Testing High Gain Antennas for Radar-Optical Axis

[mobileradio]

Hi,
I am looking to find some inspiration how to check a set of high gain parabolic dish microwave antennas. All these antennas are installed side by side on the same frame and this frame is turned into the direction towards the target by a massive turn table. I need to check whether these antennas are focussed to point to the same target or are at least pointing parallel towards the target - or whether they are defocussed. I need to check this in vertical and horizontal direction - in azimuth and elevation. The antenna gain goes up to 45dB and the distance to the object is some 5km. So, even when the antennas are just adjusted to look parallel (which may be easyer) one beam flows into the other and the target will be inside this "multi-beam".

Once I tried to transmit across a lake but the reflections on the water surface caused very bad interference at the place where we installed our test receiver. However, the transmitter and the corresponding receiver were placed just a few meters above the ground. So we "damaged" the Fresnel-Zone and this caused the interference. As most components are really heavy and bulky and I need to keep the effort low, I can not build a high wooden tower we all know from professional antenna testing fields.
Another idea is, to make the same test over a flat solid surface, maybe a piece of land where grass is growing. The reflection may be a lot more diffuse and the grass may be absorbing the microwaves touching the surface, at least partially. So the results may be better. A civil noncommercial airfield or a large field of a farm may be such a place.

These two tests may be done over such a long distance that it comes relatively close to the intended distance of 5km. A third idea has a much shorter distance: We could transmit up to a high building where we install our test antenna and the test receiver. Such a building is some 20m high and the Transmitter could be placed on various distances like 100m, 200m or 400m. The advantage of such an arrangement is that the wave reflected on the ground will be reflected by the building upwards to the sky and therefore does not interfere with the direct wave at the test receiver on top of the building.

There is one thing which make me usure concerning the third concept: The intended distance is 5km, the distance we use in the third idea may be just 200m. Therefore we are not able to adjust the antennas focussed to "the 5km-point" but we adjust them in parallel shifting our test antenna that is has exactly the same position as the transmit antenna has. We use a simple construction made from wood and have taken all dimensions from the transmit antenna assembly. One con-argument is: The test distance is much too short in relation to the intended distance. One pro-argument is: Because you adjust the antennas in parallel and all antenna beams flow into the other beams and widen by the distance, all will be well.

So, when I see the these two arguments, I developed a fourth concept: I install a simple reflector on a high mast, maybe a mast from a microwave directional radio relay we have here somewhere. This tower is some 60m high and is constructed from some lattice boom structure. I would place the transmitter some 1km away and the test receiver as well some 1km away. I would point the transmit antenna and my test antenna upwards to the small reflector. Would one be able to make such an antenna adjustment by using an arrangement like this? The longer I ponder about this concept, the more I think that one is obliged to use a LARGE reflector. However, no institution would agree in installing a reflector of 20 squaremeters or more on a fragile lattice tower.

So, I would appreciate some input and maybe some sources where I could find some helpful ideas and where I could "suck some honey".

Thank you for reading

Andy

 

[VEBill]

Install a signal source in the distant tower. Using both dishes in receive mode, peak them onto the distant source.

If one dish must remain in transmit mode, then install a corner reflector in the distant tower. You'll probably need to use gated measurements, timed to capture the reflection. Peak both onto the distant corner reflector.

Redesign the system to use a single dish. ;-)

 

[mobileradio]

Dear VE1BLL,

one dish is impossible, we need POWER for that distance.
Therefore our antenna system has several dishes.

But gated measurement is a great idea. I will check this.

73 de Andy

 

[VEBill]

Is this for a 'wireless power' transmission system?

If so, then perhaps a remote incident power measurement and reporting system. In other words, a beeping radio signal from a power measuring gadget on the distant tower, where the beeping (or data [edit]) reports back to you how much power is at the distant tower. Just peak up one dish at a time. Disable the others while peaking each one.

You'd have to account for relative phasing. You could peak up each dish, but they might sum to zero along the boresight due to relative phase. By way of example, imaging two dishes perfectly identical, but one wired up backwards to the other. Presumably this has been carefully accounted-for in the system design.

 

[mobileradio]

Dear VE1BLL,

no, we do not want to "transport" power to a distant point.
We still want to transmit signals through independant dishes.
It is just the propagation which causes the need of POWER.
If all dishes would operate in the same frequency range from
one BIG amplifier one could have arranged them in a phased
array and this would cause the same problem - I see.

73 de Andy

 

[IRstuff]

What do you mean by "focussed." Do these antennas form a parabolic array?

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[VEBill]

Okay.

I'll mention a magnetic mirror trick that I came up with many years ago.

The question arose if a Satellite TV dish was being blocked by trees or not...

I took a small thin mirror and glued it to a flexible magnet. I removed the Sat TV LNBF at the focal point and replaced it with my eye. The mirror was stuck to the surface of the reflector with the magnet, and was essentially parallel to the dish surface at that point. The mirror reflected the patch of sky where the dish was aimed into the focal point, where my eye was. It was neat how the mirror could be moved around the dish surface and it would show the same patch of sky. There were no trees showing in the mirror, so the dish aim was therefore clearing the tree tops.

