the Low Power Radio blog
Simple low power AM transmitter by
Christos Z. Konstas
Since many people are looking
for low power radio transmitter plans, I’ve decided to post the plan of
the most simple radio transmitter I know. It is a little AM «bug», a wireless
microphone really, for the Medium Wave band (but it can be used for low power
broadcast on the Short Wave band to).
attractive in this circuit is its simplicity.You can’t expect much from
such a little project, but, it’s a good and especially easy start with low power
broadcasting. It won't cost more than 5-10 USD, the components are
everywhere readily available (or, they can be cannibalized from any old AM
portable radio). It can be constructed and tested in less than 2 hours.
Diag. 1 (PNP type tranzistor)
The transmitting power is
subject to the transistor type one will use and the voltage of power supply, but
it’s somewhere around 50mW, as far as I can tell.
found that my construction works best on the 800-1200 kHz range, but with
changes to the inductor L or the variable capacitor C1 it can transmit to the
whole range of the AM dial.
The range depends mainly on
antenna length and grounding. With a simple 75cm telescopic antenna (or just a
short wire) and no grounding, it’s between 20-50m. With a full L/4 single wire
antenna and good grounding the signal can be heard several hundred of metres
away on an ordinary radio - 300-500m most likely - even further with better
radios. Not much, but with that range it can reach many people in an densely
populated urban are, or as in my case it can cover a whole village. The audio
fidelity one gets from this little transmitter is
good for AM standards.I mean that.
The current from the
power supply is subject to transistor type and the voltage and is between 1-6mA,
very low indeed! Running it from a battery (3-4.5V) it will work for a long time
before the battery is exhausted.
Diag. 2 Printed Circuit Board (optional)
It can be constructed
with only 7 components. One transistor (T), two resistors (R1 & R2), three
capacitors (C1, C2 & C3) and an inductor (L). The transistor can be of any
type. In the above diagram (Diag. 1) it’s an PNP transistor but one can use an
NPN transistor (as in Diag. 3) but notice that in this case the polarity of the
power supply (battery etc.) is reversed. There is no
need for a Printed Circuit Board (PCB), though if you want you can use one (see
Diag. 2), it is always giving better result with RF projects.
R1= 100 KOhm
R2= 500 Ohm 1/2W
C1= 50-350 pF variable capacitor
turns of 0.35mm enameled wire around a 8X140mm ferrite rod with a connection
point made at the 33rd turn
C3= 150 pF
T= AF 126 (but
also AF 127, AF 139, 2G108/109 or any equivalent)
The transistor (T) can be of any type. It doesn’t even need to
be a high frequency one. I've used so far the AF 126 and AF 127 (AF 139) PNP
type both, and the NPN type AC 127. Any low power PNP type (like the AF126,
AF127, AF139, 2G108, 2G109 etc.) will do. If you use an NPN transistor like my
AC 127 (or the 2N1711, 2N708, 2N930, 2N1893, BC170 etc. ... or any equivalent)
you’ll have to be careful with the polarity of the power supply.
The AF 127 was apparently an older germanium PNP, 32 V, 10ma 75MHz unit, and the AF 126 was probably an NPN.
The AC 127 was apparently an older germanium PNP, 32 V, 500ma 75MHz unit, and the AC 126 was probably an NPN.
US Silicon models with similar characteristics would probably be the 2N3904, 2N2907, 2N2222 and their derivatives.
It might be necessary to adjust the values of R2, depending on the transistor used.
(M= It’s an optional
«charcoal» microphone (like those used in telephone sets a few years ago).
Instead, you can put there a headphone jack and feed the transmitter with audio
from an audio mixer/a stereo unit/CD/cassette player, or whatever audio source
Diag. 3 (NPN type tranzistor)
Diag. 4 (A variation of the plan with PNP
tranzistor, C2=C1 and C1=C2 in the component list)
I‘ve used a 1000
ohm resistor as R2 (instead of 500 ohm). I don’t remember why I did that but the
unit works OK as is.
Sometimes instead of the variable
capacitor of 50-350 pF as C1 I use a fixed 150pF ceramic capacitor. With that
the transmitting frequency is around 850-855 kHz but the stability suffers a
The ferrite rod I currently use is just 8cm long
(because the, very fragile, original 14X0.8cm broke to pieces, I used the bigger
fragment). I believe that a longer ferrite rod can increase the range
I initially used as T an AF 126 transistor
with a power voltage of 3-4.5V. I fed it with more voltage to increase the
range, but it finally blew after working sometime at 9V. (Tip: don’t use more
than 4,5 V as a power supply with the above PNP transistors, or you are going to
blew them. If you do want to experiment with higher voltages - up to 9V/12V -
have some spare transistors at hand and prepare to use them).
