and neither one was really good so I went to the garage and grabbed my roll of #12 copper wire and cut off a piece about 60" long and stuck it into the antenna RCA jack and the performance improved greatly. I am transmitting a clearer signal than ever before and can overcome the adjacent station that tends to splash all over the place and also the station that is 80 miles away on the other side of the hills.

By all of this, I am concluding that my problem lies in the antenna or the circuit that connects to the antenna.

91.1 is vacant in my area and the antenna calculations for that frequency are calling for something in the range of 29 to 30 inches.

Boy am I having FUN.......

I am off to redecorate a bathroom under the supervision of my wife who ordered a new countertop.

rdljpl

 

[RadioRon]

The output stage of this transmitter is not designed to drive a standard cable connection, so putting a long coax on that with an antenna on the end is a bad thing. As you have discovered, this transmitter is designed to directly drive a piece of wire. The difference between the two situations has everything to do with impedance matching, a subject that the RF specialist becomes intimately familiar with over time. I'm glad to hear that it finally operates closer to your expectations.

I hope your countertop is a laminate type, as they are not too difficult. Doing a good job with tile is also not too bad if you are careful, plus tile can be creative and fun. Beware the idea of casting your own concrete countertop. Good luck.

 

[rdljpl]

My wife ordered Granite, so all I have to do is refinish the base cabinet, make new doors and do all of the drywall work. Granite is inexpensive, just the labor to cut it is expensive. The bathroom will be done for less than $1000.00.

I think that a piece Of #10 wire will make for a better fit in the center of RCA plug and I can begin to adjust and tune for a better output.

I will be trying to figure out how to increase my output so that I will be able to get the performance that I really want. I will have to learn about antenna ouputs and matching the antenna so that I can permanently mount the transmitter so that I can get the antenna to the attic.

rdljpl

 

[RadioRon]

When the time comes, perhaps we should discuss how to make a simple RF voltage probe. This is a simple AM detector mounted in a pen barrel, which then plugs into your voltmeter. It is a handy tool for measuring relative RF voltages and might be helpful to you.

 

[audioguru]

When the time comes, perhaps we should discuss how to make a simple RF voltage probe. This is a simple AM detector mounted in a pen barrel, which then plugs into your voltmeter. It is a handy tool for measuring relative RF voltages and might be helpful to you.

My pretty good Fluke DVM went crazy when my FM transmitter was turned on closeby. I used an RF probe connected with shiielded cable to my 'scope to see the signal strength.

 

[rdljpl]

I found one on-line right after I had better success with the longer antenna. I will print out the list of components tonight and take it with me when I go to the gym in the morning, there is a Radio Shack right next door.

Since I removed the light fixture from the bathroom, I can't work after dark.... Tonight I am going to make an antrenna that I can adjust to see the approximate length I need. Maybe just one leg of the rabbit ear antenna will do?

I did my initial voltage measurements with a cheap Sperry clamp-on with a minimal digital VOM and it seemed to do OK, I will check with my Greenlee and the old digital VTVM to see what they do. Interesting. I lost my access to a scope when I retired, never thought I would need one again. I may have to start looking more carefully at estate sales.

This is getting to be more fun everyday and I have found a reason to stay awake in the evenings instead of sleeping in front of the tube.

rdljpl

 

[rdljpl]

Interesting observation tonight while I was playing with the antenna. As usual it was inconsistant, noise would come and go along with fair to poor reception on my Yamaha tuner which was about 20 feet away.

I discovered that touching the circuit board caused a change and then by putting my fingers around the antenna close to the board caused a change also. I removed the antenna and plugged it back in and noticed an instant of clarity so I pulled it back to the point where it was no longer making contact and the output quality improved and the noise stopped. I found that by moving the antenna close to making contact with the circuit it would work pretty well but the further I moved it away the worse it got.

So by creating a tiny gap between C18 and the antenna I was able to 'tune' the antenna. Also, I could touch the line input cable without creating additional noise.

There are some possibilities. C17 and C18 are next to each other and I may have misplaced them. C18 could be bad. There could be a bad solder joint at C18.

rdljpl

 

[rdljpl]

This morning while I am at Radio shack buying components for an RF probe, I will buy a replacement for C18 and also for Q2, the output transistor. If you have any other sugestions I will probably be near Radio Shack again today while I am getting things for the bathroom redecorating.

Early this morning I did some more experimenting with the antenna and holding it within about 1/64 " from the output seemed to work best, that would seem make me holding the antenna a very large capacitor?

rdljpl

 

[rdljpl]

I finally got a little time to build an RF probe and I did some basic tests to see if it was working. I used two diffeent DVMs and the readings were similar.

TP1 Junction of R11, R12, and C20 = .009
TP2 Junction of R14, L2, and C18 = .009
TP3 Antenna side of C18 = .852

All readings were made with power on and the inputs and antenna connected.

Having never done anything with RF, I have no clue of where to go next.

rdljpl