Radio Frequency Noise Cancelling

By this time we are all familiar with the technology of noise cancellation.  The concept is that the noise is sampled along with the desired signal.  The noise is then turned 180 degrees and mixed with the received data to cancel out the noise, leaving only the intended received signal.

There are several products on the market that do this.  Here is just one device in particular that I have built from a kit.  This is not meant as an advertisement, but only a testimonial to the technology.  The QRM-180 kit by AG6YJ is a compact and clever kit.  The packaging is superb, documentation is good, and assembly is easy.

The noise cancelling device accepts two inputs.  The TX/RX antenna and a Noise Antenna input.  There is also a transceiver connection that goes to your radio.  In Transmit mode the signal from the transceiver goes straight out to the TX/RX antenna and can handle the power of a 100watt radio on HF.  Naturally a PTT is required to switch the device from receive to transmit mode.  This re-routes the RF from the radio straight out to the main antenna terminal.  A well designed device will also have RF detection to switch to transmit if a small amount of RF is detected on the radio output.  This prevents blowing out the circuitry of the device.  The QRM-180 uses tiny 6 volt incandescent lamps as fuses.  If something goes wrong, the fuse blows before the circuitry is damaged.

The receive function is what we are really interested in.  Noise cancellation will work only on local noise that you can receive with a sense antenna.  It won't be able to remove atmospheric noise, and it won't improve poor transmission quality.  If you can't eliminate a noise in your area by going around the home turning off things looking for the source, or if you have an essential device in the house that cannot be turned off, or the noise is somewhere in the neighborhood that you have no control over, then noise cancellation may work for you.  Removing noise allows weaker signals that are below the noise level to be heard by the receiver.  It may not be as effective as moving out into the country into your own private "radio-quiet zone" to escape the RF pollution in your neighborhood, but it will help.

The noise antenna can be anything that picks up the noise.  It can be resonant on the noise frequency, a length of wire, or a small telescopic mag mount.  It should not be resonant on the band that you are trying to receive on.  I small telescopic mag mount works for me on most bands.  A longwire strung outside works for some of the lower bands like 160m.  You have to find out what works for your troublesome noise.

The noise will be brought in and amplified and available for phase adjustment.  It will then be flipped in a 180° phase inverter.  The noise now becomes "anti-noise" which is the exact opposite amplitude of the noise with the same waveform.  The Main antenna will receive both the intended signal and some of the noise.  The signal is amplified and available for phase adjustment.  Tune off any strong transmissions and just listen to the background noise for a moment.  If you have an SDR you will be able to see the waterfall and the noise floor.  When the gain is turned down on both the QRM-180 and the RF gain of the radio, the phase adjustments (marked as ø in the diagram) can be made so that a null is seen on the S-meter of the radio.  In some cases the null may be very sharp and pronounced.  On an SDR you will see the noise floor across the band drop.  As the phase adjustment knobs are turned a null may even be visible moving up and down the band as the knobs are turned.  When both phase knobs are in the right position the noise floor will indicate the lowest S-meter reading.  This is an iterative process, and will also mean adjustment of the gain knob.  When the "anti-noise" amplitude matches the noise amplitude contained in the received signal the re-combination of those two signals (marked as x in the diagram) will pass the intended signal to the radio.  The removal of the unwanted noise signal is done by destructive interference.  There is a gain adjustment for the noise sense amplifier in the back of the unit, but unless the noise is unusually strong, the manual says to leave this gain up the whole way.

Now tune the radio receiver VFO to the desired signal.  Again adjust the gain knob to get the best signal with a maximum amount signal and minimal noise.  Turning the gain up too high will just make it worse again.  This seems complicated at first, but after using the QRM-180 for a few days the process becomes natural.

Reduce the gain --> find the null with the phase adjustment knobs --> adjust the gain for best signal to noise ratio.

