I decided to modify the original VK2ZOI Experimental
Changes from Original DesignCompared with the original VK2ZOI Experimental Flower Pot:
- Lengths of all segments were implemented as drawn in the VK2ZOI plan
- "Double-shielded" RG6 that used both foil and light outer braid instead of RG58
- Top coil: 4 tight turns of RG6. The total length of this cable segment was therefore changed to 630 mm.
- Bottom coil: 4 loose turns of RG6.
The plans of mice and men...My original plan involved using a bottom coil of 5 turns RG6. Using SELFRES3.EXE as the calculator, I back-calculated the self-resonant frequencies of the VK2ZOI coils: top coil from 125 through 150 MHz, and bottom coil of 135 MHz. I did not compensate for velocity factor differences and assumed solid-core RG6 has a similar velocity factor to that of solid-core RG58.
The parameters that I used for my RG6:
- Coil width 5.6 mm
- Conductor diameter 5 mm
- Diameter of coil = 42 mm OD of PVC + 6 mm diameter of RG6 = 48 mm
|# of Turns||Solenoid length, mm||Calculated resonant frequency, MHz|
Play-by-playWith the five-turn bottom coil, the SWR measured 3 at 146 MHz and ~1.2 at 144 MHz. Trimming the top of the antenna resulted in a reduction of SWR at 144 MHz and 146 MHz to 2.5; further shortening then increased the SWR to about 3 at 146 MHz and was stopped.
Because of the seemingly sharp "knee"-like SWR increase at ~145 MHz, I hypothesized that my lower coil was imparting too much inductance with high Q as the bottom coil ended up being a lot more tightly packed than I had expected. I therefore removed one turn from the bottom coil and my SWR dropped dramatically at 146 MHz to be nearly 1.0.
I used aluminum foil for coupling sleeve only on the bottom of the antenna at the time of testing.
Prelimiary result with only lower sleeve in place:
SWR 2 144 MHz, 1 @ 146 MHZ, 2 @ 148 MHz.
SWR ~ 1.5 throughout 70cm band.
Special thanks to Roger W7RC (espouses the benefits of RG6) and Bill KG7ULE for
practical advice via the 146.52 MHz Portland Graveyard Net.