Balun 1:1 for Shortwave Radio Receiver and Other Means for High-quality Reception

How to make (impedance matchers) 1:1 baluns for listening to shortwave stations on a transistor radio with a telescopic antenna, without the ability to connect an external antenna?

We will need:

discarded PC power supply

installation box or ordinary stronger plastic boxes from a spread

tin

rosin

soldering iron

pliers

scissors

multimeter

f connector adapters for cable

BNC adapter to f connector

BNC connector with terminal blocks for attaching the ends of the binding wire

bell wire

crocodile clip

car battery clamp

1 m double-wire bell wire

6 dB attenuator

coaxial cable

two binding wires (the ones with lighter green insulation) for the antenna

4 binding wires for the noise reduction box

wooden fruit box (ask your market vendor at the market)

How to make (impedance matchers) 1:1 baluns for listening to shortwave stations on a transistor radio with a telescopic antenna, without the ability to connect an external antenna?

First, check whether your transistor radio receives shortwave through a telescopic antenna and not through the integrated antenna inside the radio. This instruction applies only to radios that receive shortwave through a telescopic antenna.

The telescopic antenna on the radio has several hundred ohms of impedance, coaxial cables for the radio have an impedance of 50 ohms, and the antenna itself from the cable tie has an impedance of several hundred ohms, but of course it also differs from the impedance of the rod antenna. If we do not use a balun, not only will less signal reach the radio than the antenna is capable of receiving, but the difference in impedance also causes interference that negatively affects the signal. For example, the signal will wander back and forth over the shielding of the coax, which is manifested by increased noise. The balun can match the impedance and thus provide us with much cleaner reception. Of course it doesn't work 100%, but the efficiency exceeded my expectations. You can also use 75 ohm coax and connectors, it won't matter here. But it depends a lot on the transmitters.

Elementary, dear my Watson.

We will need two balls of binding wire, but don't buy the one with the most disgusting green insulation, it's useless. Also, rubber electrical boxes with a lid and two 50 ohm (or 75 ohm) BNC connectors. Get an old atx power supply from a PC and disassemble it. You will need the big coils, as there is the most wire around them. Carefully gut them from the power supply. I used butterfly pliers. Be very careful, capacitors are monsters. Unwind all the wires from the ferrite core. It's a hassle, but it's worth it. The ferrite cores can be yellow, blue or green, and these cores are the most accessible for us (and many of yours) and at the same time fully meet our (and your) needs. Wind 9 turns of double wire (signal and ground) around them. I used 0.8mm varnished copper wire, but in principle you can also use the ones from the original winding. Cut about 70-80 cm from it. Fold it in half, which will give you a double wire. You can twist the ends slightly to make it easier to work with. Make nine turns as tightly as possible around the ferrite. The individual turns should have a little distance between them. Use a multimeter to find out which wire is which. Everything will be in electrical installation rubber boxes. Don't forget to thoroughly scrape off the insulating varnish from the ends of the winding wires that you will solder to, it often goes wrong. This balun is great at suppressing interference, but its disadvantage is that it can attenuate some really weak stations. If you use a slightly thinner wire - e.g. 0.6mm - and make it 10 turns, it will suppress only the most unpleasant interference, but it will not suppress weaker signals as much, but you will hear the subtler background noise that the first balun suppresses. Next, find smaller coils that have small light gray, yellow or green ferrite rings in them. These will come in handy too.

We will pull the wires from both bundles of the cable tie into the box and try to fix them there, for example, using two pieces of small terminal blocks, but here the terminal block will not connect the wires together but will serve as a stop so that the wire does not come out of the box. Solder the dipole antenna wires directly to one end of the winding (one end of one and one end of the other cable tie). Another option is to use a piece of coax with an f connector (ten or fifteen centimeters of coax will be enough), and solder one binding wire to the signal wire and the other binding wire to the coax ground and fix everything properly with electrical tape or heat shrink tubing. I also screwed the other end of the binding wire, connected to GND, to the wooden board. Then connect the antenna to the balun via a BNC adapter. You can place an attenuator between the adapter and the coax. Or you can use a BNC reducer with terminal blocks. On the other side of the winding solder the ground and the signal wire of the BNC connector. To make it easier to work with the connector and to get it into the plastic box, you can solder wires to the ground and signal wires. Just carefully scrape off the varnish from the ones left over from the original winding. Mount the BNC connector into the prepared hole in the box. It's easy to do. Cut off the small circle from the inside for the cable entry. Be careful not to damage the box. Of course, the ground ring, to which we solder the wire in advance, must be inside the box. Solder both wires to the other two ends of the winding. Check everything with a multimeter.

