In my last article I talked about my ideas fo a new transceiver project beyond the QRP level. First pictures of cicuitry were also shown. In the meanwhile the transceiver has been finished, some minor changes had to be made and now it’s time to go to the details.
All construction objectives (compact size, sufficient output power to establish even DX contacts on 40 meters, good stability, good receive performance, rigidness for outdoor use) have been met so far as I can say. I had the rig with me, when I was on vacation on the Island of Jersey (GJ/DK7IH/P) from 12th to 19th of August this year. It was big fun operating the rig. Lots of stations were calling during the two days when I was on 40 meters. ODX was HL1AHS, OM Kun from Seoul. So, this was very nice for 50 watts and a vertical antenna made of a fishing rod.
First, to give an overview, let’s have a look on the completed transceiver. Cabinet size is 7.5 x 16 x 6 centimeters.
As you can see, the rig is very compact in size. The block diagram gives you an overview what is inside. Receiver section is on top, DDS can be found in the center and the transmitter is sited at the bottom of the diagram. As you can see, it’s again not rocket science and SDR-virus could not strike as well. 😉
The next posts will describe the rig in details step by step. Proceed with the receiver’s front end.
The next step in improving my homemade QRP multiband transceiver was to reconstruct the DDS VFO. This was not urgently neccessary but after some months of continously operating the rig I was not 100% satisfied with the spurious performance of the AD9850. The AD9850 is a DDS device with only a 10-bit digital-analog-converter (DAC). These ones tend to put out still a quite high number of spurious emissions aka “birdies”.”Birdies” then are detected in the receiver causing unpleasant beep tones.
The AD9951 DDS module (some general information)
Analog Devices (AD) also offers more professional DDS-chips with a better performing 14-bit DAC. The AD9951 is such a device. It is offered for about 25 USD by mouser.com and other vendors. It is also used in commercial ham band transceivers thus we can deduce that performance is improved compared to the cheaper DDS devices made by AD.
The AD9951 needs multi supply voltages, i. e. 3.3V and 1.8V. Digital inputs are 5V-compatible if 3.3V as D_VDD_IO input voltage is applied. The device can be clocked up to 400Mhz. If you use a clock generator with lower frequency, an internal clock multiplier can be used. But this deteriorates phase noise to a certain degree. For my VFO which works in the range only from 13 to 20 MHz I use a simple standard 5V 120MHz clock generator and I do not use the internal clock multiplier. To make the clock oscillator’s output 1.8V compatible a simple voltage divider has been applied.
Note that performance concerning phase noise of the DDS also depends on the voltage of the clock generator. The lower it is, the more the phase noise performance will deteriorate. To calculate exact configuration of your voltage divider keep in mind that input impedance (1.5 kΩ according to datasheet) and input capacitance (3pF) are paralleled. Input capacitance causes reactance depending on the input frequency given by the following equation:
As usual for me the DDS output consists of a balanced-to-unbalanced broadband transformer followed by an rf amplifier. For the latter one I use the MAV-11 broadband amplifier made by Minicurcuits.
This the circuit of my improved DDS device:
The first impression when I connected the AD9951-DDS to my HP 8558B spectrum analyzer was that the signal looked different compared to a AD9850 generated signal. I’m very sorry, but so far I’ve got no photos but I’m planning an article that will deal with DDS comparisons which will have photos of the spectrographic analysis of the various signals. But it was visible on the first sight, that the signal looked much cleaner than a one produced by a DDS with 10-bit-DAC.
OK, there were much lower sidetones to the main signal on the spectrum analyzer. Measurements are one side of the medal, but how would the DDS perform in the receiver? It approved to have improved when I installed the new DDS into the transceiver and listened to the bands. “Birdies” have vanished to nearly 100%, only some very weak spots are discernable. And the receiver noise also seems to have lowered by a certain degree. But I don’t have measured that so far. Measurements are still to be done. OK, let’s check it out the following days (20 meter is very quiet today!) and then I will be able to say if and how the 14-bit DAC.
So, if you want to get the best performance (low number of spurs, low phase noise) out of the AD9951, here are some basic hints:
Stay away far from the Nyquist-Frequency of the chip! Basically this is one third (33%) of the clock rate. I recommend to lower this down to one fifth. So, if you use a clock rate of 100 MHz, don’t let the DDS produce more than 20 MHz!
Don’t use the internal clock multiplier! Use a clock generator with the highest possible frequency!
Use a clock generator that will put out 1.8 Volts pp!
Keep ground leads on your pcb as wide and short as possible!
Decouple AVDD, DVDD and DVDD_I/O effectively!
