A Beach 40 DSB Rig built by AC9LF

A while back I decided to build a transceiver based on the "Beach 40" double side band rig by Peter Parker (VK3YE). This would be my first transceiver home brew project. The "Beach 40" is a 40 meter double side band direct conversion transceiver with a (originally) minimum of parts. It has some frequency agility and should put out a few watts.

I wanted to used mostly parts I already had on hand. I started off with the VXO which is a ceramic resonator variable oscillator. As you can see below I built it into an Altoids tin. These seem to be ideal for RF, it is shielded and actually very solderable. The chassis of the can was used as the ground plane, ugly type construction was used. Leads were kept short as best I could.

Start of the oscillator for the DSB transceiver. -AC9LF

I decided on using varactor type tuning instead of going with a variable capacitor. I did have some proper varactor diodes but I did not get much tuning range out of them. I think I ended up using a couple of parallel 1N4001 type diodes as the varactors with good results. From the image below you can see the varactor diodes, a paralleled ceramic resonator and a small variable capacitor. Paralleling two ceramic resonators extended the tuning range of the oscillator. The variable capacitor was used to set the area of the band I wanted to tune in. In this case around 7.200MHz. I used a pot from a dead function generator for the tuning pot. I also used a voltage regulator for a stable tuning voltage across the tuning diodes.

Testing oscillator -AC9LF

Oscillator output with associated FFT. -AC9LF

I added a balanced mixer in the Altoids tin, winding the single transformer on a binocular core ferrite that I picked up from Science and Surplus for around 40 cents. The blue potentiometer is used to null out the carrier leaving only the upper and lower sideband signals.

Mixer added. -AC9LF

Output of mixer with an audio input test signal. -AC9LF

I built the rest of the rig on a piece of copper clad board starting with the low pass filter. I used toroids pulled from a dead PC for the inductors. I wound and measured the inductance before soldering them in and securing them with hot glue.

Low Pass Filter -AC9LF

I built the transmitter section starting with the mic amplifier. I then proceeded RF amplifier stage by RF amplifier stage. I made use of the binocular core ferrite as needed. I used the microswitch below to enable the transmitter for taking measurements on the oscilloscope. The center relay was put on dead bug style to switch between the transmit and receive circuits.

Beginning of the transmitter. -AC9LF

I built the audio output amplifier with an LM386, a jumper is available to switch between a fixed 20x gain or 200x gain. The screw terminal block goes to an 8 ohm speaker. The input the the LM386 has a potentiometer for volume control or "AF gain". I did use some perf board for the IC and construction of the audio amplifier. I have a heat sink on both the final transistor and the driving transistor. The driving transistor seemed to dissipate more heat then the actual final. The RF choke on the final transistor is just another scrap toroid that I wound a few turns of wire around. The output varies a bit with the number of turns on this core. For initial testing the mic and BNC were soldered directly to the PCB to be transferred the a makeshift case later.

Running out of space! -AC9LF

During some initial testing the rig seems to put out about 1 watt. I will need to see if I can squeeze any more out of it. There also seems to be a little bit of self oscillation initially at around 1MHz if I recall.

DSB RF output into 50 ohm dummy load. -AC9LF

I stuffed everything into a scrap enclosure I found. It seemed to be just big enough to fit. I still have to play around with the hardware a little more to get some more output power on it and see what it can do on the air. In the mean time I have added the schematic I based this on below. It was found on the soldersmoke blog. I did add a small filter to the front end receiver to try and remove some AM broadcast overload I was getting. I should be able to post more about this later. It has been sitting on the shelf for several months now collecting dust. I will get back to it soon...

http://soldersmoke.blogspot.com/2012/11/schematic-of-vk3ye-dsb-on-beach-rig.html

http://soldersmoke.blogspot.com/2012/11/schematic-of-vk3ye-dsb-on-beach-rig.html

Using an Xbox power supply for other things

I was walking along one day and just happened to glance in the garbage, lo and behold I found a 16.5 Amp 12 volt power supply! I have converted the proprietary connector of the power supply to something more useful to me.

I have started using molex connectors from PC power supplies for my 12V power connection pigtails on my home systems. These are pretty easy to find. The male end is a little more difficult. I spliced in this connector using the 2 inner pins of the molex connector for ground and the outer ones for +12V.

http://photobucket.com/gallery/http://s293.photobucket.com/user/B-e-t-a/media/Fan%20write-up/1_PSU_molex_connector.jpg.html

http://forums.reprap.org/read.php?219,191038

I attached a switch to the blue and red wires on the power supply to turn it on and put it back into standby. The measured standby power in is about 1 watt, not bad but not great either.

Cut off the old connector -AC9LF

New fancy switch to bring power supply out of standby mode. -AC9LF

I tried plugging in a standard IEC computer power cord into the power supply and found that there was a tab in the way. It looks like this is a 120Vac input only power supply and it may be keyed to prevent use on 240Vac systems. I didn't have a worry about that as I only have 120Vac outlets around. I snipped off the plastic and was good to go.

