Nags Head Report #7

[Andre' Kesteloot]

The following was supplied by Frank Gentges K0BRA
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Lessons Learned from the AMRAD LF Test
January 15-17, 1999

POWER

1.  Gasoline generators can be electrically quieted so they do not
interfere with low level LF reception.  Filters need to have heavy
ground straps and big clip leads to ground the filter to a ground
stake and to the generator set.  We had to make do with winding pieces
of wire around and around and around.

2.  A small 400 watt 60 Hz generator was tried out.  It seemed to have
a square wave output of 110-115 volts rms.  The effect was that the
peak voltage on each cycle was too low to charge the NRD-525 power
supply filter capacitors to normal voltage.  Problems similar to the
low DC voltage were observed.  By raising the voltage with a variable
transformer (Variac) this problem could be overcome but it raised a
problem of low impedance grounding and interference appeared where
there was none.  I would like to try a boost transformer configuration
where the integrity of the ground was carried through the process and
see if the interference is suppressed.  Not enough parts to build on
site to try it.

3.  So far [12volt DC-to- 117volt AC] inverters are a mixed set.  Some
are quiet with no filtering.  Others cannot be quieted with extensive
filtering.  The reactive load of the filters on the 60 Hz output seem
to make the switching harder and thus create more interference than
they remove.  Inductive reactance on the DC input filter caused the
inverter to go into a high frequency spurious audio oscillation.  It
was cured by adding a paper capacitor to ground on the DC input of the
inverter.  Richard O'Neill ran a small inverter of similar design from
the same manufacturer (Tripp) without filtering and did not have an
interference problem with his RAK receiver.

4.  The NRD-525 receiver is rated for AC mains operation and for 12
volts DC operation.  Testing prior to the expedition showed that it
worked down to 12.1 volts.  Below that threshold, the internal voltage
regulators start to drop out.  The most troubling impact is that the
output impedance of the internal regulator rises and allows trash from
the fluorescent display circuit to rise to higher levels and this
trash is in the LF band near, or on, some of the frequencies of
interest.   The cure is to keep the DC input above 12.1 volts even
when the receiver appears to be working below that in most respects.
The problem is that a nominal 12 volt lead acid battery under load and
without charging current will drop into the 11 volt range.  I built a
2 amp regulator that would allow operation on 24 to 18 volts obtain by
stacking 12 volt batteries or a 12 volt and a 6 volt battery.  Since
the AC generator worked well without interference, this battery setup
was not tried out.  I am confident would work well as it was tested at
home without problems.

ANTENNAS

5.  Active antennas can cope with the Loran-C pulses from the 550 Kw
transmitter at Carolina Beach.  The Burhans circuit
(Radio-Electronics, Feb., Mar, Apr 1983) was bench tested to a level
of 5 volts p-p on the input before visible waveform distortion.  A
second Burhans circuit was built and it went up to 4 volts p-p.  Both
worked well in the presence of the Loran-C pulses.

6.  The N4ICK remotely-tuned E-field probe described in the AMRAD
Newsletter Jan-Feb 1999, also worked well in the presence of the
Loran-C pulses.

7.  A couple of converted Loran-C couplers were modified to remove the
100 kHz tuning to broadband them for the whole LF band.  They worked
well in the Northern Virginia area but both exhibited overload
characteristics on the Loran-C pulses to the point they would not be
effective at 136-137 kHz.  They should have been bench tested and
modified as needed to maximize the input overload characteristics.

8.  A Sony AN-1 active antenna was tried.  It did not overload on the
Loran-C pulses but lacked enough gain at LF to reach the noise floor
on a quiet night.  A more sensitive receiver than the NRD-525 might do
fine with this antenna.

9.  In commercial and military usage, the 3rd order intercept point is
often measured and used as an indicator of the overload resistance of
an amplifier or receiver.  Should we also equip ourselves to measure
this?

10.  John Reed wrote an article in the December 1998 issue of Lowdown
on Receiver Sensitivity at Low Frequencies.  In that article he
indicated the NRD-525 was not a good performer at LF.  His concern was
the noise floor performance and thought it was probably synthesizer
noise.  He also noted some display noise which I did not have problems
with at proper supply voltages.    With some extra gain from an active
antenna, the receiver seems to perform well.  In addition, this
particular receiver has an added 200 Hz Collins crystal filter that
contributed a lot to the delectability of weak CW signals.  In
general, I found the receiver worked very well with the limitations
mentioned above and elsewhere about the power voltages.

11.  I had previously built a 3-foot long carbonyl iron loopstick
antenna. It worked well in the presence of the Loran-C pulses.  The
gain was a little low so the noise floor was not heard on the NRD-525
receiver. The carbonyl iron loopstick did not have enough excess gain
to overcome the reduced sensitivity of the NRD-525 mentioned above.

12.   I should have brought along a box with a LF amp with about 10 dB
of gain with a good high intercept point.  This would have let me try
more with the carbonyl iron loopstick antenna and the Sony AN-1.  I'll
add gain to the loopstick antenna but a separate plug-able box may
find other uses in the future.

13.  We were not able to experience really low-noise conditions, where
we could see if the big 8 and 10 foot loops reached down and received
signals when the lesser E-field probes might not have. (Bill, WA8LXJ,
has gone on several LF listening expeditions.  He indicated back in
1988 that the big loops would pull out signals at the low noise
thresholds when the E-field probes would not).  With particular care
about noise in an E-field probe design, this may be overcome.  What
the loop also does is to have a null that can be pointed at a source
of noise like power lines or strong signal like a Loran-C transmitter.