Let's look at some real results on the QRM-180.  There is something in my surrounding neighborhood that creates a fairly high noise floor.  There are a lot of houses around and who knows what noise-producing products people have nowadays!  Adjustments within the band should only be minimal if at all once the noise is nulled out in one section of the band.  Here we see the different when I turn on the QRM-180, adjust the gain down and find the null.  With the gain down the whole way both signal and noise are attenuated which doesn't help.

I see the most benefit on 17, 20, and 40 meter bands using the telescopic mag-mount antenna as a noise sense.  Here we see the signal on the Flex waterfall with the QRM-180 turned off.  The input signal from the Main Antenna goes straight to the receiver, noise signal and all.  The S-meter reading is fairly high at S5 and we see only a few signals peaking up above the ugly noise floor.

Some gain is needed to find the null.  Once the null is found the amount of gain is a judgment call for the operator.  Using the QRM-180 I have been able to hear stations that were previously buried in the noise floor.  That's the beauty of it.  As the adage is told from the days of old, "You can't work them if you can't hear them."

This device works so well, I'm not sure why modern transceivers don't come equipped with this technology built in!  Digital Signal Processing and Software Defined Filters can only do so much to enhanced the readability of a signal or reject QRM from adjacent stations.  Notch Filters are great if the QRM is very narrow.  For total broadband noise the QRM-180 has allowed me to hear stations that I would have never heard without it.
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ICOM HM-98 mic mod

I don't know why ICOM didn't design a lot of their radios to mute the mic when it is keyed from the data port in the back.  Its easy enough to overcome this problem if you have a micro-miniature relay.

I bought a couple NEC ud2-5nu 5 volt coil relays.  They are VERY small and are double-pole double-throw.  This way the PTT button can trigger the relay coil, and there are two available contacts on the relay.  The first set replaces the PTT button.  The second completes the circuit on the mic element.

This mod is not for the faint of heart.  I find that using a diamond-tipped rotary engraver tool to cut the board traces works very well.  Once the trace is severed the protective coating can be scratched away leaving enough bare copper to solder some small wires to.

I cut the board traces in three places.  One in the hot side of mic line.  Second on one side of the PTT button.  Third on the other side of the PTT button.

The coil minus side goes right to the common ground plane of the board.  This gives the relay something to hold it in place.  I put it "dead bug style" on the top of the board, since there is plenty of room in the mic case to mount it.  The + side of the coil is fed from the PTT button with the red wire.  The other side of the PTT button goes to the +5v supply, with the yellow wire back to the 100uF capacitor.

Relay contacts fed with the black wires replace the PTT button.  The other relay contacts bridge the place where I cut the mic trace using the yellow wires at the bottom of the photo.

It may not be pretty, but it works pretty well.  The Q1 transistor seems to have enough capacity to supply the hungry relay coil another 20mA of current.

Now I can run my Signal Link for our local NBEMS practice net on FM using my favorite IC-207H radio, and not have to unplug the mic every time the SignalLink keys the radio.

Amateur Packet Radio

So you may have heard about packet radio in the distant past, but what is it today?  Packet radio became a reality in the early 1980's when ASCII (American Standard Code for Information Interchange) was approved as a method for data transmission over the air.  It was in its prime until the 90's when the internet took over.

Yes packet radio is the basis for APRS (Automated Packet Reporting System) on 144.39 Mhz, but packet is more than just unconnected packets.  Packet can be connected packets finding their way around the world by a hierarchical method of location addressing.

The intricacies of packet are best explained by this series of documents written by WB9LOZ.

Packet is not dead, it is alive today and passing information for the National Traffic System, Bulletin Boards, and personal messages between hams.  Packet can get through when the internet can't.  Packet is error-free, it may have re-tries but when it does get through, the message crosscheck insures that the data is intact.

Look at all the packet activity around the world on the nodemap.  There are more than what are shown here.

http://nodemap.g8bpq.net:81

Packet operates primarily on RF via the 2 meter band for local hops, and 20 meters for longer hops.

• Typical frequencies are on FM 2 meter band: 144.93, 145.01, 145.03, 145.05, 145.07, 145.09 and 145.53 MHz running 1200 baud.

