I don’t recall ever operating in the New England QSO Party before, but several emails were circulating encouraging activity, so I thought I’d give it a go. Conditions were OK on the lower frequency bands — not a lot of noise. 15-meters even opened up for a brief time Sunday afternoon on a North/South path.
Back in March of 2013, I installed a Diamond K9000 Motorized Antenna Mount. Unfortunately it started acting up about five months ago after about 3.5 years of service mounted on a truck that leaves its garage about twice a week, and an antenna that has been raised/lowered less than 100 times. Needless to say I wasn’t happy.
I was even less happy when I opened the failed unit up and found that it obviously been filled with water at one point as many parts were heavily rusted, despite weep-holes intended to allow the water to drain. Another issue was discovering a cheaply made limit switch that turns the motor off when in the stowed position. This was very odd considering that a real snap switch was used for the other limit switch (a SPDT used in the raised position).
The unit was purchased new in early 2013. The final surprise was discovering that it had been fabricated in 2008 — meaning it had hung around in inventory for 5 years! Given the lack of quality of the broken limit switch, even having it sit around probably caused the contact to oxidize.
Given the cold weather I waited until spring to swap it out with a new one. This one appears to be new 2016 inventory. Hopefully it will last longer than the first one.
Poorly fabricated Limit Switch (Lowered Limit)
Visible Rust in shell (where bottom of motor frame is attached)
Metal Frame of Motor Bracket rusted (likely that water was in raised limit switch too)
My club, Newport County Radio Club, just completed our third and final meeting on the 40-meter QRP Pixie kit. We packaged the inexpensive Chinese QRP kit with sockets for the crystal and final transistor, two additional crystals (7.030 and 7.040) to go along with the stock Extra Class-only 7.023, and a 2N2222 to be used in place of the stock final.
We did this as a club build project, setting up multiple assembly and test/rework stations. In the end, about two dozen Pixies were built. All but four of them worked right out of the box. Two had cold solder joints. One had a solder bridge. The only “mystery” turned out to be a LM386 that was on the high-side of its maximum current spec, which caused its power source to collapse. Everyone managed to get the right components into the right locations and there were no missing parts. All of the kits produced from 300 to 500 milliwatts output power.
Another question was how well did the receiver perform. My expectations were quite low as the entire receiver circuit consists of a fairly inadequate LPF (see multiple posts about a Pixie LPF that actually meets FCC requirements), followed by a gimmick mixer (the final transistor), and a LM386 audio amplifier run wide-open gain. After testing a bunch of Pixies, a pretty reasonable performance specification would be:
- RX MDS -76 dBm (25 microvolts)
- -73 dBm is full copy but weak (this is S9)
- -33 dBm is ear splitting (S9 + 40 dBm)
- Selectivity is in excess of 10 KHz
Those are pretty bad specs, but what would one expect for a few dollars? The RX does work and many of our kit builders have used their radios on-air with great success.
February and March were pretty good for me at my current snail’s pace, adding three new countries for a total of 261, and bumping the band-point count up to 1,237.
So even near the bottom on Cycle 24, there’s still DX to be had, but 40 meters is now yielding better results, and there is very slim pickings on 10/12/15 anymore. So now I have to be content with working a station on one or two bands rather than six or eight.
First off, to be clear, I’m talking about a Diplexer, the small device that will fit in your pocket and costs < $60, that is used to combine 2-meter and 70-cm rigs to use a single antenna feed-line. However both Comet and Diamond call these things Duplexers which are usually large multi-cavity devices that cost > $1000 that certainly won’t fit in your pocket.
Some discussion on the AMSAT forum recently has centered around eliminating those pesky 3rd harmonics from inexpensive HTs that don’t meet FCC specs. Folks were complaining of having 70-cm receive desense when transmitting on 2-meters.
A suggestion was made to use a Diplexer, since it is not much more than a LPF on the low-frequency port and a HPF on the high-frequency port. I was curious if they met their performance specs and if it made a difference if the unused port was terminated or not. The executive summary is Yes, and Yes (although termination isn’t a huge deal).
I used my recently calibrated Siglent SSA3021X to test a Comet CF-4160J “Duplexer”. I normalized my SA and the interconnection cables I would be using, then hooked a Comet DF-4160 “Duplexer” into the path with the High Frequency Port terminated as seen in the photo below.
When I swept the Low Frequency Port side, the response was pretty much as expected from Comet’s specifications. There was about 0.12 dB of insertion loss and the 70-cm response was down -55 dB.
I disconnected the 50 Ohm termination and observed a rather dramatic change in the response as seen below. Note the rather pronounced dip around 300 MHz. The loss increased slightly to -0.13 dB, and the rejection was reduced by 3 dB to -52. So the advice would be to terminate the High Frequency Port.
Just for grins I swapped the Common Port and Low Frequency Port and found the results to be essentially identical to my first test, so which way you hook it up doesn’t matter, as you might expect.
In our continuing quest to evaluate the output purity of various QRP kits, Willy W1LY constructed a Bill KA1QYP Half Pint Kit.
