# Relationship between an EM wave and its current

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1. Oct 13, 2015

### tj50

Given the following:

A directional radio wave transmitting antenna which creates a beam (diameter in mm), a current in volts, and a frequency (Hz), is it possible to calculate the em wave voltage (v/m or w/m2) immediately next to the antenna? and then at a distance of 10meters?

What calculations/formulae will be required? If this is not feasible, is further information needed?

2. Oct 13, 2015

### sophiecentaur

Hi and welcome to PF
Well - you kicked off with a really difficult question there! Depending on the details of the type of antenna, there are a number of different methods for calculating near and far fields. There is no analytical method for anything other than a very basic antenna and every design will have a different approach. The (numerical) technique often involves approximating the problem by breaking the antenna down into a finite network of small elements and calculating how they will interact with each other and then takes the sum of the fields generated in any direction.
Google "finite element antenna analysis" and you will find a lot of hits but I couldn't find any free sources of information.

3. Oct 13, 2015

### Staff: Mentor

Welcome to the PF.

I'm not sure what you are trying to say with this: "a current in volts" Do you mean an antenna terminal drive voltage in volts? That would make more sense.

Very near the antenna, the field in V/m will be close to the drive voltage divided by the size of the antenna radiating elements. But as sophiecentaur says, as you move away from the antenna, the calculations get a lot more complicated.

Can you say what types of antennas you are interested in? How directional? What frequency band(s)? Why are you wanting to find the radiated field strength?

I believe there is an antenna design software package that is pretty inexpensive for the basic version. Based on your reply, I'll try to see if I can find it...

4. Oct 13, 2015

### Staff: Mentor

Here is the link to EZNEC, the package I was thinking of. The basic version is \$99 for the download:

https://www.eznec.com/

5. Oct 14, 2015

### tj50

To sophiecentaur, yes I'll look to Google the above to help answer this problem.

To berkeman, I'll need a little research time so apologies if there's no quick reply to your questions, & thanks for the EZNEC link, I'll also look into the antenna design program you've suggested.

6. Nov 1, 2015

### tj50

Further to the messages above:

To berkeman,
Yes, if by terminal drive voltage you mean the modulated AC current running from the transmitter to the antenna via the primary element.

To sophiecentaur & berkeman,

Its a hypothetical question at the moment concerning all frequencies.

After some research I've decided to begin with the omni-directional antenna (a simple metal conducting rod) for now because as you say its a more basic problem and I can progress to directional antennae later.

Can you provide a method, 'analytical or numerical', or a link for solving the above?

Thanks

7. Nov 1, 2015

### Staff: Mentor

If you're starting from the beginning, the simplest radiating antenna is the oscillating dipole. A google search for "dipole radiation" should get you started. Introductory treatments deal with the "far field", far enough from the antenna that its size is negligible. For the field close enough to the antenna that its size is significant, try something like "dipole radiation near field".

8. Nov 5, 2015

### tj50

Ok, thanks jtbell