Saturday, May 25, 2019

Low Band DX - 5 - 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 40m. I could put up a simple 40m dipole, but had this balun already.

Looking at 40m OCFD designs, none seemed to be that great. SWR over 2:1 on 40m, or different offsets to get 15m 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 20m that will take some power - up-to 1kW 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 80m center loaded OCFD design, I set a design goal of a good match (under 1.8:1) on 40 and 20m..

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

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:

40m
20m
10m
6m

MMANA-GAL file for the 40m OCFD with 33% offset.

Next step, build it and see!

Side 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!] a very good match.
  • On 40m, 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, 425pF did the trick in the model.
  • MMANA-GAL file for 40m OCFD with 33% offset and center loading.
  • MMANA-GAL file for 40m OCFD with 40% offset, this gains 15m band coverage at the expense of higher SWR on 40 and 6m.
  • More experimentation may improve things further..