Help me design an AM radio transmitter

Hi All. My name is Shelldon and I'm a complete newby here on the forums as well as in electronics.

I'm a tour guide in real life and have an idea for a project but no idea how to go about designing it. I have a large 10 seater vehicle that has a radio in it. Unfortunately the radio doesn't have a spare plug in point for a microphone so I thought about building some small "black box" I could plug a microphone into (something small that clips to my lapel) that would transmit to a spare channel on the radio (preferably am but I guess fm is fine?) and then everyone could hear me.

I could get a PA system professionally installed but it would cost a small fortune quite honestly and I don't see the point since I have a perfectly good radio and speakers already installed.

So any help/ideas would be greatly appreciated. Thanks.

Re: AM Radio Trnasmitter

Hello Dear EigerSA
I can help you to design or i can give you some circuits (i'm crazy of transmitter design) . for example see below figure please:

(top circuit is not my design)
Good luck
Goldsmith

Re: AM Radio Trnasmitter

( L1 will be the antenna coil of AM that you can buy it or you can wind it with your self and for C6 it can be the variable capacitor of AM radio . ) thus you can change the station with your desired carrier frequency !

Re: AM Radio Trnasmitter

EigerSA said:

Hi All. My name is Shelldon and I'm a complete newby here on the forums as well as in electronics.

I'm a tour guide in real life and have an idea for a project but no idea how to go about designing it. I have a large 10 seater vehicle that has a radio in it. Unfortunately the radio doesn't have a spare plug in point for a microphone so I thought about building some small "black box" I could plug a microphone into (something small that clips to my lapel) that would transmit to a spare channel on the radio (preferably am but I guess fm is fine?) and then everyone could hear me.

I could get a PA system professionally installed but it would cost a small fortune quite honestly and I don't see the point since I have a perfectly good radio and speakers already installed.

So any help/ideas would be greatly appreciated. Thanks.

Click to expand...

Although goldsmith gave a working schematic for "home brew" solution, it does sound like maybe a pre-built unit, say, a compact FM transmitter might do the job (AM is kinda 'messy' with getting an effective signal to radiate for reasons there isn't time or space to elaborate here)?

Maybe a small, pre-built FM transmitter as some vendors sell on eBay?

Jim

Re: AM Radio Trnasmitter

Heres a dead simple one, 2 components.
The oscillator can be got from an electronic component supplier such as rs components, the transformer is less common, but if you look through a couple of 70's or 80's tranny radios the red transformer is the one you want, they call it an lt700, electronics surplus houses might have them also.

Chapter 4: Radio -- Build a very simple AM radio transmitter

Some radio hams will slate this circuit as it uses odd techniques but actually its an extremely stable transmitter.
You might need to get the aerial close to the receivers aerial, that might need some creativity as if this is in a vehicle, you are basically inside a screening can.

Re: AM Radio Trnasmitter

That is an awful idea and will be certain to create a huge amount of interference.

RF_jim's idea makes most sense.

Keith

Re: AM Radio Transmitter

Hello EigerSA and jim and dr pepper and Dear keith
I'm completely agree with keith .
Well , here is a simple FM transmitter , you can try it :


Best Wishes for all of you
Goldsmith

Re: AM Radio Transmitter

Square wave transmitters - what next !!!!

I am a Radio Ham and I will slate it. The frequency stability is the only thing good about it but it transmits a stable 1MHz, 2MHz, 3MHz, 4MHz........ all at once :-?

Brian.

Re: AM Radio Transmitter

Oh, well slated.

I dont defend the circuit, except for the fact that the op saying he's a total novice probably doesnt want to build a pll harmonic filtered fcc transmitter, and a 5 v squarewave with a wire aerial less than the legal length isnt going to transmit far enough to upset anyone.

Also the op wants an AM transmitter, I see theres been a few comments about fm, these days we tend to use frequencies in the vhf band up, a squarewave fm transmitter is going to affect these probably more than a am transmitter, the harmonics will be dying down by the time they get to vhf, depending on the slew rate of the oscillator, which being fed through an audio transformer isnt going to be that high.

A good way to calm down (but not totally remove) the harmonics on this type of circuit and speaking from experience can be done with just the aerial, a wire loop and a tuning cap makes both a selective aerial and a tank circuit

I'm certainly not a radio amatuer, but I dont mean any disrespect to any of you that are.