Set DAC_RSET to a value that DAC current stays lower than 10mA. 3.9k is a recommended value.
The DDS is mounted to a small piece of veroboard using a 48-lead breakout-board for TQFP48 ICs:
Flexible wiring is used to connect the board to the microcontroller. Shielded cable is mandatory for connecting the rf feed to tx and rx mixers.
Underneath you’ll find some code snippets to set the frequency of the AD9951 device. The code has been copied 1 by 1 from my SSB transceiver’s software. Thus modification for your purposes might be neccessary.
After inspiring discussion with a reader of my blog I’ve changed the routines to optimize code concerning performance. The major objective was to avoid intense use of floating point functions because they are slow. Instead I used bitshift operations widely. But there is one floatingpoint calculation left:
fword = (unsigned long) frequency * 35.790648;
The floatingpoint constant 35.790… results from a division of 0xFFFFFFFF by f_clock which is given by the equation of the tuning word (see datasheet of AD9951). This could be converted to a bitshift operation too, if you use a programmable clock oscillator tuned to 134,217,727 Hz. Then the multiplication factor is 32 which can be easily achieved with another bitshift operation.
Thanks for reading!
Setting the AD9951’s frequency output
void set_frequency(unsigned long frequency)
// FQ_UD: PD0 (green)
// DATA: PD1 (white)
// CLK: PD2 (blue)
// RESET: PD3 (pink)
unsigned long fword;
int t1, shiftbyte = 24, resultbyte;
unsigned long comparebyte = 0xFF000000;
//Calculate frequency word
//Clock rate = 120002500
//0xFFFFFFFF / 120002500 = 35.790....
fword = (unsigned long) frequency * 35.790648;
//Initiate transfer to DDS
PORTD &= ~(1); //FQ_UD lo
//Send instruction bit to set fequency by frequency tuning word
//Calculate and transfer the 4 bytes of the tuning word
//Start with msb
for(t1 = 0; t1 < 4; t1++)
resultbyte = (fword & comparebyte) >> shiftbyte;
comparebyte >>= 8;
shiftbyte -= 8;
//End transfer sequence
PORTD |= 1; //FQ_UD hi
//Send one byte to DDS
void spi_send_byte(int sbyte)
// PORT usage
// FQ_UD: PD0 (green)
// DATA: PD1 (white)
// CLK: PD2 (blue)
// RESET: PD3 (pink)
int t1, x = 0x80;
for(t1 = 0; t1 < 8; t1++)
PORTD &= ~(4); //Bit PB2 set to 0
//Set respective bit to 0 or 1
if(sbyte & x)
PORTD |= 2; //SDATA Bit PB1 set to 1
PORTD &= ~(2); //SDATA Bit PB1 set to 0
//SCLK line set to 1 // = set clock line to RISING edge to store bit in frequency word
PORTD |= 4; //Bit PB2 set to 1
x >>= 1; //Shift bit to divide x by 2
Resetting the AD9951-chip:
(A reset must be performed once immediately after your program was started and before you
transmit the first instruction to the AD9951 DDS chip)
// FQ_UD: PD0 (green)
// DATA: PD1 (white)
// CLK: PD2 (blue)
// RESET: PD3 (pink)
PORTD |= 0x08; //Bit PD3 set
_delay_ms(1); //Hold reset line hi for at least 20ns.
PORTD &= ~(0x08); //Bit PD3 erase
This article is outdated! Please read about the new version of the antenna tuner here!
After I had finished my 5 band multibander for QRP SSB operation I had 2 choices becoming QRV on the hf bands – either to build up something like an antenna farm (which would have caused severe problems with my familiy 😉 ) or to build a multi band antenna capable of being used on all short wave ham bands. In order to survive the next months with more or less stable health I have decided to take the latter alternative.
Browsing my good old antenna handbook by Rothammel I came across an untuned multiband dipole. This is known sometimes as a “doublet antenna“. It’s a simple dipole, fed by an open wire feed line which has to be matched to the transmitter for each band of operation. The length of one branch of this antenna is about a quarter wavelength of the lowest operating frequency which makes it a half-wave dipole for in my case 80 meters. On the internet I found lots of users of this antenna speaking positively about its performance, so I decided to give it a try. Antenna construction is very simple and the doublet antenna does not spoil the optical impression of your ground property seroiusly.
But one major shortcoming of this antenna should not be ignored: The “doublet” antenna can not be operated directly from coax line, it has to be matched for each band seperately to the desired frequency with its specific impedance to come as close as possible to the 50 ohm of the transceiver. Open wire feed line must be used because there is always a high VSWR on the feedline due to the fact that the dipole is not tuned to the operating frequency. But as losses are low in double wire line even if SWR is high, this is not a problem. Be sure that more than 95% of your transmit power will reach the antenna.
