| 'What
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:
"As radio
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.
In the case of the National HRO the designers 'went back
to the drawing board', and returned to the system used by
early home-made designs, with plug-in coils. The RF
section was turned sideways so that the coils could be
built into one unit, plugged in from the front. The four
gang tuning capacitor evolved into a gear-driven design
driven at a right-angle from the centre, using a worm
drive and a micrometer dial with an effective length of
twelve feet. This guaranteed accurate logging, although
absolute frequency was a matter of interpreting a graph
on the coil pack. The resulting receiver was simple and
(by the standards of the day) compact, and performance
was maintained up to 30MHz with ease."
The HRO, as designed by James Millen, first appeared
around 1935, and the price of $300 made
it a luxury item until its adoption for extensive wartime
use. Today, unrestored versions of the 'classic' HRO-M
and HRO-5 command good prices.
Last updated 11th November
2002
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