HFU HF Underground
Technical Topics => Part 15 AM and FM Station Operation => Topic started by: tybee on June 05, 2025, 0318 UTC
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It kind of surprised me to see this 1947 Adaptol Phono Oscillator on ebay that has a broadcast range from 1100-1700.
https://www.ebay.com/itm/197193149980
(https://i.ebayimg.com/images/g/AnwAAeSwOyZn9qL3/s-l1200.webp)
At first I thought that was odd, since the expanded band didnt come into existence until the 1980s(??), but 1940s advertisements confirm it - then I got to wondering if maybe they simply splattered across the entire AMband (for about 70 feet) and you just tuned to whichever.. like tunnel radio did.
I know there were phono-occilator designed for specific AM bands, but apparently (seemly anyway) also those that covered the entire band?.. I mean surely this thing wasn't tunable, was it?
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If I remember right, 1700kHz was police dispatch in the 1940's. This 'lil OCS, one-tube-circuit, looks very simplistic. I'll bet that hum, drift, and electrical shocks, all came from this. However, still neat to check out.
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So apparently it wasn't uncommon for the public to ease drop on police communications during the 1930s but the police had already quit using 1700 AM by1940. According to Evolution of AM http://www.antiqueradio.com/Evol_AM_Ripley_12-99.html
".. Many antique radios can pick up this expanded band. Back in the 1930s the so-called "police band" included the 1600-1700 kHz space, and a lot of the old AM dials were able to tune up to 1700 or even 1800 kHz..." And at the Radio Historian http://www.theradiohistorian.org/police/police.html
"... By 1940, most police radio systems had converted to FM with Motorola equipment..."
So anyway I guess it's not so odd there was part 15 AM on 1700 during the 1940s since the radios were capable of receiving it. - But it still strikes me odd. Well just for kicks here's a glance at every AM expansion date:
1921
The AM band featured one frequency at 833 kHz
1922
Two more frequencies were added, one for special reports (619 kHz) and the other for entertainment (750 kHz)
1923
The band expanded to 550-1360 kHz
1925
Expanded to 550-1500 kHz
1941
Expanded up 550-1600 kHz
1952
Expanded down 540-1600 kHz
1995
Expanded up and down 530-1700 kHz
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Of note: In 1977 when TIS came into being they were granted permissions for 530 kHz (and 1610) but, if Im interpreting correctly the band wasn't officially expanded until 1995