Due to the fact that my antenna installation requires only 5 to 6 meters of feedline from my roof window to the antenna’s feed point, I first built a ladder line from flexible cable and homemade acrylic spacers. But this tended to be a little bit service-unfriendly. Always whenI tried to pull back the antenna for modifications, the line (the spacers to say more detailed) got entangled in the roofing-tiles. So I bought commercial 450-Ohm Wireman line that does not show this problem.:
After having installed antenna and feedline, I first I built a simple L-match tuner for test purposes. If this antenna had not worked with me I would not have lost much time and money. But no need to worry. After having set up the antenna and the tuner I was quite satisfied. Performance was beyond my expectations. But with using this setup more often, I found out that retuning after having QSYed was not that nice. I had written down the settings for capacitor und inductance on a piece of paper so that they could be recalled relatively quickly. But it was not overwhelming conveniant. Thus my idea was: If you have the settings for each band on a paper table, why not putting them into a microcontroller and let the micro do the tuning?
The basic concept of the tuner
The tuner is an L-Match tuner that can match high Z antennas. Long antennas tend to have higher impedances than usually the 50 ohm transceiver’s output. Integrated into the tuner is also an SWR measurement circuit (directional coupler from old CB radio) so that an external SWR meter is not neccessary. The tuner is tuned manually. After matching has been achieved, the values for L and C are stored by pressing a button and can later be recalled by bandswitch. Retuning is easy. Just press buttons for L+, L-, C+ or C-, retune and store the modified settings. After that: Have fun in a QSO!
There are 6 coils in the tuner, switched (shortened) by 6 relays. The inductances are doubled (more or less) from one coil to the successor, so that in combination nearly each inductance between 0.5 uH and 39 uH can be switched. It turned out that this concept works fine. The single inductances are:
20uH, 10 uH, 5 uH, 2.5 uH, 1 uh, 0.5 uH
Coil data: Each coil except the smallest one is wound on a T80-2 Amidon toroid. Use 0.3 mm diameter copper enameled wire. The winding data:
L1 (20uH): 60T.
L2 (10uH): 43T.
L3 (5uH): 30T.
L4 (2.5uH): 21T.
L5 (1uH): 13T.
L6 (0.5uH, wound on T50-2 core): 10T.
Another coil must be wound because the antenna has an unbalanced circuit inside (1 coil, 1 cap). The feed line on the other hand is balanced, so this has to be kept in mind. To convert the unbalanced output of the tuner circuit to the balanced feeder line, I use a 1:4 balun at the output of the coil chain (T1 in circuit diagram). Wind 12 turns bifilar of 0.8 mm twisted (about 3 twists per cm) enameled copper wire to a FT114-43 Amidon core using this pattern.
Hint: On the picture below there is a balun transformer that I previously had installed. It was a Guanella balun but it did not perform satisfyingly. Tuning could not easily be achieved on 10 and 15 meters.
The tuner uses a motor driven air dielectrical and butterfly shaped capacitor with max. 220 pF capacity. The advantage of a butterfly type is that it needs only 90° to turn it from minimum to maximum capacity. The motor (a 5V dc version) is connected via a 240:1 gear drive by TAMIYA. The drive has two outlets providing one axle at each side of the drive. To one of the axles I connected the capacitor, the other one connects to a potentiometer to report the current swing angle to the microcontroller.This allows precise feedback of the capacitor’s current position which is essential for setting it to the desired value. The value of this variable resistor does not really matter since it is only a simple voltage divider. Anything between 5k and 100k should fit. Make sure that you use a piece that is easy to turn to minimize friction. To connect the axles I used PVC tubing with an inside diameter of 3 mm.
This is the setup:
Here’s the schematic of the tuner:
Right at the 50 Ohms input where the transceiver is connected you can see a 120 kOhm resistor. The purpose of this one is to keep static electricity away from your rig. Antennas with open ends (like the “doublet”) can build up high voltages particularly when strong rainfall coincides with heavy winds. The raindrops build up an electrical potential during their way through the clouds. I once have seen sparks of 1 to 2 cm of length in my shack when I brought the feed line wire of the antenna close to a grounded metal cabinet.
First impression of performance
I have been using the tuner for three weeks by now. My antenna has got a total length of 40 meters, height is from 10 to 12 meters above ground. Feed point is approximately centered. The antenna with this tuner allows operation on all the 5 bands of my QRP multibander. Signal reports are usually good. On 20m I receive 5 and 9 reports very often. Transmit power only is 10 watts. On 40m and 80m the same is true. Even 15 meters, usually not so fine on a 40 meter long doublet, works fine. 10 meters has not been tested yet due to band conditions.
