Steve Quest's Transmitter
In various newsgroups Steve Quest has described an AM transmitter for mediumwave
and shortwave that has produced great results and is quite inexpensive. Here is the
verbal description he offered:
Author: Steve Quest
Date: 16 Jul 1998 13:57:24 EDT
> For God's sake, publish the schematic of your AM transmitter before you
> depart. People are starving for amplitude modulation out here...
I need to get around to that. However, for now, just picture a
standard Pierce oscillator, crystal controlled on your frequency of
choice, with the output connected to a standard class C amplifier stage.
The only modification to the standard class C amp stage is the insertion
(series, so cut the trace and insert) of a modulation transformer between
the power supply line and the isolation inductor on the collector. What
works great is the $1.95 at my local audio shop, SPECO line matching
transformer. Connect the transformer backwards! In other words, the
input side goes to the class C amp, and the output side, normally the side
connected to the speaker becomes input. :) Transformers work both ways
you know. ;) Use the 500 ohm and common taps on the class C amp side,
and the 8 ohm and common tap on the input side.
Now hook up a power audio amplifier to the (now) input of the transformer,
and a mic or other audio source to the line input of the power audio amp.
Adjusting the volume on the audio power amp will increase and decrease
your MODULATION! Use a scope to set your modulation, just lay the probe
near the antenna to see the AM waveform. Adjust to about 80% modulation,
don't overmodulate, it sounds like crap AND is quite inefficient.
*IMPORTANT* Use a chebichev filter between the output of the class C and
your antenna! If you don't your harmonics will be over a watt for sure!
This configuration, when powered by 12 volts will generally give you an
input power of about 5 watts. That's based upon the class C amp giving
you a times 10 gain, and the input power from the Pierce oscillator of
half a watt. Sloppy design of the Pierce, and poor tuning on the class C
will cause the power to go DOWN. However, if you're good, you can get
10 watts or more by tweaking the Pierce up to a 1 watt output into the
x10 class C stage.
About a 4 to 5 watt input signal to a 150 foot random-wire antenna on
the 41 meter band during the average sunny winter day resulted in a
clear signal copied at greater than 200 miles and a weak signal copied
at 1000 miles. This range example is reproduceable by several
successful tests.
Now there it is, it might have been easier just to draw it. :)
In another message, he gave a few more details of his circuit:
... since the rig operates in the 41 meter band, I just used
generic CB radio transistors. The Pierce oscillator uses a generic CB
radio driver transistor, and the final is a CB radio final. These
transistors are cheap, readily available, and probably will never be
outlawed (like I suspect VHF RF components will eventually be). Since CB
radio is 27 Mhz, running them at 41 meters (7 Mhz) is very easy on them.
Any type NPN, TO-220 package CB radio driver and final will work
for this application. There are probably _better_ transistors out there,
but these are cheap, available at the local radio shop, and already in
stock for those of us who fix a few CB radios from time to time. :)
And now a schemaic is available! Thanks, Steve. Here it is:
Parts List:
R1 = 47k 1/4w
C1 = .001uF
C2 = 470pF
C3 = 470pF
C4 = 100pF
C5 = .01uF
C6 = 470pF
C7 = 820pF
Y1 = 7.3728 Mhz series type (off the shelf, CPU application)
Q1 = ECG or NTE282 or similar
Q2 = ECG or NTE235 or similar
T1 = Speco type T7010 70 volt line matching transformer
L1 = 28turns around 1/4" ferrite overwrapped with 9turn secondary
L2 = 28turns around 1/4" ferrite
L3 = 1.5uH choke
To make L1, tightly wrap 28 turns, then wrap 9 turns over that to
make the secondary. Space the windings out to evenly cover the
full length of the 28 turns. Use light gauge enamel wire for all.
To connect the modulation transformer, wire backwards using the
normal secondary as the primary. Connect the red wire of the
input side to Vcc and the black wire to the class C amp choke.
Connect the output side white wire to the amplifier speaker jack
positive, and the black wire to the speaker jack negative. Use
the amplifier volume control to adjust modulation level.
And here's some additional detail:
Date: Wed, 9 Dec 1998 14:01:59 -0500 (EST)
> Say Steve Q., I have gotten most all the parts gathered up for the
> transmitter, but am still having trouble finding 1/4" dia. ferrite.
> Went to jap shack and all they had was stuff that was to skinny
> and to short. (assorted chokes that I could of torn apart) and all
> my searches have turned up nothing suitable for the perimeters set
> in the destruction.
I get my forms from a distributor, and my inductors as well.
However, I know there are ferrites that are "close enough" in the Radio
Shack assortments. :) You should be able to find one fairly close.
> The question this time is could one substitute say 5/16" core for L1
> & L2 and adjust the windings to come out with the same results. If the
> answer is yes, then could you recommend a good starting point to make
> the alterations?
Sure, use a 5/16" core, and the same number of turns.
Remember, I didn't specify the wire gauge, and I should have said
"about 1/4" ferrite rod" instead of being exact. Use a heavy enough
enameled wire that you don't vaporize it with current flow, I used the
middle gauge out of the Radio Shack 3 pack for my windings. This fact
should probably be added at the website. :) You may have to adjust
secondary turns between the oscillator/driver and the final to tune for
maximum power. Don't glue the windings and leave plenty of extra (not
looped but curved up) so you can do that. Once you find the right
amount, glue them down. I use wax usually, but have also used shellac,
varnish, silicone, you name it. :)
On the inductor in the final, an extra few turns will not hurt, but
don't take away! Same goes for the primary on the osc/driver. However
if you add turns, you'll have to adjust the secondary turns and
capacitors. One thing to remember with HF verses VHF, the windings
are NOT as critical. 3 turns in an air wound VHF oscillator tank coil
will get you, let's say 88 Mhz, and one more turn and you're down to
64 Mhz, and another and you are in the 40's. It isn't like that with HF,
it's not so touchy. :)