Why does an 80m OCFD require an additional RF Choke at its feedpoint, even when using a good dual-core 4:1 Guanella (current) balun, when the 40m OCFD  [usually] does not?




Although the Common Mode Current (CMC) on the 40m OCFD antenna is as bad as that of the 80m OCFD, the balun itself is not as effective on the lower frequency as on the higher frequency.


Technically speaking, its Common Mode Impedance (CMI) is high enough on 7 MHz but insufficient on 3.5 MHz.  Therefore an additional choke is required on 80m.





Normal baluns, whether voltage or current types, use a hunk of ferrite, usually in the shape of a doughnut, and called a Toroid.


Toroids come in different sizes and with different ferrite mixes.  We’ve often heard the terms, #61, #43 – sometimes heard about #31 and occasionally about #52.  These all look exactly alike but have very different permeability.


Typical “Beginning Permeability”:


61 = 150        43 = 850         31 = 1500       52 = 250


Assuming same size toroids and an equal number of turns on each toroid, a higher permeability yields a higher inductance.  As we know, inductance impedes the flow of alternating current;  Remember, RF is high frequency alternating current.


Therefore a higher permeability results in a higher CMI.


DOWNSIDE:  a higher permeability results in a lower Curie point.  As a reminder, the Curie point is the temperature at which the ferrite toroid loses its permeability and its CMI performance goes to hell in a hand basket! 


Common Mode Current:  CMC is an RF current and when its level is high enough, it can warm/heat the toroid significantly, even to its Curie point.  When the CMC level is high enough, it can cause the Toroid to break down, crack, or even shatter.


IMPORTANT:  The level of CMC is also affected by height above ground as well as surrounding objects.  In my two test environments, the 80m antenna was only 2m (6' 6") higher than the 40m antenna so there was probably a higher level of CMC present.  But this is probably typical of most installations.


The level of CMC at the feedpoint is also affected significantly by the length of coax.  For an explanation on this, read G3TXQ's web page on RF Chokes.


CMI Behavior:  As Toroids heat up, their CMI level declines.  This is dramatically true for transformer baluns (i.e., 4:1, 6:1, etc.) and to a much lesser extent, also for 1:1 baluns.  Therefore a 4:1 balun which may perform well at QRP power levels, or perhaps even 100w levels, often breaks down at higher power levels.


IMPORTANT:  With identical core an number of turns, the CMI of a 4:1 Guanella balun/choke is only half that of a 1:1 Guanella because its two cores/inductors are effectively in parellel.  In really hard-neck cases of CMC, the 4:1 alone may not suffice.  More Info:  G3TXQ Basic Baluns  (last 2 pages)


Note: CMI, like any impedance value, is complex, consisting of a resistive and a reactive component.  In general it is better to use a choke/balun with a higher resistive value than the reactive value, but if the resistive value is not high enough to reduce the CMC level to a very low value, the core can heat up significantly.  MORE INFO (by G3TXQ)


Permeability vs. Frequency:  For a given size toroid and a specific targeted CMI, higher frequencies require less permeability to reach this value.  Lower frequencies require higher permeability. 





If a high level of CMI is required, it is impossible to cover the entire hf spectrum (1.8 to 30 MHz) with a single ferrite mix.  Therefore it is impossible for a single balun to have the same high level of CMI across the entire hf spectrum. 


However, low permeability ferrite mixes are more broad-banded than high permeability mixes.  Many Balun Manufacturers build baluns with  #61 ferrite and claim they have a high CMI from 1.8 to 54 MHz, but if you look carefully, you will find the CMI to be only about 1000 or 1500 Ohms.  Although this is high enough for many applications, it falls short by a factor of 4 to 6 for hard-neck cases of CMC, such as the OCFD antenna!


Permeability vs. Power:  For a given size toroid, the higher its permeability, the lower its power rating.  Since higher permeability toroids break down at lower temperatures, it is obvious that they will also break down at lower power levels.





These guys are in business first and foremost to make a profit - which is OK, that's what every buisness exists for, but one should never forget this point.  But it also means that the buyer must do his own due diligence.   





The worst thing that can happen to a Balun OEM (original equipment manufacturer) is to have one of its products break down in the field.  This is bad for their reputation and hurts their business.


The best way for the OEM to prevent a balun from breaking down in the field is to use a toroid whose ferrite mix has a low permeability: #61 and occasionally #52 are the OEMs' mixes of choice - regardless of what the application might potentially need.  For this reason the misleading and confusing their customers.


In many applications, especially with symmetrical antennas such as center-fed dipoles or yagi antennas, the CMC problem is so small that these low permeability toroids produce enough CMI to cope with the situation.


Unfortunately some applications, especially OCFD (sometimes called Windom) antennas produce so much CMC that a balun with these low permeability ferrite cores has no chance of coping with the problem.   It breaks down and its CMI goes to hell in a hand basket.





However there is one Balun OEM who is learning, listening to the members of Serge’s Windom group and responding by changing his recommendation on his web site and giving us additional products for our special needs.  This is "Balun Designs."  This is why on the [rare] occasions when I purchase a commercial

balun, I buy from Balun Designs.


As a buyer, you should be prepared to “tell” the OEM what you want; do not ask him what you need.  He doesn’t know.  Beware of recommendations such as “most people buy this one.”  When it comes to baluns, most people have no clue what they are buying.


THE  BOTTOM  LINE  (and FULL answer to the Quiz Question):


The 80m OCFD antenna requires an RF Choke in addition to its good Dual-Core Guanella (current) balun because the balun alone does not produce enough CMI on 3.5 MHz to cope with the heavy CMC produced by this antenna. 


Its CMI level on 7 MHz will usually suffice.  Therefore a choke is usually not needed on 7 MHz, “if” you build the proper balun. 


I know of no commercially sold balun which I consider to be good enough (even for 40m) under all circumstances, but the Balun Designs Model 4115 usually is. 

Sometimes it too will need a choke on 40m.  This is because the CMC level also depends on the surrounding environment.


We could of course build a Dual-Core 4:1 Guanella balun using #31 toroids.  These would likely suffice without a choke, however its CMI would drop off rapidly on the higher bands. 


IMO it is best to use #43 for this task and add a Maxwell or Guanella [1:1] choke if necessary.


Or . . . 


Use a [good] "Hybrid Balun" !

Then there is no need for the additional choke. 

I will soon disclose my recommendation and details of this Hybrid Balun.






This outstanding chart produced by Steve, G3TXQ, shows us what we need for given CMI levels on different frequencies.




I  hope this page was helpful.


73 - Rick, DJ0IP


I would ask you to "Like" this page but unfortunately my web page CMS has no Like feature!  :-(