HF WIRE ANTENNAS MADE IN GERMANY
HF WIRE ANTENNASMADE  IN  GERMANY

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Photo:  DJ0IP

What's Wrong with other OCFD's?

 

This is a fair question. 

  • Lots of other manufacturers and vendors sell OCFD antennas
  • Many of their customers are perfectly satisfied with the operation and performance of their antennas.
  • What's wrong with that?

 

BACKGROUND:

 

The Off-Center-Fed Dipole (OCFD), also called "Windom" by many hams and vendors, is one of the most highly disputed antennas in the world.

  • Thousands of users love this antenna
  • But thousands of users curse this antenna and warn people not to use them.
  • Most of the world-renowned antenna gurus do not like these antennas and warn people not to use them.

 

So who is right?     Answer: They are ALL right!

The correct answer is, it all depends.

 

Primarily, it all depends on each individual installation.

REASON:  Common Mode Current (CMC)

 

Because it is fed off-center, the OCFD antenna is not a balanced antenna.  As a result, it has inherently a large amount of Common Mode Current which the balun must deal with.  "THIS" is the "Lynchpin", or the "Archilles Heel" of the OCFD antenna.

  • Different balun technologies have significantly different amounts of Common Mode Impedance (CMI) - which is their ability to impede the flow of CMC down the coax.  (See Balun Technologies BELOW)
  • The amount of CMC varies with height above ground (the lower to the ground, the higher the tendency to have CMC on the feedline).
  • The amount of CMC varies from one installation to the next,
    • Due to neighboring objects near the antenna.
    • Due to how the coax is run away from the antenna.
      • Best case, it should be run perpindicular to the antenna wires.
      • Many times it is run diagonally away from the antenna, which significantly increases the amount of CMC.

With all of these variables, it is fair to assume that no two installations are exactly the same.  Some are better than others, some are worse.

 

DIFFERENT RESULTS:

  • If we have an OCFD antenna with a poor balun (low CMI), but erect it high and in the clear, in the middle of a wide open field, we probably won't have any trouble with CMC at all.  Unless it is a dreadful balun design with a lot of loss, it will be a very good radiator!
  • If we take that same OCFD antenna and errect it in a small typical city yard, it will be near several houses, which cause reflections.  CMC will rapidly increase.  This can cause all sorts of problems in our own home or with the neighbors.  Never-the-less, the antenna is often still a good radiator!
  • If we take that same antenna and run one leg over a garage with a metal roof, we may incur lots of CMC problems! 
  • If we are only running QRP (5W), none of the above may make any difference.  We probably won't notice any problems.
  • If we are running just 100W, perhaps the above will cause problems, perhaps not.  We may or may not notice problems.
  • If we are running 500/1000/1500 Watts . . . ALL HELL MAY BREAK LOOSE!
  • On the other hand, if we have any of the cases above, but are using an OCFD antenna with the purpose designed "Hybrid Balun" used in our Aerial-51 Model 807-xx antennas, AND are running up to 500w, you should not incur any trouble at all.  It will be a good radiator.
    • Running up to 1000 Watts 'probably' will be OK too . . . except in the case of the metal garage roof.
  • Running 1500 Watts:  Not Tested; it is twice our legal limit here in Germany.  Do your own due dilligence!

 

GRANTED:  ALL OF THE ABOVE is perhaps a lot of information to digest.

Read it again; and again.

 

Now it should be apparent why some people are quite happy with even the cheapest OCFD antenna, and others despise it, calling it a very dirty antenna!

 

DJ0IP has spent 6 years researching the Common Mode Charistics of the OCFD antennas, under every thinkable configuration (exept the metal garage roof).

In this process, more than 1500 field measurements were made, in order to determine what works and what does not work in a less than perfect installation.    

 

WE ARE CONFIDENT THAT THE 807-xx

IS CURRENTLY THE BEST OCFD ANTENNA

ON THE MARKET. (October 2018).