Re: AM Radio Transmitter

dr pepper said:

Oh, well slated.

I dont defend the circuit, except for the fact that the op saying he's a total novice probably doesnt want to build a pll harmonic filtered fcc transmitter, and a 5 v squarewave with a wire aerial less than the legal length isnt going to transmit far enough to upset anyone.

Also the op wants an AM transmitter, I see theres been a few comments about fm, these days we tend to use frequencies in the vhf band up, a squarewave fm transmitter is going to affect these probably more than a am transmitter, the harmonics will be dying down by the time they get to vhf, depending on the slew rate of the oscillator, which being fed through an audio transformer isnt going to be that high.

A good way to calm down (but not totally remove) the harmonics on this type of circuit and speaking from experience can be done with just the aerial, a wire loop and a tuning cap makes both a selective aerial and a tank circuit

I'm certainly not a radio amatuer, but I dont mean any disrespect to any of you that are.

Click to expand...

dr pepper.

I must have missed where the op expressed a preference for an AM transmitter solution; I would have not presented an FM solution otherwise except perhaps to draw the distinction as to the problems one can expect with AM, such as transmit distance of about three feet/1 meter using a flexible rubber-duck style of antenna.

The wavelength at 1 MHz is 300 meters whereas at 100 MHz it is 3 meters; in the first case a 50 cm length rubber-duck antenna represents 1/600 Lambda and in the second case 1/6 Lambda. In the first case the antenna efficiency will be less than 1/10 percent and in the 2nd case maybe 30% efficient (rough numbers).

Regards,

Jim

Re: AM Radio Transmitter

Dear Jim
Hi
There is no problem with the size of antenna , we can increase the power to compensate it . isn't it ?
Best Regards
Goldsmith

Re: AM Radio Transmitter

goldsmith said:

Dear Jim
Hi
There is no problem with the size of antenna , we can increase the power to compensate it . isn't it ?
Best Regards
Goldsmith

Click to expand...

Short answer, Goldsmith, no. We cannot equip our tour guide with a 10 watt transmitter (think of the batteries required!) to achieve an ERP of 10 mW with a .1% efficient antenna even. Better to equip our tour guide with 50 mW or 100 mW FM transmitter and achieve 20 to 50 mW ERP. And I have not considered the efficiency of the receive antenna yet.

I would recommend some practical tests (or reading of good white papers by Wheeler and Chu from the 1940's) some time if you do not accept my short answer!

Regards,

Jim

Re: AM Radio Transmitter

Dear Jim
I'm completely agree with you , but i think for his application it is no problem because there is not long distance between receiver and transmitter . but i'm agree with your latest post.
Respectfully
Goldsmith

Re: AM Radio Transmitter

goldsmith said:

Dear Jim
I'm completely agree with you , but i think for his application it is no problem because there is not long distance between receiver and transmitter . but i'm agree with your latest post.
Respectfully
Goldsmith

Click to expand...

Goldsmith,

I almost completely failed here, for I forgot to also include "waveguide beyond cutoff effects" on the propagation of a long wavelength inside an enclosure, such as being inside a small bus or van.

Propagation only takes place in free space or waveguide of certain minimum size 'walls' (such as TE10 mode: 1/2 WL x 1/4 WL) ... and clearly our longer wavelength AM signal will not 'propagate' rather but only 'set up fields' inside a large vehicle even!

Using this calculator I calculate the Fco (lower cut off frequency) for a bus-sized interior of 50 to 80 MHz, therefore AM will not propagate (Fco >> AM band):

WAVEGUIDE CUTOFF & LOSS


Regards,

Jim

Re: AM Radio Transmitter

Dear Jim
But i tested it for many times ( with distances about 100 meter it was very good ) . and i don't thing that his distance be more then 10 meter .
Thanks for your patience .
Best Regards
Goldsmith

---------- Post added at 22:13 ---------- Previous post was at 22:10 ----------

By the way , what is your idea about navigation transmitters with 127 KHZ carrier frequency ? ( in sea telecommunications )
Why we can not see very long antenna for that frequencies , on the ships ?
Respect
Goldsmith

Re: AM Radio Transmitter

goldsmith said:

Dear Jim
But i tested it for many times ( with distances about 100 meter it was very good ) . and i don't thing that his distance be more then 10 meter .
Thanks for your patience .
Best Regards
Goldsmith

---------- Post added at 22:13 ---------- Previous post was at 22:10 ----------

By the way , what is your idea about navigation transmitters with 127 KHZ carrier frequency ? ( in sea telecommunications )
Why we can not see very long antenna for that frequencies , on the ships ?
Respect
Goldsmith

Click to expand...