First, mount the prepared BNC connector into the box, as with the first balun, and solder both winding inputs to it. Solder winding outputs to appropriately long insulated copper wires with a solid copper conductor (they are sold as a pair, so separate the wires from each other so as not to damage the insulation), the grounding clip should be the big one for a car battery, the signal clip can be a completely ordinary alligator clip, because you will attach it to the telescopic antenna of the radio. Definitely avoid cables from an extension cord and the like, use wires with a solid conductor. Pull the remaining ends into the box and fix them with terminal blocks. Use a multimeter to find out which wire the winding should be soldered to and solder them. Check everything again thoroughly with a multimeter. Be sure not make to knots on the wires, this would create an unwanted coil that could negatively affect the signal. Close both boxes.

If you have the antenna outside (probably on a balcony rather than in the garden, because it would need special grounding there), the signal from the antenna should be routed through a room where there is a lot of electromagnetic interference (electrical wiring, LED bulbs, fluorescent lamps, etc.) using a well-shielded 50 or 75 ohm coax equipped with BNC connectors. It is not that big of an investment. If you want to connect the antenna directly, you only need a second balun, to which you connect the attenuator and the antenna via reductions. (The attenuator can also be made using resistors. Search the net for diy attenuator.) For the second balun, it is necessary to attach the grounding terminal to some large metal object, such as a metal chair frame (try it, something will work out in the end), which will serve as grounding, otherwise the radio will be subject to a lot of interference. That is why a larger clip is soldered to the grounding wire.

Now comes our smaller ferrite rings. Since the wire with the crocodile clip, which leads to the antenna, is still subject to slight interference, it is a good idea to suppress this interference as well. But we will be careful here, because here it will only work for part of the band. Make 6-10 turns of another piece of wire that you have left over from the twin wire on this small ferrite, but this time we will use only one wire. And solder the crocodile clip to the shorter end of the wire. You will attach this to the telescopic antenna of the radio. On the other end, where you remove about 1 cm of insulation, you will click the crocodile clip from the balun. It works well for part of the band, but in other (often lower) parts of the band, the ferrite unnecessarily suppresses the signal. You can make more similar adapters and experiment with the number of turns. I have my Grundig music boy 175 transistor radio in an anti-interference box (fruit box), with several coils of binding wire wrapped around the sides. The box stands on its side, the bottom of the box is free. The radio stands on the side (under which there are wires). This also ensures comfortable access to tuning. It's not crazy. It's the principle of a Faraday cage, which prevents interference from various devices in the room from reaching the radio. On short waves, therefore, the radio will not have a signal without an antenna connected via an alligator clip, or by pulling the telescopic antenna out of the box. On medium waves, on the other hand, the box improves the signal slightly (hence the unwired bottom) and also serves as a loop antenna, although it does not reach the quality of professional loop antennas. I call it, according to one review, the Trafalgar box. I power the radio with batteries, because a lot of interference from the mains gets into it via the 230V plug, and the entire setup we described would be as good as a bicycle for a fish. Put a 6dB attenuator on the coax to the first or second balun, to prevent the radio from being overwhelmed by the signal. This will help you pick up weaker stations. On shortwave, stations often cross each other. Just to be on the safe side, buy 10dB and 20dB attenuators, but in most cases 6dB is perfectly sufficient.

With 50 Ohm RG58 coax cable - diameter 5mm, since it is cheap, you have to work a little magic. Cut off a centimeter, a centimeter or so of the tubing so that the ground wire lump remains intact, pull it down to the insulation. Cut off the white insulation as shown in the picture. Solder the wire at the ends of the cable, trim the excess tin with scissors so that the wire still holds there and put an f connector on the cable so that the ground wire lump touches the connector sleeve, but not the signal wires - secure the connector with a few layers of electrical tape. Screw a connector jumper into the connector, preferably a straight 50ohm bnc to -f connector reduction. Check both ends with a multimeter. It takes a little practice, but it will work in the end. You can also use classic 75 ohm coax and put reductions to -f connectors on them. Which is a better way for a beginner.

Shortwave, or also international, is a wonderful space where you can take a break from the annoying media landscape and the internet madness. Anyone who likes world music, English DJs from the original British pirate stations (they now broadcast legally on leased frequencies), will find an oasis in the desert here. A good radio is then like a camel that will lead you to the oasis.

This guide was partly the result of me persuading the open source application deepseek to help me build this system. It often listened and expanded on my initial ideas, or explained certain physical phenomena that I didn't fully understand at first. Otherwise, my process was a classic example of trial and error.

I also want to thank Radio Workshop G4NSJ for his help with the antenna. The idea of ​​two binding wires came to me after watching his brilliant video shows.