378
Item nr.
Anti Parasitic magnetic loop antenna
| Production | France, 1950. |
|---|---|
| Bands | GO, PO, OC. |
| Tubes | EF41. |
| Cabinet | Bakelite. |
| Power | Powered from radio. |
| Documents | Michel's article on the Rex P3. |
Because loop antennas give various advantages over wire antennas,most modern radios contain their miniature counterpart, a ferrite rod. Some radios from the early fifties had magnetic coil antennas, but all radios from 1950 and before just had connections for a wire. For these sets, the Rex was a nice set top addition that could improve reception greatly. The signal from a magnetic antenna is weaker but cleaner. First, the antenna is directional so it can be rotated to have an interfering station in a null. Second, it reacts to the magnetic field, while most home appliances interfere mainly in the electrical field. Finally, an external loop antenna usually comes with an additional tuning circuit, reducing interference from stations at other frequencies (adjacent channel or mirror).
The Rex antanna consists of just a single turn to pick up the magnetic field, switchable coils to combine this turn into a LW, MW, or SW coil, and a penthode amplifier, whose output is connected to the antenna jack of the radio. No power supply is needed, as the necessary B+ and filament power are taken from the radio. (For this reason, these coil antennas come in separate versios for AC or ACDC radios.)
| Obtained | 6/2014 from Marc Chaerle. |
|---|---|
| Condition | 7. |
| Disposed | Sold 4/2016. |
| Sound sample | PLAY SOUND  This sound clip demonstrates the directional effect of the Rex antenna, connected to a Philips BX594A. You hear a voice (Mike Oldfield singing Moonlight Shadow) from moderately far away (Waver in Belgium, RTBF1), carried away by thunderlight shadows. After about 20sec, I rotate the antenna and you hear the station disappear and parasitics coming in. With a non-directional antenna, you would hear both all the time and not be able to separate the two. |
Inside is a small chassis with the EF41 tube. I have replaced all paper capacitors, and introduced a modification to allow power to come from an external supply. When the antenna is rotated, the entire tube chassis rotates with it in the box. Standard connection of the Rex is a three-wire cable that connects to a special base inserted between the output tube of the radio and its socket. This base taps the filament power (6.3V) and plate voltage (250V) from the radio circuitry. But, because the grounded filament connection differs from radio to radio, the filament of the EF41 tube is kept floating in the Rex: originally, none of the two blue filament wires connects to anything else in the Rex. So it receives B+ from the special base, and its zero (also for the plate supply) from the antenna/ground connector. This is all logical in 1953, when people had just one radio and were not worried about safety. But in 2014, if you have 20 radios, one cannot get 20 different connector bases, which lead me to a modification.
Caution: In my Rex, the black wire in the power plug is connected to the black ground wire of the output. I modified my PlaatLader to contain a three wire power supply: black is common ground, blue is 6.3V AC, red is 250V. I can connect the Rex to this supply, and then use the output of Rex for any radio that I want.