audioguru's FM transmitter, antenna design.

[theimperia]

Hi everybody!

I, as almost everyone else, has built the FM transmitter (Mod 3) by audioguru. The difference is that I connnected a high-gain (constant volume-output) microphone circuit, powered by its own 1.5V battery to it, instead of the microphone and preamp.

It's working very nicely. Now, again as almost anyone who built it, I need to find a good antenna design for increased range.

I used a #24 single-strand wire, ~60cm, at first. Gave me a range of about 60-70 meters.

Then, I used a folded dipole, each one of "dipoles" being approx 70cm in length. I attached the Ground (of a coax cable) to one "dipole" and the signal to the other one. Got a range of about 300-400m. The dipoles are made of 2 hollow-tubed retractable antennas (from old TV).
I'm sure that there's a way to increase it with a better antenna. The connection to the circuit (from the dipole) is direct (with a short, 15cm, coax cable), without any matching chokes.

The circuit is NOT in a metallic box, but the stability is very great. I might add one later.

Measuring the battery, it gives me about 6.8 (instead of 9) volts.

I'll try to figure out a better antenna and I'll post here.

Audioguru, didn't you say that you get about 2km range? Could you post the photo of your antenna? Or maybe a drawing with dimentions? Thanks.

TI|CP

 

[audioguru]

Hi The Imperia,
Sorry, I don't look over here very often.
My transmitter has a piece of wire 80cm long for an antenna, hanging vertically (I didn't put a telescopic whip antenna yet) the same polarization as the whip antenna on my car. My car radio is very sensitive and I measured about 1km line-of-sight, from one hill to another hill without anything in between. The range would be more if I had a vacant channel since I had to use the same frequency as a low-power broadcasting station somewhere on the other side of my city.

The battery's voltage is very important for good range, mine started at 9.2V and was 8.2V a couple of hours later. Your battery is nearly dead. :lol:

 

[electech2u]

If you can sacrifice the omni-directional pattern of your antenna for a directional pattern you can get a great increase in range,and keep the polarization matched to the receiver or you will lose about 20 Dbs.

 

[electech2u]

with a omni-directional antenna your pattern is something like a doughnut shape or flower shape, the relationship of the antenna length compared to the wavelength of the frequency changes the pattern in an omni-directional antenna. A 1/2 wavelength vertical needs no ground radials also, with a little gain for a 5/8 wavelength vertical because of a lower angle of radiation mostly. A good antenna choice would be a commercial fm antenna like you use for your stereo, it would already be tuned, it dont have to be an expensive one, since you wont need the bandwidth of a log periodic and if its a yagi 3 or more elements should work fine unless you plan on trying moonbounce.

 

[HiTech]

In my college days I built a 2-transistor FM transmitter. Along with the music I broadcasted, I injected a 19kHz tone to fool the stereo decoder in FM tuners to trigger. I used a center-fed dipole antenna about 15 feet above ground. I received my signal as far as 2 miles away. I even connected a 10" open reel deck to play mixed rock songs on Friday eves. so the neighborhood drugheads had something to zombie out to!!! LOL

 

[rdljpl]

What I am attempting is to use an FM Transmitter to transmit my music throughout my house and yard to FM Receivers in different locations

I just assembled a LKG industries FM Stereo transmitter kit (80-060) and it does work, but poorly. I connected a dipole antenna to it and I can receive a weak signal if I put the receiver's antenna within a few inches of it. I get a lot of noise from far more powerful brodcasts.

I am using a cheap 12 vdc adaptor from another device and at first I tried the whole works in a metal building with foil backed insulation, so my next move is outside and maybe a different power supply.

I am very new at this and my thoughts are from what I have read on the internet.

Is there a difference between a transmitter and receiver antenna?

Eventually this, or another, transmitter will be mounted in a closet with an antenna mounted in the highest attic space.

Thanks in advance

 

[rdljpl]

OK: I tried an 80cm solid wire antenna just plugged into the antenna jack using no ground. I connected to the line output of a cd player thinking maybe that I could recognize the CD. NOT. It appears to maybe work a bit at around 100 on the FM radio with a single wire antenna also. I was able to overpower a very weak station and create a bit of sound when I bumped the antenna but was never able to find it again. I am thinking that next I will try a DC output from the car parked right next to the transmitter and attempt to find the CD player output on the car radio which is probably better than the FM receiver I am using. Also I will try to get a better antenna connection rather than a # 12 wire pushed into the transmitter's antenna jack.

 

[rdljpl]

Seems like I am talking to the wind, lol, but maybe I will figure it out myself by the time everyone gets back from vacation.

