Introduction:

As with many senior citizens, I am morbidly fearful that I may need to move to a care facility, HOA community or other place where it might be difficult to enjoy my ham radio hobby. This has led me to preoccupation with portable antennas in general and small magnetic loops in particular.  Recently I acquired a model HF-319 SignalLoop as described on http://www.antennas4less.com  It is very light, easy to put together, moderate cost,  and -- most important of all -- within 10 minutes of receiving my antenna, I made a contact from inside a room that has aluminum siding. I have no doubt that this antenna works under portable ultrastealth conditions. 

My main problem, as forewarned in the instructions, is that the tuning is very "touchy." Tuning uses a simple knob that covers rather large ranges ( 3 to 5 and 5 to 19 megahertz) over about 180 degrees of rotation for each range. Furthermore, the tuning seemed  rather sensitive to proximity of my hand, and the rotation feels somewhat loose. For me, personally, tuning was  specially tough because of poor sensation and coordination in my fingers

Disclaimer:

I am sure you have seen the message, "Your mileage may vary."  Please ignore any illusion of high precision that may be implied in this article. For example, a small difference in the alignment of the extension shaft with the tuning capacitor could result in a large offset between my vernier settings and those you might find.  Similarly, I have been quite extravagant in employing decimal places. I think I could have left out a large part of the math, but I don't know which part.   Throughout this article, I use the term, "Vernier," even though it does not have a vernier scale.  On the other hand, that is how the dial is described in its packaging and advertising. I suppose I can hide behind a secondary definition,  " small auxiliary device used with a main device to obtain fine adjustment" (https://www.merriam-webster.com/dictionary/vernier)

Construction:

Among my DIY treasures, I had a Philmore S38 Vernier Control Knob (available from Amazon) and I decided to try it as a way to mitigate my tuning difficulty.  This dial provides an eight to one ratio and has a 0 to 100 unit calibrated dial over the 180 degree rotation of the controlled shaft. Thus one division on the dial is roughly equivalent to 1.8 degrees of shaft rotation with 12.5 degrees of knob rotation.  The feel is firm with no discernable backlash. I did not want to dig into the guts of the tuning box or make any irreversible changes. At the same time, I wanted to add a moderate amount of isolation from my hand.  My "design" uses a section of 1/4" Plexiglas rod and a length of 1/4" ID polyethylene tubing as a coupling to make an extension between the vernier dial and the shaft of the tuner variable capacitor.  Similarly, pieces of the same 1/4" Plexiglas rod form two standoffs as shown in the pictures.  Small self tapping screws through the two holes in the vernier knob housing enter holes drilled into one end of each standoff. The other end is secured to the front of the tuning box with methylmethacrylate cement. I don't believe that the exact dimensions are critical. With the three points of support, the dial feels very stable and steady.

Initial Experiment:

The antenna was set up with a four foot length of co-ax cable to an antenna analyzer (Comet CAA 500). For both the lower (5-9 MHZ) and upper (9-19 MHZ) ranges of the antenna, the vernier dial was set in a pseudorandom way to the eleven major calibration marks (0, 10, 20....100). These settings were not in any intentional order. For each range, two readings at each major mark were taken. A data point  ("sweet spot") was recorded for a stable dip in standing wave ratio (SWR) with my hands off the antenna analyzer and with standing wave ratio (SWR) lower than 1.2:1, and  with the impedance indicated to be close (+/- 2 ohms) to 50 Ohms. The results of this experiment are shown in the following figures:

Detailed Charts for Ham bands:

For the 75/80 meter and 40 meter bands, random sequences of 20 points each using the major and minor divisions of the vernier dial over  the range of frequencies in that band, as predicted from the initial data. A  similar process was followqed; that is, the vernier dial was set and the data was plotted, including the parameters of the linear trend lines,  as shown below:

The slopes and intercepts for the  least-squares trend  lines were then used to generate a tuning chart, as shown below.  For 60, 30, 20, and 17 meters; the bands  are covered over only a very few vernier dial divisions, so no interpolation was deemed practical. However, these are also included in the following  tuning chart.   

Further Observations:

The magloop and tuning chart were used on numerous (well, at least 8 so far) occasions with my KX3 radio. On each occasion, the antenna had been disassembled and then reassembled. Once a target frequency was set on the radio,  the vernier dial of the magloop was set to approximate the closest corresponding value in the chart. On all such occasions (so far) the SWR reading on the radio was less than 2:1 and could be adjusted lower within one or two vernier divisions.

Conclusions:

From these rather limited experiments, it seems that the relationship between the vernier dial readings and the frequencies at which the SWR dip was seen was stable and follows a regular monotonic curvilinear relationship. Another observation is that the tuning range for each of the amateur radio bands is quite narrow; such that the added "band spread" afforded by the vernier is very useful.