I believe the main function of the shielding in this design is to improve isolation between the signal source and the SA612 receivers (so, to W5DXP¡¯s question, I doubt there¡¯s much improvement at HF, but this would need to be verified).
Other comments...
Mike you can make the dual shields and install them fairly easily. Make one small rectangle to cover one of the sections, and then make a similar shield, but with one of the long sides bent up, that will cover the other section.
Solder the first shield to the board per qrp rx¡¯s Soldering recommendations, then place the second shield on the board, solder it¡¯s 3 sides that touch the board to the board, and then solder the lifted (fourth) side to the top of the first shield.
It¡¯s worthwhile investigating qrp rx¡¯s suggestions. I will note:
1. Capacitive coupling to ground via the shield might be an issue, but a big question I have is how much difference in noise you are seeing. For example, if you were to short either c26 or c27 to ground, I would expect the noise floor to increase by 6 dB ( or is it 12 dB?) after recalibration. Attenuation due to capacitive coupling to ground will be less than this. So if you are seeing a large increase in noise, I would doubt that it is capacitive coupling. But do not rule it out as the cause without first verifying.
2. Similarly, I am hesitant to say that the shield itself, because only three corners are tacked to ground, is somehow injecting noise. The many ground visa should already be providing a low impedance ground path. Could there be a ¡°slot antenna¡± effect due to the gap that is somehow injecting interference? Possibly, but from experience with my own designs and testing for EN61000 (and FCC) radiated and susceptibility compliance, I haven¡¯t come across a poorly installed shield being worse than no shield. Still, do not remove it from your list of suspects until you have verified that it is NOT the problem ¡ª while I haven¡¯t seen this as a problem, myself, QRP RX might have come across it in their designs.
3. Also, before each test, make sure you verify that the calibration is correct ( I have had some strange calibration effects). Verify that for S11 the SOL loads appear on the Smith Chart where you expect them, and verify that S21 looks correct with and without the thru jumper. (In other words, don¡¯t rule out operator error!)
4. Finally, just a note that, although you or I might think the designs are identical, they need not be. Parts have tolerances (and might even be counterfeit). There could be manufacturing errors, etc.
- Jeff, k6jca
