Home Up What's New Contact Us Literature Req.

Combline Bandpass

Download a printable pdf

Combline Cavity Filters

ALLEN AVIONICS CMBF series of Combline Bandpass Filters are the answer when you need superior performance in a small package for Bandwidths ranging from 0.5% to over 20%. Available over the 500 MHz to 26.50 GHz frequency band. These filters can be supplied with 3 to 8 "high Q" resonant sections. Each filter is custom designed to your exact specifications so that you get optimum performance at the lowest cost. Allen Avionics Cavity Filters are normally constructed using Silver Plated Brass and Aluminum for improved electrical characteristics and current flow. Allen Avionics cavity filters can be supplied with your choice of connectors. For connector styles see chart below.

Allen Avionics Combline Bandpass Filters are fixed filters that feature sharp stopband rejection and lower loses than Discrete Element or Tubular style Bandpass Filters. This filter type offers high-Q with reduced size and excellent Frequency response. Filter performance is easily predicted using our microwave filter design software. Manufacture takes place in a state of the art CNC machine and fabrication center where custom filters can be completed in days not weeks. The CMBF series of filters are rugged and well suited for severe environments.

Available Frequency 500 MHz 26.5 GHz
Available Bandwidth 0.5% Up to 20%
Insertion Loss (Depends on the number of sections and BW)
Stop Band Attenuation see graphs
Number of Resonator Sections Available 3 to 9
VSWR 1.50:1
Dimensions See drawings below.
Consult Factory for exact dimensions.

The curve to the right displays the Relative Attenuation as a function of the Normalized 3dB Bandwidth for narrow bandwidth filters.(0.5%) The graphs supplied are designed to give an approximation of the frequency response for the CMBF series of Cavity filters. Using these curves and a few simple formulas shown below you can predict the attenuation for a given number of sections. Curves for narrow bandwidths (0.5%) and wide bandwidths (5.0%) are also provided. There are many variables that affect the Frequency Response, Attenuation and Insertion loss of Waveguide filters so when exact information is required contact the factory at  (516) 248-8080 or send email to sales@allenavionics.com


The Frequency response curves we supply show the approximate attenuation as a function of the normalized 3dB bandwidth.

Using these graphs we can calculate attenuation at a given frequency for a specified number of sections or determine the number of sections required to meet a set of given attenuation points.

(Find required # of sections)

We want a Bandpass filter centered at 10.0 GHz (FO) with a 3dB bandwidth of 100 MHz (FC). We have a stopband requirement of 60dB at 9.80GHz and 10.30GHz.

First we need to determine the percentage bandwidth (%BW) using the following formula to find the correct graph to use.

With a 1% bandwidth the graph for .5% above would give us the best approximation. Now we have to look at the two stopband requirements. The low side 60db attenuation point is 9.70GHz and the high side is at 10.30GHz. Now we need to calculate the number of 3dB bandwidths using the following formula.

Checking the above graph we can see that 6 sections will have well over 60dB attenuation at both stopband requirements.

EXAMPLE #2  (Find minimum attenuation at specified frequencies)

We want a Bandpass filter centered at 2.50 GHz (FO) with a 3dB bandwidth of 250 MHz (FC) and we are thinking about using 6 sections. We need to find the minimum attenuation at 2.00GHz and 3.00GHz for 6 sections.

Using the same formula above we first find the %BW to determine the correct graph to use. In this problem the %BW equals 10% so we can use the 16% BW graph to the right. Then from the second formula we used in the first example we find that we have four (2) 3dB bandwidths on each side. Checking the above graph we can see with 6 sections we will have greater than 40dB attenuation at both 2.0GHz and 3.0GHz for a filter with 250MHz 3dB bandwidth. Three plots are provided, 0.5%,

5.0% and 16%, which can be used to 20% so the approximate response can be determined for any bandwidth in the range of .0.5% to 20%

Allen Avionics has advanced simulation and modeling software that our experienced Microwave engineers use in the design and manufacturing of Combline Cavity Microwave Bandpass Filters. This software combined with our Numerical Control Machining Centers gives us rapid design and production capabilities. Our Automated inspection equipment insures every part will meet their specifications.

