By Simon Favre
For quite a few years, some AM radios used a ferrite rod antenna, also known as a ferrite loopstick antenna. Ferrite loopsticks are usually a round or rectangular cross section rod of ferrite material with one or more coils of wire wound on them. Sometimes in the course of radio repair it is necessary to make adjustments to the loopstick antenna. This usually only happens when the loopstick has to be replaced with an alternate, or if an adjustable loopstick has slipped. Some radios do have instructions for aligning the loopstick. Many do not. This article presents a technique and homemade tools for this purpose.
Fellow CHRS member, Arden Allen commented on the CHRS forum about his “magic wands” for radio tuning. I duplicated one of his and used it to retune a loopstick that was a substitute for one that was missing in a Grundig radio. The tool is simple to make. A radio grade ferrite is easily salvaged from a scrapped radio. The one I used is from a small transistor radio. I mounted the small loopstick on a phenolic rod with hot melt glue. The wire should be stripped off the old ferrite, but it didn’t seem to affect the usefulness of it. On the other end I mounted a closed loop of wire. The rod needs to be nonconductive and long enough to keep human body capacitance from influencing the antenna.
How is it used? When the radio is basically working, and the normal alignment is complete, the wand is placed near the loopstick to determine if adjustment is needed. If the radio has a magic eye or other tuning indicator, that is a great way to see the effect of the wand. The other way is by monitoring the AVC voltage, which is what tuning indicators do anyway. The ferrite end will increase the inductance of the loopstick. The wire loop will decrease the inductance of the loopstick. The loop of wire has to be placed around one end of the loopstick to see its effect. The ferrite end concentrates the magnetic lines of force thereby increasing inductance. The wire loop acts as shorted turns thereby decreasing inductance. Both ends of the loopstick need to be checked for the effect of the wand. If the ferrite end makes the signal stronger, the loopstick needs more inductance. If the wire loop makes the signal stronger, the loopstick needs less inductance. If the loopstick is correct, neither end of the wand will make the signal stronger, instead both ends may make it worse.
How do you adjust the loopstick? Some have two coils mounted so that one can be moved up and down the rod. The original Grundig loopstick had two coils, but this was missing. Some loopsticks have one coil that can be moved up and down the rod to adjust it. Some have no adjustment.
In the case of the Grundig, I was consistently finding that the different loopsticks I tried had too much inductance. I settled on one that fit the original mount and had the least inductance. This loopstick was non- adjustable with the winding sealed in wax. The “loop of wire” principle was adapted to become an adjuster for the fixed loopstick. A loop of stiff wire was formed to fit closely around the loopstick. The end of the loop was attached to an insulating cardboard piece that could slide up and down the mounting for the loopstick. The wand had revealed that one end of the loopstick was more sensitive than the other. I made the wire loop as a coil of two turns soldered so it was closed all the way around. Once the optimal position for the wire loop adjuster was determined, the loop was fixed in place with wax. With the adjuster in its optimal position, both ends of the wand gave no improvement at either end of the loopstick.
In the case of a loopstick that needs more inductance, a small coil could be put in series, or more wire could be wound on the loopstick. There will be interaction between the loopstick adjustment and the antenna trimmer capacitor adjustment. Repeat the adjustments until there is no further improvement. The trimmer may be found mounted next to the loopstick or connected to it under the chassis.
There you have it. You can use magic wands to fix radios, and you don’t have to take classes at Hogwarts!
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