Reversing the inductor in the Chinese QRP antenna tuner makes all the difference

Like many I was disappointed with the Chinese antenna tuner kit "QRP manual days" back in 2014 and I never really got it to work. 

That was even after I had replaced the two polyvaricon capacitors. The two screws for mounting each of the variable capacitors were too long. The result was that they interfered with the moving plates of the capacitor and I ruined both of them. There was no warning of this in the manual, so beware anyone who buys this kit.

Despite fixing this, it didn't really tune anything. I later learned that it probably works for low bands like 80 m as it is, but I didn't notice that. I thought maybe the toroid core of the inductor was of inferior quality, but never really investigated it, so it sat unused on the shelf for 7-8 years.

QRPGuys LA3ZA Zero Beat Indicator

Image: QRPGuys.com

Some time ago QRPGuys approached me and asked if they could make the low-power zero-beat indicator that I made many years ago available as a kit. I was surprised and very honored by this request. It's been available from them for a while now and here is a description from their page:

The QRPGuys LA3ZA Zero Beat Indicator gives you a visual indication when you are tuned to the callers frequency, based on the tone of the received signal you hear on your transceiver. The theory is simple. The device takes into account the amount of built-in offset your transceiver has, usually around 300-800Hz, and illuminates an led when it detects that specific tone you hear when you are tuned to transmit on the callers specific frequency.

The Ultimate WSPR Spot - part 2

Being spotted by the designer of my WSPR transmitter must be the ultimate WSPR spot! Perhaps only surpassed by being spotted by the designer of the mode himself, K1JT, Joe as I was four years ago.

It is Hans, G0UPL, of QRPLabs who is the designer of both my U3 to the left (turned off) and the U3S to the right. The latter was the active transmitter at the time of the spot below.

By the way, this is a line from VK2TPM, Peter's excellent WSPR Watch app for Ipod.

QRSS experiments: FSKCW and Slow Hell

These last few days I've been experimenting with my QRPLabs Ultimate 2 and Ultimate 3s transmitting on 7 MHz. In addition to WSPR, the modes transmitted have been FSKCW with 6 second long dots, and Slow Hell with 17 second long characters. The result as received this morning can be seen on the display from the grabber of Les, G3VYZ in Northumberland, UK. This is a stack of 6 consecutive 10 second frames as can be found on the QRSS grabber site of AJ4VD.

FSKCW and Slow Hell reception of LA3ZA at G3VYZ

My signal is on 7,039.870 kHz and has been set up with a FSK shift of 6 Hz. Power output was 0.2 W and the distance is about 890 km.

It works but the reception is much less reliable than for WSPR, which is not so unexpected. At the same time the WSPR signal was received all around Northern Europe (G, GM, DL, ON, OE, LX, LA, OY, OH, PA, SM) as well as on the Canary Islands, 3930 km away.

Even better low-pass filters for transmitters

The last issues of QEX have featured two interesting articles by Gary Cobb, G3TMG. He outlines the advantage of using Zolotarev designs for the harmonic suppression filters of transmitters, giving even better suppression of the second harmonic than the more common Chebyshev or quasi-elliptic filters.

Chebyshev low-pass filter from the GQRP data sheet (issue 1)

My interest in this was triggered by the test of the Ultimate 3 QRSS/WSPR kit from QRP Labs in the Nov 2016 QST. The review was positive overall, but it was remarked that the harmonic suppression does not meet FCC requirements (-43 dBc or better). I am not sure whether this is due to PCB layout issues, or if better filters can alleviate it, but I note that the design uses the simplest filter of the ones I have listed here.

The evolution of filters for use for harmonic suppression follows at least these three steps:

So you want to play with a Pixie 2?

My own surface mount version of the Pixie2

Here's a guide in table format to minimalistic single-band amateur radio transceivers. The Pixie 2 and related kits are fun to build, yet they perform well enough to be used, although with some effort, for real contacts.

The idea of using the power amplifier transistor as a mixer seems to come from George Burt - GM3OXX - whose five transistor FOXX was described in 1983 in SPRAT. The basic design of the oscillator, PA/mixer and the simple keying has been more or less unchanged since Oleg Borodin - RV3GM - described the four transistor Micro-80 in 1992 in SPRAT. Then Dave Joseph - WA6BOY - replaced two of those transistors with the LM386 audio amplifier in the Pixie 2 (QRPp 1995). Most later versions are variants of these designs.

