THE FUTURE OF OCFD:  "CURRENT SUMS ANTENNA" (CSA)

The Current Sums Antenna (CSA) is an antenna that is relatively unknown outside of German-Language speaking hams.  It was introduced to the ham community in 2000 by Karl H. Hille, DL1VU (SK) in his book entitled "Windom- und Stromsummen-Antennen").   [Eng: "Windom and Current Sums Antennas"]

In this book, DL1VU introduced a new method of designing OCFD antennas; specifically, a method for determining the best position along the half-wavelength of wire to feed the antenna.  He proposed using the sum of the currents of all the bands that the antenna is being designed for;  thus "Current Sums" Antenna.

Karl also wrote a program in BASIC for using this method of designing the antenna, but it never really became popular, due to the need to enter all of the command lines manually into the computer.   

About 10 years later, Klaus Warsow, DG0KW wrote a program for Microsoft Windows^TM based on DL1VU's BASIC program.  This program, which may be downloaded in this web's section on "OCFD Antenna Modeling", is easy to download, install and run.   Unfortunately, it is in German.  However I have translated the user manual into English, which enables most hams to use the program.

OCFD vs. CSA

An OCFD antenna is a CSA antenna, and a CSA antenna is an OCFD.

The only difference is the design goal for positioning the feedpoint.

The Original Concept of the OCFD Antenna was to find a feedpoint position where the impedance of all of the bands the antenna should cover are equal (or at least very close to being equal).  This position occurs at the point where the RF current curves along the antenna intersect.

It was found in the original design of the antenna that the currents of ALL of the HF antenna bands intersect at a position that is 1/3 of the overall length from the end of the antenna.  This was in 1949.

The original OCFD antenna does not work on 15m.  Why not?

In 1949, the 15m band had not yet been allocated for amateur radio use.  It was not introduced until 1952 (CW only) and 1953 (for SSB).

For the next 40 years, hams happily accepted the idea that the OCFD does not work [efficiently] on 15m.  We will see why that is, below.  

Of course it is very easy to make the OCFD antenna work on 15m, even without losing efficiency on the other hf bands.  We will see how, below.

We cannot know who first found a way to add 15m to the OCFD antenna, but in his 1996 book entitled HF Antenna Handbook, by William I. Orr, W6SAI, Bill introduced a method on page 3-7*. There he wrote:

For 80-meter fundamental and harmonic operation the OCF antenna should be about

136-feet long.  For a 50 Ohm line with a 4:1 current balun, the tap point [feedpoint]

lies about 20% from one end, depending upon antenna height and location.

Bill went on to say the common mode current was an issue with this antenna and that a ferrite line decoupler [RF choke] is necessary just below the balun.

{*Note: my copy of Bill's book is the fourth printing in 2005. 

  The page number may be different in the original printing.}

This is only one of several feedpoint positions that enables the OCFD to cover all 5 "classic" hf bands. A good method of finding the best feedpoint for an all-band OCFD (or any OCFD) is to take a close look at the sum of the RF current for the bands of interest at each position along the antenna.

The Concept of the CSA Antenna is to find a feedpoint position along the antenna where the sum of the RF Current of each of the bands is maximum.  The question is, how do we do this?

I will show you how to do this using the OCFD modeling software from DL1VU/DG0KW, but first my own personal story of how I did it in 2011:

Designing CSA/OCFD Antennas using Software

Before we begin, we need to understand one simple point about the flow of RF Current along a wire antenna.  As we all know, a full cycle of RF current requires a full wavelength of wire.  A half wavelength dipole is only half as long, thus only one half cycle completes along this wire.   The second harmonic band completes a full wavelength; thus both halves (the positive half and negative half complete).

RF radiation from the antenna is strongest at the point of maximum current and weakest at the point of minimum current.  On the fundamental band, the strongest point is the physical middle of the antenna; the weakest points are its two ends.

On the second harmonic (i.e., 40m for an 80m antenna), there are two current maximums, one positive and one negative.  The strength of the radiated signal is the same for both the positive and negative current peaks.

In the interest of examining the strength of the radiated signal, we can use the absolute value of the currents, thus displaying all peaks as a positive.  This will enable us to better understand the radiation pattern of our antenna.

The graphs below depict this:

The graph on the left ("RF Current Flow") shows how the RF Current for each band really flows. Remember, the negative half cycle on 40m radiates the RF signal just as strong as the positive half cycle.  However, since we are searching for peaks and points of intersection, it is beneficial to invert the negative half cycle and display it as a positive (absolute value).

