Hacker News Reader

Comment by darkmighty | Hacker News Reader

Those are intentional jamming devices, I hope unintentional sideband interference wouldn't be treated remotely as harshly? The kind of power to achieve this kind of jamming would be pretty large too -- I'd judge at least 50W (depending on the jamming bandwidth I guess). A few miliwatts from an SDR transmitter isn't going to do much disruption. Not to mention there are pretty significant interference sources from defective devices mentioned elsewhere -- cheap usb power supplies, defective lightning, and more.

That said, anyone should indeed be filtering their output unless transmitting more than extremely low amounts of power (perhaps someone with radio experience could give a hand of thumb? Up to hundreds of microwatts sounds quite safe) as a matter of civility. Besides, you get to learn basic electronics by building a simple passive filter!

View on HN

There are two problems:

A few milliwatts can easily block the sensitive input of a Police Repeater on the other side of town.

FWIW, your "Hundreds of Milliwatts" are routinely used by hams to communicate world wide.

And no, a simple bandpass filter WILL NOT clean up this rubbish. Its output spectrum is the base-band signal repeated over and over, all the way up to VHF. It would take a very capable filter to pick out the single product required. This is NOT the way to design a clean transmitter.

It will not only cause interference, but it will interfere with EVERYTHING within range.

Most output filtering is a simple Low-Pass Filter which is designed to remove harmonics. But by definition, harmonics are an octave removed from the fundamental. This horrid device puts out a closely packed comb of spurii from DC to daylight. A simple passive filter will not even come close to cleaning it up.

You first start with a sound design, then add filters to clean up the last of any unwanted emissions. Not the other way around.

And because these faults have been well documented, this equipment would most definitely be classed as an "intentional jamming device".

Thanks for the reply! I'm quite interested in this.

> FWIW, your "Hundreds of Milliwatts" are routinely used by hams to communicate world wide.

Read again, I've written "hundreds of microwatts" :)

And that's for a reasonably wideband signal! A narrowband signal with high power has a lot more potential to interfere with a specific application. I seriously doubt anything of that order specially with a rough bandpass (passive) filter around the frequency of interest can cause significant interference. Passive filters have <1 gain, so they shouldn't risk narrowband amplification of the weak signal. The background noise should be within this order of magnitude, no?

(I don't quite have the time to give numbers right now, but here's a source from quick googling: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=7833115 -- the ambient noise should be on the other of .1 uW / GHz; at a few hundred meters away this competes with our wideband microwatt-scale transmitter?) I'll see if I can do some actual calculations later

Here's an example of unintentional interference http://www.arrl.org/news/view/amateur-radio-sleuthing-pins-d...

> "The troublesome transformer was not replaced, but the building owner agreed to turn off the sign should problems arise"

> I'd judge at least 50W

GPS is quite easy to jam with very low power. Something as low as 500mW could easily disrupt a fairly wide area. The reason being that GPS signals are generally already below background noise as measured from the ground.

Indeed. When I was experimenting with GPS spoofing, an output of 20dBm (100mW) was enough to make devices think they were in the middle of the ocean in a 200m radius (line of sight). With a quickly improvised yagi antenna I was able to push it to 500m, even after losing 3dB from switching from circular to linear polarization(!). IIRC, GPS satellites have 25W transmitters, that's insanely powerful, but they're 20000Km away, so it's rather easy to overpower their signal from close due to the inverse-square law.

Even with the first tests with 10mW in my room, iPhones at my house picked up GPS signal to know the time and timezone, which caused my flatmates to miss appointments and many websites stopped working due to certificates being expired by "2030".

> IIRC, GPS satellites have 25W transmitters, that's insanely powerful, but they're 20000Km away

Both numbers are correct, but 25W isn’t “insanely powerful”. The spec for GPS satellites lists the antenna gain at 13dBi. 25W fed through a 13 dBi antenna is just shy of 500W EIRP. Sitting near me is a 2W 2.4ghz amp and a 24 dBi parabolic antenna. That combo is just over 500W EIRP. The reason you stated (distance) is the big factor, over 12,500mi, the free space path loss is huge (182dB assuming flat gains). 500W is just shy of 27 dBW, so 27-182 is -155, which is a very very very weak signal.

The old AM border blasters were legitimately "insanely powerful". John R. Brinkley's XERA was transmitting at 1 megawatt EIRP back in the 1930s. It covered almost the whole of the US and parts of Canada, even on a crystal set.

Today, the Solt transmitter in Hungary operates at 2 megawatts.

https://en.wikipedia.org/wiki/Border_blaster

https://en.wikipedia.org/wiki/John_R._Brinkley#Brinkley_and_...

https://en.wikipedia.org/wiki/Transmitter_Solt

Yeah, my wording wasn't the best. It's insanely powerful, (the transmitter leaving the antenna aside) in this sense:

Low earth orbit satellites are much closer to the Earth, so transmitters need less power to cover the range with high quality signal. For example, NOAA satellites are able to send images of the earth at 5W that you can reliably receive with a dipole and a $5 amplifier.

Geostationary satellites are significantly higher, ~35000 Km, but stationary (duh), so they benefit from high gain, very directional antennas both on the satellite and the ground. They send high data and symbol rate signals for HD video of hundreds of channels, and each transponder sits at 15 to 150W (depending on many factors: band, area to cover, etc). They tend to have multiple transponders, though.

GPS satellites sit in the middle, pretty high up, so there's a ton of free space loss, in a non geostationary orbit, so high gain antennas are useless, plus receivers are supposed to be portable, hence there's no room for helix/axial antennas or arrays on the ground. So for this reason they have to ramp up the power to geostationary, TV broadcast levels, only to push a very, very slow data rate (50 bits/s).

Reply on news.ycombinator.com