Experimental 2 meter / 70 cm dual-band high-gain flower pot antenna using RG6 coax

I decided to modify the original VK2ZOI Experimental

Dual Band Flower Pot Antenna to work with RG6 coaxial cable and utilize a ten-foot-long section of 1+1/4" PVC pipe that is found endemically here in the United States. This resulted in an antenna with SWR of about 1.0 on 146.52 MHz simplex, and 2.0 on 144 and 148 MHz.

Changes from Original Design

Compared 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
3	17	180
4	22	143
5	40	135
5	28	122

Play-by-play

With 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.