W3NQN 600 Hz CW Filter
Product Evaluation Report
Jack Ritter WØUCE
While talking with well known filter expert Ed Wetherhold, W3NQN, about trying one of his 160 meter Low Pass Filters in hopes of overcoming a suspected BCI noise problem when using separate receive antennas on Top Band; Ed happened to mention he had recently completed constructing several new CW Audio Filters based on design information in the 2006 ARRL Handbook. While his CW Filter was not related to the problem at hand, I became interested as Ed explained his passive CW filter had 3dB bandwidth and should provide enhanced selectivity and noise reduction without ringing.
As a die hard CW operator I asked what he expected the filter would do to help work weak signals on 160 and when Ed explained the filter’s capability to reduce noise and improve selectivity my interest peaked. Later in the conversation ED asked if I would consider evaluating one of his new CW filters and in turn provide him an operator perspective of resulting performance. Ed said he would set the filter center at 600 Hz which is close to my preferred CW sidetone frequency. My ham radio focus is solely operating CW versus other modes or design engineering aspects of our hobby, thus I was somewhat skeptical about what an audio filter could provide versus the DSP capabilities in my ICOM 756 Pro II and Pro III transceivers. I previously used a commercial audio filter with selectable settings but put it aside after purchasing an Icom 756 Pro II and later a Pro III which have excellent DSP capabilities, however, I agreed to give one of Ed’s filters a try.
When the filter arrived, I opened the box and immediately noticed the device was packaged in an oblong cardboard box and appeared to contain what looked like telephone inductors which raised a skeptical viewpoint about audio filtering versus DSP. Then the question “Why take a step backwards in technology?” popped into mind. My memory went back to long-ago days when I was working in outside plant engineering with a Midwest telephone company which, like many TELCOS of the day, used 88mH inductors in wire line trunk design and I said to myself, “this is just not going to do much to enhance CW reception compared to DSP.” The filter description Ed provided states the 600Hz CW filter is designed for use with an 8 ohm audio system and has 3dB bandwidth at 260 Hz with 2.9dB insertion loss. The filter consists of seven inductors, two 8/2000 ohm transformers, a DPDT switch, phone jack and connector cable with a ¼” audio plug which connect into a receiver’s ¼” headphone jack.
Initial Filter Tests
The W3NQN filter is certainly plug and play… I simply inserted the filter input plug into the receiver headphone jack on the Pro II and selected switch settings labeled Filter In / Out to engage the filter. The first few times I tried the device conditions on 160 were fairly decent, some QRN and not much band activity was present. Using the filter with my 756 Pro II while opening up the selectable DSP filter settings to wide, I tuned across the band and immediately noticed the normal background noise and QRN crashes were significantly reduced. I then switched the filter to my Pro III which has improved receiver capabilities over the Pro II. I was quite surprised to find Ed’s filter made the Pro II receive selectivity and noise reduction equal to the Pro III.
Over the next few weeks I used the filter in conjunction with and without the DSP capabilities in both ICOM rigs and my enthusiasm for the filter increased. I also used the filter on 80 meter CW and found I could equal, and at times, enhance noise reduction and selectivity over using DSP; the filter not only allowed me to hear but clearly copy weak signals through QRN and QRM. For months a Pirate Commercial Station had been operating on 3567 Khz, starting at 02:25 UTC, calling on CW using split operation; and once the outstation answered, the Pirate went into RTTY mode. At times QRM from the Pirate interfered with 4th region net to the point it was necessary to shift frequency. Using Ed’s filter I have been able to completely eliminate QRM from the Pirate station when it operated CW or RTTY which enabled me to conduct the net even under noisy conditions.
Proof is in the pudding – “Open mouth, insert foot”
After three months of using W3NQN’s CW filter on 80 and 160, it was put to the test during the recent FOC Marathon and CQ DX Contest. I have to honestly admit Ed’s filter performance under contest conditions ended any and all skepticism. I certainly put my foot in my mouth by thinking “what can an audio filter do over and above DSP.” Had I not used the filter during the FOC Marathon there were any number of stations I would not have copied reports from on the first attempt due to member activity on upper bands where several stations were calling and making exchanges in close proximity. The W3NQN filter was responsible for me working a large number of weak multipliers on 80 and 160 meters; it was especially beneficial on 20 meters during the ARRL CW Contest with such crowded conditions that I otherwise would have been unable to hear weaker stations let alone work them.
Late in the contest on 20 meters I had difficulty finding a spot to call CQ. I tuned high in the CW portion of the band, turned the LPDA toward Europe and found a narrow opening between two “Big Guns”. I called CQ which produced immediate response from multiple weak Europeans some of them signing /QRP. I had a nice run going until a US station just a state away came close to my operating frequency and started calling CQ. Despite sending QRL with 1kw several times the station either did not hear me or chose to try and run me off the frequency. He or she did not succeed! A flip of the W3NQN Filter In switch and the persistent “invader” no longer prevented me from working weak European responses to my CQ.
Saving the best for last.
What is even more amazing about the filter took place during the CQ 160 meter contest just after I received the filter from Ed and I was able to work many weak Europeans even under 20dB over 9 US stations operating nearby. Early Sunday morning of the contest weekend I was doing search and pounce trying to find a multiplier or at least a station I had not yet worked. As I tuned up from the bottom of the band, I was thrilled to hear a JA calling CQ. Just as I was about to enter his call in the logging program a very loud US station started calling CQ very close to the frequency which completely obliterated the JA. A flip of the switch engaging Ed’s CW filter coupled with a slight tweak of the tuning knob on the Pro III and the JA was perfectly readable. I hit the F4 key to send my call and to my delight the JA answered on the first call. Not only did I work the JA under heavy QRM, it was my very first JA in the log since I relocated to North Carolina. I have W3NQN to thank for this special log entry…
Evaluation Summary
W3NQN’s 600 Hz CW filter may employ long-forgotten 88 mH TELCO Inductors, be packaged in an oblong cardboard box and have but one In/Out Switch; one thing I know for sure, this filter has become a most welcome and appreciated CW Operator’s tool in my shack; it is going to stay here despite all the technology in my current radios. In fact I have re-configured station audio input / output cabling so with the flip of a switch I can use Ed’s CW filter with any of my three transceivers be it either 756 Pro III or the recently acquired TS-480.
Bottom Line
Follow up conversations with ED indicate he may offer his CW Filter to the amateur market. Regardless of the number of rigs, make or model of any transceiver or receiver in your shack; even if they have DSP or not, and you want to enhance operating pleasure and receive capabilities, I highly recommend investing in one of Ed’s CW Filters.
Ed’s contact information is:
Mr. Ed Wetherhold, W3NQN
1426 Catlyn Place
Annapolis, MD 21401-4208
Telephone: 410 268 0916
e-mail: faye.ed@comcast.net
A First Look at the filter
