Why the QX and QX Pro were discontinued..
Several months ago I was confronted with a dilemma. Two critical components that I had been using for years in my loops were now unavailable. For years, I had been using a 2-ganged variable capacitor that provided a large capacitance range and a MOSFET transistor that had proven to be a very good and reliable performer. Both of these components were no longer being carried or made which sent me on a search for suitable replacements.
Variable capacitors, as many of you know, have become quite rare these days. I was able to find a fairly large supply of suitable replacement capacitors but their maximum capacitance and capacitance range were not as great as those that I had been using for years. Rather than just quit making loop antennas, I decided to redesign the tuning section of my loops. This required redesigning the coils and adding a third tuning range position to the range switch. That is, instead of just a High/Low range switch, the new loops required a High/Medium/Low switch. At first appearance, this may seem less convenient. In fact, however, the greater frequency resolution per switch position permits easier Q-multiplication use. So, nothing significant was lost and greater ease of use was gained. So far, so good. In fact, being forced to redesign the coils had an added advantage...on the v2.0 series of loops, I was able to offer individual loopheads each optimized for three different bands (LW, MW, and the Tropicals). The performance improvement was most noticeable re: the QX Pro where two coils were located on the same ferrite rod; using separate loopheads allowed me to optimize the coils for each band.
Resolving the MOSFET issue was more daunting. I had always used Motorola transistors when possible; they were simply the best, in my opinion. When Motorola abandoned semiconductors a few years ago, after my stock was depleted, I had to find a substitute. I tried "new old stock" supplies, NTE and ECG equivalents, etc. and found that while some individual samples were quite good, many were junk. I test each transistor and select only those that meet or exceed specs and found that, at times, I was forced to discard three or four transistors for every one I would put in a loop. The Motorola MOSFETs cost less than $3 each when I first started using them and now their "replacements" were up to as much as $13 each, due to their scarcity. Discarding three or four of these losers to get one good one quickly became cost prohibitive. Even the good ones sometimes required custom tweaking to get the best out of them.
(NOTE TO QX AND QX PRO OWNERS: If you happen to notice a rise in the noise floor on your loop, remove the bottom of the base cabinet and check to see if there is a trimmer located on the bottom of the circuit board (in the middle). This trimmer adjusts the gate control voltage and directly affects the gain and noise floor of the amp and it's possible that over time, or after rough handling, that it may need to be readjusted. With the loop plugged in, adjust this trimmer for the best balance of signal and noise (don't adjust for the greatest signal, a balance is better) If you loop doesn't have the trimmer, you are good to go.)
My only option, if I wanted to continue making loops, was to redesign the amplifier. So, instead of using a JFET/MOSFET design, I switched to a JFET/JFET design which I was further able to tweak. This amplifier change resulted in a somewhat lower S-meter reading but a significant improvement in the noise floor of the amp. Plus, the S-meter reading could easily be increased by using the Q-multiplication control. In practical terms, there was no loss at all in signal gathering ability compared to the earlier QX and QX Pro models and, in fact, with weak signals the lower noise floor was an improvement.
I also decided to incorporate into the new v2.0 loop some of the features that were popular on the QX Pro model. Namely, an on/off LED light and an external antenna connector so that the v2.0 could be used as a preselector for an external antenna.
After designing the new JFET/JFET amp, I happened to locate a small supply of absolutely prime Motorola MOSFETs (not the same model as use earlier, however) that perform exceedingly well. I decided to incorporate these MOSFETs in a new JFET/MOSFET amp that is being offered in the v2.0 Plus+ model. This model provides more gain than the regular v2.0 model with only a slight increase in noise floor. The v2.0 Plus+ model also incorporates a continuously variable Q-spoil control (instead of the three-position switch on the v2.0). The value of this feature is that is allows more precise adjustment of the Q during phasing and permits a finer adjustment of the signal level of any external antenna that is attached to the loophead. I personally think that this is the best loop I've ever made when signal-gathering ability, nulling, and general ease of use is considered. It's the one I now use for DXing. Please don't infer that the regular v2.0 is a slouch compared to the Plus+ model though. This past DX season many transAtlantic stations were logged with the v2.0 and I was even was able to pick up a few transPacific carriers from here in Florida, so it's a very competent loop. The v2.0+ just adds a bit more "oomph" and better Q-spoiling control.
Thanks for your continued support over the years and I hope this season finds you logging much DX.