As G3TXQ pointed out, for the purpose of CMI, the 4:1 Guanella balun has only half as much CMI as each of the two 1:1 Guanella baluns. The problem is, you can't fit enough turns of transmission line onto each core to obtain sufficient CMI for use in most OCFD antennas.
The only exception is a 40m OCFD with a feedpoint position very close to the center of the antenna (i.e., >40%). Remember: CMC increases when the feedpoint is moved farther away from the center, thus the amount of CMI required increases.
As a result, an additional 1:1 Guanella (choke-balun) is required in addition to the dual-core 4:1 Guanella balun. Or . . . better yet, a Hybrid Balun
(but not just any old hybrid balun!)
Slide 24:
Additional Loss Due to SWR
Any edition of the "ARRL HANDBOOK" has a section with the title "Myths about SWR".
In the Index in the back up the book, look up "SWR" and under SWR, find "Myths".
This topic is located in the Transmission Line chapter of every HANDBOOK.
A third and perhaps even more prevalent myth is that you can’t “get out” if the SWR on your transmission line is higher than 1.5:1, or 2:1 or some other such arbitrary figure. On the HF bands, if you use reasonable lengths of good coaxial cable (or even better yet, openwire line), the truth is that you need not be overly concerned if the SWR at the load is kept below
about
6:1.
(Note: they did not define "reasonable length" but using the chart in the handbook for "Additional Loss Due to SWR", under 100 ft. (ca. 30m) seems to be a reasonable length.
HHG2 OCFD: Part-1 COMMENTARY
THE INFORMATION BELOW is support information for Part-1 of my 2-part presentation on OCFD antennas, entitled
The Hitchhiker's Guide to Off-Center-Fed Dipoles.
It pertains additional, supporting information - or links to supporting information - for the slide number shown in BOLD BLACK.
SLIDE-4: BEFORE WE BEGIN
RECOMMENDED RESOURCES FOR INFO ON BALUNS:
G3TXQ - Steve Hunt (SK)
GM3SEK - Ian White
K9YC - Jim Brown
VK1OD - Owen Duffy
W1HIS - Chuck Counsleman
---------------------------------------------------------------------------------------------------------------
W7EL - Roy Lewallen (Beginners should begin with this document first!)
---------------------------------------------------------------------------------------------------------------
W8JI - Tom Rauch
Tom's site contains a wealth of information on baluns and chokes.
It even includes its own Google-Search box:
This search engine can search the entire web, or just Tom's own site. Be sure you are searching just Tom's own site (W8JI.COM - highlighted in yellow above).
Then type either "BALUN" or "CHOKE" into the search box.
SLIDE-10: Alternatives to OCFD
The following document describes several multi-band antennas that could be considered as alternatives to the OCFD antenna. I show the antennas, then list "my personal opinion" of the advantages and disadvantages of each antenna. These are based on my interest in operating and the "Eagle-Eye" of my XYL and neighbors. (Your Mileage May Vary)
Slide-11: Keep it Simple, Stupid: OCFD
It has long since been known that EVERY OCFD antenna should have an RF-Choke connected directly to the 4:1 balun at its feedpoint. Unfortunately, people seem to have ignored this recommendation.
Read the article in QST, 25 years ago, below; "Choke The OCFD":
Slide-13 OCFD's SHADY REPUTATION
THE primary cause of problems in OCFD antennas is Common Mode Current (CMC).
You will find a fairly simple explanation of what it is and what causes it: HERE
Slide-18: The Balun is the Lynchpin to Success
THIS is one of the most important point of this entire presentation!
THE PROBLEM is, the Balun OEM's of the industry are selling
4:1 current baluns that DO NOT WORK! There are few exceptions!
The source of this problem is Jerry Sevick. In his (otherwise) excellent book on Baluns and Ununs, Sevick suggested that the 4:1 balun may be built in two different ways: on just one single ferrite core, or on two ferrite cores.
Here is a document showing a discussion between Steve Hunt, G3TXQ (SK) and Tom Rauch, W8JI, on the several problems with Sevick's book on baluns. DOWNLOAD:
FACT is: The 4:1 single core Guanella (current) balun DOES NOT WORK in HF antennas! NOT AT ALL. It not only fails to offer common mode impedance, it actually generates common mode current.
The LINKs below to work by Steve Hunt, G3TXQ (SK), Tom Rauck, W8JI and my own, Rick Westerman, DJ0IP, clearly show that the single-core 4:1 Guanella balun DOES NOT WORK.
