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How can I connect 4 antenna to 1 receiver ?(to increase gain) Can I use power combiner? What is the effect of Loss of combiner? What about NF of the total system?
Well, basically at a distance from a transmitter there is a certain ammount of power/meter². If you have one antenna and it picks up X watts of power, having four antennas (spaced out properly so they subtend fresh solid angles of space) then you will have 4 * X watts of power received. That is why satellite dishs are sooooo big.
It is easier if you do it at baseband. Also it is depend on what kind of environment do you have. It it is very far and LOS exists with no multipath. You can just use a combiner and you antenna will be just antenna array. If the mutipath exists, the story is different. You might need to do it at baseband with some kind of adaptive algorithms.
You can use two stages of two input Wilkins combiner.
1. Get some 70 to 75 ohm coax.
2. Determine the velocity factor.
3. Cut six pieces a quarter wave long at your frequency.
4. Connect two to the ends of the 50 ohm cable from two antennas.
5. Connect the other ends in parallel. Call this point X
6. Do the same to the other two antennas.
7. Take the final two quarter wave sections and connect each one to one of the point X. Put the other ends in parallel. Run more 50 ohm from that point to your receiver.
There is no need for the balancing resistors as it is highly unlikely that the purely mechanical antenna system will fail.
just look at minicircuits, or k&l, or any other RF componnets comp.
buy a 4->1 RF combiner (splliter), it will most of the time be phased macth,
then just use equal length cables and connect the 4 antennas to the splitter,
if the phase match isn't 100% than you'l have a tillted beem, so just have to move the array to the other direction (no bigi).
you should put the antennas at about 0.7lambda.
in order to eliminate GratingLobes your element separation should be less the 1lambda, the lobes are eliminated entirely at 0.5 lambda but that may be to close so mutual coupling is big, 0.7lambda is the best compromise and almost in any array this will be the case.
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