Modern Ham Problems

Our typical RF environment

The work that I’ve been doing on this new transceiver has been very helpful in my Project Yamhill work as well. Not only has it been a great way for me to get back into the groove of making a radio after quite a few years of down time, because a lot of the work is parallel to the needs of Project Yamhill, it’s allowed me to find weak spots in potential Project Yamhill circuits that need to be corrected now, before I get further into the design of it.

The last thing that the new 80 meter CW transceiver needed was some low-pass audio filtering, as there was just too much high-frequency AF hiss to make listening to the radio pleasant. In the spirit of the general theme of this radio, I decided to try something I haven’t done before: a passive audio filter made from high-value inductors and film capacitors. The older ham homebuilder literature regularly features these types of filters, but it seems they have mostly been deprecated in favor of op-amp filters. I am planning on creating some op-amp AF filters for Project Yamhill, but I wanted to investigate if it would appropriate to include a simple passive AF filter board as well.

So I combed my book collection for some example circuits, but eventually decided to create a design from scratch using the Elsie filter design software. The design ended up being a very typical 7-element Chebyshev low-pass filter, which enabled me to use the 100 mH inductors that I already had on hand in order to get a cutoff frequency of about 1.2 kHz, which would be fine for this application.

100 mH Power Inductors

After squeezing the new filter onto the existing audio board of the new transceiver, I did a rough measurement of the frequency response with my waveform generator and oscilloscope and it performed as I expected. However, listening to the audio was an entirely different story. The new filter injected a ton of power line hum into the audio, and a fair bit of other hash too, probably from the various chargers and LED lighting systems installed in our home. This clearly was a big problem.

I tried isolating various stages and components to see if I could tell exactly where all of this unwanted noise was ingressing the receiver, and it seemed pretty obvious to me that the 100 mH power inductors were the source. In retrospect, I shouldn’t have been too surprised. These parts are not shielded, and are designed for power supply use. Is it that shocking that a bunch of free-floating RF in the vicinity coupled into these devices?

While I was in the process of troubleshooting, I was taking measurements with my scope and inadvertently touched the probe tip with my finger. This is what I saw:

Yeah, that’s the RF environment in our newly-build home. I have no idea exactly where that is coming from, but it’s nasty. Perhaps that’s anomalous, but I bet a lot of folks have similar conditions. Plus, I’m in a rural location now, and most people using a radio will be in urban or suburban conditions. I know this topic has been beat to death in our circles, but the proliferation of LED lighting, switching PSUs, and all manner of battery powered devices with their associated charges and BMS means that we have to design our way around such a hostile RF environment.

I even ripped out the offending LPF and designed a high-pass filter using the same inductors using a cutoff frequency of about 300 Hz to see how these inductor would react. Would the filter do its job? As it turned out, that filter caused just as much unwanted noise as the other one. It seemed as if just having these devices in the audio chain was enough to induce massive amounts of mains hum and hash, even if configured to theoretically filter out such noise.

Clearly, I was going to have to abandon circuits using such devices. I’m sure they did the job before the advent of the previously-mentioned circuitry, but it doesn’t seem to be viable now…or am I wrong? If any of you have successfully used these inductors in audio filters in rigs in use now, I’d love to hear about it.

New 80 Meter Transceiver On The Air

Because I didn’t want to use any integrated circuits, I decided to replace the passive AF filter with an active filter, but instead of using a unity-gain op-amp, I used NPN transistors in emitter-follower configuration. This is pretty much the same filter I used on the qrp-l Group Project transceiver (any of you remember that?) 15 years ago. That allowed me to use cascaded RC networks without the massive loss you see in a passive RC network, which seemed to do the trick pretty well. No more mains hum. The roll-off of this filter is a bit gentler than the passive filter, but it cuts out the most offending high audio frequency noise.

Last night, I hooked the rig up to a newly installed antenna which I had inherited from my uncle, WB6JAD (SK), a Chameleon CHA EMCOMM II end-fed. This antenna will allow me to get on the air on the lower bands, something that I couldn’t do here at the new QTH before because all I had up was a Cobweb antenna that covered 20 meters through 6 meters. The center of the receiver passband is at 3.5815 MHz, which is the W1AW CW frequency on 80 meters. I haven’t tried to listen to W1AW on 80 meters here in Oregon for a long time, and I recall it being a bit of a crap shoot, which isn’t surprising given that we’re on opposite sides of the continent. To my delight, at 7 PM local, I was able to hear the code practice transmission at about RST 539 with some moderate QSB. You can hear it for yourself, along with some adjacent digimode transmission another kilohertz or so up the dial at about the 25 second mark:

Not bad! The background noise is a combination of a little bit of residual RF hash and atmospheric noise. You definitely get the telltale increase in noise when you plug in the antenna to the rig, denoting that the sensitivity is good enough for on-the-air use.

Very soon, I’d like to try to make a QSO or two some evening when it’s not ARRL code practice time. I may self-spot on qrpspots.com when I give it a try here in the evening on the US West Coast. If any of you in or near western Oregon would like a sked, please let me know, I’d be honored to try to work you!

What’s Next

Now that I know not to bother with a passive AF filter, the next board design coming is an op-amp AF filter design. That shouldn’t take too long to bang out, and should be the topic of my next post. After that, I’ve got a few things to tweak on a few of the existing designs, and then I’m going to lay out a board for the new transceiver. Once those are done, I will be ready to order some initial PCBs from the fab, after what seems like quite a while. Exciting times!