Portable 23cms ATV.


For some years now I have been planning how to make a 23cms ATV station small enough to pack away in a small case, that would be readily available for local contacts, say on a day out or on holiday. This summer (1998?) I have been making my first prototype for the receiver part. My old version of the Forrester (G4WIM) TX is probably going to be a useful companion for it for the time being.

I have divided this page into sections. Click on the one you want:

Receiver
preamp
addition
Transmitter
Integration
& Improvements

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updated 23/10/01
RX Part.

To keep things simple (otherwise they don't get built!) I decided to base the receiver on an existing satellite tuner. The smallest one, that I knew about at the time, was the Tatung tuner that Bob Platts uses in his Dove receiver, and is known to work ok. The rest of the circuits are conventional and realised in surface mount components where possible. Exceptions are things like large value electrolytics, too expensive for their space savings, integrated circuits and tunable coils.

It is housed in a standard diecast box, 90 x 110mm (3½ x 4¼ ins). Below are pictures to prove it really exists!

Receiver

Complete receiver with colour viewfinder monitor.

Sockets on right hand side:
2x phono = A & V. 3.5mm Jack = ext LS. SMB = prescaler o/p

Sockets on left hand side:
sub min 3-pin = Viewfinder(5v & V). 2.5mm = 12v in. 3.5 Jack = 'S' meter. F = Antenna.

Front Panel:
10T dial = Tune. LED = On/Off. Switch = tune range. Knob = On/Off + vol.

The mess inside. A received signal.
Inside Off screen.
This shows the type of construction and components used. The Hastings repeater as seen on my 14ins monitor, plugged into the mini RX, during good ducting conditions.

Here's a block diagram of the receiver, drawn in MS Word:
Block diagram

EF = emitter follower


Addition of a Preamp.
(13/8/08)

Inside

The most needed addition has been a low noise preamp. Thanks to John, M1BAI, I now have a smaller satellite tuner made by Sharp which gives space for a preamp within the receiver case. It also has more gain & better signal to noise performance than the previous one.

I have made a preamp on a pcb, based on my previous design but modified to exactly fit the space available and with the SMA antenna input in the same slot in the case as the previous tuner's 'F' socket.

In the photo on the Right:

Antenna input is at the top left, feeding the first GaAs FET via a crude matching circuit. The three tuned lines are between the first and second stages. These two stages did not provide enough gain so I had to squeeze in a MIMIC to give a few more dBs; luckily it is very small and only required one more capacitor and one resistor!

GB3IV has now improved from P3 to P5.

Close up of preamp.


Another improvement I hope to make is to the 'S' meter circuit as it is useless the way it is at the moment!

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This part started on 13/05/03.
TX Part, Based on The G4WIM Design and in the same sized diecast box as the RX described above.
"24 cm FM Amateur Television Transmitter" by Tim Forrester G4WIM, Elektor Electronics February 1994
Also Described in CQ-TV 165 pages 9-19.

The controls On the Left hand side of the box:
Synthesiser frequency control input socket with simple 2-frequency switch plugged in
"Out of lock" LED
Variable control of RF power output.

On the front panel:
"Mute" switch
Phono AF input
DIN socket for combined AF & Video from camera
BNC Video input

Inside.

The wires I added between the synth IC and the Cannon 'D' socket for frequency control, are clearly visible at the lower centre right. Note the copper foil under the PA module to improve the thermal contact to the diecast box as well as better RF grounding to the PCB.

The out-of lock red LED is on the two white wires.

Inside

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Integration of the Parts.

For possible antennas look at my Skeleton Slot or a version of 'Flat Plates' as made by locals.

The 'rest' is yet to be thought out, done and then tested!

I thought it would be nice to be able to assemble it into a hand portable station with its own small 12v rechargeable dry battery. It would also be useful to rearrange the RX & TX boxes with a higher power PA to run from an external power source, monitor, antenna and so on, for static /P or /M operation.


Future additions or modifications.

TX
Add a manual frequency setting circuit, such as thumbwheel switches, EPROM or better still PIC controlled!
Add a mike preamp or VOGAD in the space behind the power control pot.

RX
A more usable 'S' meter circuit
Update resistor values for tune range with this new sat tuner
and a DC feed/isolator for an LNB.

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