SIGNAL PROCESSING

14.  The use of the Toshiba 425CDT notebook computer with the FFTDSP
program by Mike Cook, AF9Y, was a real asset for finding weak
signals.  The alternative would be to slowly tune a few Hz at a time
while listening carefully for the weak signal to appear out of the
noise in a narrowband filter.  As the filter is squeezed down in
bandwidth, all noise tends to sound like a weak signal making narrow
filters a mixed blessing that the FFTDSP does not suffer.

15.  The FFTDSP program decimates the spectrum into 2 Hz cells.  The
ability to reduce this might help to improve signal to noise ratio in
the LF case.  The program was originally designed for weak signal
detection on EME systems and the 2 Hz was probably dictated by the
signal Doppler spread on an EME path.  For LF a fraction of a Hz cell
could function effectively as the Doppler spread is less than with
EME.   So far we have observed slow propagation fading which would
indicate low Doppler spread.  The ability to integrate multiple frames
(up to at least 99) looked to be a useful weak signal feature to
retain.  The program is compatible with the built-in sound card of the
Toshiba 425CDT notebook.  It is in fact the only program we could find
that was.

16.  The N4ICK frequency marker generator which will appear in the
March-April 1999 AMRAD Newsletter was very useful at having a means to
independently verify frequency calibration. This particular
construction featured a 5 second on-and-off keying rate. In fact, a
variable rate, from 1 to 5 seconds, would be useful.  If we are
integrating with FFTDSP then the four second on and four second off
rate might be needed.

17.  On January 17th, some storm-related static was heard.  It was
occasional and quiet periods in between could be used to ascertain if
a signal was present.  The FFTDSP program seemed to work well in the
presence of these and the display did not show a major impact.  On
January 18th, the tornado-bearing storms west of the site created an
almost continuous string of static where it was not possible to hear
any signals in between impulses.  Receiver AGC speed can have an
impact on performance with these impulses.  Generally, fast AGC
settings seemed to recover sooner from an impulse than slow AGC
settings.  Running with the AGC setting off and the RF gain backed off
seemed to work best in terms of impulse recovery.

18.  A narrowband audio filter was built.  The filtering was provided
by some surplus frequency division multiplex FSK teletype filters
being about 85 Hz in bandwidth.  It did not contribute much that the
narrow IF filter in the NRD-525 and the FFTDSP program could not.

19.  The Toshiba 425CDT notebook computer has no external DC power
mode that would allow long term use directly on batteries. The battery
is only good for about 2 hours of processing and the Lithium Ion
batteries are expensive.  That particular laptop has a built-in AC
switching power supply to run the computer off of line voltages and to
charge the Lithium Ion battery.  This supply generates moderate levels
of LF interference. The noise seems to come out of the notebook at the
AC power line connector.  Future use needs to look at some filtering.
A minor roadblock is that the AC connector is only two pins without a
chassis connected pin.


MISCELLANEOUS ISSUES

20.  It is an interesting happenstance that the azimuth to Worthing UK
(G3LDO's QTH) is 181 degrees from Carolina Beach at the Oregon Inlet
site.  The result is that if a loop antenna is turned to null the
Loran-C transmitter at Carolina Beach, its other 180 degree null will
be pointed almost directly at Worthing.   It was not evident that this
unfortunate occurrence materially affected our results since the
E-field probes did not pick up even a hint of the G3LDO signals from
Worthing or any other signals from the UK at the noise level.

21.  At the Oregon Inlet site, we were some 1/2 mile from the power
lines.  We could not hear 60 Hz noise or see the characteristic
squiggly lines on the FFTDSP display.  We must wonder how close we
could get to the power lines before it would become a problem.  What
power line criteria should we have for other future sites?

22.  Early tests at the house at Nags Head demonstrated the value of a
transformer in the RF line to isolate the AC power currents from
flowing to the antenna.  A transformer was wound on Saturday morning.
It had 20 turns of 30-gauge wire-wrap wire on both the primary and
secondary using a FT-37-75 core.  It worked fine at passing on the
signals and blocking the ground currents.  When it was removed at the
receiving setup at the Oregon Inlet site the power from the generator
became an interference problem.  A transformer such as this should
become a regular component in an active antenna coupler.

23.  It is a feature of beaches that they tend to have much sand  (!)
which  gets all over everything.  Don't go to the beach without taking
some precautions.  A box of plastic trash bags was taken along.  They
were used for trash, of course, power, antenna and other cables after
rolling them up from the beach.  A doormat or two might help reduce
tracking sand in at entrances to tents and vans.

24.  Bring a towing strap so that, if one of us gets stuck in the sand
we that we may at least try to pull him out.

25.  The propane "Mr. Heater" heaters worked fine in the tent.  They
produce too much heat to use in a vehicle such as a van.  We need
something that can be throttled down to produce less heat with the
convenience of these propane heaters.

26.  The Internet connection we used on Saturday and worked fine was
dead on Sunday.  It would not validate our password.  We need to go on
these trips with multiple alternatives for Internet connection.  We
missed some worthwhile information by missing our Sunday email dump.
A shell account would do just fine for our needs.

27.  A LF quiet room would be useful for testing setups before going
to the field.  It would allow testing of the inverter problem to
localize the source of interference and verify at least to a gross
sense if it is fixed.  In past incarnations, we used to build aluminum
foil rooms by stapling rolls of aluminum foil on the ceiling and
walls.  They were not full 100 dB screen rooms but would make 50 dB
that was adequate for many uses.

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