(Most of these links are also shown in the PDF DOWNLOAD above.)
G3TXQ:
VK1OD:
W8JI:
DJ0IP:
In every antenna book I have ever seen, the 4:1 Guanella balun is always shown using two Toroid cores. Sevick is the only reference suggesting it will work with a single core; but it clearly does NOT.
Slide-19: The Balun is THE Lynchpin to Success
In most descriptions of the OCFD antenna, it calls for a 4:1 current balun. The better articles will point out that it MUST be a Dual-Core 4:1 Guanella (current) balun. Unfortunately, this information is incomplete. In most OCFD antennas with just one exception, the Dual-Core 4:1 Guanella balun has insufficient Common Mode Impedance (CMI) to completly remove ALL Common Mode Impedance from the coaxial feedline.
The amount of CMI a balun has depends on THREE factors:
The drawing (above) shows how a 4:1 Guanella balun is constructed. Basically, it consists of two 1:1 Guanella baluns with their inputs wired in parallel and their outputs wired in series.
The chart shown above is an excerpt out of Steve Hunt's (G3TXQ - SK) excellent document "Basic Baluns". It shows the various types of baluns and their Common Mode Impedance (called "Zcm" here) at various frequencies.
NOTE the text highlighted in yellow.
Link to original document: Basic Baluns (page 9)
From the chart on the right, we see that a 1:1 Guanella balun wound with 17 turns has about 8k Ohms of CMI on 80m but just 4k Ohms on 160m. The 4:1 Guanella balun has only half as much CMI on each of these bands.
As G3TXQ pointed out, for the purpose of CMI, the 4:1 Guanella balun has only half as much CMI as each of the two 1:1 Guanella baluns. The problem is, you can't fit enough turns of transmission line onto each core to obtain sufficient CMI for use in most OCFD antennas.
The only exception is a 40m OCFD with a feedpoint position very close to the center of the antenna (i.e., >40%). Remember: CMC increases when the feedpoint is moved farther away from the center, thus the amount of CMI required increases.
As a result, an additional 1:1 Guanella (choke-balun) is required in addition to the dual-core 4:1 Guanella balun. Or . . . better yet, a Hybrid Balun (but not just any old hybrid balun!)
Slide 24: Additional Loss Due to SWR
Any edition of the "ARRL HANDBOOK" has a section with the title "Myths about SWR".
In the Index in the back up the book, look up "SWR" and under SWR, find "Myths".
This topic is located in the Transmission Line chapter of every HANDBOOK.
A third and perhaps even more prevalent myth is that you can’t “get out” if the SWR on your transmission line is higher than 1.5:1, or 2:1 or some other such arbitrary figure. On the HF bands, if you use reasonable lengths of good coaxial cable (or even better yet, openwire line), the truth is that you need not be overly concerned if the SWR at the load is kept below about 6:1.
(Note: they did not define "reasonable length" but using the chart in the handbook for "Additional Loss Due to SWR", under 100 ft. (ca. 30m) seems to be a reasonable length.
Slides 29/31: Adding More Bands In Theory: Computer Modelling Software
There really is no point in trying to model the OCFD antenna yourself. John Young (K8BA) has modelled these antennas, showing thousands of data points. He has presented his work into several Excel files.
These are found on my web site, here:
OTHER SOURCES OF INFORMATION
OCFD GROUP (about 1300 members): https://groups.io/g/ocfd
This is an excellent group for meeting others and sharing experience with sourcing components, building and using the OCFD antenna.
COMMON MODE CURRENT FIELD TEST (40m dipole and OCFD antennas) by DJ0IP:
A study of the CMC characteristics of OCFD antennas, with a detailed look at the differences between different baluns and chokes. This study compares about 500 pairs of field measurements, showing exact measurements of Common Mode Current on the feedline along with its affect on the shape of the SWR curve across the 40/20/15/10m hf bands.
Battle of Baluns Field Test (80m OCFD) by DJ0IP:
A real life comparison between a dozen different baluns used in an 80m OCFD.
This includes about 500 field measurements.
Windom-und Stromsummen-Antennen
A textbook written exclusively about Windom- and Off-Center-Fed Dipole antennas.
By Karl H. Hille, DL1VU (SK)
As the title suggests, this book is written in German.
Those of you interested in this antenna are invited to join the OCFD Group on groups.io. We have nearly 1300 members of this group and share a wealth of experience with these antennas.
See: The Off-Center-Fed Dipole Group