A Duo of AM DX Receivers
                                                for the LF and MF Bands

This project began with a pair of old Collins Aircraft (Radio) Direction Finders (ADF-650A). Airplanes can use aircraft directional beacons in the LF band for fixing their
position. The ADF's also are capable of receiving AM broadcasts (MF), since these commercial (fixed-land) radio stations are marked on aircraft flight charts. The AM
broadcasters' antennas (transmitter sites) are a good source of directional location and positioning.
What the original product looks like ... times two.
I decided on coupling the pair with a center module and end ears so that it would become a single space (1U) rack mount device (1.75" x 19"). I disconnected all the
direction finding electronics (it used a servo circuit to work the "sense" antenna against the "DF" antenna for indicator readout of positioning). I also replaced the
grain-of-wheat lamps with blue LEDs, added IF filter switches (5K / 2.5K), audio tone control (LP/Norm/HP), a 2-station antenna feed multicoupler (splitter), audio
mute/standby switches, and an 8-Watt audio power amplifier.
For the photos, I set the radios to the extremes of their receive capability; 200 KHz to almost 1800 KHz. The frequency is set by the use of decade rotary switches. The
Collins filters are superb, but what else would we expect!
The "A" and "B" receivers are indicated on the center module
by red/blue LEDs. Red indicates power 'on/standby,' and blue
indicates 'active.' Above, as you can see, the B receiver is on,
while the A receiver is in standby. The uses (or need) of dual
receivers are up to the user's imagination, but the one real
advantage is that they can be set up in true diversity fashion ...
like the Signal Corps did back in the 1950s ... not antenna
diversity, but true receiver diversity. Feed each radio with
similar antennas located about 0.1 wavelength apart and fading
(QSB) seems to go away - at least, significantly reduced.
Some of the center module details. behind the front panel are the Power and Master Gain controls along with the A & B indicators. Behind that is an 8-Watt (TDA-type) power amp.
At the very rear is the RF antenna splitter/coupler.
There are no Signal Strength meters, although there is the provision for them. It is possible to take the relative differential voltage from the "sense" and "DF" antenna amplifiers and
use that for a relative signal strength indication. I can do that without too much trouble - on another panel - but I think I've probably 'gilded this lily' way too much already.

I have a long history of doing late-night DXing of the AM b'cast band and some NDB (non-directional beacon) hunting -- it's very relaxing. I suppose that none of that sounds
particularly exciting on the surface, but I'll warn you, if you've never been involved in either, be forewarned. It's captivating and compelling.
If you have a decent low band receiver (LF and MF), and an adequate antenna system, then try these exercises:

1. Try to identify an AM broadcaster every 10 KHz (in North America -- 9 KHz in other areas) throughout the band. You may find that it's easier to list the
channels that you don't hear anything, because that's a pretty active place after dark.

2. Slowly scan the LF band looking for NDBs. Decode the CW (Morse) markers and see how many you can log.

Warning: Select a time after the grayline has passed and allow more time that you planned -- it's kind of infectious.

There are many station ID resources available for free on the Internet. I like "Radio locator", "AM Logbook", and "Where's That Station".

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