Members of AMRAD travelled to Nags Head NC over the weekend of January 15-18 1999 to conduct LF listening tests. These tests were to test antennas, receivers, filters in an ideal listening environment. Noise mitigation methods were tested to see how the ultimate LF noise floor might be reached.
European amateurs who are now licensed to operate on amateur LF frequencies will be transmitting at coordinated times for the tests to include amateur transatlantic listening tests. These tests are reminiscent of the 1921 transatlantic tests conducted by US and European amateurs. Hopefully, we can report the first transatlantic amateur LF reception for over 70 years.
Participants brought various parts of an LF receiving system to allow test and side by side comparison leading to a "best of breed" set of components under almost idealized conditions. This lead all participants to better understand the unique nature of LF and how to best build a good receiving system and avoid pitfalls.
One potential problem with the Outer Banks is that a 600 kW Loran-C transmitter operates at Carolina Beach NC which is about 150 miles south of Nags Head. It transmits a pulse signal and there is a potential problem with the signal having residual sidebands at 136 kHz where the European amateurs operate. Loops with deep side nulls were expected to work satisfactorily with this. Omnidirectional e-field probes could have a problem. Once these pulses overload the front end components of a system they will be all but impossible to remove with downstream filters.
Another problem is power line noise from high tension lines that run the length of the Outer Banks. Insulators tend to get salt encrusted from the sea water spray. Special insulators that seem to be designed to mitigate this problem are installed so the problem may not be as bad as in the past with conventional insulators. If a problem is due to a single errant insulator we can move up or down the beach to get to a more quiet area.
We set up on the beach at a site near Oregon Inlet that is about .5 to .75 miles from any power line and in a desolate area free of most manmade noises except for the power lines
Some of the things we brought:
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.
2. So far 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 OíNeill ran a small inverter without filtering and did not have an interference problem with his RAK receiver.
3. Active antennas can cope with the Loran-C pulses from the 550Kw transmitter at Carolina Beach. The Burhans circuit 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. The N4ICK tuned efield probe also worked well in the presence of the Loran-C pulses. A couple of converted Loran-C couplers were modified to remove the 100 kHz tuning to broadband them for the whole LF band. 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. 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?
4. 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 detectability of weak CW signals. In general, I found the receiver worked very well with the limitations mentioned above and elsewhere about the power voltages. We overcame these limitations.
5. The carbonyl iron Loopstick antenna 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.
6. The use of the Toshiba 425CDT notebook computer with the DSPFFT program was a real asset for looking for 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 DSPFFT does not suffer.
7. 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. It 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.
8. The DSPFFT program from Mike Cook, AF9Y, 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 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.
9. We were not able to get really low noise where we could see if the big 8 and 10 foot loops reach down and get signals when the lesser efield 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 efield probes would not. With particular care about noise in an efield 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.
10. It is an interesting happenstance that the azimuth to Worthing UK 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 efield probes did not pick up even a hint of the G3LDO signals from Worthing or any other signals at the noise level.
11. 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.
12. 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, 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 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.
13. 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 this problem could be overcome but it raised a problem of secure 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. I want one of these generators.
14. We were some Ĺ mile from the power lines at the Oregon Inlet site. 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?
15. 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 an 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.
16. 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.
17. The beach has so much sand. It gets all over everything. Donít 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.
18. 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.
19. 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.
20. Get a short beach chair for use in the van. A regular chair has me hitting my head on the ceiling all the time.
21. Bring a towing strap so if one of us gets stuck in the sand we can pull him out.
22. The N4ICK frequency marker was very useful at having a means to independently verify frequency calibration. In the future, smaller, lower power (TCXO?), built in batteries that will run all night. Keying rate of one second on and one second off would be useful. If we are integrating with FFTDSP then the four second on and four second off rate might be needed. Perhaps a switched set of rates would be useful.
23. A box with an LF amp with about 10 dB of gain with a good high intercept point. This would have let me try more with my carbonyl iron loopstick antenna. Iíll add gain to that antenna but a separate plugable box may find other uses in the future.
24. An 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. 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 which was adequate for many uses. I have a metal sound booth that may suffice.
Frank Gentges K0BRA