Shortwave Radio Converter

I built this about 27 years ago to receive international shortwave broadcasts through a car AM radio. This circuit tunes the 9.5 to 10 MHz and 12 - 12.5 MHz shortwave bands. Back in the day I would listen to the BBC World Service, Radio Canada, Radio Moscow, Deutsche Welle, Radio Australia, and many others. Radio France gave excellent reports on the radiation cloud that rolled across Europe from the Chernobyl mishap in 1986. Several times daily, they would read off a list of all the major cities in Europe, along with the radiation levels in each one. I was "glued" to my radio following the progress of this. The information was of good enough detail and quality one could actually plot it on a map, graphically monitoring the progress of the situation.

The VFO and mixer circuits were taken from the 1979 issue of the ARRL Radio Amateur's Handbook. I built them from the publshed schematics, then tweaked some of the component values for better performance. The VFO coils are wound on glass 3AG fuses that had their elements removed. I heated the end caps with a soldering iron, removed them, took out the fusable element, and glued the caps back on. Then I wound the coils using #28 magnet wire, using the metal caps to solder the wires onto. I used Bohning "Fletchtite" archery glue to secure the windings so they wouldn't move. The variable capacitors were very old, surplus units bought I-can't-remember-where. Looking at the left-hand-side of the photo: The control on the left-hand front of the box is the band selector switch. Next to that is a variable capacitor used to tune in stations. On the rear of the box (in the right-hand-side of the photo) there is a solitary variable capacitor used to tune the antenna for whichever band one is listening to. The whole unit is built in modular form - the VFO has its own board and the mixer is on a separate board. This allowed me to develop and refine the system one module at a time. The VFO is built using "ugly" or "dead-bug" type construction. Its circuit board was made by scoring a copper-clad circuit board with a knife into a grid pattern. The mixer board was actually laid out, then etched using ferric chloride solution.

After many years of sitting unused in an RV, being subjected to all kinds of temperature extremes and even a roof leak, this little device still works. Not only does it still work, the tuning has held calibraton!!! It is within 5 or so KHz of where it was when built. This attests to the quality of the components I used to build it, as electronic components generally drift in value with age.

If I can ever find the original schematic I drew, I'll post it in a future blog entry.

Within the past several years, I have heard China Radio International, Radio Australia, various Spanish-speaking stations, etc. on this unit. Regrettably, the BBC has gotten away from broadcasting quality news programming on shortwave, instead focusing on satelite broadcasts aimed at various target audiences - telling them either what they want to hear or whatever the Powers That Be WANT folks to hear.

To use the converter, simply tune the car radio to an empty spot on the AM dial between 1.40 and 1.55 MHz. Connect the converter between the external antenna and the antenna input on the radio. The converter runs on the car's 12 volt supply; an internal zener diode regulates the supply voltage for the circuit to 9 volts. Use the band selector switch to choose the band to listen to. Reach around the back of the box and turn the peaking control for maximum signal. Then simply tune the right-hand front panel control to tune around for a station. The lower band - the 9.5-10 MHz band, offers the WWV time station from Ft. Collins, Colorado. These WWV signals are very useful not only for checking your watch, but also the RF and audio signals provided are extremely accurate and may be used for equipment calibration. WWV may be heard on 5, 10, 15, and 20 MHz - these broadcasts are done around the clock, 365 days a year. Again I mention those because they are an excellent way of checking the calibration and even the operation of a shortwave radio. Basically, if you can't hear WWV, you will likely NOT hear much else, either.

Soon, in a future post I will delve into the features to look for when buying a shortwave receiver and what to listen for on your radio.

Ten-Tec Short Wave Receiver

Short wave and Amateur (ham) radio are good for keeping in contact with the outside world during emergencies, and as an alternative source of news and information. One only has to look to the recent events in Egypt to understand the need for other means of staying informed on breaking news. This, and the recent events in Japan, impelled me to obtain a general coverage shortwave receiver. Years ago, I built a converter that allowed me to receive the 9.5-10 MHz and 11.5-12 MHz bands through a car AM radio. This converter still works, but I wanted more bands and the capability to receive CW (morse code) and SSB transmissions from amateur radio operators. The problem is that unlike 20+ years ago, decent short wave radios aren't available to the average American consumer for an affordable price. Nearly all have digital tuning that mutes the audio while you are tuning. That is practically worthless when you are tuning around looking for a particular station, as one generally does when listening to short wave. Most of my listening does NOT involve typing in a known, published frequency on a keypad; it involves tuning around, looking for an unpublished station. The few cheap radios with analog tuning are likewise so bad as to not be worth having, in my opinion. As a 'starving' grad student, I need something that works but doesn't cost $1000+ . I could build one "from scratch", as I did the converter, but I don't have the time to invest in designing, testing, troubleshooting, re-work, ... that such a project would entail. And so I looked into kits.

After much searching on the net, I found Ten Tec. Their products are built in the USA, and they have a number of relatively inexpensive kits for both short wave receivers and amateur radio gear. They make a 9-Band short wave radio, the Model 1253, as well as a less expensive Model 1054 which covers 4 bands. Both the Model 1054 and the 1253 are a Regenerative type receiver. This design, when carefully engineered and constructed, offer reasonable performance for a cheap price, and were often used years ago. I built one from a Radio Shack kit when I was in grade school. While that worked, it was very unstable and it was quite prone to "hand capacitance" effects. I was skeptical of building another one at first, but a demonstration on you-tube of the Model 1253 convinced me to give Ten-Tec a try.