• HF BBS in North America, you can find the "Network 105" BBS on 14.105 MHz LSB running 300 baud.

• There are backbone links between BBS's on 220 and 440 Mhz as well, but most of the local activity for users is on 2 meters.

So how do you get in on this technology?  All you need is a 2 meter FM radio and some knowledge of where the packet activity is in your area.  You can use a SignalLink, or any sound card as a modem.  Sound card modem software packages are SoundModem and Direwolf.  You can use a Terminal Node Controller that was purchased at a local hamfest or fresh off the shelf of your amateur radio product dealer.  The favorite TNC is the Kantronics KPC-3.  This TNC contains the appropriate firmware to maintain a Bulletin Board System (BBS) within the TNC itself, requiring no external devices other than a computer with a terminal program to access the data over an RS-232 serial port.

What if you want more?  What if your QTH is the only one with access to the BBS backbone of the packet radio network?  In that case you may want to become a SYSOP (System Operator).  This is the next level of BBS system.  It can accept messages and traffic from BBS's via the Internet or from an RF source.  It can distribute the internet gathered bulletins and pass messages back through the BBS backbone.  In this case you want to look into BQP32.  Written by G8BPQ this fully featured software suit will enable your node to become a fully featured BBS net/rom.  And there are versions for Linux and Raspberry Pi as well.

If the documentation seems overwhelming, or unclear there is a shortcut for Linux users.  Run the bqp-config configuration software from AC0KQ.  Run it once, get the settings roughly the way you need, and then don't run it again.

Still confused, need it all step-by-step.  NL7OM has the guide for you.

Here is an excellent guide to each parameter in BPQ by AG6QO.

So after all that configuration you are ready to get online.  My node is CLYPA:N3FIX on 145.030 Mhz, part of the Underground Packet Network of Central Pennsylvania.  This node acts as a digipeater and serves to forward BBS messages and serve the South Central Susquehanna Valley from Cly, PA near 3 mile Island.  The antenna is a classic Ringo-Ranger fully restored and mounted on top of the roof fed by LMR-400 coax.  The rig is a simple 25 watt Azden PCS-4000 courtesy of KA3LJL.  The TNC is a PK-232 MBX running KISS mode operated by a first generation Raspberry Pi running PiLinBPQ software.

• N3FIX-1 is the BBS server
• N3FIX-2 is the CHAT application

 

The humble Ringo-Ranger

The humble CLYPA alias node in the basement running unattended.

K3CHB had a good idea to install the Raspberry Pi INSIDE the PK-232.  This is a pretty good idea, and I may try that sometime.

Resurrecting a "Watt Weasel"

The Cly Institute has been given the opportunity to resurrect a VHF amplifier based on the February 1971 design in QST magazine by W1SL.  Over the years this design has become known as the "Watt Weasel".  It employs two 4CX250B tetrode tubes for an achievable output of 1 kw when driven hard with high plate voltage.  Since the first 500 watts is what counts, the goal of this restoration is to achieve at least that output linearly.  Our benefactor K3WHC has used this amp 30 years back routinely putting out 800 watts.

The original components consist of the PA deck and a Screen / Grid power supply.  The original B+ supply has been lost to time and smoke.  Therefore, the first big missing piece is to design a new B+ supply with at least 2000 vdc capability.  Since our experience at the Institute is with Heathkit SB series HF amplifiers our first thoughts are to employ the technology that we know and trust.  An SB-200 runs roughly the same plate voltage and current, therefore a Harbach Electronics PM-200 kit will work nicely here for the B+ supply.  It is a voltage doubler circuit that contains all the parts we need to rectify and produce the B+ voltage.  The power transformer chosen was an Antek AS-8T800 toroidal-wound transformer.  This should give us 800 vac at the high voltage secondary, (with a tertiary winding available for 12 volts or 6 volts if needed.)  The transformer can be air-cooled to work into its 1.2 safety factor on voice peaks.  Supplying just about half an amp of current will be attainable, and more than we need for our minimum goal.

