There are a lot of flaws in the system right now, but I am working on fixing them as best I can.
The foremost in my mind is that there are currently 100-ft Ethernet cables at each point in the network providing power and data to the Bullet modems, which at the two endpoints cause signal attenuation at 0.24 dB per foot- quite a high loss rate. If there is a midpoint in our network to relay the communication between the two end nodes, I'm not sure what a 100-ft Ethernet cable at that point would do to the signal. Since there's no new communication originating at the midpoint node, it's possible that the signal wouldn't be attenuated there at all; the 100-ft Ethernet cable would essentially act like an extension cord for powering the Bullet.
That being said, the better thing to do here is to use some low-loss cable such as LMR400 or higher, just long enough to minimize the guaranteed attenuation but at the same time make the cable unlikely to be too short (which would actually not be disastrous, given that a long enough extension cord could probably make up the difference). So the question is how much new cable do I need to get?
Assuming I'm circumventing the Bullet on the broadcasting side (which I probably can do because I think the TP-Link 3G router is just as strong a transmitter as the Bullet, but I'll have to check that), I only really need a small amount of cable to reach the router, which can be kept in a weatherproof box on the roof close to the antenna with an extension cord running up to power it. On the receiving end, I will actually need a large enough length of cable so that the Ethernet cable from the Bullet to the Netgear router can be really short. (This means the whole Ethernet cable will be indoors, and since I already have a nice 3-footer, I won't have to buy a new one.) That will probably mean between 30 and 50 feet of coax cable (with one N-male end and one N-female end, of course!).
The silly thing is that this wouldn't be a problem if I hadn't already wasted money on the 100-ft Ethernet cables. This would be a non-issue if I wasn't on such a limited budget provided not by some huge corporation, but by MIT community member donors through the Public Service Center. But to trace the whole situation back to my own failures as an engineer, I had no idea that you could lose so much data over Ethernet cables. As I now realize, Ethernet isn't a type of cable, it's a protocol that runs over particular types of cable- namely Cat5/Cat5e/Cat6/Cat6e, which consists of four twisted pairs of copper wire set up in a particular way (twisted to minimize inter-wire interference via magnetic induction). Moreover, this cable isn't formulated to be particularly lossless, unlike the better insulated LMR-higher number coax cables and above. Stupid, stupid.
The other changes in the system I've probably already mentioned in the rant above, but here they are more clearly stated:
One of the engineering goals in this project is to minimize components and costs so that the system can be affordably implemented elsewhere. I want to try the system without the first broadcasting Bullet, so the TP-Link router is broadcasting the signal directly out of its RP-SMA female antenna jack and out the directional antenna, instead of supporting the link as a user on one of its Ethernet ports. I think this should be an improvement because I think (and I'm not sure why; I think it might be written on the device somewhere, but I could be wrong about this) the TP-Link's transmit power is greater than 20 mW, which is what the Bullet's transmit power is. I'll definitely have to have the answer to that by tomorrow morning, since it determines how I'm going to configure the Bullets in my network (with 3 or 2).
That brings me to the next change- to add a midpoint consisting of a 12 dBi omnidirectional antenna and Bullet to act as a relay. In case there's a whole row of trees situated along the line of sight- we simply can't have that. This relay will require another mounting pole, grounding wire, grounding rod, and Ethernet cable (or coax cable + extension power cord, if attenuation matters) in addition to the Bullet, POE injector, and powerful omni antenna (which are really expensive, long, and unwieldy! Mine is almost 5 feet long). Expensive, but unavoidable without adequate knowledge of the terrain. This project would be better done by thorough scouting in-country first, then buying all the parts and going back. But plane tickets are expensive.
Finally, the 19 dBi antennas are being replaced by more powerful 24 dBi antennas, because the old ones worked fine in open air but even a few trees in the way would attenuate the signal to nothing. The new antennas (from Zda Communications, a wireless hardware manufacturer based in SC) are each less than $30 more than the old ones and have much better directionality (narrower beamwidth); we'll still need to have line-of sight, but we'll be able to avoid trees more easily, and hopefully the stronger signal will be able to penetrate that one tree we simply can't avoid. I should have invested the extra money to get them in the first place, but didn't in an effort to cut costs early on. I guess this is why researchers make the heavy investments first to get a product working, and then try to cut costs afterward.
More on failures as an engineer: Next time, I'll have to make sure to buy the antenna with the CORRECT GENDER connector so we won't have to waste another 3 dB of attenuation on the connector converter.