I later used the AF 127 (2G type) instead. With that, the range was a
bit reduced, and they were too sensitive to voltage increases: don’t use more
than 6V with those (but feel free to experiment with higher voltages -7.5/9V -
only don’t forget those spares!). With more than 6V this transistor is
overheated, so a heatsink can save it, for sometime, with 7.5 or 9V.
The NPN I currently use, the AC127, it’s absolutely immune to
higher voltages. In fact with less than 9V the range is very limited (just a few
metres), but with 12V or more (I use an 18V power supply at the moment, for best
My next project will be to increase the
voltage to 24V and even 35V if the transistor survives! Only, I don’t have a
spare AC127, so I won’t try anything until I get a couple. I believe that (with
a decent antenna) 24V will give me an effective range of 1 Km and even more with
I also plan to use a 2N3055 tranzistor and see
Other Possible modifications (Haven’t tried them myself except
There are a number of possible modifications that
can improve the performance of this little transmitter.
1. You can use an electrolytic capacitor 25-50 microF/25V at the input
of the power supply to eliminate any interference (50/60 Hz "hum") from the
power supply, if you are on mains (no need for that when using a
2. A matching transformer like that old
AD9014 or any equivalent, at the input of the Audio feed might improve further
the audio fidelity, though as I said above it’s already good enough as it
3. The use of a power transistor with a
heatsink as transistor T will definitely improve the range, as it will allow
4. I think that a lengthier
ferrite rod (one about 19cm long) will also improve the radiating power and,
hence, the range.
When your construction is
finished earth (ground) the unit, connect the antenna, switch on your audio
source, tune an AM radio on a clear channel between 800-1200 kHz, make sure that
the polarity of the power supply to the transmitter is the right one and then
switch on the transmitter. Adjust the C1 var. capacitor until you find the your
signal on the radio.
Performance and Use
Pic. 1 (possible construction, sorry for the quality
of the pic)
operated, with the simple «charcoal» microphone as an audio source (as in the
diagrams), and a short (50cm or so) helical wounded wire for an antenna this
little unit can be used as a «wireless microphone» with a range between 5-15m. A
75cm telescopic antenna is better if someone want to use them as AM walkie
talkies along with small portable AM radios. The construction can be put inside
an emptied plastic 4.5V torch case for such a use. The range will be somewhere
If used for low power broadcasts
instead of a microphone, connect the unit to the phone jack of a stereo unit, an
audio mixer or a PC sound card(!). I obtained best results when taking the audio
feed directly form the loudspeaker wires of my home stereo unit, but from a
«line out» or a «phone» output jacks the results are quite similar. Use a longer
antenna if you want to be heard in some distance.
Antennas and Grounding
require long antennas and good grounding to perform at their best. If you want
maximum performance use an L/4 single wire antenna, calculated to your desired
frequency (i.e. the antenna length (ANT) should be equal to the Wavelength of
the desired frequency (L) divided by 4). A 5/16 wavelength antenna might be even
better but it will be more lengthy. You can calculate the particular wave length
(L) of any given frequency (f) if you divide the light speed C in metres (300 *
10^6 m/sec) with the frequency f in Hertz. Practically 300/F in MHz will give
you the wavelength in metres.
If your chosen
frequency is 1250 kHz then, Wavelength L=300/1.250=240 metres. Thus the optimal
antenna length (ANT) in that case will be ANT=L/4=240/4=60 metres of 0.3-1mm
single strand wire, usually suspended with insulators between two buildings, or
trees (except if you live on a building 60m tall. In that (unlikely) case, just
hang down the wire out of your window, to have a true 60m vertical
Grounding must also be good for best results.
There are several simple ways to «earth» your transmitter.
One way is to connect it with a lead to a metallic water pipe or a tap.
Another is to connect it with the «ground» of an electrical socket but avoid
that option, especially, if you are unfamiliar with electrical wiring. Mains
voltage can kill. A third way is to take a wire from your transmitter’s ground
and connect it to a single or a number of metallic rods stuck in - preferably
moisturized - soil. For those perfectionists out there,
there are more elaborate (much more effective) ways to earth their TX units.
Grounding frames, grounding radials etc. A number of ground radials of L/4
length (2,4,6...16 etc.) is one such a way, but if you take the trouble to
construct such a grounding system, do yourself a favor and get a more powerful
transmitter! For more info on AM/MW antennas see
related web sites.
That's the most important aspect
in transmitters, is it not? Well, as I said, don’t
expect much from such a simple transmitter. To get the maximum out of it,
experiment with the higher voltage possible for the transistor you are using and
use an appropriate antenna for your chosen frequency.
The chart below will give you an idea from my experience («L/4 ant» is
the optimal antenna length for the chosen frequency, and «antenna» is the actual
antenna length I used. In the AF126 case the 6m antenna was of 1mm braided
insulated wire. In all other cases was of 0.3 single stranded insulated
||range (SW harmonic
||200m 3750 khz
I estimate that an AC127 with 18V at 1250-1251 kHz with a 60m
antenna will have an effective range of 500m (for reception with an ordinary
portable AM radio) or 700m (for better AM receivers).
