|The original design in my book for a flat-top |
optimised 40m OCFD (click for larger image).
Update: Well, I've been using it for a day and have worked Cuba (CO), Moldova (ER), Qatar (A71), Saudi Arabia (HZ) and Ceuta & Melilla (EA9) on 15m where it seems to go great guns. 40m has given strong contacts into Europe. I like it.
Update, February 2016
I've now been using the antenna for about four months so here is an update. It works very well on 40m, and is consistently equal to or down about 0.5 S point on my W5GI. I haven't used it much on 10MHz, but it looks to be slightly better than the W5GI. It works very well on 14MHz and 18MHz and is really good on 21MHz. Again, I haven't used it much on 12m, but it plays on 10m, although it is consistently slightly worse than a dedicated half-wave dipole by about 1-2 S points. Overall, not bad for a multi-band antenna.
|A typical 4:1 two-core Guanella balun.|
The cores are then put back to back and wired with one side in series and the other in parallel. If the characteristic impedance of the twin wire is about 100 Ohms this gives the required 200 Ohms/50 Ohms = 4:1 impedance transformation.
The current thinking is Ferrite Type 61 for the top end of the HF range; Type 43 for the bottom end; or Type 31 for the best compromise across the range. The image is not of my balun but it is of a similar design. Mine is housed in a box with all the joints sealed with tape for waterproofing purposes.