Tube🌿Timeever wonder how you get cell service inside large buildings? they install antennas on the roof and use air-line coaxial cables, like this one, to connect them to antennas on the inside.
here's the same air-line coax cable but i've cut it in half. unlike a common TV cable box coaxial that uses a plastic dielectric, this cable uses air as the dielectric, and the spacer just centers the signal line.
this is also used for ERRCS (emergency responder radio communications system) to allow public safety folks to use their radios even while inside larger buildings.
there are both passive and active systems. passive systems are just two antennas with this air-line coax. active systems use inline amplifiers to boost the signal.
Gary @N8DMT@tubetime They also make cables with slots in the outer conductor to allow a certain amount of leakage (radiation) from the cable along it’s length. Just having one antenna at the end of the coax inside the building is pretty limiting compared to a leaky radiator stretched across a given floor. Amplifier are typically used for the slotted cable systems. 
Григорий КлюшниковGary, this must be how cell service works in some subway tunnels in my city
crzwdjk ✅ 
Michael Katzmann🐈@n8dmt @tubetime Usually done in tunnels using leaky feeders ... https://en.wikipedia.org/wiki/Leaky_feeder
kaPOTTke
DamonHD
Andrew Jorgensen@tubetime
Can hams use their radios inside equipped buildings too? Or are the frequencies the systems carry too specific to emergency responder use?
Can hams use their radios inside equipped buildings too? Or are the frequencies the systems carry too specific to emergency responder use?
@ajorg probably depends on the antennas used in the installation. if it works with public safety at 150MHz or so, then it should work with the ham bands right below.
ThatKomputerKat 
@tubetime I’ve run that stuff in buildings.
Mark Bessey
DJ🌞
@tubetime Can't help but wonder, how do they manufacture this?
@infosecdj there was a How It's Made about it: https://www.youtube.com/watch?v=eMJk4y9NGvE
Plumbus: How They Do It | Rick and Morty | Adult Swim
@infosecdj more seriously, it's probably very similar to making regular coax: https://www.youtube.com/watch?v=ooTie9V_bFg
How Coaxial Cable Is Made: Messi & Paoloni Factory Tour Pt1
@tubetime Love the two strategically placed dish sponges
Michael Cook@tubetime What’s the benefit of swapping out the plastic dielectric?
@foobarsoft much lower losses for air dielectric
s0 Is Not A Sysadmin@tubetime I work in a building with multiple active Distributed Antenna Systems, for 4G/5G as well as safety-critical UHF radios (multiple P25 trunking systems and one or two DMR). Worth mentioning lossy/radiating coax as well! That’s one of the major antenna types we have, primarily in long underground corridors.
Luka Rubinjoni@tubetime Just passive antenna-coax-antenna setup?
@rubinjoni sometimes yes, other times they will include an amplifier
Timon 🛠
Michael K Johnson@tubetime are they typically just passive antenna pairs, and use air-line for low loss?
ever wonder how you get cell service inside large buildings?
I usually don't, unless you mean that 2G that's just strong enough to show up as such but not enough to load anything 😅
Billy O'Neal@tubetime Hmmm... I've seen some stuff like that in my building and thought it was a defunct Thicknet installation :O
Wulfy—Speaker to the machinesI've asked our infrastructure guy why is there cell reception, in the elevator (lift: to you terrorist colonials).
A steel cage, in a ferro concrete shaft, 20 stories underground.
And he said "No personal communication devices on base, Ill have to breach you"
No, actually he said "gibber jabber I have no clue bla blh, and that's how!"
Antennas, it's always about antennas.
vxo@tubetime is this pictured line "Radiax", a leaky feeder?
It looks kinda like LDF4 that's been sanded down to reveal the inner secrets but that usually doesn't have a white jacket.
Most of the big beefy line I use in the broadcast world looks like with an air core dielectric, and can be pressurized to exclude moisture.
@vxo this is the regular kind, not the leaky kind.
@tubetime since passive systems are being mentioned here, i want to share how amusing passive outdoor microwave reflectors are. here are some examples: https://wchsutah.org/miscellaneous/telephones.php
if you ever see what looks like a blank billboard up on a mountaintop or ridge, it's likely one of these guys. It basically just bounces the signal like a mirror.
georgebmillion.com@tubetime that's very interesting
Andrew Ball@tubetime Does that qualify as radiax?
@kb9ylw no this doesn't have the apertures along the shield. this isn't a leaky coax, unless you count the opening i made in it for the cutaway view lol.
Mister Moo 🐮@tubetime So they could just not do that in movie theaters and bam, problem solved.
Garrett Wollman@tubetime Or they just install microcells in the building. (Probably using the same sort of transmission line.) When our building was built we had both GSM microcells for the GSM carriers (then AT&T, and T-Mobile) and a CDMA repeater for Verizon. I don't think any of these work any more since the 3G shutoff. They also had Nextel iDEN on the system for the facilities crew. Little conical emitters about 3" diameter all over the basement and lower four floors.
fruchti@tubetime Is the wavy shield for rigidity only or does it have an electrical benefit?
@fruchti for flexibility. electrically it isn't the best but it probably averages out.
Elizabeth@tubetime @fruchti You need the waves to make it bendy, but it is possible to tune the periodicity of the outer conductor’s surface to the operation frequency and get less loss through the length of the cable. That can be fairly finicky and expensive, but you absolutely do not want to hit a bad resonance accidentally. My guess is that they don’t really care about optimizing signal here so it’s not worth the cost of precision engineering, but the period is likely on the order of a quarter wavelength or much much smaller than a wavelength so that they don’t see much additional loss.
Bluefox Longtail ΘΔ@tubetime This is so cool and explains why I don't get service in one of the older concrete hospital buildings around here that probably didn't have this because cellphones weren't a big deal when it was made, and they never retrofitted it, but do get service in the newer building in the same system. I thought it was just a matter of construction techniques and materials not actual hardware carrying the signal inside.
J.P. Stewart@tubetime Yeah. DAS design : installation is a huge thing. Depending on frequency, sometimes multiple systems are needed. I’ve been in buildings with 4 separate cabled das systems. Three of which with similar internal antennas but the fourth with a directional antenna structure.
Also the reason for different systems is if they can’t be amplifiers together or they are on a different fiber backhaul or to a different egress antenna. It’s wild. And interesting. :)
AValles