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Overview

Before putting my recently completed homebrew single conversion 20m mono-bander on the air, I borrowed another radio to listen to the transmission and make sure everything was OK. It turns out, it definitely wasn't and all I could hear on the receiving radio was a high-pitched squeal. I'm hoping it's a simple matter of tuning the BFO on transmit to place the audio correctly in the crystal filter passband (I can pick up stations fine on receive), but it could be something else of course.

Question

Which brings me to the question. Are there time-honoured ways to test the quality of a transmitted SSB signal before putting it on-air? Obviously, having another radio is the easiest way: transmit into a dummy load with a small pig-tail on the receiving rig. But, what if you don't have another radio or a spectrum analyzer with a RBW fine enough to see the sidebands properly (I have a TinySA, but it's hard to see the sidebands)? Are there methods available for those with more basic tools: signal generator and oscilloscope? Happy to accept all methods (including additional receiving radio).

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An inexpensive RTL-SDR with direct sampling capability can monitor the full bandwidth of an HF RF signal (an entire amateur HF band as well), and also be tuned to higher frequencies to check for multiple possible harmonics. Various software apps allow zooming in to the spectrum to inspect signals in fine detail, such as examining for the width of opposite sideband splatter. You can use an airgap between transmitting into a dummy load and receiving with a short sniffer wire to help prevent overloading the receiver, although some care or distance may be required to reduce spurious RF conveyed though any common computer, power, or ground connections. Some transmitters can cause a nearby sensitive receiver to scream, even if the signal out the antenna connector is clean (but the USB or ethernet cable noise isn't). A step attenuator can be added to the receive path to reduce the likelihood of overload, and better determine relative signal levels. You can also use a more expensive SDR receivers such as an Airspy HF+ Discovery or SDRPlay unit, et.al.

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I've heard of people using an old style tungsten filament light bulb as an improvised dummy load.

Up side is, they are (or used to be) easy to get and fairly cheap, and they can handle a bunch of power (100W bulbs are very common). You can get a socket at the local hardware store, make up the wire to connect to your transmitter output, and spend under $20 for the whole project (my dummy load cost about three times that, as I recall, and it's rated for less than a minute at 100 W).

Down side is, you may still radiate more than the FCC would like, because the filament and standoffs in the bulb, plus the split part of the wiring from the coax to the socket, will act as a (very poorly tuned) antenna -- plus your SWR will likely be rather large and reflected power into your transmitter may cause problems.

Since, from comments, you have and are using a dummy load, and your oscilloscope shows modulation, you probably need to add an attenuator to the receiver you're testing with. Even a dummy load will radiate slightly, and if you've added any kind of antenna to it it's very likely you're wildly overloading the receiver's detector. If you can, try bypassing the receiver's RF gain, or install some kind of attenuator between the receiver's antenna and RF section.

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  • $\begingroup$ thanks for the quick response. I'm familiar with dummy loads and indeed I have a pretty good one, that's not really the issue. In my case, I can transmit into the dummy load and (via the oscilloscope) see an obviously modulated signal. Problem is, on a receiving radio, it sounds like garbage (high frequency squeal). So, I guess what I'm asking is if there's an easy way to characterize the output other than listen to it on another rig? $\endgroup$
    – Buck8pe
    Commented Oct 21, 2022 at 14:30
  • $\begingroup$ You need an attenuator on the receiver. Even the fraction that radiates from your dummy load and its wiring is overloading your receiver. I'll edit this in. $\endgroup$
    – Zeiss Ikon
    Commented Oct 21, 2022 at 14:32
  • $\begingroup$ that's really good to know. It just shows, you tend to overlook the simple reasons! But, I'll leave the question here all the same. It will be interesting to know if there are clever simple circuits or clever tricks with sig gens that would allow you to "hear" or otherwise understand the transmitted SSB signal (if that makes any sense). $\endgroup$
    – Buck8pe
    Commented Oct 21, 2022 at 14:41
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    $\begingroup$ Well, I don't know if you could hack a modern radio to do this. That Halliwell is an old "All American Five" tube circuit, but with a solid state rectifier replacing the rectifier tube in the original design. Then again, any simple regen receiver (like the one-tube circuits that are all over YouTube) can do this trick too -- but you probably want to include an RF stage to prevent the receiver radiating when you tune the regen a little too high. $\endgroup$
    – Zeiss Ikon
    Commented Oct 21, 2022 at 14:49
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    $\begingroup$ Well would you believe it! youtube.com/watch?v=kmiiM-Mv1s4. Thanks for the inspiration. $\endgroup$
    – Buck8pe
    Commented Oct 21, 2022 at 14:54

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