View Full Version : Answers to some antenna questions plz
Can someone explain to me how the dish antenna setup works?
I've been looking at the prices and the dishes see, to give the greatest gain compared to cost (I need a better connection btw)
Do you take the dish dBi and add it to the feed dBi to get the actual gain or what? Also it looks like it's going to really tricky to get the direction setup properly (I can see myself falling of the roof)
Comments from anyone welcome
Thx
there's alot of different opinions to that question but most of them comes down to: no they don't count the feeds with the dishes gain because of the sidelobe radiation and therefore it's impossible to give the exact dbi for a feed + dish. its really not not that diffecult, aslong as u have an idee where to point it to and the rest is just fine tuning afterwards. just remember that some ap's uses horisontal and other vertical polarization
so if the feed is 9dbi and the dish is 21dbi then the overall gain would be something between 21 and 30ish
I though of another question.
How can you relate antenna "strength" to signal strength?
In other words, if I get an antenna with say 10 more dBi would that relate to a signal strength increase (or is that decrease) of 10?
I'm trying to figure out what kind of antenna/dish I need for a decent signal ('cause atm it's k#k)
think of the antenna as an amplifier cause that's what it does, it focus and amplifies the signal going in and out ur radio
that would be a increase from say -80 before the antenna to -70 after u add the antenna
savagedavid
05-01-2006, 21:47
An antenna is not an amplifier. Think of it more as a focuser. Picture for example your wireless system compared to a hosepipe. Your transmitter puts out a fixed power e.g. 100mW or 20dB. This is like the water pressure in your hose. Just like the hose pressure is constant and cant be increased or decreased so is the output power constant. Now picture fitting an antenna onto your system, like putting a control nozzle on your hose. Turn the nozzle one way and you get a very broad spray covering a big area at a short distance. Similarly a low gain antenna (1 - 5 dB) will be virtually omni-directional but will have a close range (100m - 1km or so). Turn your nozzle the other way and you get a very tight beam of water that has more dstance but covers a small area. Similarly a high gain antenna (18 - 30dB) will have a long range (12-50km+) but will have a narrow focus beam, requiring very careful setup and alignment.
So to summarise an antenna cannot amplify or attenuate a signal, it merely focuses the signal in the manner that you require.
savagedavid
05-01-2006, 21:49
Pox you need to give us more info about your setup and what you are trying to achieve in order to give you better advice on the best route to go.
yes, just like turning the volume knob on ur stereo - hehe
now i confused: so an antenna does not amplify ur signal?
My main problem is I'm sitting far away from everyone else so my signal strength is borderline-bad at best. If I ever want to hook up to the ctwug network I'm going to need a major signal boost.
Linksys WRT54G with a 18dBi flat panel
My main problem is I'm sitting far away from everyone else so my signal strength is borderline-bad at best. If I ever want to hook up to the ctwug network I'm going to need a major signal boost.
Linksys WRT54G with a 18dBi flat panel
Rather change your flat panel for a more directional antenna like a grid, you can even try a yagi. This should eliminate your problem. How far are you away from the AP?
about 8km to the platt-tv AP :)
savagedavid
06-01-2006, 15:39
Pox, it sounds like the problem is not so much your antenna as much as it is the distance from the highsite. 18dB should easily get 8 km (i have used them succesfully over much longer distances than that). The problem is the highsite signal needs to get back to you. Most highsites are designed with a focus radius of about 5km around the central transmitter. Beyond that it starts becoming a problem to guarantee reliable transmission. That said, if you have very clear line of site and good open space between you and the transmitter, than 8km should not be a problem (I have a reliable 7.5km connection to Wireless Online using a 21dB grid for example), but if you have trees, water etc between sites than it will weaken your signal.
savagedavid
06-01-2006, 15:39
Correct Eben, an antenna cannot amplify signal, it just focuses it in a certain way.
k, so what you're saying is there receiver is not up to the job. That would explain why i get far more outgoing packet loss than incoming.
savagedavid
06-01-2006, 21:07
That would explain why i get far more outgoing packet loss than incoming.
If that is the case it is more likely that your antenna is not aligned properly
thanx for that info - everyday is a learning experience for me
POX: the distance from me to the highsite is about the same distance as yours, and i do get 12mbps + running it on "g", only differance is that i have perfect LOS, only clear sky. the antenna on the high site is a 10.5dbi 120degree sector and the same on my side - i can even get 5mbps throughput with a homemade biquad antenna so if i were u i would consider taking out some buildings and trees
sorry david i tend to diisagree, i did some googling and did a search on the mikrotik forum and a antenna is considerd as the best amplification for wireless tx and rx side
An antenna is an amplifier in a sense. It is the antenna that catches the signal from the air, much like a sail of a ship catches the wind. Any piece of metal will serve as an antenna, and the bigger the better if you use the sail/wind analogy. But regardless of size, some work much better than others. Why is this? Resonance. Some bits of metal are the same size in one dimension as the radio wave it's catching. This causes the wave to be felt stronger by the metal, resulting in the a stronger electrically induced current. It actually works well if you are some even multiples and fractions of the wave's size. Quater wave length segments are very common and useful.
savagedavid
07-01-2006, 15:07
To understand why I say an antenna is not an amplifier you must understand what it does. It really converts an electrical signal from the radio card into radio waves that can be transmitted through open air. On the other side the antenna collects the radio waves and converts it into an electrical signal.
The effect of an antenna may be likened to an amplifier, but it is not capable of increasing the amplitude of the electrical signal (it is not an active device, merely a piece of wire or some metal plates shaped in a certain way)
Think of something like a magnifying glass. It collects sunlight and focuses it into a tight beam. Bigger magnifier = more sunlight collected = more powerfull beam. A magnifying glass cannot however increase the amount of sunlight radiating to earth.
In the same way a bigger / higher gain antenna = more radio waves collected / transmitted = more powerfull signal. The antenna however cannot increase the amount of electrical power being sent from the wireless transmitter.
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