VHF Band 1 Amp Design

[rigg]

I have a tv modulator, ch0 46MHz, and I want to boost its output with a
simple one transistor amp connected to a short antenna so it will transmit
up to about 20 metres. I was thinking of using a 2N3866 in a class C
configuration with a 12v supply. Can someome give me advice on how to select
values or provide a simple circuit so I can do this? I am not sure how to
pick values when using a short antenna of 30 cm. I assume matching
impedances must come into this somehow... I have seen lots of circuits on
the web but they are for much lower frequencies. Thanks Rigg

 

[Tam/WB2TT]

"rigg" <riggdm@hotmail.com> wrote in message
news:4192c4ff$1@news.alphalink.com.au...

I have a tv modulator, ch0 46MHz, and I want to boost its output with a
simple one transistor amp connected to a short antenna so it will transmit
up to about 20 metres. I was thinking of using a 2N3866 in a class C
configuration with a 12v supply. Can someome give me advice on how to
select
values or provide a simple circuit so I can do this? I am not sure how to
pick values when using a short antenna of 30 cm. I assume matching
impedances must come into this somehow... I have seen lots of circuits on
the web but they are for much lower frequencies. Thanks Rigg

I have used the 2N3866 at 50 MHz. So, it should work just fine at 46. You

can not use a class C amplifier, it has to be linear for video. For the
extreme low power you need, just build a class A amp.

Tam

 

[Rich Grise]

On Thu, 11 Nov 2004 12:34:37 +1100, rigg wrote:

I have a tv modulator, ch0 46MHz, and I want to boost its output with a
simple one transistor amp connected to a short antenna so it will transmit
up to about 20 metres. I was thinking of using a 2N3866 in a class C
configuration with a 12v supply. Can someome give me advice on how to select
values or provide a simple circuit so I can do this? I am not sure how to
pick values when using a short antenna of 30 cm. I assume matching
impedances must come into this somehow... I have seen lots of circuits on
the web but they are for much lower frequencies. Thanks Rigg

With proper antennas on each end, what you already have should be able
to cover 20 meters distance, with no problem.

Well, except that a 1/2 wave dipole is a little over 3 meters long.

Good Luck!
Rich

 

[bin]

such questions are good questions and large signal RF amplifier design is an
involved subject. I've never met many (any?) people locally with these
skills and I have only limited ability in this area however, I suggest the
following;

1. Perform a class A dc bias design.. .ie... probably no collector
resistor... only an emitter resistor setting max collector current.
2. Choke the collector supply line with a RF choke (10uH probably will do).
( you could also use a tank here... but dampen it with a resistor for a
wideband signal)
3. Bypass any emitter resistor you have with a suitably high capacitor (1nF
will probably do)
4. determine from the spec sheets the equivalent input/output impedance of
the transistor at the supply current levels set in the bias design at the
frequency of interest.
5. design an output matching circuit to match the input impedance of the
antenna under test (eg. if using a 50ohm whip) to the transistor output
impedance via a dc blocking cap (1nF again). For a short wire, impedance may
be high and you could probably dispense with the output matching circuit by
feeding directly to the collector.
6. design an input matching circuit to match the 50 ohm driver to the
transistor input impedance via a dc blocking cap (1nF again).

(no 4 is the trickiest as not all manufacturers include that info)

bindy

"rigg" <riggdm@hotmail.com> wrote in message
news:4192c4ff$1@news.alphalink.com.au...

I have a tv modulator, ch0 46MHz, and I want to boost its output with a
simple one transistor amp connected to a short antenna so it will transmit
up to about 20 metres. I was thinking of using a 2N3866 in a class C
configuration with a 12v supply. Can someome give me advice on how to
select
values or provide a simple circuit so I can do this? I am not sure how to
pick values when using a short antenna of 30 cm. I assume matching
impedances must come into this somehow... I have seen lots of circuits on
the web but they are for much lower frequencies. Thanks Rigg

 

[Phil Allison]

"rigg"

This is sounding a little tricky for me...I don't suppose someone has some
simple circuits I can work with. Like I said I am using a rf modulator on
channel 0, Australian channel 46 MHz and want to feed a short
antenna...any help is really appreciated...Rigg

** How about you say what the application is ???

Camera mounted on a toy vehicle by any chance ??




............. Phil

 

[rigg]

A camera mounted on a robot for inspecting heating ducts under my house....



"Phil Allison" <philallison@tpg.com.au> wrote in message
news:2vifjcF2m8s4lU1@uni-berlin.de...

"rigg"
This is sounding a little tricky for me...I don't suppose someone has
some simple circuits I can work with. Like I said I am using a rf
modulator on channel 0, Australian channel 46 MHz and want to feed a
short antenna...any help is really appreciated...Rigg




** How about you say what the application is ???

Camera mounted on a toy vehicle by any chance ??




............ Phil