80m 10 channel Transmitter

Please note: The ICS525-01 used in this transmitter is no longer manufactured.
and may become increasing difficult to source.

Photo above shows the transmitter enclosed in a Sistema 200ml food box for a good degree of weather protection.

This 80m ARDF transmitter was designed to be multi purpose, suitable for Classic (1W minimum) , Sprint (300mW-1W) and FoxOring formats.
Compact and lightweight and easy to deploy.
Low component count.
Multipurpose socket for charging/syncro/tune-up.
Internal ATU for matching to a short wire antenna.
The battery pack is mounted below the pcb.

CIRCUIT DESCRIPTION

The ICS frequency configuration was derived from a design by SM0KON. This allows for a selection of a frequencies between 3510 Khz and 3610Khz at 10kHz steps by setting the high/low status of just 7 configuration pins on the IC. The other 12 configuration pins are fixed in the PCB layout. The 7 pins are set high or low by the PIC IC, reading a 10way rotary BCD channel selector.

POWER OUTPUT CIRCUIT

A pi network provides a 50 ohm match at the TP1 test point. Then a L network to match a long wire (see text below). C15 and L2 provide a trap to reduce 2nd harmonic radiation.

The output matching was optimize by trial and error. The filtering is minimal, but in the context of ARDF deployment, is considered acceptable. If this design is used or modified for any other purpose the user should be satisfied the filtering is sufficient to suppress harmonic or spurious emissions.

The ATU section of CT2 and L3 is the L-network designed to match a 6m long wire with a 6m counterpoise.
A set of pre-cut antenna wire is required for the ARDF event, so the user should decide the preferred length of the wire antenna and modify the turns on the toroid (L3) to suit. 8m wire length is a popular traditional choice, but the author finds 5 or 6m lengths work well enough and are easier to deploy. Of course, other (usually shorter lengths) can still be connected and tuned but the ATU may not be optimal. The counterpoise socket is mounted on the aluminium ground stake. The author has found multiple shorter lengths of counterpoise wire more practical and just as effective, than a single length. eg 3 x 2m.

CHARGING / SYNCING SOCKET

The charging socket has three functions.

1. Charge the battery.
2. Synchronize the Txs.
3. Key the transmitter (when idle). You can use the same cable for both charging and Synchronizing. Obtain a 6 way, daisy chain, 2.1mm power cable. These are readily and cheaply available on eBay (for guitar amplifiers!). Synchronizing requires an external shorting switch connected to an inline power plug.

Schematic

(Open image in new tab to view in full resolution

Parts List

Resistors and capacitors are generally SMD 1206 format, but 0804 format parts should just bridge the pads.

Battery Box

Made from two 3xAAA battery boxes with PCB pins. The pins are bent to mount the battery about 3mm above the underside of the pcb and between the pcb mounting pillars.