The PM-200 Harbach supply uses the standard 0-200uA meter movement in the Heathkit.  If we want to monitor both current and voltage simultaneously, all we need is two meters labeled as such.  A quick operation on Inkscape after scanning in the original scale, allows us to trace over it and create the appropriate scale which can be printed out 1:1 scale and put on the back of the original scale with contact adhesive.

Easily removed with four screws so we can apply new artwork

The Harbach boards and toroidal transformer work nicely in the 3U rack enclosure box.  Cutting out the front panel is the fun part which required a 2" diameter hole and lots of elbow grease.

The power supply layout allows for the low voltage windings to be used for a case circulation fan through the back of the rack enclosure.  5/8" standoffs were used to mount all the boards.  As you can imagine the center of gravity WAY to the left side.  It produces a healthy 2.1 kilovolts with no load;  right on design target.

Here are some photos of the expertly crafted PA deck.

Modern Technology dictates that a pair of VDR's be installed at each screen to cathode.

Before

 

Shaft coupler repaired and rotation stops added

After

The next task is to insure that the Screen / Grid power supply is operational.  This supply as it originally came to us, has a full wave bridge and unregulated Screen voltage which balances out around 310 vdc.  In doing some research the Screen voltage around this value may be ok for Class C operation, but not for Class AB1.  Some others have insisted that good regulation is needed on the screen supply, whereas this supply has none.  The voltage is metered, but the current is not presently hooked up.  This Screen current is a good tuning method and should employ the hole in the cabinet for one with a centered needle.  A better regulated supply needs to be employed to hold the Screen supply stable with load changes and reduction of secondary emission.  That means that the original Screen supply will need to be completely re-worked.

The Grid supply as it arrived to us, is apparently half-wave rectified.  It is unregulated and produces about -130 vdc.  (Other articles indicate that this voltage should more more around -55 vdc for Class AB operation.  There is an adjustment pot for the grid supply, but it only changes a few volts.  This seems unsuitable.

To aid in neutralizing the tubes, it will be essential to install a pair of grid current meters to individually monitor grid current.  Presently there is no instrumentation on this line.  A spare conductor run to the 8-pin Jones-Cinch connector will do well to provide individual grid current measurements.  I found some nice edge-reading mA meters for the individual Grid Currents.  They will work in the remaining panel real estate.

The final and perhaps the most important issue is the control and interlocking of all the components.
The VHF / UHF DX Book is an excellent publication to refer to on the requirements of feeding the 4CX tube family.  The control requirements are like this in order of priority.  The availability of the first item on the list permits the second item to start as a permissive.  The Grid is negative, so it doesn't matter.  Danger exists when the Screen supply is on before the B+.  If the Screen starts acting like a Plate, then there will be quick destruction of the tube.  If the Grid or B+ fail, the Screen supply must shut down.

1) Air Flow
   - Engage Fan and wait for flow switch.
2) Filament Voltage
   -  (2 minute warmup time)
3) Grid supply
4) B+ supply
   - interlocked with relay to Grid supply 
5) Screen supply
   - interlocked with relay to B+ supply and Grid supply

The more research that was done on regulation of the screen supply lead to a the need for a better screen and grid supply, which would also incorporate a sequence / protection system.  Tetrode Boards by GM3SEK offered a ready-made solution with everything that was needed according to my research.  The manual is extensive, and requires the boards to be stuffed with the appropriate parts based on the tubes used and design intended.  Ian produces a set of printed circuit boards which offer exactly what is needed for tetrode operation and protection.  This was the right thing to do.  The screen regulator is a shunt type, which can absorb as well as deliver current to the screen.  In an unfortunate case of upset and potential flashover, the tube is protected by absorbing energy through the regulator.  Zeners are too noisy and cannot absorb the energy.  Neons can absorb in the reverse direction but are not precise enough, and cannot guarantee a tight voltage regulation required to eliminate IMD on the tubes caused by variations in screen voltage.

Rough wiring before the final tye-wrap job.