Pic. 2 (the coil for the inductor L).
Not any particular problems
will occur with this transmitter. Attention should be paid to the careful
construction of the inductor L, to the right connection of the transistor legs
and the battery polarity. If there is "hum" in the audio, check your mains
adaptor, choose a grounded one to avoid the "hum". Sometimes, when I use a portable CD player for audio source odd things
happen. The CD disk stops or starts going backwards. To avoid such problems I
connect the CD player to an Amplifier and take the audio from there, however I
haven't been able to resolve the problem permanently. My solution some times
works an other times fails.
No annoying spurious
signals are likely to be detected in other positions of the AM dial. In my
experience even on a radio sitting just next to the operating transmitter only a
couple of a faint signals can be detected few 100s kHz above and below my chosen
frequency, and those disappear completely 2-3 metres away from the transmitter.
So it is unlikely that any neighbors trying to listen to their favorite AM
station will complain for interference from you if you choose an unoccupied
channel to transmit. Splatter is also unlikely to cause
interference to adjacent channel reception for more than a few metres from the
unit, but if someone nearby is trying to listen to Short wave, well there might
be a problem because of harmonics...
Harmonics are inevitable and
this transmitter is no exception. In fact this little goblin is a true garden for harmonics! I’ve found out that some of
the harmonics produced can be received from greater distances than those in the
basic frequency. So, if you are going to use this transmitter in a highly
populated area (especially in the night hours) use a low pass filter to filter
out the harmonics because it can cause interference to Short Wave reception. The
3rd harmonic and oddly the 9th seem to be the more powerful ones. There are also sub-harmonics that can be received on the Long
wave band. When transmitting somewhere between 1300-1400 kHz I also received my
signal on Long wave 320-340 kHz.
Odd behavior: A bonus?
the transmitting frequency to 1080 kHz I found out that the 9th harmonic at 9720
kHz on Short wave was so powerful that it could be received in distances 3 times
greater (150-300m and more!) than the MW signal in 1080 kHz (50-100m), (6 to 15m
antenna scenario). I don’t know why this happens with this harmonic but one
might consider it an additional bonus if one wants to transmit on the SW band.
In that case just calculate your antenna for the 31m band and your signal will
be heard much further than the MW! The same applies for
the Long wave sub-harmonics. Use an appropriate antenna for LW and broadcast in
the LW band if you weirdly desire so! However if you
just want to transmit on MW do filter your signal in order to avoid causing
troubles to nearby DXers who try to log on a distant LW or SW station just where
your harmonics are.
Strange things have
occurred during my experiments with this transmitter (TX). If switched on and
no audio fed to it I could turn the C1 knob and hear
in the radio, coming through my transmitter (?!), on my preset frequency other
AM radio stations with their signals amplified! It was working as a AM/RF
amplifier & tuner!? But the most weird thing
happened when, once, I switched on the audio before switching on the
transmitter. Even though the transmitter was switched off, the audio was heard
from the radio, weak but audible! Startled I checked the power source and
confirmed that it was disconnected indeed! Ghost transmissions? Well, I figured
out that the electrolytic capacitor I was using to filter out the 50Hz mains
interference was discharging slowly and was behaving as a battery! As the
current going through this transmitter could be as low as 1mA, even a charged
electrolytic capacitor could emulate a battery!!!
Summary of transmitter characteristics
||Transmissions on the LW, MW, SW bands are feasible with
||700-1600 kHz (actual)
||... or only minor modifications to the LC tank and/or
||3-4 VDC (AF126/127 case)
||tested up to 18V (with AC127, depends on tranzistor
||0.3-10mA (depends on tranzistor type)
||ranges from 10 to more than 50 mW
||good for 9kHz channels
||better in the case of 10kHz channels
||is affected by the quality of the C1 var.
||150m max with 1/10 antenna
||more than 500m with a full L/4 0.3-1mm single stranded
Perhaps that was a too
exhaustive article for such an insignificant AM transmitter. However many things
included here also apply to better and more powerful AM transmitters. And one
can gain useful «broadcasting» experience with it before moving on to higher
powers! I first saw the diagram of this little
transmitter in a, Greek, primary school circulated magazine, in 1975, when I was
10. I was immediately hooked to it and started having dreams about making my own
that little AM radio-microphone of my childhood days and decided to build it
just to have the pleasure of hearing my voice on the MW/AM band too. The results
of that project are described in the above article. I dig out that little AM
«bug», with it’s AF 126 transistor «burned». I quickly replace it with an AF 127
(and later with the present AC 127 when all my three AFs where sacrificed to
experiments), an I was «playing» with the airwaves again on the AM
Christos Z. Konstas
the Low Power Radio blog
former operator of
«FM radio Corsair» in Thessalonica, Macedonia-Greece
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