Here's the table of Foxx, Micro 80, and Pixie 2 kits:

Disappointment with Spice and the QRP-er's favorite, the LM386

The trusty old LM386 audio amplifier from the 70's is still used a lot in low power and portable equipment. Recently some ultra high gain circuits have been recommended that I wanted to simulate with Spice. I started with the datasheet examples for checking the quality of the model. The result was surprising.

In the data sheet one can find a minimum parts circuit, a high gain circuit, and a bass boost circuit:

Gain = 20 (26 dB), minimum parts

Gain = 200 (46 dB)

Bass Boost

The simplest possible AM transmitter

Here's a design for a 1 MHz amplitude modulated (AM) transmitter. I've been looking a while for something like this, a simple short range AM transmitter for the medium wave band, as I needed something for demonstration of my collection of old radios.

The result is the AM transmitter shown here in an Altoids tin on top of a Radionette Kurér radio. This is a portable tube radio from the 1950's. Several hundred thousands were produced, and it was exported from Norway to 60 countries. It is still popular among collectors.

The transmitter is as simple as it gets. The heart of it is a 1 MHz crystal oscillator in a can. Its 5 Volt power is modulated via an audio transformer, one taken from the output of a transistor amplifier (primary 147 ohms - secondary 3 ohms). I drive the modulator from my cell phone into the low resistance side of the transformer and get good audio when the phone's volume is set to maximum.

Cosmetic K2 Upgrade

My Elecraft K2 which is now more than 11 years old (serial #2198) has served me well, even earning me QRP DXCC (100 countries with 5 Watts) some years ago. It is still cosmetically in mint condition, except for that single item which sticks out like a sore thumb: The tuning knob.

This became very clear to me a couple of months ago when LA8OKA's and my K2 were displayed side by side at our stand at Oslo Maker Faire. His is less than two years old, serial #7224, and they were similar except for the scratches in the faceplate of my tuning knob. If I am allowed to generalize from my knob only, it seems as if the faceplate had poor quality paint and has been replaced by a better quality version over the years.

Anyway I ordered a new one and paid the price which at present is $30.37 plus shipping. The difference is really striking and I feel like I have a new K2 now, well worth the price! The K2 now serves as my second rig, and I think it has many years of good service still to come.

Related posts:

• Elecraft K2 do's and don'ts
• Five LEDs in buttons for the Elecraft K2
• My Elecraft K2 Modifications

1 Volt/2 Volt Transceivers

Transceivers with a power supply of 1 and 2 Volts, how much can one achieve with that? Well, actually quite a lot according to DL2AVH, Helmut, who together with DL4ALJ, Gero, wrote two articles about that in the German QRP-Report in 2011. I am impressed by the output power, up to 200 mW with one battery cell (1.5 Volts) and 0.5 Watts with two cells.

I wrote about this in April last year where I also mentioned that the 1 Volt design from 2000 later had been corrected. Those corrections can be found in the article in QRP-Report 3/2011: "Niederspannungs-Schaltungstechnik - der 1-V- und der 2-V-transceiver" (Low voltage circuit technology - the 1 Volt and the 2 Volt transceivers). The improvements are concerned with better input filtering at 14 MHz with a quartz crystal in the front-end filter and better efficiency in the mixer and removal of an audio stage in the direct conversion receiver. This design only uses bipolar transistors and no ICs.

Whatever happened to the 1 Volt QRP Transceivers?

Output stage of the 1 Volt DL2AVH transmitter
(from http://www.lichtnetzwerk.de/1volttxvr_dkumentation.pdf)

I am intrigued by minimalist transceiver designs like the Pixie 2 which I built some years ago. Therefore the "1 VOLT Challenge" from Dayton 2000 is also something I wish I knew more about. It had these winners:

1. Duncan Walters, G4DFV - The HAMEOBA - A 100 mW Single Cell CW Transceiver (30m)
2. (tied) Helmut Siefert, DL2AVH - A 30m 1V QRP transceiver
2. (tied) Charles Fletcher, G3DXZ - An 80m CW QSK Transceiver
4. Jim Roberts, NC9H - A 20m 1.5V Transceiver

Now, what happened to these designs? I'll try to figure it out from sources on the web.

