NB: Requires external output transformer of 7,000 ohms impedance and external speaker.
Essentially, the HRO-M was the civilian HRO Senior model.
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Makes the HRO Special?'
A classic military receiver, first built in the early 1930's, the HRO was much favoured by the intelligence community for its high sensitivity and high selectivity - the dial giving an effective scale length of 12 feet (or 3.65 Mickey Mouse Metres). It became a wartime standard and many found their way into private hands as post-war surplus.
The National HRO series of receivers are like no other, at least in appearance! Their distinguishing feature is the use of a removeable coil pack for bandswitching and a large, heavily weighted tuning drive wheel above that gives 10 full turns of indexing. There is no frequency readout althought the front of each coil pack gives a dial/frequency graph for that particular band. However, each chart is calibrated to a particular receiver which does not allow for coilset interchangeability.
But how did it get to look this way? John Hoare, G3PJI, says this:
communication developed in the thirties, designers were
confronted with the conflicting demands of increasing
congestion, higher frequencies and increased demand for
sensitivity. In high performance receivers, the
conventional approach of ganged wavechange switches, plus
banks of trimmers, led to large and clumsy 'front ends'
with ensuing stray capacitances limiting the receiver's
performance. Add to this the need for one or often two
stages of tuned RF amplification, and the solutions
became difficult and expensive.
Last updated 11th November 2002