Saturday, May 9, 2009

Dipole leg trimming :

Is there a way to measure the electrical length of each half of the dipole - individually - not as a "set?"

In the ideal case, there would be nothing proximate to the antenna. But in the real world, quite often my antennas need to be positioned where some part is in closer than ideal to another metal object.

I know this affects the antenna resonance point as the element is moved closer/further away from the metal object. And usually, the antenna is not symmetrical with the metal object. Which means the amount of coupling to the structure by that leg is different than with the other half. And that should imply that the ideal case of a balanced antenna is no longer balanced.

I remember reading about trimming elevated radials for equal currents - but it's been too long to recall where I saw it. The half of the dipole should be similar to the equalization of radial currents case.

If there were an easy way to measure the electrical length of each half individually, the lengths could be adjusted so they are the same. And from that point, trim the lengths for resonance in the normal way (equal length trimming from each element).

There are of course many ways to force a more equal current distribution - transmission chokes, baluns, etc - but for this question, I want to focus specifically on ways to obtain balance through a measurement of the wire alone. Perhaps a method involving the MFJ-259B or similar.

Appreciate any comments you may have and many thanks.

1 comment:

  1. I saw this post a couple days ago and almost wrote a reply, but I decided to wait and talk it over with some other hams who are better at antennas than I am first, so please forgive the delay in commenting.

    The gist of the discussion is that there's not much value in attempting to tune the dipole to balance the currents; you'd be better off just using a choke to cut off the common mode current (assuming that they're actually unequal enough to be problematic) than trying to tediously trim the dipole to get equal currents on each arm.

    We did talk about the idea of measuring the electrical characteristics of each leg, but came to the conclusion that while you could do this, the results you would get would not correspond to the electrical characteristics of the full dipole that would allow you to predict how to best cut the antenna. It's possible that, with a lot of simulation work and careful experimentation, someone could figure out a way to measure the electrical characteristics of each leg and from those numbers predict the characteristics of the full dipole system, but none of us have ever heard of anyone doing it. The comment that stuck with me was that it might be an interesting exercise from an academic standpoint but it isn't likely to be of much practical value.


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