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Coaxial_Tank_VHF_Filter_Designer.php   7953 Bytes    03-08-2014 15:34:16

Coaxial Tank V.H.F. (Bandpass) Filter Designer

Sometimes also referred to as : Cavity Resonator, Cavity Filter, Topfkreisfilter

Coaxial Tank V.H.F. (Bandpass) Filter

Frequency [MHz]

Rod Diameter [mm]

Rod Length [mm]

Tap Height [mm]

Box Width [mm] (square)

Box Height [mm]

Box Diameter [mm] (round)

Diameter Disc D [mm]

This Calculator is fully experimental. It is inspired by an article in QST, October 1964. With the "Law of Scaling", dimensions are calculated to the frequency of interest. As the title suggests, this is useful in the VHF range. Consider it as a starting point for optimisation :-)

On public demand, some words shall be added here. In order to gain some insight, it is the best way to design and build such a thing. Our center frequency is f = 1090 MHz, whilst the rod diameter shall be 10 mm. The assembly was not that challenging, as we used standard pipes made of brass. The bottom and top plates were made of copper. On the top plate a M6 screw with soldered nut is for the adjustement. We used sma connectors. (farnell #146-3066)


Fig: 1 :: View inside the bandpass-filter


Fig: 2 :: Fully assembled


Fig: 3 :: Insertion loss / amplitude response

Even the height was slightly higher (60 mm) than calculated, the disc-diameter of 20 mm was too big. The disc could be removed completely. The capacity made out of the M6 screw and the 10 mm (diameter) rod was sufficient to tune to the desired frequency. We measured an insertion loss of 0.8 dB with the network analyser and 0.9 dB with the spectrum analyser. The bandwidth was about 18 MHz (1080.95 to 1099.2 MHz), measured at -3 dB. (about 1.65 %). In order to make sure, that the tuning screw would not touch the rod, an 8 mm dowel was pressed into the rod. This may have been not that smart, as the overall Q presumably dropped by this.

Similiar projects with some building information :


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t1 = 3913 d

t2 = 582 ms

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