اوك شد في هذاك الخيط
ليكن في علمك حاليا B-2 الطائرة الوحيدة التي تمتلك RAM أكثر كفائة ضد vhf-band هل تعلم لماذا ؟ هل تعلم كم عرض و سمك ram في مقدمة b-2 ضد vhf-bnad ؟ عندما تجيب عن هذه الاسئلة ساستمر في نقاشك .. مع كل ماذكرت يمكن رصد b-2 بواسطة رادارات vhf خاصة مع تعديلات على هوائياتها و الكترونياتها و برمجياتها سيكون المدى أقل لكن أفضل من لا شيئ
تفضل اقرأ قليلا ... العالم كامل يعرف هذا عن اف22 و اف35 إلا هنا في هذا المنتدى
Russian industry publications state quite bluntly that US stealth designs have been largely optimised to defeat decimetric and centimetric band radars by shaping and the use of absorbent materials. The Russians have also observed publicly that the long wavelength of the Russian VHF band radars makes defeat by shaping difficult if not impossible, and that the thickness of absorbent coatings required to defeat VHF band radars would be prohibitive.
These are correct observations. The shaping features of US stealth fighters are dimensionally such that they fall into the resonance and Raleigh scattering regimes for VHF band radars, where shape has little impact on the Radar Cross Section. Their observations on skin depth are also correct, as resistive and lossy materials always exhibit greater skin depth than conductors such as aluminium skins.
So a fighter with stealth design features faces an interesting challenge when confronting a VHF band radar. Most of the carefully faceted, serrated and blended shaping features so effective at beating decimetric and centimetric band radars will appear as blobs to a VHF radar and reflect across a wide range of angles, losing their capability to scatter impinging radiation away from the radar illuminating the aircraft. The shaping of the planform is also less than entirely effective, unless it is very disciplined in the manner of the Northrop YF-23A or B-2A designs.
The issue of skin depth is also of concern. Surface coatings designed to attenuate creeping surface waves in the decimetric and centimetric bands will typically be too thin to cope with VHF band radiation, which will penetrate through to the underlying skin panel and structural materials, which will likely be electrically conductive. While a well shaped stealth fighter will have a lower VHF band signature than a legacy fighter festooned with external fuel tanks and radars, it will certainly not be a marble or tennis-ball sized refle
ctor.
For instance, let us consider the F-35 JSF in the 2 metre band favoured by Russian VHF radar designers. From a planform shaping perspective, it is immediately apparent that the nose, inlets, nozzle and junctions between fuselage, wing and stabs will present as Raleigh regime scattering centres, since the shaping features are smaller than a wavelength. Most of the straight edges are 1.5 to two wavelengths in size, putting them firmly in the resonance regime of scattering. Size simply precludes the possibility that this airframe can neatly reflect impinging 2 metre band radiation away in a well controlled fashion.
The only viable mechanism for reducing the VHF band signature is therefore in materials, especially materials which can strongly attenuate the induced electrical currents in the skins and leading edges. The physics of the skin effect show that the skin depth is minimised by materials which have strong magnetic properties. The unclassified literature is replete with magnetic absorber materials which have reasonable attenuation performance at VHF band, but are very dense, and materials which require significant depth to be effective if lightweight. The problem the JSF has is that it cannot easily carry many hundreds of pounds of low band absorber materials in an airframe with borderline aerodynamic performance. Some technologies, such as laminated photonic surface structures might be viable for skins, but the experimental work shows best effect in the decimetric and centimetric bands. Thickness again becomes an issue.
The reality is that in conventional decimetric to centimetric radar band low observable design, shaping accounts for the first 10 to 100 fold reduction in signature, and materials are used to gain the remainder of the signature reduction effect. In the VHF band shaping in fighter sized aircraft is largely ineffective, requiring absorbent materials with 10 to 100 fold better performance than materials currently in use. In the world of materials, getting twice the performance out of a new material is considered good, getting fivefold performance exceptional, and getting 100 fold better performance requires some fundamental breakthrough in physics.
One possible strategy which may be viable against VHF band radars is active cancellation, where the aircraft emits a waveform identical to the impinging threat radar waveform, but out of phase so the two cancel each other out. This is not feasible in the upper bands, but is feasible in the lower bands, assuming a suitable VHF antenna can be integrated without compromising microwave band radar signature. The drawback of active cancellation is that the opponent can deploy VHF band passive receivers and simply listen for the sidelobes emitted by the active cancellation system to track the aircraft.
As the JSF, like the retired F-117A, is totally reliant upon stealth for survival, in an environment with VHF threats it has no simple fallback technique other than staying on the ground in its hardened shelter. This is unlike the F-22A which has supersonic cruise to fall back upon, and is harder to acquire by missile seekers since its microwave band stealth is so much better.
What is abundantly clear is that VHF radars will have much better detection performance against fighter sized stealth aircraft, compared to decimetric and centimetric band radars.