Simple DC 80m RX

A SIMPLE 80m DC RECEIVER FOR ARDF

This receiver has a VXO using a ceramic resonator. The VXO is very stable, and the frequency can be pulled down to 3.51MHz using a variable capacitor. It can therefore cover the whole frequency band used in international ARDF events - 3.51MHz to 3.60MHz.
However - Important - Ceramic resistors are very variable in their suitable for a VXO. Please see my post on VXOs before attempting this project.

Photo above
The RX on the left uses a ferrite rod antenna and is tuned with a polyvaricon capacitor. The RX on the right uses a loop antenna and is tuned with a varicap capacitor.

The polyvaricon makes it easier to get a wide frequency by from pulling the ceramic resonator, but the tuning band-spread is over 180 degrees.

Varicap diodes Compared to a air tuned capacitor, they give a limited capacitor swing. A reverse voltage of 1v-10v is typically required to provide the full capacitance variation. In this reciever a modern, but extremely small, BBY66 is used which only requires a maximum of 5v for the full capacitance swing.
Together with careful selection of a series inductor the full ARDF band of 3.51MHz-3.60MHz can be covered but, again, please refer my post on VXOs.

Schematic. Open in new window to view in high resolution

The RF amplifier has more than enough gain to lift a weak signal the above floor noise experienced on 80m. Too much gain and noise will not help when detecting the null signal direction and reduces the useful range of the gain control ie. the higher end of the gain control becomes unusable.
The maximum front end gain is partly limited by the toroid transformer L1. A 10:10 ratio is recommended, which will give adequate gain. If for some reason more RF gain is required a 56pf capacitor can be added to the primary winding of L1 or the windings increased to 16:16.

Photo This shows a RX with a loop antenna mounted in an enclosure, initially prepared for a rod antenna. If designed for a loop from the outset, the entry holes are located slightly nearer the top of the box giving more clearance from the PCB.

The audio amplifier uses a TL431 rather than the more generic LM386 to save space in the relatively small enclosure which also has to house the antenna.

The latest version (ROX80-VT3) uses through hole components. It is straight forward to build apart from two aspects, which are both detailed on separate posts.

1. Adjusting the vxo oscillator to give the required band-spread.
2. Building the loop or ferrite rod antenna.

If PCBs are available for purchase, they will be listed on EBAY