Defeated tab for regular IEC use. -AC9LF

I connected this power supply to my 2 meter VHF radio and was pleasantly surprised to find that the power supply did not produce any noticeable RF noise on the 2 meter band. I have yet to test it with my HF radio running, but so far it seems to be pretty clean.

New connector plugged into VHF radio. -AC9LF

QRP magnetic loop antenna for 40 meters

I know I will have to start doing a bit more with circuits on this blog of mine. Lately I have been focusing more on antennas. This post is no exception. I have been sitting on this magnetic loop antenna for some time. It has been near complete in my basement since early winter. The reason I have not completed it is that final tuning and testing needed to take place outside. Winter makes it cold and miserable out and there is far too much metal in my basement for it to couple to and throw everything off.

This weekend was great, yesterday was sunny and around 80F. I did try taking this thing outside once before but I made the loop too large to fit through the stair well. The original circumference of the main element was about 18 feet with original testing and experimenting done in the basement during the winter. I had to cut this down and make it closer to 16 feet.

I used this site to calculate the antenna characteristics: http://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

Magnetic loop leaning on tree - AC9LF

I used a 20 foot roll of 1/4" copper tube from the local Lowe's store which cost about 10 bucks. The antenna isn't very efficient per the calculation from the above website at 5 watts:

Note that these numbers are a bit off from what I measured:

Antenna efficiency: 23% (-6.4 dB below 100%)
Antenna bandwidth: 15.1 kHz
Tuning Capacitance: 157 pF

Capacitor voltage: 580 volts RMS
Resonant circulating current: 4.11 A
Radiation resistance: 0.034 ohms
Loss Resistance: 0.114 ohms
Inductance: 3.12 microhenrys
Inductive Reactance: 141 ohms
Quality Factor (Q): 477
Distributed capacity: 13 pF

I measured the main loop with my LC meter which turned out to be about 4.6uH, The variable capacitor I used is of the butterfly type. This allows lower loss because there is no mechanical wiper connection. It also allows higher voltages across the capacitor. The trade off is that the capacitance is lower and butterfly capacitors go from fully meshed to fully unmeshed with a 90 degree change in rotation where as standard variable capacitors are 180 degrees. For my purpose this turned out to be perfect as I was only interested in covering the 40 meter band which is 300khz wide. Measuring the capacitor it is roughly 5pF to 20pF. 

Close up of capacitor and connections -AC9LF

To resonate the main loop at 7Mhz with 4.6uH of inductance I needed roughly 113pF of capacitance. With only the small capacitance of the variable butterfly capacitor I needed to add some capacitance in parallel. I chose to go with RG58 a/u as my extra capacitance. This coax uses solid polyethylene insulation which increases its maximum voltage rating over foam insulation. According to a datasheet the max voltage is about 1,400V RMS. This is well above the 580V RMS calculated at 5 watts. Capacitance per foot is around 30pF so I cut a length of slightly over 4 feet to bring me a bit above the 100pF needed for resonance. 

I trimmed the coax away slowly which shifted the frequency up to 7Mhz with the plates of the capacitor fully meshed. I used the receiver of the VX-7R to follow the frequency with the highest noise level while pruning the coax. The frequency with the highest noise level is the frequency of resonance. This was confirmed with my Dip Meter. Once satisfied I switched to my BITX 40 to perform final transmitter SWR testing.

Here is the data sheet for Belden RG-58 a/u: http://www.belden.com/techdatas/english/8259.pdf

I used something a bit different but should be pretty similar in characteristics.

Coax used as parallel capacitor - AC9LF

This combination gave me a calculated tuning range of a bit more then 300Khz which as stated before fits the whole 40 meter band. It also allows for easier tuning as having a wider tuning range makes it harder to hit the resonant "sweet spot" at about 50 ohms for the transceiver to be happy. I used a hard plastic straw on the shaft of the capacitor to decrease coupling between me and the antenna when tuning it.

The frame is made out of remnants from a wood trellis that I scrapped. The inner driven loop is made out of the same RG58 a/u coax cable with one end of the shield connected to the center conductor. The center conductor on the opposite end of the driven loop is connected to the center conductor of RG-174 which has a SMA connector on it. The shield of the RG-174 goes to the Shield/Center conductor side. Connecting the outside shield to the inner conductor on the driven loop helps to reduce electric field noise pick up. The circumference of the driven loop is 1/5 of the outer loop. 

Using the setup below I was able to get SWR close to 1:1 on any frequency of interest on the 40 meter band. Even though fully meshed to fully unmeshed of the capacitor only covers a little more then 300Khz hitting the "sweet spot" was still a little tricky. I plan on putting a stepper motor on this in the future for remote tuning. This will also allow micro stepping for more accurate and easier tuning of the capacitor.

QRP setup -AC9LF

Even though the XYL says it isn't, this really is a small antenna seeing as a dipole on 40 meters would be around 60 feet long.

I still have yet to make a 40 meter contact with the BITX40. My operating skills must not be up to par for QRP.