Greater Harmonic Suppression and a Narrower RX Filter for the SST20

My scratch-build of the Wilderness Radio SST20 has been a great success. I’m not a very active operator but have had 18 QSO’s on it so far, running it into a horizontal loaded dipole (a Buddipole) at 25 feet. The furthest was Hawaii at about 2350 miles distant, followed closely by Midland, PA at about 2200 miles away, as the crow flies. I previously thought the output power was 2.25W, but it looks as if it’s closer to 1.5W. I’ll explain why in this post.

Ever since completing it, I had been having uncertainties about the low-pass filter on the output. I understand that spurious emission requirements had used to be a little more lax for QRP transmitters, specifying that for transmitters under 5W, spurious emissions needed to be greater than 30dB below the level of the fundamental emission. This is no longer the case, as the requirement is the same for all HF transmitters in the amateur service, and is found in 97.307 –

“(d) For transmitters installed after January 1, 2003, the mean power of any spurious emission from a station transmitter or external RF power amplifier transmitting on a frequency below 30 MHz must be at least 43 dB below the mean power of the fundamental emission.”

Although I am not set up to make spectrum measurements on transmitters, a very rough test using the S-meter of my K2 indicated that my SST was emitting a 2nd harmonic that, at best, was only a few S-points below that of the main signal, It looked very much as if I wasn’t even fulfilling the older requirement of a minimum of 30dB suppression of spurious emissions. Digging around on the internet a bit, I found this paper on the GQRP site, giving practical values for low-pass filters for 50 ohm systems for all the HF amateur bands that I thought should satisfy current requirements. The suggested LPF for 20M was this –

As well as greatly improving harmonic suppression over the original SST filter, the use of this LPF in the receive path also improves image rejection as the image, with this receiver, is on the high side of the VXO.

 

Fitting it into the same space that the previous 2-stage LPF had occupied was a tough call, but I managed it. I did have to break one of my own rules of decorum, and allow one of the toroids to be slanted slightly, so that the layout looked less uniform (Oh, the horror!) but I was happy that it fitted into the space at all. The yellow T37-6 toroids of the new LPF are easily visible in this close-up view –

The first thing I noticed was that despite careful peaking of C28 (at the output of the TX mixer), the output power as measured on my OHR WM-2 wattmeter was now only 1.5W. Given that the 28MHz harmonic was previously very possibly only 2 or 3 S-units down on the main emission, I’m thinking that a significant portion of the 2.25W I was measuring at the output before was 2nd harmonic energy (and perhaps some unwanted TX mixer products too). The good news was that listening to the 28MHz 2nd harmonic on my K2 now revealed it to not even register S1, when the fundamental at 14Mhz was S9 +40dB! That is an excellent result, and one that falls well within FCC requirements. I observed a very similar result with the VXO signal at 18MHz. My K2 doesn’t cover the unwanted TX mixer product frequency of 21.932MHz (VXO freq + 3.932) – at least not with full sensitivity, and I don’t have a general coverage receiver with an S-meter unfortunately, so I can’t check the level of that unwanted emission.

The levels of the harmonics emitted from the antenna jack now easily comply with FCC Part 97. Due to the previous harmonic level I measured, I’m thinking that a significant level of 2nd harmonic is being delivered to the input of the PA and being amplified, before then being attenuated by the LPF. It would be preferable for that harmonic to be filtered out before the PA stage, and I may take a look at doing this in the future. In his SST40, JN3DMJ added an extra stage to the bandpass filter after the TX mixer to increase the level of spurious attenuation. You can see it here, under the heading “Upgrading of the filters”. With a better BPF in place, it may be possible to get an honest 2 – 2.5W from the final, with all of that power consisting of 14MHz energy.

As if to confirm to me that all was well, the little rig gave me a brief daytime QSO with KD3CA in Midland, PA – 2200 miles away. Not bad for 1.5W into a detuned dipole with an SWR of nearly 6:1!

The other thing that I took a look at was the crystal filter on the receiver. I had used the values of C6, C7, C8 and C9 suggested by Rich K7SZ on QRP-L for a wider response than even the values given in the manual for a wider filter. The stock values give a particularly narrow filter, and not all users will want that. I changed the values to those given in the manual as a mod for greater width. Does the way I’m explaining it make sense? I was going from extra wide to moderately wide, so to speak. Here are AF response curves taken by measuring the AF response of the entire rig at the headphone jack. The program used was Spectrogram.

Using K7SZ’ “extra wide” values of C6, C9 = 47pF, and C7, C8 = 120pF (the red vertical marker is at 400Hz – the sidetone I use) –

and using the “stock mod values” from the manual, for “regular wide response” – C6,C9 = 68pF and C7,C8 = 180pF (the red line represents an aggregate of all the peak values taken over a 16 second period, while the blue line is a response in one instant in time) –

Not sure if I’ll decide to go narrower with the filter, or leave it as it is. I need to experience a few more contest weekends before making any further decisions 🙂

On an entirely different tack, I have been using a new camera for the one photo in this post, and all the photos in the previous post about my SST build. It’s much lighter, and more compact than the camera I used for the photos in all the other posts on this blog. I’m still trying to decide whether the image quality is up to par for me. It’s a different lens, with a slightly wider focal length, a different sensor, and I’m using different software to process the images. I am not quite yet used to using this particular lens in order to “see” my subject the way I want, and so there are a lot of factors in deciding whether it’s going to cut the mustard for use in this blog. However, it’s a great and relatively inconspicuous camera for carrying around with me. Occasionally, I like to do what one might call street and candid photography, and it excels at that. This is one of the things I do when I’m not slaving over a hot soldering iron –

But, for the most part, I prefer taking photos of radios. They are very relaxed and compliant subjects, and don’t give me a hard time when I point a camera at them, unlike some members of the general public (gee, I wonder why they would do that).

I’ll get back on topic and talk about radio in the next post, I promise. In the meantime, as I am getting the “neither here nor there” figure of 1.5W of power out of my SST20, I turned the drive down to 1W, to make it a nice, even figure. I am looking forward to sending “PWR 1W” during QSO’s!

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