Saturday, May 25, 2019

40m OCFD

After previously re-considering verticals or dipoles, I decided to build an OCFD using ON4AA's design and ordered a Model 4116 balun from Balun Designs LLC.

To start with, I have decided to keep things simple by comparing verticals and dipoles on 40 m. I could put up a simple 40 m dipole, but had this balun already.

Looking at 40 m OCFD designs, none seemed to be that great. SWR over 2:1 on 40 m, or different offsets to get 15 m resulting in worse matches on other bands.

Having recently acquired a used Ameritron AL-80B from VA7ST, I also now need an antenna with a good match on 40 and 20 m that will take some power - up-to 1 kW PEP. My LDG remote ATU is rated to 600W PEP, at high(er) power levels I'd prefer to have properly matched antennas available.

I also bought An Introduction to Antenna Modeling with my $10 birthday coupon from the ARRL, this has good info for getting started with antenna modeling. With some ideas in mind from the book and what ON4AA described with their 80 m center loaded OCFD design, I set a design goal of a good match (under 1.8:1) on 40 and 20 m..

The process I followed:
  1. MMANA-GAL, using real ground, and a height of 12 m / 39 ft (my situation).
  2. I started by finding the resonant length for a 20 m dipole at 14.2 MHz = 10.45 m.
  3. Double the length to 20.9 m.
  4. Tried various offsets between 17% and 40%.
  5. Found 33% is the best compromise between 40 and 20 m.
  6. An added bonus is low SWR on 28.5 MHz and 50.1 MHz.

Results:

07.15 MHz, R 144, jX 74.6, SWR 1.72, Ga dBi 5.58, Elev 51.1
14.20 MHz, R 122, jX 2.39, SWR 1.64, Ga dBi 8.12, Elev 24.5
28.50 MHz, R 162, jX 14.8, SWR 1.25, Ga dBi 9.44, Elev 12.3
50.10 MHz, R 202, jX 32.1, SWR 1.17, Ga dBi 11.2, Elev 7.00

I have rounded the numbers above slightly for better formatting. Now, time for some pretty charts:

40 m
20 m
10 m
6 m



Next step, build it and see!

Notes:
  • The averaged R across the four bands is 157 ohms, using a 3:1 balun would get [in theory since this is a model after all!] an improved match.
  • On 40 m, resonance is just below the band. Borrowing from ON4AA's center loaded design using a capacitor in the center to resonate the antenna on the lowest band, 425 pF did the trick in the model.
  • Moving the feed-point to 40% enables 15 m, but results in higher SWR on 40 and 6 m bands.

Model files for conventional 33% offset, center loaded, and 40% offset can be found here: https://qsl.net/kl3no/Models/OCFD/