The unit I really wanted was their model 1253 9-Band SW Receiver kit, priced at $99 USD. They told me that they are not currently selling these because a particular diode used in the band switching circuitry is unavailable. So I defaulted to the Model 1054 4-Band receiver priced at $39 US. These were in stock; I ordered one on a Friday, and had it within about 4 days. I also ordered the matching $7.99 aluminum case, but that is on back order and will (hopefully) arrive in a few weeks.



The Kit and Technical Details:
The manual was easy to read and follow, though there were some corrections and addendums that looked almost "pencilled in" in some instances. The circuit uses 3 FETs, 2 bipolar junction transistors, and an LM386 audio amp IC. One of the FETs acts as an RF amplifier. This is a good design feature - isolating the antenna from the rest of the circuit and mostly prevents it from radiating RF through the antenna. Tuning is accomplished with a varactor diode controlled with a potentiometer. The regeneration control is also a potentiometer. Another trimmer potentiometer serves as an attenuator to keep very strong signals from overloading the 'front end'. The circuit uses two 9-volt type batteries: one for the RF section and the other for the LM386 audio section. As explained in the manual, this was done to minimize problems for beginning kit builders in getting the circuit to work properly. Band switching is accomplished with two push-button switches. These are operated in four different permutations to select the different bands. (See the picture of the front panel - it's pretty straight forward) Note the blue LED I added to the front panel as a power-on indicator lamp. Though I had to drill another hole in the front panel to accommodate the LED, the PCB has a spot for the LED and dropping resistor. The LED is powered by the audio amp battery through one of the DPDT power switch contacts.

I was able to fully assemble the kit within two and a half hours.

Testing, Use, and Operation:

A VERY USEFUL thing to have around is another (borrowed) short wave receiver OR a crystal oscillator with crystals that fall within the bands of the receiver - you will need one or the other of these to properly calibrate your radio and know what parts of the bands you are tuning.
I was able to use my old converter to check the alignment on Band #3 - the 8.5 - 10.2 MHz band. That still leaves the other three bands in question, as far as calibration goes. I can use my 40 meter ham transmitter with a 50 ohm dummy load for checking Band #2.

This receiver does work. Using a 25 foot random wire antenna lying on the floor inside my house, I have received WWV time signals from Ft Collins, CO; China Radio International; many Latin stations; and numerous religious stations. That said, here are some more detailed observations and critiques:

1) The regeneration control is quite "touchy". The manual tells you, as part of the initial setup, to adjust a certain trimmer potentiometer such that the front panel regeneration control will start regeneration at about half of its rotation. Mine starts VERY ABRUPTLY at between 1/8 and 1/4 turn - I cannot get it to do better than this. I double-checked to make sure I got all the right parts installed in the right locations and in the right direction, in the case of polarized components.

2) The audio amplifier is an LM386 IC. These have the advantages of being cheap, use a low parts count, and will drive a small speaker to a nice volume level. On the flip side they are electrically noisy, and suffer from high distortion levels. I'd much rather have a decent quality audio section based on a NE5532, or similar, audio op amp (these are used in audio mixer panels). I'd GLADLY give up speaker volume, using headphones only, to have a better audio circuit!

3) One VERY GOOD thing that struck me was the stability - I didn't notice much thermal drift at all, and "hand effect" is minimal. Ten Tec got that right!

4) Sensitivity is good, though you have to get the 'feel' of the regeneration control to maximize sensitivity and selectivity without distorting the audio. This tends to be true of any regenerative type radio.

5) The silk screened labels on the front panel and PC board are of excellent quality. If the case ever does arrive, this kit will have a very professional appearance.

6) Tech Support is lacking - I wrote tech support an email regarding the regeneration control almost a week ago, and still have NOT heard ANYTHING from them. NOT ZESTY AT ALL - I expected MUCH better from these people!!

7) A SAFETY NOTE - if you have the volume turned up to a normal listening level and the radio goes into full regeneration, the squeal is deafening - especially when wearing headphones!! You can DAMAGE your ears if you are not careful. An audio limiter circuit or automatic volume control would be VERY nice here. Until I can do something about this, I turn the volume down to almost minimum while adjusting the touchy regeneration control.

8) Come on, Ten Tec. The manual I got with my kit has a 1994 publication date! You guys can afford a 2011 update to your manual with all the corrections, revised part numbers, ... so it doesn't look like the notes I've scrawled during college lectures. I'd be very happy to write you a proper manual for reasonable compensation.

Conclusion:
All in all, this kit is, in my opinion, worth the $39 US spent on it. It's a great introduction to radio and electronics for the beginner, and is a very worth-while father-son (or daughter) project. With certain improvements I plan on making, it will be a usable radio for emergency situations or for getting alternative news.

I intend to do some modifications such as adding a dual 9-volt regulator circuit that can be fed from an external 12 volt battery, and possibly adding a band spread control for better selectivity in crowded bands. I want to build my own improved audio section based on a decent audio chip. I would also include an active filter for CW reception and a limiter to prevent hearing damage from loud noises.

Here is a link to their website:
http://www.tentec.com/

For my follow-up post, see http://inkarlslab.blogspot.com/2011/05/ten-tec-model-1054-shortwave-radio.html