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Old 19th June 2006, 10:20 AM   #1
diyAudio Member
 
Join Date: May 2005
Location: Mumbai
Default FM -- Antenna and Box/PCB construction

Hello everyone..!

I have a few doubts about the antenna and box/pcb construction of a simple FM transmitter.

I have designed a stereo FM tx. with a low power common base colpitts VHF osc. followed by a single RF power amp. stage.
The o/p power of the RF amp. is around 250-300mW when it sees a load of around 200 ohms. I intend to use a quarter-wave monopole whip antenna whose Rrad is 30-35 ohms and couple it to the RF stage by simple low-hi LC matching n/w.

Here are my doubts :

a) Is there any chance of power loss due some components radiating EMW. If yes, what should I do to reduce such stray effects.

b) How should the whip ant. be kept from outside i.e in which position/angle if the ground plane would be provided by the fully enclosed metallic body in which I intend to place the transmitter.

c) How to space the components..?

d) How efficient would the monopoles radiate energy.?



Waiting for a reply.

With regards,
Prasanna Rao.
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Old 7th August 2006, 09:16 PM   #2
gideon is offline gideon  Israel
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Join Date: Oct 2005
hi xitronics,

i would be much gratefull if you could post your schematic, since i am also looking for some time to build a stereo transmitter.

i am not sure if i can be much help to your questions, but here i go:
- well i am not sure if the frequency of the fm dail is high enough to cause power loss.

- best is to keep the oscilator sheilded from the rest of the circuit, maybe by a different compartment or aluminum foil around it.

- i once built the fm transmitter from the esp site (mono one) and i found that the most important thing for stability of the signal is to make sure there is no contact between the "grounded" body of the transmitter and the antenna.

and i think best is the antenna raises over the box for the least interferences.
altough from my short experience the sound quality is most of the time surprisingly good and clear even when you use a peice of wire for the anttenna. pre-emphasis is very important.
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