Small voltmeter for a QRP rig

AT Sprint rig in an Altoids box, itself contained in a video case.
Digital voltmeter in lower left corner shows 12.0 Volts.
(Click for larger image)

Every battery-operated rig needs a voltmeter. I had a small analog voltmeter for my AT Sprint QRP (= low power), 4 Watt rig, but after it failed I wanted to upgrade it to a digital one. I found a miniature meter on Ebay which is perfect for a QRP rig and which I recommend.

QRPp: Ultra low power operation with the Pixie 2

QRPp is radio amateur jargon for communication using a transmitter with less than 1 Watt power output. That’s less than a handheld GSM mobile phone (max wireless range ~35 km) or a flash light. My Pixie 2 transceiver has so far contacted 15 different countries on 3.5 MHz CW. The first version from 2002 was made in an Altoids tin (picture later), but in 2010 I repacked it in one of the nice clear top tins from the 4SQRP (picture below).

My repackaged Pixie 2

AT Sprint (the original)

Photo: LA8EKA

The AT Sprint is a very nice limited edition surface mount CW transceiver designed by KD1JV Steve Weber. The printed circuit board measures 85 by 54 mm. It was quite a challenge, but a fun one, to build! 

It was designed to be a rig of a size fit for camping. It covers the 40, 30, and 20 meter bands with plug-in modules. The whole transceiver fits in an Altoids tin and outputs about 4 W. The rig in the tin weighs about 85 grams. It uses a DDS - Direct Digital Synthesis - chip for the VFO, it has a superheterodyne receiver with a four-crystal filter, audio AGC, audio filter, and a very efficient class E output stage. It also has a built-in memory keyer and a morse code frequency annunciator. It can take any battery voltage between 5.5 and 13.8 Volts.

25 part transmitter for the QRP Minimal Art Session

My Minimal-Art-Session rig shown here was made for class B of the QRP-MAS competition which is in May every year. In class B the transmitter should have a maximum of 50 components, and the fewer the number of components, the more points one will get per contact. With such simple rigs, only CW is viable.

This rig was made by combining ideas from several designs that I had seen and it was finished the same day as the competition. I ended up with 22 components in the transmitter except for the output filter. The rules say that no matter what, the output filter counts as three components. I guess this is in order that no one is tempted to simplify the filter too much and start emitting harmonics. 

QRP Antenna Tuner

My 75 meter horizontal loop antenna is tuned by a home-built Z-match antenna tuner, a slightly modified version of the Norcal BLT (Balanced Line Tuner), with a resistive SWR-bridge using a super-bright LED. I could tune my 75 m loop on all bands up to and including 20 m, but at higher frequencies I could not get the series capacitor small enough. The cure was to add a switch to choose between the original primary coil with 8 + 8 turns or taps at 4 + 4 turns, after an idea by DL8RAM. This is selected by the 80-20/20-10 m switch and comes in addition to the switch between Hi-Z/Lo-Z on the secondary (the output). This tuner also tunes a 13.4 meter doublet well on 40-10 meters.

I designed a frontpanel layout for the tuner. It was generated from a text file using the program of WB8RCR. I would recommend this program for any homebrewer who wants to improve his panel layouts. I print mine on paper,

Unleashing the LM386

From Wikipedia Commons, author 'Samba pa ti'

The LM386 must be one of the most popular audio output amplifiers among radio amateurs, despite having been around for a long time. It's an IC which even has its own Wikipedia page. The LM386 can be obtained in both dual-in-line and surface-mount packages and outputs 325 mW in the standard version that runs from 4-12 Volts supply voltage.

Its voltage gain of 46 dB is in many cases too little, especially in direct conversion receivers. When I built the Pixie 2 with the LM386 audio stage, it struck me how the sensitivity of the receiver was limited by the audio gain. I asked myself if it would be possible to increase the gain and add some filtering in a simple way. The result is a gain of more than 70 dB and an audio bandwidth of a few hundred Hz by only adding 5 passive components to the standard circuit: 3 resistors, 1 capacitor and an inductor.

QRP Zero-beat Indicator for the Elecraft K2

This is a circuit that builds on KR5L’s zero-beat indicator circuit. He suggested trying a CMOS version of the 567 instead of the standard bipolar version, and here it is: the QRP version of his circuit. It draws only 0.9 mA at 5V compared to about 20 mA/8V for the original circuit.

Compared to the original circuit, C2 and C3 have been divided by about eight, and C1 has been divided by about two, give or take values found in my junk box.