Maximum distance so far was to W2YP (East coast NY) with my rig having 10 watts output on 25th February receiving a 57 report.
I used a simple plastic box that is available for eletrical installations. RF leads are on top:
I’m currently doing some improvements concerning software. If you are interested in the software, please drop me a mail: peter.rachow(at)web.de !
Enhancing the tuner
This tuner mainly for matching long wire antennas which have a comparatively high Z in relation to the 50 Ohms of a radio tranceiver. If you also want to match low Z antennas (shorter than 1/4 wavelength), the capacitor must be put to the opposite of the coils:
If you would like to have the capacitor in place for switching between hi an low Z antennas another relay will do the job. The microntroller has still plenty of usable ports.
OK, I admit it: Drawing schematics is not among my favourite hobbies. 😉 But it’s a must if you run an amateur radio blog. 😉 Sadly, I sometimes make mistakes. And I appreciate that attentive readers sometimes find the things I’ve lost sight of. Two minor changes thus had to be made in the schematic of my portable handheld rig for 14 MHz: REF pins at the AD9835 are now connected correctly to ground and the ATMega328 is now correctly power supplied. Thanks to Hellmuth, DF7VX, for his annotations!
Recently I have revised the rod antenna for my handheld QRP SBB transceiver. The main objective was to simplify the matching circuit. As I’ve pointed out before in the antenna article, one of the major problems with shortened antennas is the low feed point impedance. The first version of the antenna matcher used a capacitor and a coil to form part of a PI-filter. This new circuit uses an autotransformer made of a linear coil.
The tap is at about 1/4th of the total windings which transforms the feed point impedance of about 10 ohms to the coxial cable with 50 ohms. A 5 meter wire acting as the second part of the dipole should be used to increase performance of the rod antenna. All dimension concerning antenna length have remained unaltered, please see respective article!
For my handheld QRP transceiver I have developed a rod antenna for outdoor use when cycling or hiking. In this article I’ll first give the reader some basic considerations on shortened antennas and afterwards the pracital consequences of these.
A full-sized vertical antenna usually has got a length of a quarter wavelength. Some special constructions of a 1/2, 5/8 wavelength and others are also commonly known. But for a rod antenna the basic construction is a 1/4 wavelength based circuit. Full-size 1/4 wavelength antennas that have the correct length (1/4 multiplied by 0.95 shortening factor) have got a feed point impedance of between 40 to 50 ohms. So they can be directly connected to a 50 ohm coaxial cable. The problem: For 14 MHz this antenna would have an overall length of about 5 meters. Not very appropriate for a handheld transceiver except you were Arnold Schwarzenegger.. 😉
When the mechanical length of an antenna is shorter than 1/4 of the wavelength that it is desired for, a mismatch can be obeserved. The antenna not longer is resonant to the operating frequency. An additional capacitive reactance appears. This must be compensated by an inductive reactance. Therefore shortened antennas have an integrated coil either at the bottom (bottom loading coil), anywhere in the middle (center loading coil) or top (top loading coil).
Another problem must be faced: Feed point impedance is usually much lower than for a full size antenna. Sometimes only between 10 and 15 ohms. Thus some sort of impedance matching circuit must be integrated, too,
Last, degree of effiency will also decrease. So, don’t expect too much from a shortened antenna!
To sum up the things that have been mentioned before: We have to face 2 general electrical problems with short antennas:
A correct loading coil must be found, and
proper impedance matching must be performed.
Into my antenna I integrated 2 loading coils. One at the bottom to serve as an impedance matcher and one slightly below the center to compensate capacitive reactance.
The antenna is mounted to a standard male BNC-connector.
Mechanical deimensions and coil data:
Bottom coil (L2):
L2, the bottom coil, is 50 turns of 1 mm diameter enameled wire wound on a 8mm diameter plastc tubing from the local hardware store. From the bottom end of L2 a 120 pF capacitor is lead to the ground potential of the BNC plug. This, together with L2 sets up low pass filter serving as an impedance matcher.
Edit: Another impedance matching circuit I’ve described here.
The plastic tubing used for L1 and L2 is hollow. Into this tubing a 6mm diameter aluminium rod fits in exactly. The length of the rod between the two coils is 45 cm. Following is another piece of the plastic tubing carriyng L1.
Center coil (L1): L1 is 45 windings of 0.6 mm diameter enameled wire.
The top rod of the antenna is a telescopic antenna with an overall length of 120 cm that can be bought on the internet. It’s outer diameter is also 6 mm, so it also fits into the plastic rod.
