Way back in 1965, country music star Connie Smith sang about
a "tiny blue transistor radio". The "trannie" was then the height of desirable
consumer hi-tech. The ability to take pocket music and news with you was
near-revolutionary in an era when almost everything electronic was wired to the
mains.
These early portable radios were AM (Amplitude Modulation)
only, covering the 520kHz –1650kHz medium wave (MW) broadcast band, with a
significant part of their appeal due to the inbuilt radio-signal-concentrating
ferrite rod antenna. Mains powered MW radios prior to this era had used bulky
wire loops or lengthy external aerials, neither of which suited portability.
Ferrites are iron-based magnetic materials and an aerial coil
wound around such a rod could be brought to resonance via a variable tuning
capacitor within the radio circuitry itself. They’re convenient and very compact
and usefully offer good broadside directivity, arising from response to the
magnetic component of the radio signal.
However, their efficiency is much less than a traditional
antenna, a fact now often forgotten. Although ferrite rods are further hindered
by an upper frequency limit of just a few MHz, almost all portable radios made
in the last 50 years have used them for MW reception.
1. Medium Wave (MW) AM broadcast band loop antenna. Built using cheap 4-pair (8 wire) telephone "ribbon" cable ( Jaycar WB-1625), and (optionally) housed in cheap garden 13mm irrigation plastic hose. The more rigid self-supporting version is better suited to serious use, as it can better null offending local noise or stations and even DF (direction find) when rotated towards remote signals.
2. The compact version allows easy storage – suitable for portable and traveling needs. Three metres of cheap 8-wire cable will resonate nicely over most of the upper 500kHz -1.7MHz MW Broadcast Band with a common 60-160pF miniature variable tuning capacitor (eg, Jaycar RV-5728). However you should use longer lengths for stations at lower MW frequencies OR add a second capacitor in parallel to the variable.