Allen Avionics has the ability to manufacture prototypes and short runs quickly and reduce lead times on large production runs. Our Microwave and Mechanical engineers are always available to help with custom designs or applications. The sales force at Allen Avionics is ready to help with information and prices. They try to answer all quotes in 48 hours or less and are familiar with all our microwave products.

Code Description
1 Series
2 Number of Sections
3 Center Frequency (GHz)
4 3 dB Bandwidth (MHz)

Connector Code(Input/Output) (see chart)

6 Material (AL) (BR)

1 2 3 4 5 6
CMBF/ 5/ 20.25/ 101.25/ FS/FS/ AL

COMPLETE Part Number=
CMBF / 5 / 20.25 / 101.25 / FS/FS / AL

Contact Factory With Your Specific Needs For:
-Electrical Performance
-Mechanical Configuration
-Insertion Loss


2.4 mm FEMALE 24F
2.4 mm MALE 24M


The CMBF series of Microwave Bandpass filters is normally constructed from Aluminum but when better temperature response is required brass can be used.

When bandwidths exceed the 20.0% limit for this filter type or when insertion loss, length rejection required or some other parameter cannot be met, Allen Avionics may have other Filter constructions available which could be used to solve the problem.

Contact our sales department for assistance. (516) 248-8080 or sales@allenavionics.com

Frequency GHz .50 - 3.0 3.01 - 6.0 6.01 - 10.0 10.01 - 26.5
Multiplier (H) 2.00 1.50 1.00 0.500

% Bandwidth (Fc/Fo) 0.50 - 7.0 7.1 - 14 14 - 20
Multiplier (A) 0.30 0.75 1.00

The number of sections, Center Frequency and the % bandwidth along with the cavity structure are the basic parameters that determine the overall dimensions of Combline Cavity Microwave Filters.

Using some basic formulas we can approximate the length for a given Frequency,  and width and number of sections.

To simplify this approximation we are assuming that width and height are the same.

From EXAMPLE #1. We had a Bandpass filter centered at 10 GHz with 1.0% BW. From the table above we see for 10GHz the (H) multiplier is 1.0. From the same table we see the % Bandwidth multiplier (A) is 0.50. Our previous calculations done for example #1 Indicated that we needed 6 sections to meet the required attenuation. With the number of sections (N), height multiplier (H) and % bandwidth multiplier (A) we have all the information we need to calculate the approximate length (L).

N = number of sections.
H = Height and Width in inches
L = Approximate length in inches
A = %Bandwidth Multiplier

ALLEN AVIONICS INC manufactures many other types of microwave Filters. Our engineers will help you pick the right filter to meet all the requirements you have for performance, size and cost. Our advanced simulation and modeling software will design and do optimization of Circular Post Coupled filters, Metal-Insert Filters, Iris Coupled Filters, Combline and Interdigital Filters. All of our Filters can be custom designed for your application and we offer fast prototypes. No need to wait five or six weeks to test your idea or check your system.

Allen Avionics Inc. started in 1960 And has 50 years experience in design, Production, quality and they use it on every product they make. Check out this web site for all their products.

Allen Avionics Home | Products | Video Capture & Playback | Video Hum and Noise Eliminators | Analog & Digital Delay Lines | Offset Marking Machine | Video Delay Lines | Microwave Filters | Video Filters | Brickwall Low Pass Filters | Filter Facts | Mechanical | General Purpose Delay Lines | Sales, Prices & Representatives | Glossary | About Hum | NTSC Video | Audio Ground Loop Isolation Transformers | Copyright | Feedback Page | What's New | Search | Site Map

Allen Avionics, Inc.
2727 Clinton Street
River Grove, IL  60171
Phone: (708)-453-3238
Fax: (708)-453-0297
E-Mail: Sales@AllenAvionics.com

We are pleased to accept

It's your choice