Trying to build decent AM transmitter

[Mike Bates]

The am broadcast band isnt hardly used around here anyway, and i can only
pick up with 3 different recievers about 4 stations. thats it.

I have built the transmitter from the WRPDJ design, except I am using a big
modulation transformer, and an IRF510 mosfet with 19vdc driving it.

i dont have the parts to build a VFO, thats why I built the crystal
controlled CMOS divide by 4 circuit. it runs at approximatly 1610khz. (also
an empty freq).

I found a board from the WRPDJ clone, and modded, and used it insted since
it had a matcher/harmonics suppresor on it. so the system does go farther,
and stays on one frequency now.

Here is my problem. im only getting about a watt out of teh IRF510. The
mosfet is capable of putting out about 7 to 15watts.

How would I get my 1watt out to about 7 to 10 watts out?

Thanks!

 

[Joerg]

Hi Mike,

Not to spoil the broth here but this is most likely not legal. I believe
FCC part 15 transmitters are limited to 100mW input power of the final
amp and 3 meters antenna length, with some exceptions such as tunnel
radio etc. You can look it up:

http://www.access.gpo.gov/nara/cfr/waisidx_02/47cfr15_02.html

Regards, Joerg

 

[Andrew Holme]

Mike Bates wrote:
[snip]

I have built the transmitter from the WRPDJ design, except I am using
a big modulation transformer, and an IRF510 mosfet with 19vdc driving
it. [snip]
Here is my problem. im only getting about a watt out of teh IRF510.
The mosfet is capable of putting out about 7 to 15watts.

How would I get my 1watt out to about 7 to 10 watts out?

Some or all of these may work:

1. Increase drive level to MOSFET.
2. Change the output matching network so the drain "sees" a smaller load
resistance (Power = V^2/R).
3. Increase DC supply voltage.
4. Add a linear amplifier.

 

[Joel Kolstad]

Not to spoil the broth here but this is most likely not legal.

He probably knows that and figure he's going to risk it anyway; the pirate
radio community is alive and well. I do think you can make a good case that
there's been a coordinated effort by commercial broadcasters to keep 'little
guys' (e.g., <=50W stations) off the air due more to concerns about
competition/loss of advertisements than any technical issues.

I do wonder where he is that the AM band is so dead. That used to be the
case in many locales, but talk radio created a huge revival some 5-10 years
ago.

What will be interesting to see is whether or not some form of 'AM digital'
catches on. Although there already are digital broadcasters on FM, AM has
the somewhat unique advantage that it's almost trivial to keep compatibility
with traditional AM transmitters and receivers while adding much higher
fidelity from a digital add-on.

---Joel

 

[Joerg]

Hi Joel,

I do wonder where he is that the AM band is so dead. That used to be the
case in many locales, but talk radio created a huge revival some 5-10 years
ago.

Sometimes it's just that the receivers are dead. The art of designing a
proper AM front end seems to have been lost. The only good AM reception
I can enjoy is with very old tube radios or professional comm receivers.
The AM part of almost any 'modern' stereo I have seen is junk.

What will be interesting to see is whether or not some form of 'AM digital'
catches on. Although there already are digital broadcasters on FM, AM has
the somewhat unique advantage that it's almost trivial to keep compatibility
with traditional AM transmitters and receivers while adding much higher
fidelity from a digital add-on.

I believe digital AM is pretty much dead. Certainly if it weren't 100%
compatible. Heck, even HDTV doesn't want to catch on. Most if not all of
the lower priced TV sets we can buy here in California are analog only.
I don't know anybody who can receive the offered HDTV-on-air broadcasts
here in town. So, while there is programming the audience is lacking for
technical reasons. I don't think that was the FCC's plan.

Regards, Joerg

 

[Paul Burridge]

Mike Bates wrote:
[snip]

I have built the transmitter from the WRPDJ design, except I am using
a big modulation transformer, and an IRF510 mosfet with 19vdc driving
it. [snip]
Here is my problem. im only getting about a watt out of teh IRF510.
The mosfet is capable of putting out about 7 to 15watts.

How would I get my 1watt out to about 7 to 10 watts out?

