The mystery of the "Bandpass Radome"

[Chogy]

Food for thought...

Inside a fighter's radome is the heart of the machine, a powerful AI radar. The material of the radome must be transparent to RF (radar) energy, so the signals can exit, echo, then return to the radar.

This is easily done on fighters like the F-16 and earlier by making the radome out of some composite material, like fiberglass. If the radome is metal, it will prevent the radar from operating.

Now, the mystery - on a stealth airplane like the F-22, the radome somehow lets the Raptor's own radar out, and also lets it return. Yet it must be opaque to an enemy radar. If an enemy signal were to penetrate, it'd bounce around on the radar hardware, exit, and make a strong echo.

So how do they do it? With a technology called a bandpass radome. Materials are selected that make this happen, but for the life of me, I can't figure out how.

How do they make a material that is transparent to the onboard radar, yet opaque/reflective to an enemy radar?

Googling "Bandpass Radome" provides a lot of hi-tech stuff, but nothing I can quite decipher. In the end, it appears that modern radomes are both extremely sophisticated, and also top-secret in materials and construction.

Any engineers want to explain how this is done?

 

[VCheng]

The pilots are the skyjocks and heroes, but geeks and nerds rules the skies!

 

[solid snake]

Very interesting topic of discussion, but one in which I lack knowledge, despite being a final year electronics engineering student. Is there anyone here who can explain this technology?

Actually, to understand this, you will have to extensively study the internal construction of such radars. I'm assuming the calculations of the waves are based on vector calculus, while one must have some knowledge of materials science. The arrangement (distance, angle, thickness) of various components in relation to each other affect the usefulness of the radar system.

I make these observations based on the textbook Frequency selective surfaces: theory and design By Benedikt A. Munk which is showing up in the google search result you provided.

I hope Pakistani military is training officers in this field of study, and conducting its own research into such advanced radars.

 

[Chogy]

Thanks for the info. It truly is a very exotic science, one little thought about. And I would guess that an F-22 radome (for example) is TOP SECRET in terms of materials and construction.

A very big leap from a fiberglass radome on an F-106!

Something else to consider - radars today are frequency agile, and might hop through many gigahertz, meaning the radome would have to be transparent throughout the range. I'll bet a cookie that these radomes are active in some sense - perhaps they apply a charge through a mesh, turning transparency ON and OFF in time with the radar pulses themselves.

 

[krash]

Thanks for the info. It truly is a very exotic science, one little thought about. And I would guess that an F-22 radome (for example) is TOP SECRET in terms of materials and construction.

A very big leap from a fiberglass radome on an F-106!

Something else to consider - radars today are frequency agile, and might hop through many gigahertz, meaning the radome would have to be transparent throughout the range. I'll bet a cookie that these radomes are active in some sense - perhaps they apply a charge through a mesh, turning transparency ON and OFF in time with the radar pulses themselves.

But wouldn't turning transparency ON and OFF still leave windows for the enemy's radar as well?

 

[Chogy]

But wouldn't turning transparency ON and OFF still leave windows for the enemy's radar as well?

I have no idea... I'm just speculating.

 

[zavis2003]

the fact is that every material has the properties which make it different form others , so same with transmission properties or dielectric properties
the thing which rules here is that how you intelligently use the transmissions and reflections coefficients wisely and that the point which distinguish between a confused one and a capable engineer

i can give you just hint that when electromag signal passes through radome it faces same trans and reflec coffiecients and that part of signal which is passed have to face some total internal reflection

important point is total internal reflection

use your mind now onward

ok

 

[Jango]

Maybe thats the reason F-22 is not for export!!. But truly, a very intruiging topic you touched Chogy.

 

[SQ8]

From an RF perspective.. Its also possible that the radar itself absorbs part of the incoming hostile radar signals.. perhaps the APG-77 isnt JUST a radar .
Considering AESA radar's can potentially scan through hundreds of frequencies in one thousands of a second. The material must be permissive to them all.
What is the Transmitted power of a T/R module compared to a traditional antenna?
Perhaps the radome allows for a particular pattern of waves to exit and traps traditional scattering waves??

 

[gambit]

All,

Instead of "radome bandpass" or the likes, I recommend 'ansoft frequency selective surfaces' for your word search. Slide 20 is applicable to the construction of a radome, especially the 'stealth' version. A basic layman understanding of FSS is the start for this topic.

 

[Safriz]

My vote goes for what Gambit says....

Another alternative may be various small receivers scattered across F-22 instead of both transmission and reception done by the same radar in the nose cone.
The radar beam would be transmitted by the radar in the nose cone but received by scattered receivers across the body of F-22..That way the nose cone only needs to be one way..It allows transmission but does not allow reception as it absorbs and scattered any waves hitting the radome surface..
The reception is done by small antennas all over the F-22.
The nose cone shape of the F-22 suggest a similar arrangement..Its shaped to scatter the incoming radar waves bur outgoing radar waves remain unaffected..
The Radome may be coated by radar absorbent material..part of the outgoing radar waves may be absorbed by the coating but that can be compensated by notching up the strength of transmission..
But the incoming waves will not reach the radar inside the radome because they are much weaker than outgoing waves..and will be scattered and absorbed...
So for the reception bit the F-22 must be relying on its much famed "Antenna farm" across the fuselage.
These little antanna may not be stealth at all..but since they are small and at various angles..the enemy radar beam will be reflected in different angles and will be rendered useless for the enemy radar.

 

[pshamim]

Radar works on about 12 different frequencies bands. All Air Forces have the info as to what frequencies the adversaries may be using based on intelligence or from open sourse. Based on that information certain frequencies will be blocked from reflecting by several method namely, FSS, PVC foam, reflective clay glasses and frequency cancellation electronics. This allows the aircraft radar working on certain frequency while blocking some or all other frequencies from outside source.

Hope this answers the question.

 

[razgriz19]

this is one of the most intresting thread!
chogy for starting this.

 

[gambit]

But wouldn't turning transparency ON and OFF still leave windows for the enemy's radar as well?

There is no 'ON/OFF" switch for the "bandpass/bandstop capability of the radome. It all depends on the design and the combinations of FSS layers that will pass thru or stop a freq.

 

[Safriz]

There is no 'ON/OFF" switch for the "bandpass/bandstop capability of the radome. It all depends on the design and the combinations of FSS layers that will pass thru or stop a freq.

I think it will be simpler than that...as in one way radome which only allows transmission but no reception....

Or filtering by wattage not frequency as that way it will be simpler...Radar signals upto certain watts are absorbed by the coating and scattered by the shape of the dome..Any waves above certain watts will still be scattered and absorbed but part of it will make it to the radar receiver...
The antenna will start with bursts of energy in selected directions and if targets picked .. it will continue to transmit high power concentrated radar waves in that direction instead of scattering waves all over the place.
Either that or a one way radome.....Dont know..just speculating like evrybody else.