Hacker News Reader

Comment by lolc | Hacker News Reader

Reads like a good intro into how beamforming works. One detail I found dubious is this part about bigger antennas:

> To stretch our non-mathematical metaphor well beyond its breaking point, a "bigger sphere" will contain more signal as well as more noise, so just expanding the size of your region doesn't affect the SNR. That's why, and I'm very sorry about this, a bigger antenna actually doesn't improve your reception at all.

In my understanding that is incomplete because it ignores amp noise. When your antenna is larger, you get more signal (yes including more noise), so you need to amplify it less before the signal can be sampled. That means less amplification noise and the effective SNR is better. Thus antenna gain, not just antenna directionality, will improve reception.

View on HN

Larger antennas help even though both signal and noise are amplified. The difference is that former is more amplified due to coherence: the antenna is shaped in a way to add the signal parts in the same phase. On the other hand, the noise is added together in a non-coherent way, with different phases, which makes the result smaller.

Think about it as the difference between summing two unit vectors in the same direction - with a resulting vector of magnitude 2 - and doing the same thing with two ortogonal unit vectors - the result will have a magnitude sqrt(2).

The article does later get into "changing the shape of the scoop", meaning a more directional antenna. And the designs for directional antennas are generally larger than omnidirectional ones.

So I'm curious as to why UHF and VHF require larger antennas. My parent's house was about 100 miles from the WTC and the antenna was massive. People in the city though used smaller sized ones.

UHF and VHF (that is, Ultra High Frequency and Very High Frequency) are the 3GHz-300MHz range and the 300MHz-30MHz range, respectively.

That translates to a 10cm-1m wavelength for UHF and 1m-10m for VHF. Minimal effective antenna size for a basic wire antenna is 1/4 the wavelength, so you end up with big antennas for low frequency / large wavelength.

https://udel.edu/~mm/ham/randomWire/

Note that different organizations use the same terms for slightly different bands. In the IEEE nomenclature for radar, UHF is 300MHz to 1GHz. 3GHz is solidly within S-Band. And L-Band is in between them.

You brought back a fun memory. When my family moved upstate, about 125 miles north of the city, my dad was determined to watch the yankee games on channel 11, and my grandfather wanted to see his Mets.

He special ordered a massive antenna at Radio Shack. This thing disassembled was the full length of an old full size Chevy station wagon, I’m guessing 12 feet in the box and a bit longer assembled. Dad and grandpa mounted the thing to a pole that was also quite high attached to a device that would rotate the antenna.

When it was time for a game, you had to rotate the antenna to hit a specific gap in the mountains, which varied for different channels. Good times.

VHF requires larger antennas than UHF.

most of the reason bigger TV antennas were a thing was the "bigger is better" illusion. They make fancy stacks of elements and pt "+28 Dbi" numbers on the box but at any specific frequency those could be outperformed by a simple single cut wire. (they weren't utter horseshit: considered across the whole band they could be better for being bigger, but thats a technical and theoretical "win")

There are places where "more metal harvests more signal" in antenna design, but they're little dots on a broad map, not where you'd expect them to be. Herringbone antennas at WiFi freqs are fun.

The single cut wire will only be better at its resonant frequency. The TV antennas are designed to cover their entire bands.

Yagi arrays are a real thing, and they do take up a lot of space.

People closer to the transmitter receive enough raw signal power that they can get away with a really insensitive antenna. Farther away, the antenna needs to be more optimal to pick up enough signal to be usable. It's not that the giant antenna is magically better, it's that the smaller antennas are mediocre.

Every frequency has a wavelength, as in a literal length of wave, which means a literal length of perfectly sized antenna. There's a formula. The right size for FM (100mhz) antenna is something like 4 or 5 feet, but for wifi (2.4ghz), it's the size of a water molecule. That is not an accident, as the FCC did not lease 2.4 out permanently since water absorbs it, hence we can just plop an unregulated router/AP in our house. This paragraph is probably vague and generalized, and possibly wrong-ish towards the end, but close enough to aid general understanding.

Fluid water absorbs it very well, but the wavelength for 2.4 GHz is 0.125 m.

Yeah antennas need to resonate, not just absorb. A litre of water wouldn't be a very good antenna at 2.4, but it would warm up due to absorption if it is a trong signal. A vhf antenna might make a terrible antenna at 2.4 as well, but it would also warm up in a strong enough signal. Don't belive me? Shove one in a microwave (please don't really).

“length of a water molecule (at an accuracy of 1 ft)"

Reply on news.ycombinator.com