In the graph on the right, we see that the two curves intersect in two places.  Thus, their impedance is [nearly] the same at each position.  Why it is only "nearly" and not "exactly" the same will be explained elsewhere in the OCFD section of this web site.

As we model different versions of CSA antennas, we shall always examine the absolute values of the RF Current.  In doing so we can easily see why some feedpoint positions work and others do not.

In our first EXAMPLE, we shall use the modeling software to see why the original (classic) OCFD antenna does not work on 15m and learn what we must do to make it work on 15m.

Here we have modeled the antenna for just 3 bands:  80m, 40m, & 15m.

Reminder:  The Impedance (measured in Ohms)  Equals Voltage / Current   ( R = E / I )

• If the current is low, impedance is high
• If the current is high, impedance is low

With an 80m dipole, the current is highest in the middle (50%).  We know it has a typical impedance of 50 to 70 Ohms, depending on height.  However, on 40m and 15m, the current is very low (nearly zero).  Therefore the impedance is sky high (in practice it is a few kilo-Ohms.);  thus the SWR is sky high!

At the feedpoint of a classic 80m OCFD (i.e., 33.3%), the 80m and 40m current curves intersect;  thus their current is about the same, and their impedance is about the same.  However, the 15m current curve at this point is very low. Thus its impedance and SWR is very high.

(Note: in theory, it is zero (in free space) but here on earth it is slight higher.  We'll see why, elsewhere in the OCFD section of this web). 

When the current is low, the impedance and SWR is very high. 

This is why an 80m OCFD fed at 33.3% does not work well on 15m.

If we would move the feedpoint to the 25% point, the three current curves are close together - as clearly seen in the peak of the Current Sums (red curve).  Thus, we would have high current  and relatively low SWR on all three bands. 

• Their currents are not all equal, but their SWR all fall into a usable range. 
• Thus, the antenna will work just fine on 80, 40, & 15m.

It is no coincidence that the optimum feedpoint position is exactly the point where the current sum of the three bands of interest is at its highest peak.  Thus the name:  "Current Sums Antenna".

The example above shows how easy it is to move the feedpoint to bring the 15m band into a useful range.  Clearly, the balun and OCFD vendors who claim this antenna cannot work on 15m, simply do not understand how this antenna works.

In practice, we also want the antenna to work on 20 and 10m. In that case, the 25% feedpoint is not a good choice.  There are two good choices that give us all 5 bands (20% and about 29.x%).   Which of these two we choose and the exact value of "x" will also depend on which WARC bands we want to include in the antenna.  More on this, elsewhere in the OCFD section of this web site.

In our second example, it gets interesting:

Suppose DJ0IP has a 4-element monoband Yagi antenna for 20m.  He is looking for an second antenna that will cover 40m, 15m, and 10m.  The classic OCFD antenna covers 40m, 20m, and 10m, but not 15m.  Let's use the OCFD Modeling Software to design an antenna for these three bands.

In the graphs above, we find two feedpoints that will work well on the 3 chosen bands (40/15/10m).  The feedpoint at 42.6% is slightly higher than the feedpoint at 14.7%, so we would probably choose 42.6%.  But that is only half of the story! 

We must consider Common Mode Current (CMC).  For a half wavelength antenna, CMC is lowest at the center of the antenna.  As we move the feedpoint away from the center, CMC rises, being maximum at its two ends.  For this reason we would definitely choose 42.6%!

If DJ0IP did not already have a 20m antenna, we would have modeled the antenna optimized for 4 bands.  This would result in different recommended feedpoints.

Another feature of the OCFD Modeling Software is, we can design the antenna for 3 bands, and show the resulting curve on one or more other bands - albeit, the other bands currents are not used for calculating the feed point.   Just for fun we shall run the same model as above, but show 20m.

In the graphs above, we see our 40/15/10m CSA again with the same two suggested feedpoints, and we also see the current curve on 20m (yellow curve).  Note that the two originally suggested feedpoints are no longer optimum for a 4-Band CSA.  We would have to move the feedpoint a farther away from the end to get the best feedpoint.  In this case we would choose 41%.

A WORD OF CAUTION:  The models shown here were using specific wire with specific insulation and specific I.D. and O.D.  Using a different wire would give slightly different results.  In addition, the height above ground (AGL) and consistency of the earth below the antenna would change the results slightly.

At this point, it is time to build the antenna and erect it in the field . . . 

then prune it overall length and feedpoint position for optimum results.