Shortwave Listening With The K3

I would love to see the K3 receive the attention it deserves from the too-often ignored hardcore shortwave and mediumwave DXers and believe that with a few minor software tweaks the K3 could fill a huge void in the high-performance communications receiver market since it has been all but abandoned by major manufacturers.

Twenty years ago when I first started out in the radio business there were a good number of new desktop shortwave receivers to choose from including the Yaesu FRG-8800 ($639.95), Icom IC-R71A ($799,95), Kenwood R-5000 ($859.95), JRC NRD-525 ($1189.95), not to mention many used late-model receivers such as the JRC NRD-515, Drake R7, Yaesu FRG-7700 and Icom IC-R70. With sadness I've watched as radios like these have been slowly disappearing from the market, replaced by small portable radios aimed at the broader consumer market while serious hobbyists have fewer and fewer choices other than prohibitively expensive mil-com HF receivers, or amateur radio transceivers which more often than not perform awfully in the AM mode.

With the K3, I see hope; a basic 10W K3 costs only $1600 (or $1400 if self-assembled), less than the soon-to-be discontinued NRD-545 is currently selling for, and even with only the stock 2.7 kHz roofing filter the K3 would likely run circles around the '545 not to mention most of the classic receivers mentioned above. However, it will take a little fine-tuning by Elecraft for it to realize its potential as a top-notch SWL receiver.

AM Mode Filtering: There was talk on the Elecraft reflector a while back about the way the filtering works in the AM mode, specifically the 3 kHz maximum "audio" bandwidth. From an SWL's perspective, this makes no sense; it is counter-intuitive and non-standard compared to all other high-end communications receivers (I'll use the NRD-535D as a reference, since I'm most familiar with JRC equipment). With the '535, the filtering in AM mode works like one would expect; no matter which filter is selected, when the passband shift (PBS) control is centered the filter passband is centered on the carrier frequency and shifting the passband shifts tha actual i.f. passband, not the audio passband. So if a 2.4 kHz filter is used, it will pass 1.2 kHz of each sideband when PBS is centered. Offsetting the PBS 1.5 kHz to the right, for example, will move the entire filter passband to pass only the upper sideband while rejecting the lower sideband. This is useful to the SWBC DXer in cases where there is a strong signal 5 kHz below a weaker station that I'm trying to receive and the lower sideband suffers from adjacent channel QRM but the upper sideband is clear. And vice versa, if the interfering signal is 5 kHz above, tweaking the PBS to the left passes the lower sideband while rejecting the upper sideband and the QRM.

I'm not sure what actually happens in the same scenario when using the K3. If I set the filter width to 2.4 kHz and shift the passband to the right, it seems to work as expected - audio frequency response gets higher; but when the passband is shifted to the left, frequency response gets lower, which indicates that the filter passband is either not centered on the carrier, or it is split similar to the way it works in RTTY mode, and passes equal slices of both passbands. Whatever the case, the way it currently works is at odds with what is expected by a true SW or MW broadcast DXer.

Synchronous Detection: There is also the unanswered question as to whether the K3's forthcoming AM Synch mode will allow independent sideband selection. With synchronous AM detection, the ability to select sidebands independently is pretty much mandatory, otherwise the synchronous mode is useless on all but the strong, clear signals which frankly sound just fine without synchronous detection. The NRD-535 selects AM-USB and AM-LSB with the ECSS (exalted carrier selectable sideband) button. Most other receivers that have this feature work similarly. The Sony ICF-2010 does it a little differently - it lets you select sideband by adjusting the main tuning dial up or down slightly, and a little LED indicates which sideband is selected. The Drake R8, by comparison, does not directly allow selectable sideband; its synch mode was DSB, and although you could move the passband shift it isn't as effective as the JRC and Sony systems which totally reject the unwanted sideband.

Joe, W4TV, gets it; in one of his reflector postings on AM filtering he pretty much describes how the JRC ECSS system works in principle (the exception being that JRC doesn't do it with DSP):

Given the DSP demodulation in the K3, it's a shame that there isn't an "offset" option to do "vestigial sideband" demodulation (offset the AM filter to the upper sideband or lower sideband) and demodulate carrier and one sideband for better fidelity.

I'm not sure if Joe is talking about syncing the carrier and replacing it with an internally generated one, but that is what a good synchronous ECSS system does in a nutshell. Without selectable sideband capability, synchronous AM detection is like tits on a bull.

SWL Mode: Another easy software mod that would be welcome for people who wish to use the K3 solely for SWL purposes would be to include a setting to disable the transmitter, similar to the TX TEST mode but without requiring it to be set each time the radio is turned on. This can be done by at least making the TX TEST mode persistent so that the K3 would remember it was in test mode when it was last powered up. Preferably there would be an additional CONFIG menu parameter that would put the radio into SWL mode in which the TX indicator will be turned off (not flashing like in Test mode); if the PTT line is keyed while in SWL mode the K3 would display a message like "SWL MD" and there will be no RF output - basically extending the out-of-band transmitter disability to K3's the entire frequency coverage while in SWL mode.

And that's it - just modify the firmware code and send a K3 off to Larry Magne for testing! The rest will be history.