The implication of these boards are that additional voltage is required to allow the regulator to work.  The transformers that came with the old supply would not deliver the voltage or current needed.  A new transformer was required.  Ideally a transformer with a 400 volt winding and two 120 volt windings.  An industrial control power transformer is ideal which is why I selected a Hammond 75VA PH75MQMJ.  One 120v winding will be used as a primary.  The 480v winding is used for the screen supply voltage.  To take the stress off the screen / grid regulation circuits, I put an auto-transformer in front of this transformer to dial in the exact voltage required to the Grid / Screen supply.

Additional markings are needed on the tuning knobs.  This will allow a repeat of initial tuning by setting per the dial.  These dial markings were made in Inkscape and then printed out and adhered to the plate behind the knob.

I may not leave the fan where it is mounted in the photos.  It is loud!

The TX and RX relays are both Tohtsu coaxial units.  The TX relay has to handle the full power, so its got N connectors.  I chose an RX relay as well instead of running separate lines from my XV144 Elecraft transverter.  This allows me to run barefoot with the transverter.  The limitation of the transverter is that is has no ALC input.  So in practice watching the drive level will be critical.

Initial power-up test into a dummy load were good.  The amp was able to idle without internal oscillation or unbalance.  The neutralization was not disturbed in the PA deck, so it must have been effectively neutralized in the past.  The long wait of the warm-up timer before the HV enable relay kicked in was an anxious moment.  All the magic smoke stayed in.

The first on air test was done December 18th, 2019 with WA3USG and K3TEF.  Other than a little mic hum the signal reports came back clean.  The amplifier is operating well within its design limits at 600w out, with no spurious emissions.

The initial relative knob settings are:

Grid Tune = 115

Grid Load = 40

Plate Tune = 134

Plate Load = 13

Kenwood TS-830S - Bandswitches

A customer came to us with this Kenwood TS-830S right out of the 1980's.  This hybrid radio isn't something we see everyday.  It is clean and well cared for, however the band switch failed to select the 40 meter band with any reliability.

There are lots of notes on how to do this, so I won't go into a lot of detail since someone else has documented this procedure very well.  https://www.k4eaa.com/bandswitch.html

• Pull the final tube nearest the center of the radio, and the driver tube.
• turn the bandswitch to 160m and loosen the setscrew
• turn the bandswitch to 30m and loosen the setscrew
• pull the shaft straight out
• unsolder the 3 wires, and disconnect all connectors
• undo the mounting screw and pull the board out.

Suffice it to say that the band switch contacts were not well attached electrically to the board traces.  Many were corroded, or had loose rivets.  I saw suggestions on how to attach them with conductive paste.  This didn't seem like a good solution, when solder is king!  One contact was pretty badly twisted, so that had to be addressed with some fine mechanical adjustment to get it just right.

A fine diamond-tipped engraver was used to clean the corrosion from each contact, rivet, and trace.  It was then easy to flux and solder the contacts directly to the board trace.  I fixed EACH and EVERY contact in ALL the bandswitches, not just one.  They would all eventually all succumb to failure if left alone.  Each switch contact slider was treated with De-Oxit and checked for continuity.  This fix will outlast the rest of the radio.  The plastic parts have already begun to crack.

The RF board wasn't too hard to take out, but there were three wires that had to be unsoldered.  One was hidden under the board.  (Thanks a lot Kenwood!)

All back together and right on frequency.

SB-200 input impedance correction on 10 and 15 meters

I had known that both the 10 and 15 meter band input circuits on my old Heathkit SB-200 linear amplifier were not well matched to my transceiver.  Using this amplifier with a newer radio, would cause the transceiver to fold back on these bands with high SWR.  Referring to the schematic we see that these employ an L-network to attempt to match the input impedance to standard 50 ohm load.

Going through the mathematical exercise for a PI network, which I won't trouble you with here, it stands to reason this would probably work better just like it does on the lower bands.  This more sophisticated matching network filter can have more loss, but that's not something I was worried about.  The limited number of turns on the 10 and 15 meter coil meant that they required some fairly small capacitors to attempt to make the matching come out correctly.  The problem is that those size capacitors are about the same value of some of the stray capacitance I believed to be in and around the wiring.