Last night I tried a rabbit ear antenna from an old TV with an impedance matching transformer. I just plugged it into the antenna jack with an adaptor. Now I get a weak signal at around 100Mhz on a radio that is about 10 feet away with a single wire antenna. Then I replaced the power supply with a connection through the lighter socket on the car, which is parked within five feet and also tried to tune it in on the car radio. I still got no improvement. I tried turning the antenna and it did not improve the signal that I could hear. The second car, which was about 30 feet away, got the same reception.

Next thoughts;
Match the antenna impedance to the transmitter? I have a Digital VOM.
Boost the power output on the transmitter? I do have a schematic.
Replace the transmitter?

Thanks

 

[audioguru]

My FM transmitter goes for miles with just a piece of wire for an antenna.

Maybe yours is tuned to the image frequency of your radio so its reception is poor. The IF frequency of FM radios is 10.7MHz so the image frequency is 10.7MHz higher or lower than the desired frequency.

My high quality home stereo and car radio receive my FM transmitter perfectly with a range of 1km to 2km.
My cheap Sony Walkman FM radio has a range of 400m.
My extremely cheap scanning FM radio from the Dollar store has a range only across the street.

 

[rdljpl]

antenna

Thanks for your answer

I am not sure I understand how to tune away from the image frequency. My transmitter has a pot that I can use to adjust the transmitter frequency ( I think that is what I am doing). There are two pots to adjust the audio to the max without distortion.

I tune the radio to an area where there seems to be no station or a very weak station and try to tune to that point using the tuning pot listening for the CD that is playing on my CD player with line outputs to the transmitter inputs. I do live in an area where FM stations abound.

I don't know much about radios and antennas so that is why I am looking for simple pointers

 

[RadioRon]

Because you get so very little range from your transmitter, I think that it is not working properly. If it was, you would have a very strong signal to your FM receiver across the small distance of one room, like 15 feet or so, even with practically no antenna at all. I recommend that your carefully inspect all your components to see if they have been installed the right way (for transistors and diodes for example) and to see if your solder joints are all good.

Can you post a closeup photo of your board for us to help with the inspection? I tried to find a schematic but could not.

 

[audioguru]

Many radio projects do not work because they are made with a big mess of wires. A circuit made on a breadboard is a big mess of wires. I laugh when I see one.

 

[audioguru]

Aren't we talking about my FM transmitter circuit that has its frequency adjusted with one trimmer capacitor?
Then why does your circuit tune the frequency with a variable resistance pot?

Why does your circuit have two pots to adjust the audio to the max without distortion when my circuit does not have any pots??

 

[rdljpl]

My transmitter is a kit on a printed circuit board, I will see if I can post the schematic for it after I clean up from digging sprinkler line ditches. After that I will inspect all of the solder connections and component placements with a glass. It very well could be that I lost a few soldering skills after not doing any for over 30 years and now having old eyes. Thanks for all of your suggestions. I am sorry I did not find Audioguru's schematic before I bought this kit.

But I am having fun......Thanks

 

[audioguru]

The schematic for a kit is frequently on the internet.
What is the manufacturer and model number?

 

[rdljpl]

Here is the Schematic for thr StereoCaster FM Stereo transmitter kit Datakit # 80-060 This morning I checked all of the soldering and cleaned up a few and connected it again, still with the same results. Looks like I may have to go back and check every connection. After I finish the sprinkler system repairs.



Roger

 

[audioguru]

The Stereocaster looks like a fairly old but good circuit. Its range should be a few thousand meters to a good radio or across the street to a cheap radio.

 

[rdljpl]

I managed to get some time to get back to this transmitter. First I tried different power supplies and there was no change with 9v, 12v, or 15v power supplies. The instructions say that I can use anything between 9 and 15v.

Then I removed the 75-300 ohm adaptor from the antenna circuit, connected about 20' of coax with the center wire exposed at the end and connected that to one 'ear' of the rabbit ear antenna.

My power increased considerably and the distance increased to about 30 feet.

I have discovered that I do want a more stable tuner so that I am sure of where I am transmitting.

Still not enough power to get me in trouble with the FCC or to pick up my signal in any location in the house and yard.

 

[Vikky]

hi audioguru,

Can you post your FM transmitter schematic again?

I have search the forum but was confused about which one is to use.

Regards

 

[audioguru]

hi audioguru,

Can you post your FM transmitter schematic again?

I have search the forum but was confused about which one is to use.

Regards

Maybe some parts will not be available in India. Here is my FM transmitter again: