Noise, The Hunt for RFI

I posted earlier that I have decided that I have power line radio frequency interference which is obstructing my amateur radio reception. The task now has turned to isolating the source.


Step 1. Isolate if source is in my house.

I set a battery powered SW radio to AM mode and tuned it to a frequency in the 80m band where the noise was particularly undeniable. Then I went to the breaker panel in the house and turning off the MAIN circuit breaker.

No luck, The noise level at 3630MHz was at Signal Strength 5 with or without power in the house.


Step 2. What Next?

Directional antenna? and What band?

These are not independent. A directional antenna can be used but only on VHF, and VHF also requires the noise source to be close by. For example a 440MHz Beam and a 440MHz hand held AM mode would do the trick. Examples are captured below I will have to decide if I need to go this route.

Cushcraft A4496S 6 Element 440MHz Amateur Radio Antenna


The Cushcraft A449 6S is an effective, economical choice for your 70 cm FM/Packet needs. Great for vertical or horizontal polarization configurations. Mast shown is not included.

Specifications:

• * 3dB Beamwidth, Degrees E-Plane 60
• * Boom Lenght ft.(m) 2.9(0.89)
• * Forward Gain, dBi 10.5
• * Frequency, MHz 440-450
• * Front to Back Ratio, dB 18

Yaesu VX-7R Tri Band Handheld Amateur Radio Transceiver


Yaesu VX-7R Tri Band Handheld Amateur Radio Transceiver is a triple band (6M, 2M & 440 MHz) HT with extensive receive coverage, and leading-edge features. Power output on 6 meters, 2 meters and 440 MHz is 5/2.5/1/0.5 Watts. It receives the AM and FM broadcast bands, shortwave (AM), VHF and UHF TV audio, VHF Air and public service frequencies.

Features:

• * Receiver coverage 500 KHz to 999 MHz (Cell Blocked)

Step 3. AM Radio from the Truck.

I decided to give the radio in the truck a try. I tuned the AM radio to 1700kHz, the very top of the AM broadcast band. Then proceeded to take a drive around the neighborhood.

My drive route took me past several locations where I thought I may find my noise source; street light standards, the well lighted neighborhood grocery parking lot, several hydro electricity transformers, (the neighborhood has under ground electrical distribution).

What I found was very odd.

There were a few highly localized hot spots quite far from my house.

One was on a housed street about 500m from home, the noise source had the typical 120Hz sounding signature but the source was very localized, as it only presented it self over a <20m class="blsp-spelling-corrected" id="SPELLING_ERROR_11">noise was remarkable strong was; (in descending order)

• In my next door neighbor's driveway
• In my own garage
• In my other next door neighbor's driveway

more to come...

Operating Digital Mode

Over the past six months I have been predominately operating PSK31 mode. PSK31 is a half-duplex digital phase shift keying modulation mode with a signal bandwidth of 31.25Hz. Its uses a varicode to represent ASCII characters and the net speed is about 50 words per minute. PSK31 has a low power requirement because if it's compact bandwidth and low snr requirements.

• Compact spectrum controlled through cosine pulse shaping and modest linearity requirements.

• The baud rate of 31.25Hz is conveniently related to the popular 8kHz sample rate of basic sound card circuits.

• The Varicode used supports the entire 127 cahracter set of the ASCII code.

A bit error rate of 1 in a 100 is achieved at a signal to noise ratio of about 5dB. After propagation loss, if 50W PSK transmission achieves a workable 5dB SNR then a 50W SSB trnasmission with 100 times the bandwidth (3kHz) would achieve a dissapointing -15dB SNR. This simple example serves to present why PSK is well suited for DX operation without the need for significent kWatt transmit powers and beam antennas.

Within the 3kHz pass band of the audio signal, there is enough "space" for up to 40 parallel QSO's. Although this density would be quite uncommon, I have easly worked 20M band conditions where there have been >20 with no difficulty. The program running on the PC will copy all the QSO's and hilight callsigns and CQ calls. Without touching the rig's VFO you can monitor all 20 QSO, and move between them for transmission.

PSK31 was developed by Peter Martinez G3PLX.

Rig and Hardware

• FT2000D or IC-7000, Well yes you need a transceiver. The transceiver needs to be set in USB mode, regardless of the band you are operating. Also, because linearity considerations, it is important that compression be disabled. Usually on the IC7000 I can run 50W PEP comfortably, the FT2000D can easily double this but is not usual operation.
  

• SignalLink USB, the USB to Analog converter, provides interface between the rig and the PC. In this case, the adapter provides the analog converter, and PTT control. This device also provides isolation from Rig to PC. Transmit drive level is easily adjustable during QSO withe the front panel control. This is especially handy when you want to reduce power or give that 459 a bit of a boost.
  

• HP Pavilion Slimline PC, PC used for Ham Radio Deluxe and USB

Software

• Ham Radio Deluxe, Is the heart of the PSK31 modem.

Setup

The setup is quite straightforward, The radio is connected to the Signalink USB with a connector providing audio input and output signals as well as a PTT control to key the transmitter. Connection to the PS cannot be simpler than the single necessary USB cable.




Operating

• Frequencies

Frequencies of operation are agreed as follow

160m - 1838.150 kHz
80m - 3580.15o kHz
40m - 7035.15 kHz
30m - 10142.150 kHz
20m - 14070.150 kHz
17m - 18100.150 kHz
15m - 21080.150 kHz
12m - 24920.150 kHz
10m - 28120.150 kHz

• QSO's and Macros

Many people use macros to help reduce the amount of typing involved. This can be a blessing and a curse of the mode. I personally think is it important to use these to help in the exchange of information, but it is also important to leave room for some personalization outside the scope of the operator's name.

A quiet 40M band example

The image below is of the program DM780 which is part of the Ham Radio Deluxe package. This screen shot shows;

• Vertical window on the left which shows QRZ info for the call sign which was identified.

• Horizontal window across the bottom which is a representation of the 3kHz audio signal from the rig. PSK transmissions show as vertical traces, after a while you can learn to recognize the characteristic shape, as well as those of other digital modes. Clicking on any one of these will set the receiver to that frequency and if a transmission is made it will be on that half duplex frequency.

• The horizontal window on the top is a copy window in which the decoded PSK ASCII will appear.
  

• The horizontal window below it, is the outgoing message window, This is where you can enter the CQ CQ CQ or the reply you wish to send.

• Logging is very slick because it is a one click operation, and information from QRZ is automatically inserted if you are on line.
  

Operation of this digital mode is relatively straightforward and is very rewarding from a DX perspective because of its key properties. Bands can get very busy at times and there are no shortage of new calls. I have had spent as much time calling CQ or QRZ and driving long strings of QSO's as well as looking around and answering calls from others. South America, Africa, all of Europe, the Middle East and Russia are all within reach from my wire dipole. I'd suggest this mode to anyone who thinks that they might enjoy it.

RFI is the worst kind of noise.

Last winter during the evenings there would appear a noise level in my receiver which would practically render my radio useless. I started to realize that something was wrong when I could not hear stations on a local net which others in the city were clearly able to copy. I gathered the following experience;

• Noise was most notable in AM mode
• LSB and USB modes were also effected
• S9+ levels were regularly "achieved"
• Noise was everywhere regardless of VFO's frequency
• Most significant in the evenings but not there in the mornings.

I suffered through the winter and in the spring it seemd to dissipate. Now that winter is back so is my old firend.

I decided to take a fact based approach to tracking this down and this is the start of what I hope is a very short journey, but some how doubt it.

• Noise is most noticeable in the evening hours.
• S9 noise levels are generally exceeded.

The plot above is the result of reading my rig's S-Meter every 10minutes over a period of 16 hours. I measured 2 frequencies 3.940 and 7.058 MHz. (This was done with some programming. I did not sit in front of my rig for 16 hours). Readings started at 11:44PM. 3.94MHz is in Blue, 7.058 is in green. Rig was set in AM mode. I note that at 30 minutes in, the the noise at 7MHz dropped >2 S-units (>12dB) , but then at about 6 AM the noise level creeps up again. 4MHz does quiet to a limited extent but never drops below S9.

• Typical noise waveform has period 120Hz
  
• Harmonics at 240Hz and every 240Hz

The plots above belong to the the audio output of the Rig when tuned to 7MHz. Time domain signal is very typical of that I hear and see. Notable signature is the high frequency content "spikes" which occur at a rate of 120Hz. (There's a good hint) The spectrum of the audio signal demonstrates the harmonic content of the noise. Very broad band and is spread throughout the audio band. This sample was taken just before the 2S unit drop.

• Receiver output becomes free of noise on next 10min sample
  

On the next audio sample after the S-Meter had dropped 2 units, I can see that the noise is completely gone. Humm,... something got turned off? Hold that thought.

• Blocking (filtering) AM broadcast has no impact

PAR electronics makes a handy HPF which is used as a AM broadcast block filter. The image above is the frequency response of this filter from PAR's website. This filter provides >50dB of rejection of frequency content below 1.6MHz. The application of this filter makes no difference to the noise level.


Other things I have noticed...

Source

• Switching FT2000D to Ant2 (w/ no antenna connected) S-Meter goes to zero
• Disconnecting antenna, S-Meter goes to zero

Radios Effected

• Effects battery powered SWL radios same (w/ internal antenna)
• Effects I-COM 7000 and FT2000D the same.
  

House Power

• Tripping the main breaker in the house has no effect.

Linearity

• Goes as >1.5dB/dB of front end loss

General Coverage

• Occurs from 1.7MHz to 17 to 18MHz.
• Tuning VCO generally has no impact
  

Inter modulation

• VRF Filter has no impact

Stabbing in the dark

• Adding common mode choke at Radio has no impact

Final Words

I have decided that what I am looking at is power line noise. and have concluded this after reading the ARRL's description of power line noise. Filtering at the receiver will be of no help as the RFI is broad band and extends across the entire 160M to 40M bands. The period of the noise waveform agrees with this conclusion. As to the intermittent nature of the RFI, I suspect that the interference generator is temperature realted as this would explain the sesional and perhaps even daily cycles. The only remidy is to physically locate the source and seek a remidy at the source.

Next steps will focus on locating the source.

QSO's around the world

The image below was created using Google Earth in conjunction with Ham Radio Deluxe, it shows in a simple form the contacts I have made with my station over the past few months.

A successful 2 way radio contact or "QSO" is a short exchange between 2 operators where each exchanges their Station Call, Name, Location and signal report. The image above shows aproximately 340 QSOs all made from my home in Ottawa Ontario.


Countries to which I have established QSOs are listed below.

Austria
Belgium
Brazil
Canada
Canary Is.
Colombia
Costa Rica
Cote d'Ivoire
Cuba
Czech Republic
Dominican Republic
Ecuador
England
Estonia
European Russia
Fed. Rep. of Germany
Finland
France
Greece
Greenland
Grenada
Guadeloupe
Guatemala
Guyana
Honduras
Hungary
Ireland
Isle of Man
Italy
Jamaica
Lithuania
Martinique
Mexico
Netherlands
Northern Ireland
Norway
Panama
Peru
Poland
Puerto Rico
Scotland
Serbia
Slovak Republic
Spain
Ukraine
United States of America
Venezuela

VE3IAC Station

---- QSL cards


I have finally setup myself with QSL cards. The design was made up in PowerPoint and I print up 4 to a sheet and then slice & dice. This may be the long and hard way, but for now this is it. I've started sending cards to as many of my QSO's that use direct. For the moment, I'm quite interested in gathering these tokens of the great distances I've been able to communicate.

Thanks to all to QSL back.


---- Updated Shack


In the fall I updated my radio shack to provide for a bit more desk space. Radios are the Yaesu 200w FT2000D, truly a basestation radio, the I-COM 7000 a nice 100W all mode HF/VHF/UHF mobile transceiver, and a classic, not quite a boat anchor, Kenwood TS-850AT.

---- FT2000D




I purchased my FT2000D late summer 08. I spent many hours pouring over as much material as I could find to help me make this decision knowing that this would be the radio I'd have for many years. I also looked at the IC756PROIII. In the end I decided to go with the FT2000D.