The plan was to double the number of turns and increase the capacitance required on the Input side of the network.  I then stumbled on this article by PA0FRI who came to the same conclusion.

I stripped the 10 and 15 meter coils from their windings and re-wound with 1mm diameter magnet wire.  Scrounging through the junk box yielded the capacitors that I calculated and were confirmed by PA0FRI.  I didn't have any tuning caps available, so trial and error yielded appropriate values for the output capacitors which had to factor out the stray capacitance in the wiring.

The trick was how to validate and tune the slugs in the inductors.  I've lost the source from a forum post on eHam, but the procedure is to simulate the resistance of the tubes per their data sheet.  So we temporarily place a 220 ohm resistor between the filament lead and then to ground on the chassis.  While manually pressing in the transmit / receive relay an antenna analyzer is used to measure the impedance at the input connector.  After few tweaks of the slugs both bands yielded acceptable impedance at less than 1.5:1 SWR reading.

The final results are below:

Proficiency in "Radio Sport" helps me at work

For those not familiar with the term “Radio Sport”, it is the Amateur Radio activity of contacting as many other stations as possible in a set period of time.  This contest activity involves being able to hear distant stations through noise and interference, as well as exchanging a precise set of information with the other station.  The exchange of information must be exact with no mistakes and no misunderstandings.  The information is logged for future reference to be submitted to the sponsoring organization for points.  Points are for bragging rights, and the sense of accomplishment for success.  The proficiency is good practice in the event of supporting communications during an actual emergency.

In the field of Technical Support, it is important to be brief, but friendly, and always precise.  In the heat of the moment flowery language is not appropriate, sticking to the subject with a balance of courtesy is essential.  The skills I learned in Radio Contesting apply well to situations where asking the right questions without being misunderstood is of primary importance.  The client requesting assistance is sometimes flustered or panicked.  Letting them know you are listening to their problems and calmly providing answers puts the client’s mind at ease.

Speaking on the telephone with a client while they are in a noisy equipment room, is not significantly difficult in comparison to a radio conversation on single sideband with other conversations a few kilohertz up or down the band bleeding into your own conversation.  It does require a good ear for a person’s voice.  Sometimes during a tech support phone call, the person on the other end of the conversation may have an accent.  This is commonplace in radio when speaking with radio operators from around the world.  Getting the questions and answers accurately exchanged when trying to repair equipment using technical terms can be a challenge.  This is where using the International Phonetic Alphabet to spell things out becomes a great tool to ensure accuracy.  If a part or model number is confused it could mean sending the wrong replacement to the client.  Misinterpretations resulting in confusion and delay are not acceptable when a client’s investment is in question.  One should not be afraid to ask the person to repeat themselves or ask them to explain something using different words.  Repeating what you have heard is also a good tool to confirm that the message is received without error or misunderstanding.  This practice is commonplace in radio conversations where noise and interference can often lead to misunderstandings.  Being a parrot provides this crosscheck of accurate information.

In radio conversations the conditions of the band are usually called to blame for any misunderstandings that may arise, whether that is the actual cause or not.  The person’s equipment on the other side of the conversation is not called into question, because that would be confrontational.  The same goes for Technical Support, in that blame is not placed.  Avoiding overuse of the word “YOU” keeps the situation from becoming personal.  Always refer to the problem as a technical task to be overcome, and never succumb to emotional responses when conversations turn away from objective conversation.  My mentor would always say, “They can’t argue with the science.”  If the support engineer remains objective with an attitude of solving the problem logically, the person requesting help usually responses with an attitude of gratitude. 

When time allows establish a good rapport with the person at the other end of the conversation.  Letting them know something about you, or listening to something about them, goes a long way to keeping the conversation friendly.  Listening before speaking goes a long way to let them know they have your attention and best interests in mind.

As it is done in radio the conversation is ended with a word of thanks and good wishes.  Express your gratitude for their patience while working through the problem.  Apologize when required and stick to the science.

This is Eric N3FIX.

73