My loop prototype learnings this summer are mainly about waterproofing and reducing intermittent performance. It’s time to move on to the final build.
This summer, I have been playing around with my prototype wideband magnetic loop with the LZ1AQ amplifier. As described previously, I built a surface mount version of the LZ1AQ amplifier, and use a one meter diameter loop made from aluminum core PEX. For mounting, I use 3D printed structures. Performance is pretty good through to 15 MHz using 100′ shielded CAT7 transmission line.
So, what are my loop prototype learnings to date?
First and foremost, I must do a better job of waterproofing the amplifier box at the loop base. One of my boards suffered water damage, as shown above left. While I cleaned up the board with alcohol and a toothbrush, right, I has never performed well since. My board was mounted very close to the plastic floor of the amp box. Not sure if the problem was condensation or outright leakage.
Second is intermittent performance with the undamaged board. From time to time, the amp goes a bit crazy with reduced gain but lots of IMD. Driving me crazy as the individual parts work perfectly in the lab, but not when assembled outside.
Third, I got to wondering about voltage drop over 100′ of CAT7. At present, I am regulating 10.4 volts in the shack with an LM317 and feeding it to the amplifier. This should give me 9.7 VDC at the amplifier with a 7% voltage drop over 24 AWG pair. Perhaps the intermittent performance relates to additional voltage drop.
My fourth item in the loop prototype learnings is that connecting the CAT7 shielding to ground at the receiver end only does a great job of reducing local RFI and improving loop performance.
Loop Prototype Learnings – Moving On
I am going to redesign the LZ1AQ amplifier board with the addition of a shielded RJ45 connector and 10 volt regulator. Up until now, I have used screw terminals to attach the CAT7 cable. My changes should reduce the chances of intermittent connections and clean up any stray voltage drops by putting the regulator right on the board.
Next, the 3D printed amplifier box will be made watertight, supported by silicone caulk when assembled. The CAT7 connector will no longer be exposed to the elements.
Finally, I am switching to a passive video balun for the receiver interface, above lower. These are normally used for CCTV camera installation. I see the specifications say they should match 100 ohm twisted pair to 75 ohm coax, and are rated for DC to 6 MHz. My experiments show they work fine throughout HF.