Reduce the range of a Scosch FMT4 FM Transmitter


Mr. Man-wai Chang

Wrap tin foil around it?

The device:

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Mr. Man-wai Chang said:

The thread here, says "no". They used foil
to actually increase the range :) Hilarious.


Generally, if you "do something stupid" with
a radio, the exact opposite of what you wanted
to happen, happens. For people with cantennas,
their output goes down. For people wrapping
stuff with foil, their output goes up.
An amateur cannot predict, without testing,
exactly what will happen. It's because what
an amateur does, is so uncontrolled. They
don't really know what they're affecting
(coupling, attenuating, leaking, wrong
impedance, reflections, you name it).

On real transmitters, fooling around can create
a high standing wave ratio (SWR). Which is not
good for the transmitter (i.e. when the transmitter
is dealing with high powers, and the power is
reflected back to the transmitter). On the self contained
device you have, they've already taken care to
match the antenna to the transmitter.


You will have to open the unit up, to see
what is possible. Simply disconnecting the
antenna, would reduce the range. The transmitter
then, is looking into a high impedance.

The antenna has a characteristic impedance.
A way to quiet down a circuit, is to not only
disconnect the antenna, but in it's place,
use a carbon composition resistor which
matches the impedance. Standard impedances
used for things like this, are 50 ohms, 75 ohms,
(or higher values for various balanced twin
conductor setups). The old TV sets for example,
used 300 ohm wiring (also used for FM radio).
The highest I've heard of for twin lead is 450 ohms
for ladder leads. Once the resistor is placed
across the two terminals of the RF section,
emissions will go way down.

A proper termination resistor, a professional one,
is surrounded by metal on the ground side (for an
unbalanced termination). And that ground would also
be joined to chassis ground. Using a resistor in the
open, with no metal around it, wouldn't be quite as
good a method. But in your case, you still want a
little bit of signal.

You could also use an attenuator, which would have
four terminals on it. Two terminals on input.
Two terminals on output. That's another way of reducing
power. The intention there, is to continue driving
an antenna, just at reduced power levels.

On a real radio transmitter, power levels are significant.
And such a resistive solution for absorbing power is
termed a "dummy load". Your unit is limited by FCC
rules for unlicensed FM transmission, so it's going to
be a relatively low power unit, and chances are the
resistor of your load is not going to fry.

The hardest part, will be locating a carbon composition
resistor. There are many kinds of resistors, and the
carbon composition has the lowest parasitics. An RF
guy at work was saying "they're flat to 1GHz", to give
some idea, when connected to a network analyzer, how
much L or C the resistor has. Which is why it makes
a good RF load. It looks like a pure resistance,
up to around 1GHz. As long as the leads on the
resistor are kept short. They also make film resistors,
and if you shop at an arbitrary shop, you're likely
to get film resistors instead. Carbon composition
is how they "used to make them".

This picture is an example of an RF attenuator for a
real transmitter. It isn't a dummy load. It's a four
terminal device. "Looking into" either end of that
thing, you see a standard 50 ohm or 75 ohm or whatever
value. Notice the heat sink fins on the outside. As
an attenuator for a powerful transmitter, the
attenuator gets warm. As less power ends up on the
antenna which is connected to one end. The impedance
looking into one end, is only correct on that one,
if the matched impedance is connected to the other end.
Like this.

50 ohm ----- Power ----- Antenna
impedance ----- Attenuator ----- Twinax

In your case, with a transmitter at the 100mW level,
just about any carbon composition resistor you can
find (likely 500mW a.k.a. half watt ones), will not
even break a sweat.

So look for balanced attenuator designs or dummy
loads for RF. Take the cover off the design,
and see what's possible. If you add lengths
of wire, they'll function as an antenna, so even if
you were crafting a dummy load, chances are the
transmitter will still be sending a signal.


You can also experiment with a Faraday cage, if you
want another idea. Use a ferrite bead around the
audio signal wire, where it comes out of the
Faraday cage.

A Faraday cage is like your "foil" idea, only
conditions are more controlled. If you look at
that picture, the walls are made from copper wire
based hardware cloth. At the corners of the
faraday cage, is copper foil flats. The hardware
cloth (i.e. screening) is soldered to the
copper foil flats. When the door closes (tight),
the copper flats touch one another, for a low
impedance connection. Since the copper flats are
very wide, it's like the contact surface on a
microwave door, when the door closes on the chassis.

The reason the foil doesn't work, is because there
are all sorts of gaps and bad connections. You can't
really solder foil, to make it gap free. And the
foil could oxidize, making performance unpredictable

Even ordinary computer equipment, uses caging
for portions of the Wifi gear. Engineers
use a "can" which fits over something
needing protection, but the "can" is soldered
down to the PCB. The idea is, when completing the
sixth side of the can, to make sure the electrical
joints don't have gaps. Soldering the gap
ensures a tight fit.

(Tin can soldered over MAC chip)

But if you just wrap foil around stuff, and end
up touching one of the conductors on the radio,
it's just as likely to end up transmitting.
If you can place an MP3 player plus the transmitter
inside a properly constructed Faraday cage, nothing
is going to come out. By opening the door on the
cage a tiny bit, you get a tiny bit of signal. Etc.

So your foil idea wasn't entirely wrong. You don't
want the cage touching metal conductors on the
MP3 player or wireless transmitter. You want
the seams to be "RF tight". They solder the copper
flats to the copper screening, for that reason.
Having a door on a Faraday cage is a compromise,
and the idea of the flats, is to give the best
solution for conduction when the door is closed.

On gear we used to make at work, we'd use beryllium
copper spring contacts, for things needing RF quality
grounding. As to meet FCC on equipment, sometimes
springs were needed so removable hardware would
remain RF tight. You can see in the following
small picture, how springs on the chassis meet
the cover, and provide an RF tight box. Beryllium
apparently keeps the contact surface "bright" and
helps protect the copper.


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