Thread: nec2 questions

Is anyone here familiar with the nec2 program? I have a few questions about what some of the numbers in the output mean. My main question though is which numbers are important for a receiving antenna? Does it matter if the impedance matches, or is it only important to keep the SWR down?

Originally Posted by MagiMaster

Is anyone here familiar with the nec2 program? I have a few questions about what some of the numbers in the output mean. My main question though is which numbers are important for a receiving antenna? Does it matter if the impedance matches, or is it only important to keep the SWR down?

I'm not familiar with that particular program, but your question is largely independent of software. First, SWR is a measurement of impedance, so a good impedance match automatically implies low SWR, and vice-versa.Because impedance match/SWR only tells you something about how much power from a source gets transferred to the antenna, it is only part of the story. The radiation pattern is very important. How much maximum gain does the antenna provide (relative to, say, a hypothetical isotropic radiator, or relative to a dipole)? What is the directivity (essentially a measure of gain in different directions)? What is the polarization? Etc. All of those factors matter.

Good to know. 4nec2 does handle all that, and I think I know how to extract those details from a given model. Gain itself is one of the more obvious numbers in the program, and it can separate that into vertical and horizontal gain, which I assume is the polarization. (I've heard that you want high horizontal and low vertical for TV.) Is directivity directly relevant for a TV antenna, other than raising the gain in that particular direction?

I've also read that the complex impedance should be near 0. Should I be paying attention to that, or is that something else that's already covered by keeping the SWR down?

The program only outputs raw gain (at a given spherical angle). For a receiving antenna, you have to subtract off the SWR losses from that to get the net gain, right? Is there anything else that affects the net gain?

Originally Posted by MagiMaster

Good to know. 4nec2 does handle all that, and I think I know how to extract those details from a given model. Gain itself is one of the more obvious numbers in the program, and it can separate that into vertical and horizontal gain, which I assume is the polarization. (I've heard that you want high horizontal and low vertical for TV.) Is directivity directly relevant for a TV antenna, other than raising the gain in that particular direction?

It's also a measure of how well it discriminates against unwanted signals in other directions. So you'd like high gain in the preferred direction, and low gain in other directions.

I've also read that the complex impedance should be near 0. Should I be paying attention to that, or is that something else that's already covered by keeping the SWR down?

Achieving a good impedance match (achieving near unity SWR) automatically implies that the imaginary part = 0 if you are matching to a purely resistive load. If you are matching to a complex impedance, then a low SWR implies that you have managed to produce the complex conjugate of the load impedance.

The program only outputs raw gain (at a given spherical angle). For a receiving antenna, you have to subtract off the SWR losses from that to get the net gain, right? Is there anything else that affects the net gain?

To figure out the radiated power in a given direction, you only need to know the power actually delivered to the antenna, and the gain of the antenna in that direction. You can compute the former from knowledge of the incident power and SWR.

Originally Posted by tk421

To figure out the radiated power in a given direction, you only need to know the power actually delivered to the antenna, and the gain of the antenna in that direction. You can compute the former from knowledge of the incident power and SWR.

How do you apply that to a receiving antenna? (But the summary would be that gain and SWR are the numbers that matter, right?)

Originally Posted by MagiMaster

Originally Posted by tk421

To figure out the radiated power in a given direction, you only need to know the power actually delivered to the antenna, and the gain of the antenna in that direction. You can compute the former from knowledge of the incident power and SWR.

How do you apply that to a receiving antenna? (But the summary would be that gain and SWR are the numbers that matter, right?)

Thanks to a property known as reciprocity (which you will just have to assume applies here), the transmit properties imply the receive properties. That is, the transmit gain is the same as the receive gain, and the transmit SWR is also the receive SWR.Reciprocity would fail if the intervening medium were nonlinear, or contained magnetic materials. Assuming ordinary free-space propagation, one may safely assume that reciprocity holds.

Thanks. I think I'm getting the hang of this, though I'm mainly relying on the optimizers and evolvers to actually shape the antenna as I have no intuition as to what shape will have what effect. Knowing what the numbers mean does help set those up though.

The nec programs are good for getting transmitting antenna characteristics like impedance or the parasitics from objects sitting next to it. It doesn't seem to do much more than the lambda c thing. If your antenna indicates 50 ohms, then its at resonance. This resonance isn't an indication of gain factors above a simple wire antenna. Then if you need exact resonance to run a kw through your antenna , then nec works good.