Off-center fed long wire antenna

Question

I want to feed a horizontal wire antenna (approximately 150' long at 50' or more above the ground) for multi-band HF operation with an off-center (25-33%) feedline. The element wire will have no support between the (well isolated) ends and the end supports aren't very rigid (large, old trees with twiggy canopies.) I want to avoid suspending any large mass (transformers, baluns, chokes or other heavy devices) from the horizontal wire or feedline; ideally just coaxial or ladder/open wire type feedline from the ground to the antenna. Expected TX power level is 500W or less.

Obviously some impedance matching solution is necessary for multi-band operation. Common-mode current on the feed line is inevitable for several reasons (imbalanced load, capacitive coupling between the elements and the feed line, etc.)

The question: is there any conceivable approach that can be applied strictly at ground level where I am free to employ any device or technique that will a.) provide necessary impedance matching for multi-band operation b.) either eliminate common-mode current or obviate the impact of common-mode current (feedline radiation, pattern distortion, etc.) on the feedline and c.) keep the feedline losses low?

I believe the answer is no, but I'd like to know if I'm overlooking anything. I am (painfully) aware of myriad other antenna arrangements that can avoid some or all of the issues with the above antenna; please confine the discussion to the configuration described above. And thank you.

Answer 1

Off-center fed antennas, fed between 25% and 33% of the antenna length, are vastly different antennas. First, presuming the overall antenna is one wavelength long, at 25%, you have a 3/4λ element and 1/4λ element, which means you are feeding the antenna a current loop, which is the lowest possible impedance point of a capacitive type antenna; again, presuming a 1 wave length antenna, at 33% however, you are in-between a voltage maxima and current loop whether or not the antenna is an overall resonant length, so if coax-fed, you must use a transformer/UnUn at the antenna feed-point. Modeling your specifications, I get 450Ω impedance in the 75m band, which means you need a 9:1 UnUn, and without modeling it further, I couldn't say whether or not you would get a similar impedance at 60m, 40m, 30m, 20m, 15m, etc. You will also want to de-couple your feedline by coiling the coax at a non-harmonic interval of the feed-point (something other than 1/4λ or a multiple thereof. If your overall antenna is a non-resonant length, you are most certainly in-between a current loop and a voltage maximum, and again, you need to transform your impedance. Here is a very useful presentation that includes OCF antennas along with end-fed: https://www.youtube.com/watch?v=4lzlZxxUzM0&t=1502s

Unless this antenna is going to be for a specific frequency, it's my experience that the bandwidth of an OCF antenna (i.e. one not fed at a current loop), is very narrow, and to expand that over the lower bands in particular, you will need a tuner. Keep in mind though, the further the tuner is from the impedance mis-match, the greater the signal loss; in other words, the best place for the tuner is at the antenna, or as close to it as possible. One way around feed-line mis-match loss is to make the transformation with a 9:1 BalUn (not UnUn) immediately after your tuner (for convenience, you can at least get the coax to the outside of your building), and from there, use 450Ω window line to the antenna, and do your 33% leg and your 67% leg in opposite directions. This would be the way to keep the weight, and feed-line loss, to a minimum; just make sure you keep the window line away from the ground and conductive objects. Putting a 9:1 BalUn immediately after your tuner (outside your building), and using balanced (window) line, will also contain the inevitable mismatch from radiating very much. And while balanced feed-line needs more care in routing, it will be much better than coax, which will radiate when there is a significant mismatch.

Answer 2

Balun for common mode blocking can be put almost anywhere, but it is more effective at certain multiples of a quarter wave along the feed line. But you have to realize that if you don't put it at the antenna, the feed line in between becomes part of the antenna and may radiate. (Which is why quarter wave placement helps -- it tunes the radiating part of the feed line.) (Note: take into account feed line velocity factor when measuring that quarter wave.)

Impedance matching can also be done almost anywhere, but it works best when done at the antenna, and a sharp mismatch (at the antenna) can cause bad reflections. However, you may not care a lot about SWR in ladder line as the loss from that is much lower.

Also, it is possible to construct ladder line for a particular impedance, and you can even have a different intrinsic impedance in your ladder line at different ends with a gradual gradient in between, making the ladder line itself your matching section. Again, this works best for a section that is a quarter wave long (but for slightly different reasons).