Receive antennas or transmit antennas?

Receive is 'easy' to do; to get efficient transmit is hard to do ... I say receive is easy, but, the antenna is still 'lossy' for lack of more detailed terminology and theory as to why such as the losses in the components making up the L (wire) and the C (tuning cap or earth in near field as in around ground radials). You know, the amateur radio community is doing tests at both 136 kHz and 500 kHz as well. I have been doing test using WSPR network Propagation Map | WSPRnet recently to demonstrate 'efficient fractional wavelength' antennas on 160m, but no work at 500 kHz or 137 kHz, just becoming familiar with the theory. I should also say: I have done AM broadcast transmitter work too, so familiar with that technology as well as propagation, antennas, etc as well.

I should find those references to papers by Chu and Wheeler (and others, like Roberts) now.

Jim

Re: AM Radio Transmitter

My mean is on transmitters . ( i know that receiving is easier than receiving ) .
By the way , at some low frequencies telecommunication , we have to accept that dissipations . because we want use that frequencies and the antenna of those frequencies is not reasonable .( for some frequencies about some KM!!!!!!!!!!!!!!)
Regards
Goldsmith

Re: AM Radio Transmitter

goldsmith said:

My mean is on transmitters . ( i know that receiving is easier than receiving ) .
By the way , at some low frequencies telecommunication , we have to accept that dissipations . because we want use that frequencies and the antenna of those frequencies is not reasonable .( for some frequencies about some KM!!!!!!!!!!!!!!)
Regards
Goldsmith

Click to expand...

Yes, efficient 'radiation' (verb, not the Alpha or Beta or Gamma particle) is difficult. For the benefit of those reading this exchange, we are talking now about antennas which are only 1 or 2% of a wavelength (Lambda) as opposed to 25% or 50% of wavelength as one's Two Meter or TV antennas are 'cut' for.

On antennas used on the 'free' frequencies of 160 - 190 kHz in my hemisphere, we have a limit of about 50 feet or 15 meters and a power limitation of 1 Watt ERP (Effective Radiated Power) ... for an input power RF power of 300 Watts a radiated efficiency of 1 (one) Watt can usually be achieved .. but this is not practical for hip-pocket or purse applications of personal communications involving texting or voice!

An example of a 300 to 400 W transmitter to achieve that 1 W ERP figure:

A 400 Watt Low Frequency SSB Linear Amplifier


A quick reference to a webpage describing antenna efficiency measurements at 137 kHz:

<b>loop detail</b>


Another good, extensive read from the practical side (as opposed to theoretical side):

http://www.qsl.net/w9rb/webdoc11.htm


As with all radio engineering, one must know propagation, one must know the technology for making efficient radiation, one must work with available components and parts to make an economical solution (or customers find product too expensive, too big, doesn't work to fit needs etc).


Jim

Re: AM Radio Transmitter

Hi everyone. First off, thanks to everyone who replied so quick to my query! Just to clarify a few things...

I mentioned AM because I thought that would be easier then FM, but honestly I'm happy to go either way and have no preference.

Basically I just need it for when I'm driving. I'll have a set of headphones on with a microphone (or better yet a lapel microphone). The microphone will plug into a "black box" (the FM/AM transmitter) and I'll tune my car radio into the right frequency and then chat away to my clients. So if the black box is near me in the drivers seat, maybe just mounted under the drivers seat or just popped into a center console, then distance to car radio is less than half a meter.

The other issue I thought about last night is that the microphone I suppose would have to have an internal amp, or I'd have to add an amp to the transmitter circuit so that I could use a pc microphone.

Re: AM Radio Transmitter

There are many solutions for a DIY wireless circuit, but getting the wireless mics to work reliably sometimes seems to require equal parts rocket science and black magic. That's why knowing these DIY devices can provide unreliable results for daily use, go and buy a cheap VHF wireless microphone on eBay, like already suggested earlier by RF_Jim, as you can get some for like 10$. Some examples:
eBay: Wireless Mini Microphone FM Transmit Receiver

Wireless Microphone FM Transmitter | eBay