Some or all of these may work:

1. Increase drive level to MOSFET.
2. Change the output matching network so the drain "sees" a smaller load
resistance (Power = V^2/R).
3. Increase DC supply voltage.
4. Add a linear amplifier.

And/or use a MOSFET driver in the gate circuit.

 

[Clarence]

Hi Joel,

I believe digital AM is pretty much dead. Certainly if it weren't 100%
compatible. Heck, even HDTV doesn't want to catch on. Most if not all of
the lower priced TV sets we can buy here in California are analog only.
I don't know anybody who can receive the offered HDTV-on-air broadcasts
here in town. So, while there is programming the audience is lacking for
technical reasons. I don't think that was the FCC's plan.

Regards, Joerg

Might it be the `$4,700 cost of the HDTV?

I know someone who has one. He watches about once a week. I can't see the
cost.

 

[Joerg]

Hi Clarence,

Might it be the `$4,700 cost of the HDTV?

Well, yes, at that price level you do get a digital receiver.

I know someone who has one. He watches about once a week. I can't see the
cost.

Same here. The last time I bought a TV it cost $250. Until they switch
off the analog stations that's what I'll use for the few times we watch
TV. By then I hope they offer decently priced converters so we can keep
using it.

These large screens I see in stores are nice but they don't even come
close to the contrast and/or viewing angle we get with the good old tube.

Regards, Joerg

 

[Mike Bates]

ive tried all of that.

ive tried a buffer stage in between, and i tried a mosfet driver for the
mosfet. it weakens the output, and becomes distorted.

ive also tried increasing the supply to 24 and even 30 volts. Range becomes
limited, and sound starts to phase out and distort.

i dunno.

Mike Bates wrote:
[snip]

I have built the transmitter from the WRPDJ design, except I am using
a big modulation transformer, and an IRF510 mosfet with 19vdc driving
it. [snip]
Here is my problem. im only getting about a watt out of teh IRF510.
The mosfet is capable of putting out about 7 to 15watts.

How would I get my 1watt out to about 7 to 10 watts out?

Some or all of these may work:

1. Increase drive level to MOSFET.
2. Change the output matching network so the drain "sees" a smaller load
resistance (Power = V^2/R).
3. Increase DC supply voltage.
4. Add a linear amplifier.

And/or use a MOSFET driver in the gate circuit.

 

[Joel Kolstad]

Same here. The last time I bought a TV it cost $250. Until they switch off
the analog stations that's what I'll use for the few times we watch TV. By
then I hope they offer decently priced converters so we can keep using it.

At least in a relatively good sized city where I live (Portland, Oregon),
HDTV receivers can be had for <$1000, and some are pushing the <$500 mark.
Given another couple of years I think sales will start significantly
shifting from the 'enthusiast' market to the mainstream. The fact that you
can already buy an HDTV receiver in Sears is a good indication of this.

I do expect that HDTV->NTSC converter boxes will be available by the time
the analog stations are shut down for <$200, and probably more like $99.

These large screens I see in stores are nice but they don't even come
close to the contrast and/or viewing angle we get with the good old tube.

The unfortunate situation is that most TVs are run at very, very high
brightness levels and almost always give up a tremendous amount of contrast,
linearity, and color purity to do so. This is supposedly necessary so that
the consumers will be 'impressed' with TV when it's under the bright lights
of a Best Buy showroom. Better TVs can be adjusted after purchase to get
back to a much more reasonable rendition of a standard; the guys over in the
home theater groups spend a lot of time talking about this, and there are
even folks who make their livings going out and properly adjusting displays,
room lighting, sound, etc. to give people a much better experience than what
comes 'out of the box.'

Maybe the MPAA is paying the TV manufacturers to make these adjustments so
that movie theatres still have an advantage over a 'home theatre.' On
the other hand, a lot of movie theatres these days no longer adhere to as
high of a technical standard as they once did either. Sad.

---Joel

 

[Paul Burridge]

ive tried all of that.

ive tried a buffer stage in between, and i tried a mosfet driver for the
mosfet. it weakens the output, and becomes distorted.

ive also tried increasing the supply to 24 and even 30 volts. Range becomes
limited, and sound starts to phase out and distort.