 

 

OCFD BALUN TECHNOLOGIES (and a bit of history):

 

For the longest time it was believed that a 6:1 balun was the best match for the OCFD antenna.  Later the trend switched to 4:1.  But THIS was not really the problem with the balun choice;  balun technology  was.

 

Originally, when the OCFD was first fed by coax, roughly in the late 1950's (i.e., by Buck Rogers, K4ABT), the balun used was a simple "Ugly Balun" made of 75 Ohm coax, with a 1:1 ratio.  And it worked.  Of course at that time an SWR of under 5:1 was considered to be OK, because the Pi-Network of the transmitter would match it just fine.

 

Early commercial OCFD antennas, i.e., the Fritzel FT-4, used 6:1 baluns.  

Later versions, as proposed by I7SWX, used a 4:1 Ruthroff (voltage) balun.

 

When W7EL proposed the terms "Current Balun" and "Voltag Balun" in his 1982 paper on "Baluns: What They Do and How They Do It", the ham radio world jumped onto the concept of CURRENT Baluns, believing these were superior in every application.   

 

 

THE DEVIL IS IN THE DETAIL

 

Unfortunately the balun manufacturers all focused on the wrong technology, due to an oversight by Sevick in his otherwise excellent book on baluns.

  • It seems Sevick showed us two ways to build the 4:1 Guanella (current) balun; Single-Core and Dual-Core.
  • What Sevick neglected to point out is, the Single-Core version only works properly when it is 100% independant of ground; in otherwords, when it is "floating" with no relationship to ground.  It works fine in floating networks, but not in antennas.
  • No HF antennas on the planet meets this requirement.  They all have a very distinct relationship to ground.  Therefore the Single-Core 4:1 Guanella Balun DOES NOT WORK with these antennas.  Specifically, it exhibits ZERO Common Mode Impedance.
  • Never-the-less, many commercial OCFD antenna manufacturers continue to build and sell their antennas with this inferior balun!  SHAME!  And this is wrong; VERY WRONG!

 

NO SILVER BULLET

 

It was thought that the dual-core 4:1 Guanella balun was the silver bullet; the answer to the OCFD balun problem.  And in some cases it actually was; unfortunately more often than not, it was not.

 

Field Tests performed by DJ0IP in 2015 proved that the Dual-Core 4:1 Guanella balun did not have sufficient Common Mode Impedance (CMI) to remove all of the Common Mode Current (CMC) from the feedline of an 80m OCFD when mounter at typical city heights (i.e., 30 to 60 ft. of height).

 

In his excellent paper on Balun Basics, G3TXQ explains and contrasts the characteristics of all of the popular balun technologies.  In that paper he points out that the Dual-Core 4:1 Guanella balun has only half as much CMI as the Single-Core 1:1 Guanella.

 

THUS, is cases where more CMI is required (i.e., an 80m OCFD antenna mounted at typical city heights), the ONLY balun technology with sufficient CMI for coping with the high level of CMC of this antenna, is the 1:1 Guanella Choke-Balun.

 

However "1:1" does not provide a good impedance match to this antenna.  Therefore the 1:1 Guanella Balun (by itself) is not sufficient either.

 

SOLUTION:  THE HYBRID BALUN

 

A word of CAUTION:  "Hybrid Balun" is just a generic term meaning two different balun technologies are combined into a single balun.  There is no exact definition of what this consists of and indeed, if you google "Hybrid Balun", you will find different examples.

 

DJ0IP has designed a Hybrid Balun whose characteristics specifically meet the requirements of a balun when used in an 80m OCFD.  This balun is used in the Aerial-51 Model 807-xx OCFD antennas.

 

Currently (2018), it is believed by us that the DJ0IP Hybrid Balun is by far the best balun technology for use with an 80m OCFD antenna.

 

THEREFORE:

WE ARE CONFIDENT THAT THE 807-xx

IS CURRENTLY THE BEST OCFD ANTENNA

ON THE MARKET. (October 2018).


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