A posting from a few days ago (https://thewakesileave.wordpress.com/2016/11/12/an-old-kits-second-chance-at-life-in-the-shack/) pointed at the use of an old “artificial ground” tuner (the Ten-Tec Kit 1251) for improving the signal received by a random vertical antenna tuned to its best match by a modern remote auto-tuner (an LDG RT-100).
In the 20m band, the remote LFD RT-100 fails to tune a quasi-vertical 50+ Ft. wire antenna (described in the previous article) to SWR values below 2.3:1. However, by adding further inductance to the counterpoise via the 1251, the SWR can be brought to almost 1.0:1. These observations only apply to this particular antenna, and more particularly, to the 20m band. Nevertheless, I thought of interest to further document the effect.
For these observations the Ten-Tec 1251 was at all times connected in series (using two cables abt. 5 Ft. long) with the counterpoise cables at the point of their connection to the unun at the output of the tuner.
The ICOM 703 was used with its own ATU by-passed. Tuning by the RT-100 was triggered by the carrier signal in AM mode using the microphone PTT with the TRX at full 10W power (a carrier signal of abt. 3 Watts). With the Ten-Tec 1251 in its lowest inductance and capacitance settings (as it happened, changing the capacitance had no noticeable effect on the SWR), the frequency was chosen at 14.0220 MHz and the RT-100 was allowed to cycle to find its best match (it stopped at SWR = 2.3:1). Further triggering of tuning cycles by pressing the “Tune” button in the RC-100 Bias-Tee did not further improved on the SWR.
For the initial set of data, the coaxial connecting the transceiver to the SWR/PWR meter was disconnected from the radio, connected to the miniVNA analyzer, and the data and graphs shown in the two top panels on the following picture were recorded. The panel on the top-right shows the same data as that on the top-left plotted on a Smith chart. The marker in all panels indicates the tuning frequency (14.0220 MHz). The two bottom-panels show similar data to those at the top but obtained after adding the inductance needed to optimize the SWR.
Another set of observations was conducted in the following manner: while keeping pressed the microphone PTT in AM mode, the induction was gradually increased in the Ten-Tec 1251 and the SWR shown in the WII meter was recorded for each inductance setting (see blue curve in the picture below). Then, a steady signal was chosen nearby the tuned frequency and the S-meter values (in the ICOM 703 screen) were recorded at increasing inductance settings (see the orange curve).
With the equipment at hand it was not possible to measure the effect that the improved tuning might have on the radiating power of the antenna. Such a test will be conducted at the first chance of a QSO in the 20m band. However, since QRP operations require a radio output at or below 5 Watts, it is tempting to imagine that with antennas like these, every single bit helps…