Here are the pictures to make clearer how the antenna is constructed:
To increase the usually low degree of efficiency of such an antenna I have manufactured a sort of “counterpoise” that represents the 2nd part of a dipole : A 5 meter long insulated cable with a large crocodile clip that is clipped to the ground potential of the metal BNC connector.
Edit: In the meanwhile I have used this rod antenna for 14 MHz / 20 meters several times during outdoor activities. What I’d never have believed: I could make lots of QSOs with it! Particularly if you are in a high place (e. g. on a look-out) you can work distances of about 1000 to 3000 kilometers right with 4 watts out of your hands. Provided the station you’re answering is strong. Then there is a realistic chance that he might hear you with reasonable signal strengh.
I have really been satisfied with my last QRP SSB rig. It performs very fine. But I wanted a transceiver still a little bit smaller. And it should be easier to set up the rig if you are outside to make quick QSOs. This and the fact that I came across a larger bunch of 9.832 MHz crystals which I thought could be an ideal basis for a ladder filter made me plan an even more compact rig for 20 m compared to the last one. Particularly for portable operation on holiday or when I am outdoor with my bicycle or hiking, I wanted a self-containing transceiver with on-board battery. So here it is…
Monoband SSB-Transceiver for 20 Meters/14 MHz.
Output: 4 to 5 Watts PEP
Frequency generation: DDS with AD9835 and ATMega8, LCD 2×8 Characters.
Transmitter: SSB Modulator for USB or LSB: NE602/SA612, 4-Pole-Ladder-Filter, TX-Mixer, NE602/SA612, Power amplifer: 3 stages, push-pull-final with 2 x 2SC2078.
Receiver: Singe conversion superhet, RF preamp with Dual-Gate MOSFET, RX-Mixer with NE602/SA612, 4-Pole-Ladder-Filter, Passive product detector, AF preamp, AF final amp with LM386. (See improved AGC circuit here!)
Built-in battery pack, also connectable to external power supply, 10-LED-bargraph-display for S and RF strength readout.
The size is about that of those older CB handhelds from the late 70s. It is abt. 18 cm long, 7 cm wide and 4.5 cm high. But don’ ask me for weight. 😉
The transceiver has, as mentioned before, a power output of 4 to 5 watts rf pep which I found sufficient for making contacts worldwide. Transmit power mainly depends on the respective power supply you use. The radio can be run either on an integrated 12 V dc rechargeable battery (1Ah, composed of 10 AAA cells) or by connecting an external dc power supply of up to 14 volts. A three-position switch allows the user to select either internal or external power supply or completely switch the rig off.
Thus the little radio is very versatile for all kinds of portable operation. The antenna is connected via a standard bnc connector. I also designed a rod antenna that you can use if there is no possibility to erect a larger aerial. I have to admit that having a qso with the small handheld antenna ist a pretty ambitious. 😉 But in recent “The King of Spain”-Contest I could work with the rod antenna from a high place over a dozen stations.
With my delta loop the rig is absolutely amazing. During the recent weeks I worked (among others) the following prefixes:
And this all was done, except from the contact to P40FN, with 4 to 5 watts pep. Only for the QSO to Aruba I had to use the 60 watt linear amplifier. The pile-up was too hard. 😉
Basic design ideas:
To make the rig not too bulky I used a “sandwich construction” in an aluminium frame. The radio mainly consists of three layers:
The RF and AF unit (mainboard)
The battery-, AGC/Meter- and switching unit
The display and push-button unit
The RF unit:
On board here you can find the DDS-VFO, the whole receiver and transmitter circuits (including power amp), SSB modulator and demodulator.
The battery and AGC/Meter and switching unit:
This board is equipped with a set of 10 rechargeable batteries (1.2 volts each), the relay for transmit/receive switching and the LED-S/RF-meter circuit (LM3915) plus the AGC device. Above this section there is another layer which keeps the 2×8 line LCD, the up/down control switches for tuning (there is no rotator tuning knob), a button for setting the tuning frequency step, selecting the VFO (4 VFOs and 2 split VFOs are software defined) etc. It is integrated in an extra housing mounted on top of the cabinet an connected via some cables. The controls are simple push-buttons. With one of these the microcontroller can be set into sleepmode to reduce the radio’s noise down to a minimum. The 1 inch loudspeaker is also mounted in here.
The circuitry itself is standard QRP design with the “usual suspects”. See the schematic to learn more:
The front panel looks like this:
Details are to be discussed in my next posting on this blog. Thanks for watching! 73 de Peter (DK7IH)