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. 

Experimenting with a low dipole on 160m

      It seems to be well known that a low dipole is a "cloud warmer," and it appears a vertical is the way to go for low band DXing, especially on 160m. But I had a hard time finding much for actual results that people achieved with low dipoles. I'd read about people trying them, and a couple being surprised how far they got on 160, thinking you'd be lucky to get 200 miles... I felt the same from what I had read about the importance of height. So I'm sharing my experience to add another reference/data point for others. 

I strung up a full size dipole for 160m with cheap 18 awg wire and a BNC to binding post connector. I only had a short 20ft RG-316 feedline, so I had to bend 1 leg of the dipole to fit in the property. One leg was about 124 feet straight, the other was mostly straight for about 50 feet, and bent about 85 degrees for the last 70+ feet. I put it up with the ends and the center about 13 feet off the ground...plenty of sag on the longer run, probably 2 ft+.  See EZNEC diagram below:

I was doing this to try out my newly built QRP-Labs QDX-M for 160m, and had it feeding Hardrock 50 amplifier. Was getting about 47w out with about 1.4w in.  

I worked 160m FT8 for 2 nights while staying at my cabin QTH where I did this experiment. Not all night by any means, but from near sundown until I went to bed, with breaks here and there, and then for an hour or so when I woke up before sunrise.

Results over those 2 nights: 161 completed contacts in 97 grids; 38 states worked as well as 4 Canadian provinces. Longest contacts were over 1700 miles to British Colombia, California and Oregon. Shortest was about 50 miles.

QSOMap of contacts:

PSKReporter shot of stations that heard me on night 1:

There was one station in Israel both nights that heard me as well, but that was the only DX station reporting me... and I heard one station in Mexico, and a couple in Spain...otherwise everything was in the US and lower Canada.

While I confirmed what we already collectively knew: a low dipole won't work on 160 for DX...it worked better than I expected on stateside FT8. On the other hand, if this were SSB, I think I would've made about 9 contacts instead of 161... Only 8 in the log had +8db reports or more in EACH direction.

So there you go... a full length, but bent dipole 0.024 wavelengths off the ground, combined with an amplifier and FT8 did OK.... Something I may keep in mind for a bit of an "extreme" POTA activation in the future. More importantly, it confirmed my QDX-M worked, and worked well. 

Some other notes: When I moved from the dipole back to my 40m Horizontal loop at 34 feet up... noise and signal levels on 160m dropped by about 12 db. And AM broadcast stations were overloading the front end on my KiwiSDR at night when fed by the large dipole...I barely see this on the 40m loop. Just interesting to me to note how much stronger receive is on lower frequencies with that longer wire!