Are you quite certain that you have matched the MOSFET's input and
output circuits properly? I mean, you have ensured that your matching
networks are appropriate at the required frequency for that particular
MOSFET's admittance/impedance parameters? Can you post a link to the
circuit diagram?

 

[Joe]

Joel,

In the middle of the country, where the population is sparse, so is the
radio coverage.

When I lived in the westernmost county of Oklahoma, there were very few
stations to receive. Without an external antenna, some FM stations from
Amarillo TX weren't heard. There was a local radio station at a town 45
miles away ... a class D station whose antenna field I suspect hasn't been
inspected in a looooonnnnngggg time and needed repair. Another couple of
stations were at a town 60 miles away.

The propagation path out there wasn't the best on the conductivity scale.

 

[Mike Bates]

i have no idea what that means.

i just well, morphed it out of my head. kinda built it from scratch with
different ideas from different circuits. it works, and sounds damn good, but
power is very limited.

 

[Paul Burridge]

i have no idea what that means.

i just well, morphed it out of my head. kinda built it from scratch with
different ideas from different circuits. it works, and sounds damn good, but
power is very limited.

Well if that's how you built it, you're lucky it works at all!

 

[Mike Bates]

well, if you want a better explanation:

I took a 7 watt transmitter design that had a VFO. well, I didnt have any
parts for a VFO, so i took a CMOS ocillator in place of a VFO. and have the
harmonics supressor on the end. Thats what I did.

I cant give you a schematic, unless i physically draw it out. i dont have a
scanner to scan a drawing.

 

[Paul Burridge]

well, if you want a better explanation:

I took a 7 watt transmitter design that had a VFO. well, I didnt have any
parts for a VFO, so i took a CMOS ocillator in place of a VFO. and have the
harmonics supressor on the end. Thats what I did.

I cant give you a schematic, unless i physically draw it out. i dont have a
scanner to scan a drawing.

Well we're not goiing to get very far with it, are we? Unless it's the
one I'm maybe thinking of in the RSGB's Radcom Handbook?

 

[Frithiof Andreas Jensen]

here in town. So, while there is programming the audience is lacking for
technical reasons. I don't think that was the FCC's plan.

HDTV took so long to roll out that people got used to the Internet and those
people are now unhappy about the slow, liniear, push-format 'offered' by the
TV media; i.e. *why* should *I* adjust my schedule to when the *broadcaster*
deigns appropriate? *why* should I sit through a boooring news item?

Not to mention that most of the programmes are crap anyway: a deluge of
quizz shows, advertising and movies - that were bad even a decade ago - the
kind of swill upon which better picture and sound quality is entirely
wasted.

HDTV is dead - some sort of Internet-based distribution will be the cable
TeeVee of the future.

 

[Mike Bates]

no. its the WRPDJ schematic thats on the net right now, I just took out the
npn final, and the 386 modulator, and replaced it with a FET and a
modulation transformer, along with a seperate B+ supply.
Thats all I did.

 

[Mike Bates]

ill better explain this:

I took this circuit: http://home.att.net/~weatheradio/wprdj_schematic.gif

and replaced the final and the modulator ONLY. Thats it. From this design
here: http://www.usinternet.com/users/kyledrake/schematic/vfo-schematic.jpg

Thats all i did. I tried using the buffer stage in the above design, but it
didnt work with the CMOS ocillator. no noticble difference.

no. its the WRPDJ schematic thats on the net right now, I just took out the
npn final, and the 386 modulator, and replaced it with a FET and a
modulation transformer, along with a seperate B+ supply.
Thats all I did.



have

 

[Mike Bates]

if that doesnt help, ill try to draw one in photoshop.

ill better explain this:

I took this circuit: http://home.att.net/~weatheradio/wprdj_schematic.gif

and replaced the final and the modulator ONLY. Thats it. From this design
here: http://www.usinternet.com/users/kyledrake/schematic/vfo-schematic.jpg

Thats all i did. I tried using the buffer stage in the above design, but it
didnt work with the CMOS ocillator. no noticble difference.