What country are you in ?

The device is made in the UK and European EIRP limit should be 10mW.

We make low power transmitters at 433 and 915MHz and find we normally
get over 20m even with a resistor dummy load across the antenna terminal.

We made a transmitter in a plastic wris****ch that the oscillator
coupled directly into a loop antenna (you need to move the collector
connection along the loop till you get best match). This unit had
radiated power of around 100uW and still achieved 100m range.

Our 10mW units achieve over 1km line of sight outdoor range typically.
I cannot imagine a range of only 5-7m (unless the receiver is extremely
insensitive or the transmitter and receiver are not tuned to the same
frequency).

Dave Platt wrote:
In article ,
Carsten Hjorth wrote:
Hello!

I just bought this: http://www.rustindustries.co.uk/remote.htm
The range is only 10-15 meters in free air, only 5-7 meters indoor.
Is it possible to connect a whip type antenna?
I have calculated a length ca 17 cm.
But i cannot see where to connect the antenna.
The transmitter has a loop style antenna, powered by one transistor and a
oscillator of some sort, i have drawn a schematic:
http://media.openbloc.com/1tpwr
I have tried to connect the antenna to the collector, but that gave me no
output at all.

I looked at the design of a similar 433 MHz nonlicensed transmitter a
couple of months ago (it was for an IR-remote-to-RF repeater system).

It looked to me as if it was using the loop both as part of a tuned LC
tank circuit, and as the radiating element. I suspect that the same
is true of the circuit in your transmitter.

Adding a length of wire to it would very probably de-tune the tank
circuit. At best, it'd oscillate and transmit on the wrong frequency.
At worst, the wire loading would de-Q the tank to the point that it
wouldn't oscillate at all.

To boost the transmitter power or the antenna gain (if doing so is
even legal under your RF-use rules) would probably require a
significant redesign of the circuit.

In your situation, would it be adequate to boost the _receiver_ gain
instead? You can probably do anything you want, legally, to the
receiver... add a collinear antenna, change to a Yagi, use a corner
reflector, etc.