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Comment by dbcurtis | Hacker News Reader

How antennas work, very high level:

1. Accelerate an electron, get a photon. A good transmitting antenna is something that is an efficient structure for accelerating electrons.

2. Antennas are reciprocal. They receive as well as they transmit.

3. Resonant structures are often used because you can keep more electrons accelerating with less energy -- the damp finger on the rim of a wine glass effect.

4. Power can be directed by appropriately phasing the radiating sub-structures to create constructive and destructive interference in the radiated energy.

The rest is modeled simply with a set of simultaneous three dimensional second-order partial differential equations.

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I admit that I have never had the nerve to use simply in the same sentence as with a set of simultaneous three dimensional second-order partial differential equations.

Also, see my comment about reciprocal: https://news.ycombinator.com/item?id=22790001

>2. Antennas are reciprocal. They receive as well as they transmit.

While true, this could be really misleading in practical application. A really good transmitting antenna doesn't always (or even usually) make a really good receiving antenna.

Honest question: in what way? As in it might make sense to have an omni-directional antenna for the transmitter but a directional antenna for the receiver rather than omni-directional for both?

The correct answer is, it’s complicated. (Of course) It depends greatly on the definition of the output, for example, which can vary by application.

Easy answer is, for a fixed load (I.e. a resistor) and frequency, one varies amplitude with power. (perhaps absolute value of area under the curve might be a better description)

For more information see here: http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/powstr.html This is for a physical string, which I think is easier to wrap your head around. The math is very similar for electrical waves.

Yes, and additionally, the return signal, for example on 160 and other low bands may actually follow a different path due to phenomenon of grey line propagation, and other more complicated paths.

I would say it's the other way around: A really good transmitting antenna is necessarily a really good receiving antenna (at least at the frequencies it's tuned for), but a purpose-built receiving antenna might make a poor transmitting antenna because of power handling and matching considerations.

Yep, like smartphones wifi antennas, get a strong signal, bur experience is bad due to weak transmitter.

Not necessarily so.

In terms of the properties of the antenna itself, it is reciprocal -- the "How Antennas Work" part. You comment is more along the lines of "How to Use Antennas", that is a different question.

The places where there is a benefit to having different antennas for receive and transmit depends on the characteristics of the channel and on the application. The classic example being where a receiving antenna that reduces reception of local noise can give a better signal-to-noise ratio than an antenna that has been optimized for the best transmitted signal footprint at the location of the other station.

So, while I agree that in practice there are plenty of times where separate receive and transmit antennas have a benefit, it isn't because of antenna physics.

Case in point is my ham radio station.

I have an active magnetic loop for HF RX. This allows me to null out near field interference (a few dB from the transformer in the back yard) and provides very broad bandwidth (VLF thru HF). It is 75 feet from the house, away from that near field interference.

TX is an inverted L, up the side of the house, with a tuner at the base. I don’t have electrical length, but I do have power, so I can trade off efficiency for a power amp, and the bandwidth with the tuner. Interference is irrelevant, but I can see some 60 Hz cross modulation on a monitoring receiver, due to coupling to the house wiring.

I think this is incorrect. See my comment at https://news.ycombinator.com/item?id=22790001

As far as I can tell, you are agreeing with me. It is the channel that is not reciprocal, which is why there is a benefit to separate antennas.

Do not confuse the analysis of an antenna in free space with real-world deployment with real ground, noise sources of various kinds, and asymmetrical propagation.

You are absolutely correct. See http://www.arrl.org/shop/Receiving-Antennas-for-the-Radio-Am...:

Receiving Antennas for the Radio Amateur Receiving Antennas for the Radio Amateur Transmitting and receiving antennas have different jobs to do. Although the fundamental characteristics of antennas apply to both transmission and reception, the requirements and priorities of receiving antennas can be vastly different from those of transmitting antennas. Receiving Antennas for the Radio Amateur focuses entirely on active and passive receiving antennas and their associated circuits. There are relatively few cases where a radio amateur cannot benefit from a separate, well-designed receiving antenna or antenna system. On the low bands, including our new allocations at 630 and 2,200 meters, heavy emphasis on the receiving end of these radio paths is essential for success.

There is a long, fruitfull discussion of the reciprocity properties of receiving and transmitting antennas.

He discusses at some length how, even at HF, variations in polarization of arriving waves can vastly influence the received signal legibility.

The reciprocity of receiving and transmitting antennas is in fact fundamentally incorrect. They behave very asymmetrically in their physics.

Ha ha, well I take issue with the “simply” part. There are several expensive, EM field solvers. If it were simply, they would be cheap.

I am glad that at least one person recognized my attempt at humor.

The cheap-skate's antenna modelling package is NEC2, which is public domain. It is a Method-of-Moment solver, not FDTD which is much more useful at microwaves. But for UHF and below, NEC2 is great as long as you know how to dodge the (reasonably well understood) deficiencies. The downside is that it is from the age of FORTRAN, so the native interface is column-sensitive punch card images. People have done free or low-cost wrappers, though, to take the misery out of it.

NEC4 fixes a number of the model bugs in NEC2, but the last I heard NEC4 fell under ITAR, so requires a license.

I don't know of a freebie FDTD EM package, though.

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