No room for radials

Question

I have a Hustler 5BTV antenna in my back yard. I do not have any room for radials, is there something I can do to improve my reception and transmission Jimmy K5wel

Answer 1

Wow, the instructions that come with the antenna are really a mixed bag of good instructions demonstrating deep knowledge, and the good ole magical thinking. More of a low-review magazine than instructions, really!

So, I think your question here is two-fold:

1. What's the problem with less/no radials and
2. what should I do to improve the situation

For the first question, I think you deserve a bit of an explanation! For the second, well, I'll tell you what I hope you'd arrive at anyways, so I'll keep that sweet & short.

1. What's the problem with less/no radials?

These radials form a ground plane, effectively. Without it, your antenna isn't the quarter-wave resonant antenna it's designed to be, and will work significantly worse. Get as much good ground as is feasible in the spot you are. Nothing else you can really do here: This is not primarily an issue of impedance matching, nor of "the lossy ground absorbing the signal" (as one of the lesser articles in the instructions claim): The ground plane, attached to the outer conductor of your coax, is simply what my RF professor would describe as a "mirror":

Think of your quarter-wavelength antenna as half a half-wavelength vertical dipole. So, imagine us cutting a vertical dipole in half, at the feedpoint (in the vertical middle):

  │
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  │
  │
  │
  └─
✂╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌
  ┌─
  │
  │
  │
  │
  │

Just cutting off one branch of a dipole doesn't work – you simply don't get the dipole properties. However, when you look at the symmetry of a dipole, all the electrical field lines, at every single instant in time, must be perfectly perpendicular to the cutting plane – because otherwise, they would need to make sharp "bend" in the middle of the air, and that's not compatible with the physics of them. (Maxwell's equations require all the fields to be differentiable within a homogeneous medium.)

So, if all electric field lines need to be exactly vertical, then adding a perfect conductor plane (say, a sheet of copper) where we just cut would make no difference to the antenna – because there's no electrical field in parallel with the conductor plane, no current would flow in it. All we need to make sure is that the whole conductor plane is at the same electrical potential as would be there without the sheet. Luckily, that's easy, because for symmetry reasons, if the dipole is fed balanced, then that'll be just the constant "middle" voltage. Suddenly, it's as if you had an electrical mirror: the half-dipole with the mirror electrically looks as if it was a full dipole.

That would mean, however, that the two half-planes of the dipole could work as individual antennas. Or, more drastically: the top half can exist without the bottom half, if we feed it unbalanced, and connect the other conductor to the conductor plane. You get a monopole antenna.

Well, that's exactly what these radials do here. They form that copper plane. Admittedly, with gaps, but these gaps are much smaller than the wavelength, and the further away from the monopole you get, the less the actual amount of copper matters (because the field strenght goes down), so that's why they're radials (whether they are as straight as the manual demands it, however, doesn't matter that much, aside from when you use cheaper materials with low conductivity).

So, your "ideal" radials would be so close to each other, they form a perfect plane, and that plane would be infinitely sized. Now, neither being infinitely rich, nor having an infinitely large space, you can never build the perfect radial ground plane, but you can get close enough that for all practical purposes it's the same.

2. What should I do to improve my situation?

Now, you say you've got restricted space for radials – so make use of that as well as you can. There's really no other way to optimize this, sensibly. Yes, you could try to raise the feedpoint and make the antenna longer, you could do elaborate EM simulations to figure out whether a slight slant of the vertical would help, but the reality is that you need to operate this thing mechanically as much as electrically, and so the feedpoint will stay at the same height, the antenna will stay the same, and you will not build an elaborate support structure to fix your antenna at 18.2993° slant (or whatever your simulation tells you).

So, do as well as you can with your radials; fill the space you have; they don't need to be permanent! If you have wet soil, that helps, too – connect it!