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I have a system with a 1/4 wave whip antenna and above the antenna there is a mess of aluminium they're not connected but are nearby. I would like to figure out what effect this would have on the radiated signal strength but I'm at a complete loss at how to do this. My actually problem is trying to figure out how far away the aluminium needs to be to have a negative effect on received signal

Just to make the situation clear;

/\/\/\/\
\/\/\/\/
\/\/\/\/
\/\//\/\



   l
   l
   l                                              R
   l
   l
GGGGGGGG

Key: \/ = aluminium
l = antenna
G = ground plane
R = far away receiver (in that approximate direction)
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In your particular case, I'd note 2 things:

  1. The aluminium is directly above the whip, where the antenna's fields are the weakest.
  2. Aluminium is a good conductor, so unless there is something about it that will increases losses, it won't decrease the radiated energy. It might change the feedpoint impedance, and it might change the direction in which energy is radiated, but the antenna efficiency must remain the same because there are no significant losses to account for the difference.

My advice would be to not worry about it too much. Probably the effect is small.

A more general rule of thumb:

  • anything 10 wavelengths or more away from the antenna is insignificant to the near-field operation of the antenna. It might have other effects by reflecting or redirecting the energy, but then the receiver could be in a Faraday cage too.
  • at 2 wavelengths, things are somewhat significant.
  • below 1 wavelength, things are very significant.

Remember "significant" isn't the same as "bad". Sometimes it can be good, look at Yagis and horn antennas, for example.

If you want to get more precise than that, some options are:

  • experience and intuition
  • modelling (EZNEC, 4nec2, ...)
  • empirical measurement
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There is modeling software you can use for this. EZNEC and 4nec2 are popular among hams, but they only do wires. You could try modeling the aluminium as a bunch of wires - spacing them, say, 0.05 wl apart. If nothing else, the model might tell you when the effect becomes negligible. Software that handles conducting planes also exists, but is AFAIK fairly pricey. May be you could get a student or trial license?

All antenna modeling software takes some learning, though. So why not just experiment. Another idea is to turn your ground plane upside down. The antenna ground plane would - at least to some extent - shield off the metal.

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  • $\begingroup$ Welcome to Amateur Radio Stack Exchange and thanks for your answer. Please don't end your posts with “signatures” — if you want to display your call sign, you can put it in your name (this won't affect your name on other Stack Exchange sites). $\endgroup$ – Kevin Reid AG6YO Sep 8 '16 at 14:18
  • $\begingroup$ NEC2 and NEC4 allow for the modeling of surface conductive patches and also larger scale conductive structures using a grid of conductors. I have modeled my pickup truck using a grid of conductive wires. But, instead of painstakingly entering the model by hand (all the GW cards), I wrote a program to generate the model given a few parameters and structure dimensions -- much better way to go. $\endgroup$ – K7PEH Sep 9 '16 at 0:29
  • $\begingroup$ Following on the above comment I just made -- the Navy has modeled entire ships using the conductive wire grid approach plus patches where appropriate to analyze antennas mounted on the ship. But note that these models use far more memory space in the internal arrays then most of the source versions of NEC2 support or have supported. You likely would have to become much more familiar with the internals of NEC2 source (Fortran) to do such a thing (ship models). $\endgroup$ – K7PEH Sep 9 '16 at 0:32
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Creating a model of your antenna (not too much different than your little diagram) with all nearby associated metal can be done with the NEC2 (or, NEC4) antenna modeling software. It is the best way to compute the effect of nearby conductors. In fact, there are no formulas or easy fill-in-the-blanks methods to do this.

NEC2 is easy to use but without a nice 3rd party output processing program, such as EZNEC (http://eznec.com) it is hard to make use of the results. These 3rd party output programs will create graphs to show SWR on the antenna and radiation propagation field and show currents on the antenna (and other things).

There are a variety of tutorials available that show you how to do this and they are easily found by googling NEC2. Note, you can probably get a Free Trial Use of EZNEC that will allow you to do the model and display the graphic results. EZNEC runs on Windows PCs only.

I personally do not use EZNEC although I did years ago. I use Mathematica as my 3rd party graphic display of result data produced by NEC2. Technically I use NEC4 but NEC2 should be able to do everything you need to do. NEC2 is freely available on the Internet, NEC4 is source licensed from Lawrence Livermore Laboratories. I purchased my single-user license for $300 about five years ago. Not sure if it is still priced the same though.

In my own antenna analysis work I always use NEC to produce the model and evaluate the results. I have found that results of NEC (NEC2 or NEC4) to be very close to actual measurements that I can make on the antenna after I put it up.

NEC4 does a better job of ground modeling and much more accurate for conductors close to the ground (inches or under) as you might use with a radial counterpoise for a 1/4 wave vertical. However, NEC2 is quite adequate for just about all uses being mindful of where the model starts to show weakness.

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