[Lowfer] rx setup

[Lyle Koehler]

A few years ago almost all LowFERs were running CW, and it was possible to
tune across the band and find signals without knowing precisely where they
were transmitting, or where you were receiving. Now everything has changed.
On the positive side, we can copy signals that are 20 dB below the level
where they could be heard by ear. The down side is that you may need to park
on a frequency all night to get a clear copy of a simple 2- or 3-letter
identifier. Very good frequency accuracy and stability are required to
receive LowFER modes like QRSS30. As you can see from some of the published
Argo displays, the range of frequencies displayed in 30-second dot mode is
only +/- 3 Hz. If you can't set your receiver frequency within 3 Hz, or if
it drifts by that amount after you set it, you simply won't see the signal
on the screen.

3-Hz accuracy corresponds to 16 parts per million at 185 kHz, which is easy
to achieve with today's digitally synthesized, solid-state equipment. But if
you shift the signal to the 4 MHz range with a converter, that same 3-Hz
accuracy is less than 1 part per million. That's OK if the transceiver or
receiver AND the converter both have temperature compensated reference
oscillators, but in general I would not recommend a converter for receiving
narrowband LowFER modes. However, you will be able to hear a lot of
non-directional beacons, which is an excellent way to get started and to
test your LF receiving antennas.

Many of the old tube-type transceivers with analog frequency displays were
lucky to give you a readout accuracy of 1 or 2 kHz, about 1000 times worse
than what you really want for modes like QRSS30. I won't go so far as to say
that reception would be impossible. If you never turn off the rig, and use a
crystal calibrator or transfer oscillator to find the frequency, the radio
might be stable enough to keep the signal on the Argo screen for an hour or
more. But in these days when cigarettes must be around 30 bucks a carton,
$500 for a good receiver or transceiver sounds like a great bargain!

Argo or Spectran display the audio frequency output of the receiver, and you
need to know how that relates to the frequency of the signal you're trying
to copy. For LowFER modes like QRSS or MT-Hell, the receiver must have CW or
SSB capability. If you get an audible tone from an AM receiver while tuned
to a CW carrier, it means that there is another signal present, either an
external carrier or a spurious product in the receiver, that is producing a
beat note. Here are some references that may be helpful. The recent article
about the frequency measuring tests tells how to make precise frequency
measurements on an unknown signal. By reversing your thinking, you can
figure out how to set up your receiver to copy a LowFER signal on a known
frequency. Unfortunately the article doesn't describe the use of spectrogram
software like Spectran and Argo, which make the process much easier and
faster. For example, instead of zero-beating an external audio oscillator
with the receiver output, you can simply look at the frequency on the Argo
screen. A local library may have the October 2002 QST. The earlier articles
are both available on-line to ARRL members.

Oct 2002 QST page 53
The ARRL Frequency Measuring Tests
Silver, H. Ward, N0AX

Aug 1991 QST page 28
What Your Frequency Display Really Tells You--Part 1
Newkirk, Dave, WJ1Z
Article: http://www.arrl.org/tis/info/pdf/9108028.pdf (1,395,975 bytes)

Sep 1991 QST page 26
What Your Frequency Display Really Tells You--Part 2
Newkirk, Dave, WJ1Z
Article: http://www.arrl.org/tis/info/pdf/9109026.pdf (2,105,365 bytes)

Lyle, K0LR