I calibrated the AA-600 the night before with the Micro-Balun set, at first, to 4:1 and using a 200Ω carbon composition resistor but had some reservations about the extra capacitance found in these "Lil Devils". It was significant enough to warrant loosing the high Z accuracy and going back down to a balun setting of 1:1 and suffer the measurement inaccuracies using the well behaved HP 50Ω load resistor. That way all of the impedance measurements are displayed directly with no need to multiply by four.
My friend Gary, KØIMJ and I went off to what my kids call "The Purple Park" because some of the playground equipment is purple, set up the 28' Jackite in the center and the 2-20' Jackites as end supports, using the Park Portable Doublet construction method described in earlier blog posts. BTW, the lawn at the park was so lush because of the recent rains and very squishy and sloppy with flip-flops on. :(
I drove an Army tent stake down and Velcro'd the 28' Jackite base to the stake, while stringing 95# paracord guys. (Obtained from Hobby Lobby).
I took readings with the calibrated AA-600 center frequency at 14.1 and span of 28.0MHz making it sweep from 0.1-28.1MHz, with 80 data points (the native data point number for the AA-600 not tethered to a computer). The measured results (cloumns E and F) were very close to EZNEC predicted values as shown in columns A through D.
Column G is the settings on the BLT where the clips are attached to the coil turn number from the center tap. I made a nemonic of the reporting of tap position with C-A-P representing where the tap points are with the (C) of the air variable capacitor tap point, (A) meaning Antenna tap point, and (P) being the switch position on the Elecraft BL2. The (+HC) is the addition of the Helper Cap in parallel with the air variable to help it out when it ran out of capacitance. The HC is an ARCO 427 set to about 75pF yielding roughly 175pF of capacitance needed to resonate the tuner on 80 and 60 meters at their respective tap points. The air variable was about 3/4 meshed.
In days prior to field deployment, I created discrete loads representative of the antenna's input impedance for each band, I used an air variable cap, a pot and some ferrite cores to create these discrete loads. I measured each load and attached it to the tuner achieving a match for each band, noting the C-A-P positions. These were my presets I used to get the tuner close when I set up out at the park. I reported only the final settings in the spreadsheet. The 'wow' factor came when the lowest SWR reading was dead on or only one turn different from the preset.
The RX graph above shows the periodic nature of the impedance (displayed in resistive and reactive componenets) as a function of frequency. Column E in the spreadsheet is color coded to show that the R or X (or both) can change quickly by varying the frequncy slighly over areas where the slope is the greatest. This accounts for the error between the calculated and measured impedances shown in the spreadsheet because the reading taken didn't line up exactly with the calculated value.
Results from modeling a Park Portable Doublet set up for Field Day 2015 are shown and align well with measured results taken with realatively inexpensive test equipment.
Carefully constructed EZNEC models can show a high degree of fidelity with the physical antenna characteristics. The model is validated by actual measurements that are in good agreement with predicted values.
Printed on Recycled Data