Off-Center Fed Dipole vs Standard Center Fed Dipole

[Jackson Richter]

I have been doing a study for a couple of years now on the off center fed dipole. I was very much in the dark on what to expect in the relationship between the off center fed (ocf) and a regular dipole. I asked many questions and most of the time I jumped to the wrong conclusions. I decided to invest into antenna software like (Eznec + ) and do a study to see what the real truths are about this antenna. I have been using this particular antenna for awhile now and have compared it to other antennas including the standard center fed dipole. The (OCF) antenna has done equally well compared to the regular dipole. I just wanted the engineering community to know that with this simple antenna configuration, this multiband ( 7 bands) antenna has almost the same far field strength as the standard dipole, using no passive parts other than a 4:1 balun at the feed point. I used a derivation of the radiation resistance formula and can predict exactly where to place the feed point for optimal usage. I would like to thank all that have offered their advice.

Jackson Richter

 

[sophiecentaur]

Is it surprising that you got your result - still nice to confirm it, though? The standing wave on the dipole will be a strong influence on the currents in it and they are what determine the vertical pattern as much as anything. Offset feed could be very useful when it comes to supporting a vertical dipole out of the way of the top of a mast., if you want to minimise the reflection from the mast and avoid dips in the horizontal pattern.

 

[Jackson Richter]

Since, both antenna radials of the ocf are different lengths, I believe the issue of common mode currents may or may not be important. The common mode currents are supposed to be reduced by a 4:1 current balun which is mounted at the feed point. Many studies have been done stating, this type of antenna, a current type balun is required. My belief, along with this authors belief, "The current balun can be thought of as an Rf transformer with an untapped floating secondary. It will force the current in the two legs to be equal no matter what their impedances are and will not allow common mode currents to pass." written by (Scientific Community Author)
All of this of course depends on the current balun in question. A balun currently on the market can support a lot of the misgivings (5kw) if the ocf is placed around and or near large objects, especially metal type may add higher levels of unequal currents in the near field. In the past, if an isolation 1:1 balun was place at the xmtr, the common mode currents were reduced. Since were talking about 32 dBm at the antenna, my question is, how difficult is it coming up with a 4:1 current balun that can create a high CMRR and can do this with the high currents at power levels of 32 dBm and be able to go from 50 ohms to 200 ohms? from 3MHz to 55 MHz. With that in mind, the actual common mode currents maybe adding to the holes in the far field. We have to investigate that anomaly later. It may come down to, the common mode currents my be needed to make this antenna effective. Where as our standard center fed dipole shouldn't have any common mode currents.

Thanks J Richter

 

[sophiecentaur]

The common mode currents are supposed to be reduced by a 4:1 current balun which is mounted at the feed point.

Is that a 'rule of thumb' for a 'typical' choice of feed point position? I can't think of any inherent reason that a suitable balun couldn't suppress common mode current to any arbitrary degree. I do realize that bandwidth would be a consideration here and the 3 to 55MHz range is a big ask. I agree with the basics, of course.

 

[Jackson Richter]

In my original post, I mentioned that the nice thing about this antenna that it is a multiband antenna (up to 7 bands) only using a wire and a 4:1 balun. The balun is placed at a certain point from the end of the half wave antenna to give you the proper impedance to operate on that band. The communication community has been doing this for years. After studying this antenna for 2 years now my major concerns were common mode currents and how well does the far field isotropic gain compare to its standard counterpart (center fed dipole)? The ocf compares closely in (far field) to its counterpart, the center fed dipole. Also I wanted to have a guide for feed point placement. With that I developed a derivation of the Rr formula to determine where the feed point should be placed. I guess at this point, I would like to quantify the ability of the balun to remove common mode currents. I would like to see reference data for CMRR on the particular balun being tested. At least this would give me a bench mark for further testing and see if truly the reduction of these currents has any affect of the far field measurements. Just by the nature of this antenna, (the uneven antenna lengths), how much CMRR is needed?

 

[Jackson Richter]

Sorry, I guess I didn't really address your question regarding the choice of location for the OCF. When becoming acquainted with this antenna, all the information was exactly that, (rule of thumb) technique. Some information on how and why's but nothing that one could predict with any certainty where to put the feed point. I discovered by using the Rr equation with a little substitution, I could plot out the complete fundamental frequencies and its harmonics and show exactly where the (feed points) could be used for multiband operation. By the way there are numerous locations available depending on length of the half wave antenna.

 

[Jackson Richter]

Is that a 'rule of thumb' for a 'typical' choice of feed point position? I can't think of any inherent reason that a suitable balun couldn't suppress common mode current to any arbitrary degree. I do realize that bandwidth would be a consideration here and the 3 to 55MHz range is a big ask. I agree with the basics, of course.

Sorry, I guess I didn't really address your question regarding the choice of location for the OCF. When becoming acquainted with this antenna, all the information was exactly that, (rule of thumb) technique. Some information on how and why's but nothing that one could predict with any certainty where to put the feed point. I discovered by using the Rr equation with a little substitution, I could plot out the complete fundamental frequencies and its harmonics and show exactly where the (feed points) could be used for multiband operation. By the way there are numerous locations available depending on length of the half wave antenna.

 

[sophiecentaur]

Sorry, I guess I didn't really address your question regarding the choice of location for the OCF. When becoming acquainted with this antenna, all the information was exactly that, (rule of thumb) technique. Some information on how and why's but nothing that one could predict with any certainty where to put the feed point. I discovered by using the Rr equation with a little substitution, I could plot out the complete fundamental frequencies and its harmonics and show exactly where the (feed points) could be used for multiband operation. By the way there are numerous locations available depending on length of the half wave antenna.

With such a vast bandwidth, the design must have been arrived at with a lot of trial and error.
Measurement of performance (radiation pattern) would be a problem. I remember sitting in on very expensive helicopter measurements of the patterns of HF broadcast Curtain Arrays (HP, of course). It's pretty important to know which vertical direction you're squirting 250kW of HF signal and most broadcasters are entirely in the hands of the suppliers. You never know what the ground is doing to your signal, without taking actual measurements.