Flexi-Waveguide and Changing of Frequency 2

[laffers]

I work on a radar system that employs flexi-waveguide between the transmitter and antenna. the rf travelling down this waveguide is at a swept frequency which is used, via the means of steering the beam, to provide us with a 3-d target (range, azimuth and height). the waveguide is constantly pressurised to 40 psi. the problem we are having is that a drop in pressure seems to be affecting the height of the target. a 15 psi drop can give us a 1 degree tilt in the beam. can anyone explain why? we have wrecked our heads here trying to find a reason why this would happen. is it due to the drop in pressure decreasing the inside diameter of the waveguide? please help.

 

[BrianG]

40 p.s.i. seems a bit high. You do not state whether there is a window at the end of the waveguide or whether the antenna/radome also operate at 40 p.s.i. Without knowing what kind of antenna you are using, could it be that you are getting small changes in antenna beam shape and side lobe with changing pressure?

 

[laffers]

the pressure at the antenna is 0.5 psi. we use a phased array antenna. i think the changes in beam shape we are getting are more than small as the height difference at max range (250 nm) is ranging between 7000 and 15000 above and below the actual height!!

 

[Higgler]

If your antenna is a slotted waveguide array, the antenna pattern (pointing direction) depends on the amplitudes and phases at each element, just stating standard array theory.
If you place a mismatch at the input to the antenna that's far away from the antenna (i.e. your flex guide), and vary its' VSWR with pressure, then the energy getting into the slots of the antenna will change due to that remote reflection. This standing wave will change the amplitude and phase energy arriving at each of your slots in the antenna and hence the antenna pattern pointing direction will change with frequency. If the mismatch is really close to the antenna, maybe things won't change as much with frequency, but having it further away changes in-phase reflections to out of phase reflections quickly.

Pressure changes could be changing the effective dielectric constant in the waveguide or the size of the waveguide (rf travels faster with a larger waveguide size, a pseudo-lowering of the dielectric constant of the air in the guide). That would shift the VSWR mixing from the source to the antenna also, producing antenna pattern shifts.

As an example;
if you were an engineer in 1965 and using a slotted waveguide VSWR measurement device on your antenna, say the antenna was 2.5:1 VSWR, and then you added a mismatch on the input side of your slotted waveguide, you'd see the Voltage Standing Wave shift, both in amplitude and phase. Pretend the location you move the probe back and forth is now the elements radiating from your antenna (a serial slotted array antenna). The amplitude and phase changes all along the radiating elements which changes the antenna pattern. Most antenna pattern shapes don't change when the VSWR of the input cable/waveguide changes, but the slotted waveguide array is different since it's a traveling wave antenna.

Or so I think.

I haven't researched this, just paused and thought about it. Feedback would be appreciated.

You could try an experiment, try squeezing or moving your waveguide a bit to see if you can detect antenna pattern shifts. Have you heard of the old trick of the strong magnet and metal ball inside the waveguide for dent tuning of waveguide. That might demonstrate things.

kch

 

[biff44]

If it is a pressurized waveguide, it is probably pretty high power, and putting a ball bearing inside of it will result in the waveguide melting! Don't do it.

Are you saying there is only one waveguide between the transmitter and the antenna, or are there a number of waveguides between the transmitter(s) and antenna inputs?

Why do you think it is the waveguide causing the squinting. I would assume that the corporate feed network in your antenna is distorting under the change in air pressure, causing differing phases to be sent to the array elements.

One thought might be similar to what Higgler is saying. Maybe there is a mismatch at the antenna, and the output of the transmitter is mismatched a little also. Therefore you might have a standing wave set up in the waveguide. As you change the air pressure outside of the waveguide, the waveguide width changes, and therefore so does the propagation velocity in the guide, setting up a changing impedance at the array elements. I agree with higgler, If you can safely grab onto the waveguide while it is operating, try squeezing the width and see if the mispointing can be induced.

 

[laffers]

Higgler and Biff, thanks very much for your posts and I totally agree with the mismatching theory. Ill just give you a little more background to our problem. we first noticed the problem around 2 months ago whereby the primary heights were differeing from the ssr heights by the figures i gave above. after much fault finding with elements controlling the steering of the beam, including temperature controllers we were about to call the big guns in when a funny thing happened. whilst watching a recording of the radar picture the heights all of a sudden snapped back to where they should be, well within specification. this occured at exactly 12 noon. now, twice a day we have to input weather data into the system. this got us thinking that maybe at certain times of the day the weather information is accepted by the system. this would certainly have an effect on the heights. just then a contractor who had been working on our air pressuriser came in and told us that the pressuriser problem had been fixed and the new pressuriser had been switched on line. at around 12 noon!!!!! so we experimented. we switched off the pressuriser for an hour and noticed that the heights went beserk. if losing pressure resulted in condensation forming in the waveguide then not only would the output from the antenna be incorrect but the vswr would be quite noticeable and arcing would occur but the vswr stayed within spec and no arcing occured. we have got to the point now where we accept the fact that the loss of pressure caused the problem but we do not know why.

 

[biff44]

There can be a hundred different things wrong. You have to search them out one by one. Perhaps someone from the original vendor (a Raytheon, Westinghouse, etc) could help you, or you need an RF consultant to come out an see what gives.

 

[Higgler]

When the one degree tilt occured,
1) is that in the plane of the frequency sweep, and
2) is that an azimuth one degree tilt?
3) Do you pressurize with air or gas? What type gas and does the gas propagate all the way down the waveguide antenna?
4) can you take an antenna pattern of the radar with various pressures. Higher sidelobes during the one degree tilt can give you a hint of the aperture distribution of the antenna. If sidelobes stay similar or vary widely, you can deduce info from that. 1 degree tilt is and added 3 degree phase change between sucessive half wave spaced elements, only 3.4% dielectric constant increase is needed.

5) for the one degree beam tilt, you know it's one degree because the frequency at which you detect the target shifts to a new value which correlated frequency change to one degree angle error - do you actually measure the frequency when the radar is in use? Any chance the RF source frequency is affected by the change in pressure?

a definite hmmn?
kch