Perhaps this sort of magnetic mirror trick might be useful for initial alignment.

Good luck.

 

[IRstuff]

One caveat is that the mirror is visible light, and does not have the same beamwidth that the RF would have. For a 20-in diameter dish, the full-width Airy disk for 2 GHZ is about 41 deg

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[VEBill]

But, in the example of satellite TV (and, in fact, most other applications), the only point of interest is the boresight. Because (for example) that's where the satellite is.

If the dish has a beamwidth of (for example) 41°, and some trees were blocking some off-axis part of that, as long as there's a clear line of sight to the distant target (by definition effectively a point), then you're done.

If your dish is large, then one should check the worst case edge. Probably the lower edge of the rim where the trees might intrude, or top edge if worried about blockage from a roof overhang. Make sure that the entire physical aperture has a clear line of sight to the distant point.

Satellite TV is at a higher frequency than 2 GHz (FSS/DSS about 12 GHz, or old 4 GHz C-band 'big ugly dish' are often 8+ feet), and the dish's beamwidth is typically less than a couple of degrees. The precise size is often chosen so that the first dip in gain before the first sidelobe is on the adjacent orbital slots (traditionally 3°, often 2° 'lately').

In the OP's case, the gain is up to 45dB (extreme), so the beamwidth in that case is tiny.

A 20 inch dish at 2 GHz is not ideal. A 41° beamwidth would be pretty wide beamwidth, and thus medium or low gain. It's likely that other antenna options might be better for almost any application. Whatever offers a gain pattern matching the application.

 

[IRstuff]

I was thinking more like a DirecTV-sized antenna, so sure an 8-ft dish would have a 1.8-deg beamwidth at 4 GHz

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[mobileradio]

@ IRstuff: 1. "focussed" means that all antenna beams point to the same target...
2. Yes, visibile light is different, but I think that we can't use this trick
anyway. However, this idea is simple and interesting. If the focus of the light
would aim on a target, the center of the antenna beam will as well, I think.

@ VE1BLL: 1. As far as I understand is the magnetic mirror trick an optical
focussing of the antenna. I am pondering if this could be of some help,
at least for some kind of pre-adjustment. Our dishes are partially offset
fed, partially center fed and Cassegrain types. It is not THAT easy but
the idea is cute.
2. Correct, "the only point of interest is the boresight". The beam widens
with the distance and will illuminate the target more or less. The 3dB beam-
width is not the only angle who is transporting power. There is a 6dB beamwidth
and a 10dB beamwidth as well and the receiver has a lot of dynamic range too.
However, the beam is narrow and the intention is, to find a relyable way to
test whether all antennas aim to the same point or are at least aiming in
parallel. I am afraid that the target will not see any Signal at all ;-)
It is easy to mis-adjust the high gain antennas that they overshoot
or undershoot the Targets elevation or have the wrong azimuth and the target
will not see any Signal from these.
Your idea with the gating is very interesting and I will check the datasheets
of COTS PIN-Diode switches and their switching speed in relation of the time
differences between the direct and the reflected signals.

@ TTFN: Sorry, I do not understand why you are a member of this discussion?
What are you going to express to whom? If anyone did do anything wrong
which is important, please tell it directly and we all surely will do it
correct in the future. However, if you can add some help to this topic,
you are welcome of course. So far I do not understand who is doing something
wrong and what was done wrong!

Yours

Andy

 

[VEBill]

TTFN = Ta Ta For Now

It's part of IRstuff's "signature", so isn't aimed at anyone.

This isn't the first time that such signatures (not his, others) have caused similar confusion.

 

[IRstuff]

You don't say how big the antennas are, so a large size complicates things. Given that you need a known, repeatable, and collimated source, I suggest figuring how you might use the Sun. It subtends ~1/2 degree, and is therefore directional, but is collimated, and is guaranteed to be collimated over any sized antenna you might be contemplating. VE1BLL's mirror approach would allow a simple verification of focus of each antenna, and since the Sun should radiate RF as well, the Sun's noise could be used to align the LNA, if it's located at the focus, as well as verify the antenna pointing accuracy itself. Hypothetically, you should be able to align the antennas to less than 1/10th of a degree, and you should have knowledge of the Sun's position to better than 1/100th of a degree.

TTFN
I can do absolutely anything. I'm an expert!
faq731-376 forum1529

 

[mobileradio]

@ VE1BLL : I confess, this was confused me too.

@ IRstuff : Yes, this is a FINE idea and seems to be simple as well! I am not absolutely sure
whether we can do this for all frequency ranges but you are absolutely right. The sun is a test
signal Generator, it is for free and easy accessable - if we do not have a heavily overcast
day. I will check with another antenna if we can "see" the sun in the intended frequency ranges.
If this is the case I will try it that way. OK, the sun moves across the sky (from our Point of
view) but I could use one antenna (and a video camera with a protectional filter) for the reference
and aim the other antennas to find the maximum power level. All in all this sounds easy - so far ;-)

Yours

Andy