That is all.
-E
The foremost in my mind is that there are currently 100-ft Ethernet cables at each point in the network providing power and data to the Bullet modems, which at the two endpoints cause signal attenuation at 0.24 dB per foot- quite a high loss rate. If there is a midpoint in our network to relay the communication between the two end nodes, I'm not sure what a 100-ft Ethernet cable at that point would do to the signal. Since there's no new communication originating at the midpoint node, it's possible that the signal wouldn't be attenuated there at all; the 100-ft Ethernet cable would essentially act like an extension cord for powering the Bullet.
That being said, the better thing to do here is to use some low-loss cable such as LMR400 or higher, just long enough to minimize the guaranteed attenuation but at the same time make the cable unlikely to be too short (which would actually not be disastrous, given that a long enough extension cord could probably make up the difference). So the question is how much new cable do I need to get?
Assuming I'm circumventing the Bullet on the broadcasting side (which I probably can do because I think the TP-Link 3G router is just as strong a transmitter as the Bullet, but I'll have to check that), I only really need a small amount of cable to reach the router, which can be kept in a weatherproof box on the roof close to the antenna with an extension cord running up to power it. On the receiving end, I will actually need a large enough length of cable so that the Ethernet cable from the Bullet to the Netgear router can be really short. (This means the whole Ethernet cable will be indoors, and since I already have a nice 3-footer, I won't have to buy a new one.) That will probably mean between 30 and 50 feet of coax cable (with one N-male end and one N-female end, of course!).
The silly thing is that this wouldn't be a problem if I hadn't already wasted money on the 100-ft Ethernet cables. This would be a non-issue if I wasn't on such a limited budget provided not by some huge corporation, but by MIT community member donors through the Public Service Center. But to trace the whole situation back to my own failures as an engineer, I had no idea that you could lose so much data over Ethernet cables. As I now realize, Ethernet isn't a type of cable, it's a protocol that runs over particular types of cable- namely Cat5/Cat5e/Cat6/Cat6e, which consists of four twisted pairs of copper wire set up in a particular way (twisted to minimize inter-wire interference via magnetic induction). Moreover, this cable isn't formulated to be particularly lossless, unlike the better insulated LMR-higher number coax cables and above. Stupid, stupid.
The other changes in the system I've probably already mentioned in the rant above, but here they are more clearly stated:
One of the engineering goals in this project is to minimize components and costs so that the system can be affordably implemented elsewhere. I want to try the system without the first broadcasting Bullet, so the TP-Link router is broadcasting the signal directly out of its RP-SMA female antenna jack and out the directional antenna, instead of supporting the link as a user on one of its Ethernet ports. I think this should be an improvement because I think (and I'm not sure why; I think it might be written on the device somewhere, but I could be wrong about this) the TP-Link's transmit power is greater than 20 mW, which is what the Bullet's transmit power is. I'll definitely have to have the answer to that by tomorrow morning, since it determines how I'm going to configure the Bullets in my network (with 3 or 2).
That brings me to the next change- to add a midpoint consisting of a 12 dBi omnidirectional antenna and Bullet to act as a relay. In case there's a whole row of trees situated along the line of sight- we simply can't have that. This relay will require another mounting pole, grounding wire, grounding rod, and Ethernet cable (or coax cable + extension power cord, if attenuation matters) in addition to the Bullet, POE injector, and powerful omni antenna (which are really expensive, long, and unwieldy! Mine is almost 5 feet long). Expensive, but unavoidable without adequate knowledge of the terrain. This project would be better done by thorough scouting in-country first, then buying all the parts and going back. But plane tickets are expensive.
Finally, the 19 dBi antennas are being replaced by more powerful 24 dBi antennas, because the old ones worked fine in open air but even a few trees in the way would attenuate the signal to nothing. The new antennas (from Zda Communications, a wireless hardware manufacturer based in SC) are each less than $30 more than the old ones and have much better directionality (narrower beamwidth); we'll still need to have line-of sight, but we'll be able to avoid trees more easily, and hopefully the stronger signal will be able to penetrate that one tree we simply can't avoid. I should have invested the extra money to get them in the first place, but didn't in an effort to cut costs early on. I guess this is why researchers make the heavy investments first to get a product working, and then try to cut costs afterward.
More on failures as an engineer: Next time, I'll have to make sure to buy the antenna with the CORRECT GENDER connector so we won't have to waste another 3 dB of attenuation on the connector converter.
That is all.
-E
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