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Construction notes:

I built the indicator in a plastic pencil box made of red transparent plastic. Since the plastic tends to pass wavelengths predominantly in the red part of the spectrum the box serves as a contrast enhancing filter, plus I avoided having to create holes and a bezel for the display.

No printed circuit board is needed, though a printed wiring board with a ground plane around the bias circuit and detector might give more sensitivity (if there is a need for more sensitivity) if it was carefully laid out. It is a good idea to have a single ground point that connects the RF and digital worlds, and have this ground point at the decoupling capacitor. Its also a good idea to position the RF detector as far away from the digital circuit as possible since this would affect sensitivity.

I grounded the crystal can by soldering a piece of wire to it and connecting the wire to the A-to-D capacitor's ground, This minimizes the signal radiated by the crystal and may help sensitivity. I'm not sure if this really made a difference, but it was easy enough to take this precaution.

The circuit is very sensitive to deferential heating between Q1 and Q2 giving up to four counts offset per degree C difference. I've noticed these thermal gradients being introduced through IR radiation such as from a light bulb (I noticed that it saw my soldering iron many cm away) and through air currents around the transistors. Since the meter only runs for a few minutes after zeroing, its only these short-term drifts sources that need to be addressed. Two countermeasures for are to put the circuit inside an enclosure and to mount Q1 and Q2 close to one-another and glue them together with a drop of Duco cement or similar adhesive.

You might have to adjust the offset in the RF detector's bias supply to get about 600 mv on the collector of Q1 so the detector output is always within the A-to-D converter's range, which is about 600 mv to ground. Bias level can be adjusted by changing the voltage divider formed by the two 39K resistors on the base of Q1.

The 220 ohm resistors used to drive the LEDs maybe changed to provide suitable levels of luminance for the particular display you choose to use. This circuit uses a common-anode display to that the controller‚ so the controller's output pins shunt the current across the LEDs. If you find that one or more segments get dim but don't go out all the way when they should, check the socket contacts since the dim segment may the caused by high resistance in series with the drive signal.

The antenna should be coupled to the RF detector through a small capacitor. I used a gimmick made by twisting two pieces of #26 insulated wire several times. This gives about 5 pf and seems to couple RF in the rage of a few MHz up pretty well. Coupling though the gimmick reduces the sensitivity to very low frequencies such as 50 and 60 Hz from the power lines and their harmonics.

The time constant of the RF detector is determined by R6 and C2. Increasing C2 will increase the time constant without affecting sensitivity.

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