WG Band Stop Filter

WG Band Stop Filter for High-Power RF Systems

Advanced Microwave Technologies designs and manufactures waveguide passive components for demanding microwave environments. Our WG Band Stop Filter selectively attenuates a defined stopband while allowing frequencies above and below to pass with insertion loss as low as 0.05 dB. Built from aluminum 6061-T6, copper, or Invar, these filters cover C, X, Ku, and Ka bands. Rejection depth ranges from 30 dB to over 80 dB depending on design. ADM maintains a 24-meter microwave darkroom for in-house electrical validation from 0.5 GHz to 110 GHz.

What Is a WG Band Stop Filter?

A product — also called a band reject or notch filter — uses resonant cavities or stubs coupled to a main waveguide transmission line. This configuration achieves high Q-factors that coaxial or microstrip filters cannot match at equivalent power levels.

Frequencies above and below the stopband pass with minimal signal degradation. Energy at the stop frequency is reflected back to the source rather than absorbed into lossy dielectric materials.

Air-dielectric waveguide construction makes this filter the preferred choice in high-power and interference-sensitive RF systems.

Key Technical Specifications

Materials and Construction

The choice of body material has a direct effect on how well it handles heat and power.

Most systems on the ground and in the air are made of aluminium 6061-T6. It has a good ratio of strength to weight and can be used with chromate conversion coating to protect against corrosion.

Copper is better at conducting electricity, so it lowers resistance loss. This is important when passband insertion loss is a key design constraint.

For ultra-narrow stopband uses, Invar (a Nickel-Iron metal) is recommended. The notch frequency stays the same as the working temperature changes because its coefficient of thermal expansion is very close to zero. In situations where small changes in temperature could let interference through, Invar is the right material choice.

Putting silver or gold plating on the inside of things lowers resistive loss and stops passive intermodulation (PIM). ASTM B571 is used to test the bonding of plating.

Application Scenarios

Satellite Earth Station Uplinks (SATCOM)

In send chains with a lot of power amplifiers, unwanted frequencies can get into satellite receive bands that are close by. A WG Band Stop Filter takes these frequencies out of the transmit path, which keeps receivers that are close together from losing their sensitivity when they are outside.

Military and Naval Radar Systems

Wideband ESM receivers on naval ships can have a band stop filter added to them to block out the ship's own fire control radar pulses. So, the ESM system can pick up information from outside sources without the front end getting too busy.

High-Energy Physics and Particle Accelerators

Higher-order modes that make beam stability unstable inside accelerating holes need to be squelched in linear accelerators. For these uses, the construction must be compatible with vacuums and the frequency stability must be proven across the operating temperature range.

High-Power Transmitter Harmonic Suppression

Harmonics are made by TWTA and Klystron-based transmitters at known frequency offsets. The filter gets rid of these harmonics without hurting the main stream.

Waveguide vs. Coaxial Construction

When the power level goes from kilowatts to megawatts, thermal breakdown and dielectric losses stop coaxial filters from working at higher power levels. Waveguide construction doesn't need insulating materials to get rid of heat because its structure is made of metal.

Dielectric loss is not a problem with air-dielectric construction. When combined with silver or gold plating, this makes passband insertion loss much lower than coaxial equivalents. This is important for system noise figure on receive chains and power efficiency on transmit.

When there are a lot of frequencies going on, like in satellite teleports or shipboard electronics, the high Q-factor makes the skirt selectivity better and the frequency separation between the stopband and passband better.

Quality Control and Testing

Every unit is electrically verified before shipment.

S-Parameter Testing (VNA): S21 confirms rejection depth and stopband bandwidth. S11/S22 confirms VSWR across the passband. VSWR at band edges is a standard inspection checkpoint.

High-Power Testing: Filters are tested at full rated power plus a 20% margin to check for thermal drift or voltage breakdown in tuning cavities.

Thermal Cycling: Notch frequency is verified across -40°C to +85°C against the rejection mask specification.

Pressure Sealing Test: Units for pressurized systems are tested at 30 PSI and checked for leakage, confirming compatibility with SF6 or dry nitrogen.

Plating Adhesion Test: Tape testing per ASTM B571 verifies that silver or gold plating remains intact under operational stress.

ADM's 24-meter in-house microwave darkroom supports full electrical validation from 0.5 GHz to 110 GHz across the product frequency range.

Customization and Configuration Options

Standard WG Band Stop Filter configurations cover common waveguide bands. Custom designs can be specified by stopband center frequency, rejection depth, bandwidth, flange type, material, and environmental requirements.

Both narrow and wide stopband designs are supported. Tuning screws for final manufacturing adjustment are standard; after production verification, they are locked and sealed to maintain VSWR and rejection specifications under vibration.

Pressurized assemblies using nitrogen or SF6 for high-power arcing prevention are available on request.

FAQ

Q: What is the mechanical difference between a product and a band pass filter?

A: A band pass filter lines up coupled cavities in a row along the direction of transmission. There are resonant cavities that shunt to the main waveguide wall or tuning stubs in a band stop filter. The energy in these resonators is trapped at the stop frequency and sent back to the source.

Q: Can the stopband frequency be changed after it has been installed?

A: Not usually, no. Some designs have tuning screws that can be used to make small changes during production. Once production is complete, these screws are locked in place and sealed with epoxy. If you do field retuning without a VNA, the VSWR and rejection specs could become invalid, especially if the system is vibrating.

Q: What limits the amount of power this filter can handle?

A: Voltage breakdown at high-field points in the resonant cavities or tuning gaps is the main thing that limits the signal. To fix this, the inside sides are rounded off and, if needed, dry nitrogen or SF6 is used to pressurise the waveguide.

Q: For narrow notch uses, why should you choose Invar over aluminium?

A: When aluminium gets hot, it expands, which changes the frequency of the notch and might let interference pass through the stopband. Because Invar has almost no thermal expansion, it keeps the notch frequency within <1 ppm/°C. This is necessary for ultra-narrow stopband designs that work in settings with changing temperatures.

Q: In general, how much passband insertion loss is there?

A: In the passband, the insertion loss can be anywhere from 0.05 dB to 0.25 dB, based on the length and number of sections. This is significantly less than coaxial equivalents, which is important for the system noise figure in receive chains.

Q: What are the most important things to look for during a check when buying?

A: Five standard inspection steps are used for aerospace and defence purchases. These are S-parameter verification using a VNA, high-power testing at rated power plus 20% margin, thermal cycling across the full working range, pressure sealing tests for pressurised systems, and plating adhesion according to ASTM B571.

Contact Us

Send your WG Band Stop Filter requirements to craig@admicrowave.com and our engineering team will review your specification and respond with a solution.