> What is more sensitive an biquad or a dipole,
>Dipole is 2dBi gain. Biquad is 8 to 10dBi. Biquad has more gain.
>> I noticed that
>> commercial parabolic wifi disks often use a dipole as a feed.
>> And what about the shielding/ reflector, in the biquad examples
>> rather large reflectors are used even in combination with a parabolic
>> disk. Why is that? does the reflector has to be a certain scale to the
>> wave length, if so how do this result in 110 mm or 123 mm?
>The gain of the feed is irrelevant in a dish. What's important is the
>radiation angle or illumination angle. Think of it in terms of light.
>If the feed were a light bulb, what configuration would give the highest
>gain from a dish? It's not a bare bulb because most of the light will
>not hit the dish or overspray the edge. It's not a spot beam because it
>will only illuminate a small part of the dish. Only a lamp, with a very
>small reflector, will illuminate the entire dish without going over the >edge.
>Same with RF. A high gain feed will just illuminate a spot. A
>hemispherical feed (like a USB dongle) will spray RF everywhere except
>the dish. Only a matched feed will work.
I don't see any problem with a feed pointing at only a part of a dish. The reflection of the dish still will go to the point where the disk is aiming to and no energy is wasted in a wrong direction. Correct me if a am wrong about that.
One other thing I like to know is the influence of a reflector on a feed mounted on a dish. As said the reflector in some examples are still 110 mm square. While a the focus of a dish should be a about 1 mm^2. Does this mean a biquad with such a large reflector has to be mounted off focus? I really don't know anything about antenna's but I do know a little about reflection. So I expect since the biquad is much larger is size then a single point, it should be mounted off focus. Maybe a fair distance towards the dish or a little distance away from the dish. Hopefully this drawing makes clear what I mean. For the ease of the drawing the dish is pointed downwards.
I S D H \\ / \\ / \\ / \\ / \\ / \\ /========\\ / === biquad ____\\/__________\\/___ ___ reflector towards dish \\ / \\ / \\ / \\ / \\ / ______\\/__________ FOCUS /\\ \\========/ __\\/____\\/_____ reflector away from dish
In receive, the dish picks up all the signal it can get and sends it to the feed. If the feed has to wide a beamwidth (such as using a hemispherical feed that picks up from all directions), the gain will be the same as with a properly designed feed. However, if the feed is a spot beam, it will only pickup the percentage of area covered on the dish by the spot, and therefore have much less gain.
In transmit, the situation is the same. A hemispherical (too wide) feed will spray much of the RF over the edge of the dish, where it is wasted. However, you're correct for the spot beam. All the RF generated by a spot beam will be reflected by the dish and sent to the remote target. In effect, this combination uses the feed to obtain its gain, and largely ignores the capture area (effective aperture) of the dish.
There's quite a bit on the subject of feed design in:
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might be a bit too techy, but try reading just Chapter 4 (21 pages) which discusses feed design.
Yes, it has to be mounted as an "offset feed". See above URL.
The problem is that the feed is so large as compared to the effective area of the dish, that any feed blockage will cause a substantial decrease in gain. For very small antennas (i.e. 0.6m DBS pizza dish antennas), this is a serious problem. The dish would need to be about
2m across before the blocking effect of the feed can be ignored.
Please note that the DBS dish was designed to work at 12-13GHz, where the feed diameters are much smaller. The scale is linear so a 10cm x
10cm biquad feed would be only 18cm x 18cm.
The problem with a biquad feed is that it's to wide and therefore blocks quite a bit of RF going to the dish. A better choice is a dipole or 1 element yagi, which is narrow and does NOT block much RF. This is what's inside one of the MMDS (2.5GHz) dish antennas.
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's a circuit board 2 element yagi feed. The traces close to the circuitry are the dipole. The wide trace with the step is the reflector. No clue what the trace in between does.
Finding the optimum location is difficult. All antennas have a phase center. That's the point where all the RF appears to be coming from if you measured it from a distance. That's also the point where you want the focus of the dish to appear.
If you mount the feed off focus, you lose more than gain. There could be frequency selective cancellations (nulls), boresight errors, and excessively wide beamwidth. If you have a flashlight with an adjustable focus (Maglight), you can see the results as you move the position of the lamp through the reflector focus.
The biquad reflector does NOT capture RF equally over its entire surface. Grossly over-simplified, the bulk of the signal is captured in a "ring" around the center coax cable feed at about a 3cm radius. (I'll see if I can grind out a model with 4NEC2). If the dish focus happen to hit on this "ring", you'll have maximum gain. If it misses, the gain will be less. If you move the biquad feed around, you might be able to see the effect.
Anyway, only with the phase center of the feed at the dish focus, do all the reflected signals (at any frequency) arrive in phase. If you move the feed to some other place, you will see some rather strange effects.
Also, please note that the maximum gain from a 0.6meter dish antenna is about: gain = 9.87 * Dia^2 / wavelength^2 * (feed efficiency) gain = 9.87 * 600mm^2 / 125mm^2 * 0.4 gain = 91 dBi = 10 log(91) = 19.5dBi The 40% efficiency (that's the 0.4) is probably optimistic.
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