What you're looking at are splitters meant for cable TV reception. Power Pass I believe means the devices will pass DC power to down/up stream devices. I would think, unless you're dealing with extremely low powered transmitters that something like these would be more appropriate.
At what frequency? You can get power dividers that are fairly narrow band and are optimized for a range of frequencies, or you can get broadband devices that cover a large range of frequencies. Also, what power level are you transmitting? Power dividers have some loss and will smoke if over driven. Also, does it have to be TSO approved?
Is your "small aircraft" aluminum body or fiberglass? If fiberglass, you install the antennas inside. However, that means that the two antennas will "see" each other, which means your antenna pattern will probably be full of nulls and peaks. Power splitting between dual antennas really only works if the antennas are on opposite sides of a a shielded (aluminum) body, and cannot "see" each other.
Power pass means that it will pass DC power. That's usually in reference to running satellite dish LMB front ends, or tower mounted antenna amplifiers for over the air TV. It's also necessary for tower mounted bi-directional amplifiers, sometimes used for Wi-Fi. Unless you have some electronics mounted in the antenna, I don't think this is a requirement for your unspecified hardware.
I'm goint to *ASSUME* that you're only interested in 2.4GHz. See:
You can also make your own:
It's fairly simple. Again, this is for 2.4GHz only, not a wide range of frequencies.
back to back, and would like to stop them 'seeing' each other by using some RF absorbing material at
which I havent looked at in depth yet but expect to be lighter then aluminium.
The aircraft will be made of fibreglass so the shielding is needed. I'm using a wifi network so the frequency is 2.4 gigs.
I dontthink the power-pass is a requirenment, although power-pass splitters seem to be lighter and cheaper. Would using a power pass enabled splitter actually cause any problems?
I also need an amplifier in my system and this one (http://www.rf- links.com/AMP8_24.pdf) looks nice and small, although I dont know if it amplifies both ways, i.e. ampllifies a trnamitted signal AND a recieved signal. Can any of you guys tell by looking at it?
Good choice but pricey. Because you have a fiberglass aircraft, the antennas can be inboard and do not need to be aerodynamic. To save weight, you can remove the fiberglass radome from the antenna. Other similar antennas to look at are:
Keep the gain at around 8dBi or LOWER so that you have a fairly wide (60 degree) antenna pattern. One antenna on each side of the aircraft.
No, not back to back. Put the antennas facing the outside of the aircraft belly, pointed in opposite directions. The antenna pattern sprays RF all in one direction, with very little going out the back of the antenna. You won't need any shielding other than what is supplied with the aluminum backing in the antennas. Also, mounting the antennas against the side of the aircraft will prevent any reflections from bouncing back into the pattern of the opposite antenna.
I'll assume that you want a downwards facing pattern. The -3dB beamwidth of the antenna is 60 degrees. Therefore, the MINIMUM angle between the two antennas is 60 degrees. I would go for somewhat more, perhaps 90 degrees. The catch is that with this arrangement, in level flight, you'll have no RF directed directly downward. If this is a problem, I suggest you try using only one antenna, facing downward. However, if you're doing acrobatics and need to communicate at any attitude, you'll need more than 2 antennas.
Think of it in terms of illuminating with lights. Each 8dBi gain antenna is a flood light, with a 60 degree wide beam. What gets lit up, gets RF. What doesn't, is in a dead area. Lower gain antennas have a wider beamwidth.
Model airplane? Manned airplane? Experimental aircraft? Human powered flight? Ornithopter?
No benefit to using a DC power pass system. There are no active devices in the antenna.
No. Amplifiers tend to be a problem. They also ost plenty and weigh a bunch. I don't want to go into details. Search this news group for "RF power amplifier" or bettery yet, search for "alligator".
I've done model airplane drone video and data systems. There was no way to do it using static antennas at 2.4GHz. The drone had to be tracked from the ground using a high gain dish antenna. The antenna on the drone was an attempt at being omnidirectional with a hemispherical pattern using just a 1/4 wave monopole antenna. That's not much gain, which explains the 24dBi tracking dish antenna on the ground. See my comments under:
What's the ground end of this video link going to look like?
the ground, I was planning to be sneaky on the fly-off day. The camera system uses an http protocol (not perfect i know, but the easiest to interface with matlab for now). It's for a model sized aircraft (about 3m wingspan). Flying range has been specified as 500m, but also within visual range at all times for safety. Im not sure how 'visible' the aircraft will be over 500m, so we will probably be flying closer, and probably no higer than 100ft too. Because the little laptop antenna i linked fits onto the screen of the laptop, i was hoping to tilt the screen so the high gain region points at the aircraft whilst a pretended to squint at the screen.
