Building a QRP-Labs QDX-M for 160m

Just wanted to document what I did for a QDX-M build for 160m. I haven't seen or heard of anyone else operating a QRP-Labs transceiver on 160m yet outside of WSPR transmissions. 

      I had come up with LPF values and BPF values to try for a 160m version, but then Hans came out with the QMX+.  I already had the QDX-M unit, and the QMX firmware isn't ready for easy completely remote digital operation yet like the QDX, so I built the QDX-M for 160, and decided to NOT reinvent the wheel, but instead use the QMX+ filter values (mostly.) 

BPF: Used QMX+ values....680 pf capacitor, and 44 turns of 28 awg on a T50-2, but then removed the 680pf and installed 1200pf to bring the peak down after initial testing.

LPF: Used QMX+ capacitor values. Inductors I used T50-2 (Instead of the T37-6 on the QMX+) and reduced the turns to 30 and 27 to try and match the inductance. 

Power output @ 12V: 3.55w. Tweaking the LPF inductors didn't really affect this at all. 

BPF values: -0.5 db from peak (now)

Image rejection: -41db

 

When I initially built it, the RF Sweep looked like this: 

 

So doing some math, I removed the 680pf capacitor for the BPF, and installed a 1200pf instead. That brought the peak down close enough for me....down ~1db from the peak instead of 5db:

 

Image sweep was good:

 

Power output was much less than I anticipated based on people's QMX+ specs, getting only about 3.5w @ 12v. Thinking this is most likely due to some flaw in how I wound the RWTST, but could be something else, too. Either way, it's nearly fine as it is, as I'm using it to feed a Hardrock 50 amplifier. I ran it at 7.5V (wound for 12v) and got about 46w out. When I increased it to 9v, I had just under 2w and got 52w or so output from the amplifier.

 

 

Specs/Mods:

• Rev 5 PCB, 1.10 Firmware. 
• 28 awg wire on everything. MUCH easier on T1 and L14 that way!
• 12V RWTST using BN43 core. (Not 61 mix like the QMX+, just what it came with.)
• TN0110 finals instead of BS170s, and Wakefield heatsink.
• Amphenol USB Connector (Jeff Moore's favorite.) 
• 1N4148 diode across L14.
• PTT resistor swap. 470 ohms was too much to trigger the PTT on my MFJ1708SDR. I removed it and installed a 100 ohm 0603 resistor.
• BPF: Used QMX+ values....680 pf capacitor, and 44 turns of 28 awg on a T50-2, but then removed the 680pf and installed 1200pf to bring the peak down after initial testing.
• LPF: Used QMX+ capacitors. Inductors I used T50-2 (Instead of the T37-6 on the QMX+) and reduced the turns to 30 and 27 to try and match the inductance. 

• Power output @ 12V: 3.55w
• BPF values: -0.5 db from peak now
• Image rejection: 41db

 

Performance: I strung up a temporary full length dipole at 13 ft up at my cabin QTH, and ran FT8 for 2 nights, and a couple WSPR transmissions. The VERY low dipole did better than I thought, netting me 161 contacts in 38 states and 4 Canadian provinces with the 47 watts or so. I'll take it! Happy enough with it. Down the road, I'll install a permanent 160 antenna at that QTH, probably something vertical for DX possibilities.  (I wrote about the dipole and qso specs here: 
https://n8btr.blogspot.com/2024/12/experimenting-with-low-dipole-on-160m.html  )

 

So there you have it, a QDX-M for 160m that seems to work well. Again, power output could be better, but thinking that's a consequence of one of my actions or component choices. Receive sensitivity seemed to be very good, didn't see any better on my KiwiSDR connected to the same antenna.