The assembled kit produced a solid 600 mW output and was one of the cleanest radios we’ve seen. The worst case was the 3rd harmonic which was -59 dBc.
The Half Pint uses a Cauer Filter, similar to a 5-pole LPF but two of the sections are tuned to suppress the 2nd and 3rd harmonic frequencies.
The Elsie Predicted response is shown below. Note the pronounced dips at the harmonic frequencies. Loss at 7.040 MHz is about 0.9 dB, and the 3 dB knee is about 8.1 MHz:
The measured performance is shown below. The 2nd harmonic is down about 63 dBc, and the 3rd harmonic is down about 59 dBc.
Willy, W1LY, decided to try using the standard unshielded 1 uH axial inductor that the Pixie uses for it’s current 3-pole filter (C5=47o pf, L2=1 uH, C6=470 pF) as the starting point for a 5-pole filter that could easily fit on the existing board. Two inductors are soldered in place of L2, standing up vertically and soldered together at the top. A cap is then soldered from that bridge down to the grounded side of C6 (Willy paralleled two caps to get the required value). See the area indicated below.
Component values are:
- C5 changed to 820 pF
- C6 changed to 820 pF
- New Cap (bridge of L2A/L2B to ground at left side of C6) 1150 pF (680+470)
- L2A (1 uH)
- L2B (1 uH)
Measuring the output of the modified Pixie indicates a passing value! The 2nd harmonic, worst of the lot, was -43 dBc. Power output was 0.3 watts using a P2n2222 for Q2.
We suspect that original 5-pole Elsie values of 1.2/1.2 uH and 910/1300/910 pF would produce even better 2nd harmonic suppression and work just fine at the lower end of 40 meters.
This is the schematic of the 7-pole LPF used in the QRP Labs 40-meter LPF:
The plot from Elsie 2.77 for the above filter looks like this:
The actual SA response curve looks like this:
The curves are remarkably similar. Knee around 8.8 MHz, about 36 dB down at 14.08 MHz. Loss at 7.04 MHz is a bit worse on Elsie, but that is based on Q values of 40 for the coils and 200 for the caps — both are probably a bit better.
NCRC had several reasons for building the Pixie as a club project, and getting it on the air is probably at the bottom of the list. Fixing flaws in the Pixie design provides learning opportunities. But by the time you fix the Pixie, would you be better off building something else? There is no clear answer to that question, but there is a “something else” that might fit the bill.
Enter the “DC40B” Direct Conversion 40-meter QRP transceiver. Notably different from the Pixie in that it produces between 2 and 3 watts of clean transmit power right off the bat, and has a substantially better receiver. It also has a real sidetone and a keyer chip built-in. All that comes at a price. It sells for about $40, and is probably 3x the complexity of the original Pixie (about 2.5x the component count, plus 5 toroids to be wound).
Willy W1LY built the DC40B and we measured the transmit spectra today. The fundamental was at 2.75 watts. The 3rd harmonic, was -65 dBc, more than meeting the FCC requirement.
The user needs to provide external jacks for the headphones & keyer, plus antenna and power connectors. It is suggested that a panel mounted cap could be used to provide some degree of RIT — otherwise a PCB mounted trimmer is used to set the desired CW offset on receive.
As has been noted in several posts, the stock Pixie 3-pole filter is hopelessly inadequate to suppress harmonics. A typical Pixie will have a 2nd Harmonic that is only about -21 dB down from the carrier. An external 5-pole filter did not improve things well enough, and ultimately a 7-pole external filter was required. In both those cases, the original 3-pole filter remained in place.
However, what if the original Pixie filter was replaced with a 5-pole design? Would that be enough? A friend of mine, Willy W1LY, set out to discover just that. He pulled the original axial leaded inductor (L2) and replaced it with a pair of T37-2 toroids. He then paralleled additional caps on the input and output side (C5/C6), and finally added a new cap between the two toroids. He was able to fit all of that in the space of the original filter plus a bit of open area next to the BNC connector. In addition, Q2, the S8050 output transistor, was replaced with a P2N2222A.
Elsie 1 dB ripple 5-pole filter. Note, it shows 2 caps between the toroids — only one cap is required
- C1/C3 input caps were set to 910 pF (standard value — Willy used 940)
- C2 cap was set to 1300 pF (standard value — Willy used 1270)
- L1/L2 were set to 1.2 uH (20 turns #26 on T37-6 core)
The results were pretty much ideal. The 2nd harmonic was suppressed by 63 dB (other harmonics were even lower). The fundamental power was increased to 25.2 dBm, or 330 mW. (Spectrum Analyzer is fed through a 40 dB external attenuator)
Given the current 5-pole LPF filter values are producing a 2nd harmonic rejection that is about 20 dB more than required, there is some room to improve the ripple response of the filter, which would probably boost the output power to 450 mW or higher. But 330 mW is more power than any stock Pixie has ever produced (usually they are around 200-250 mW out at 13.8 volts input). Another experiment would be to change the toroids out for 1.2 uH axial inductors.