Because we will be flying pretty low and looking for targets on the ground with the camera, which looks directly down and is fixed to the aircraft belly, we don't really need a signal to be transmitted downward. (we won't need to identify ourselves as targets hopefully!).
I see what the alligator term represents, big mouth, little ears. A linear amplifier amplifies a signal in ONE direction and not in the other, but does it reduce the signal in its non-amplifying direction? I guess I could use two alligators if they had big enough mouths.
That won't do much for you. Your airplane will be able to hear the signals from the ground somewhat better, but your ground station will not be able to do likewise. You would need such an amplifier at both ends in order for it to be effective.
Cleverness and sneakiness are good things. However, I think the judges are looking for good engineering.
Duh. I forgot that you were the one building the autonomous model airplane. I've been engaged in various email exchanges on the subject of flying a camera in a much larger airplane, and was thinking along those lines.
Well, I can see you haven't bothered to do the math. I'll do a dry run for you. See:
I'll assume that you're using the "Waverider" gizmo, which belches
200mw of RF (+23dBm) and a 3dB power divider with a 0.5dB port loss. I'm not sure what frame rate you're expecting so I'll assume that you can tolerate the slowest OFDM speed (6Mbits/sec) as the 802.11b speeds are generally not that useful for streaming anything. As I recall, you're using this camera:
Unfortunately, the data sheet does not offer RF specifications, so I'll conjur my own. That basic idea is to see at what range you can maintain a 20dB fade margin.
From the air to the ground: TX power +15dBm TX coax loss -1dB (includes connector losses) Divider loss -3.5dB (half the power less internal losses) TX ant gain 6dBi Distance unknown RX ant gain 2dBi RX coax loss 0dB (assumes good RF amp in Waverider thing). RX sens -88dBm (at 6Mbits/sec) Fade margin 20dB
Plugging the numbers into:
I get 0.12 miles or about 200 meters. Not very good.
The other direction, from the ground to the air is a bit different. The power divider does NOT split the signal coming in from the two ports. Therefore, the loss is less.
From ground to air: TX power +22dBm TX coax loss -0.5dB (connector losses) TX ant gain 2dBi Distance unknown RX ant gain 6dBi Divider loss -0.5dB (internal divider losses) RX coax loss -0.5dB (connector losses). RX sens -88dBm (at 6Mbits/sec) Fade margin 20dB
Plugging the numbers into the calculator, I get 0.4 miles or about 650 meters. Much better but barely adequate.
Worse is yet to come. The above guesswork is based totally on the assumption that your ground and air antennas are oriented for maximum gain. This is obviously not a good assumption as the aircraft be off axis for the majority of possible orientations. Therefore the 6dBi antenna gain may actually be considerably less when oriented off axis. For example, if you lose 3dB of gain due to orientation errors, your range will be 0.707 times as far. 6dB loss is good for 0.5 times the range.
Someone is sure to suggest that slowing down the system from
6Mbits/sec to 1Mbit/sec will dramatically increase the range. That doesn't work. Look at the receiver sensitivity chart at:
At 6Mbits/sec OFDM, the rx sensitivity is -88dBm. At 1Mbits/sec BPSK, the rx sensitivity is -89dBm. For 1dB of additional gain, it's hardly worth a 1/6th decrease in speed. That's also why I suggest you use the slowed OFDM speed.
Get a tripod. Mount a fairly directional antenna on the tripod. You don't need much gain, but you do need a wide beamwidth. Another 6 or
8dBi panel will do. Point it in the general direction of the test range. Attach to your laptop. Put a photographers blind black cloth over your head. Don't worry about looking weird.
Perfect. If you're not going to fly at much altitude, an omnidirectional antenna (monopole) sticking out of the belly, will work just fine. The radiation pattern is a horizontal donut. As long as you maintain level flight, you're fine. Do aerobatics and you'll have problems, but level flight up to about 30 degrees is just fine.
Yep. It's asymmetrical.
Most such RF amps have a receive amplifier. The problem is that it doesn't improve the receive sensitivity in any useful way. The access point (or laptop wireless card) already is running at the limit of receiver sensitivity. Adding additional gain will only add additional noise and reduce dynamic range of the receiver. What the receive amplifier will do that's useful is compensate for the coax cable losses between the amplifier and the receiver. However, for your laptop installation, that's very little coax, very little loss, and very little benifit.
Yep. Try to keep the tx power at both ends roughly the same. If you're going to fly a power splitter, you'll need 3dB (twice) the TX power output of the ground station in the airplane in order to compensate for the splitter losses.