antenna impedance matching on small TX
 

I have been designing a small transmitter on 400MHz. The output from the rf
amp is a parallel LC tank and between the collecter and +V. When measuring
the output on a specturm analyser i can get most output when centre tapped
on the inductor. The output must then be matched to 50ohm as the spectrum
analysers input is 50ohm. The anatenna for this transmitter is just going to
be a 1/4 wave piece of wire. My question is that as i dont need to send the
signal down any 50ohm cable could i just connect the antenna at the
collector rather than tapping onto the inductor and then adjust the length
of the antenna to increase its inpedance to match the output inpedance of
the transmitter.

antenna impedance matching on small TX
 

Marcus Tait wrote:
My question is that as i dont need to send the
signal down any 50ohm cable could i just connect the antenna at the
collector rather than tapping onto the inductor and then adjust the length
of the antenna to increase its inpedance to match the output inpedance of
the transmitter.

Where is your ground plane?
--
73, Cecil http://www.w5dxp.com

antenna impedance matching on small TX
 

On Wed, 5 Mar 2008 21:19:50 -0000, "Marcus Tait"
wrote:

I have been designing a small transmitter on 400MHz. The output from the rf
amp is a parallel LC tank and between the collecter and +V. When measuring
the output on a specturm analyser i can get most output when centre tapped
on the inductor. The output must then be matched to 50ohm as the spectrum
analysers input is 50ohm. The anatenna for this transmitter is just going to
be a 1/4 wave piece of wire. My question is that as i dont need to send the
signal down any 50ohm cable could i just connect the antenna at the
collector rather than tapping onto the inductor and then adjust the length
of the antenna to increase its inpedance to match the output inpedance of
the transmitter.

My experience based opinion based on Amateur TV work from many years
back says that a half wave dipole with a link coupling (not critical)
to that inductor will spare you the agony of trying to find a ground
to serve the bottom end of that 1/4 wave wire.

Don't overlook the old practice of using a micro ammeter (or modern
equivalent) with a germanium diode across an identical dipole as a
field strength meter. Best keep the diode shorted when not in use to
protect the meter!

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

antenna impedance matching on small TX
 

"Marcus Tait" wrote in message
...
I have been designing a small transmitter on 400MHz. The output from the rf
amp is a parallel LC tank and between the collecter and +V. When measuring
the output on a specturm analyser i can get most output when centre tapped
on the inductor. The output must then be matched to 50ohm as the spectrum
analysers input is 50ohm. The anatenna for this transmitter is just going
to be a 1/4 wave piece of wire. My question is that as i dont need to send
the signal down any 50ohm cable could i just connect the antenna at the
collector rather than tapping onto the inductor and then adjust the length
of the antenna to increase its inpedance to match the output inpedance of
the transmitter.

I would take a look at how remote garage door openers are configured. They
operate in roughly your frequency range. Mine has a range of about 100 feet.
A 1/4 wave antenna has an input impedance of 37 Ohms, but I suspect you will
not want to do that.

Tam

Tam