Hi, All
I got a surplus K&L MICROWAVE ,INC Tubular Bandpass Filters model 3B340-400/16-N75-SMA50 D.C.-8040 of the type described in the following URL
http://www.klmicrowave.com/ecommerce/files/Tubular%20Bandpass.pdf
The above filter was originally integrated in to a vertical dipole with a low noise preamplifier at 400 MHz and used for the Maritime Satellite MARISAT system between shore and shipboard terminals and the satellite.
Many of this filters are still available in the surplus market
Code Description
3 numbar of sections B bandpass filter 340 outside diameter 0.75" - 19.05 mm 400 center frequency (MHz) 16 3 dB bandwidth (16 MHz) N75 input connector (see note) SMA50 output connector D.C.8117 D.C. block
Note:The input of the filter is fitted with a 75 ohm N female connector to be screwed directly at the dipole ( 75 ohm impedance)
Just for curiosity I decided to unscrew both caps with connectors and pull off the filter from the tube in the hope to understand how a Tubular Filter works and I realized that this 3 sections Chebychev response filter is made by a teflon flexible rod in wich are slipped on in sequence coil-capacitor-coil-capacitor-coil
The end coils are 2 turns of enameled nr 18 copper wire while the center coil is 3 turns
Each concentric capacitor between coils is made with two brass disc as amature with inside a numbar of disc of low loss teflon foils for dielectric.
The above filter is wrapped in to a single layer of low loss teflon foil and than it is pushed inside the aluminum pipe.
The function of the teflon foil is not trivial because it represent the dielectric to form many small capacitors between each coil and each series capacitor with respect to ground as required by a bandpass Chebychev filter.
The aluminum pipe has 3 longitudinal holes each 5.5 mm in diameter and the holes are drilled just in correspondence to the middle of each coil so that the turns are visible from the outside allowing the spacing adjustement possible during tuning at the factory.
Since the difference between the original 400 MHz center frequency and our 435 MHz frequency band is very small i.e. between 1/12 and 2/12 of an octave higher i decided to see if spacing the turns a little bit it would be possible to shift the center frequency 35 MHz higher in frequency keeping as well the flatness of the bandpass as in the original filter.
The N75 female connector was then removed and replaced with an SMA female connector and the filter was connected via 6 dB 50 ohm attenuators between a HP8444A Tracking Generator and a HP8555A Spectrum Analyzer
A small tool made of insulating epoxy G10 for PCBs was shaped in the form and size of a hard and sharp-pointed toothpick with the copper removed from it and it was found to be OK to fine adjust the spacing of the coils.
During retuning step by step it was observed that changing the spacing of the central coil it mainly shift up and down the whole band while adjusting the end coils mainly changes the flatness and the shape of the bandpass.
In addition it was observed that introducing or removing the insulated tool in to the holes of the filter while spacing the coils it do not change at all the previously made adjustement.
If the adjustement of the spacing in the coils is carefully made it will converge with easy to translate the original 400 MHz bandpass center in to the new 435 MHz center frequency with the same flatness in to the same 16 MHz bandwidth as in the original filter.
Here are the results after retuning.
Center frequency 435 MHz 3 dB bandwidth 16 MHz from 427 to 443 MHz Response 0.05 dB flat from 430 to 441 MHz Insertion loss center frequency 1.5 dB VSWR center frequency 1.5:1
Measured attenuations:
MHz dB
270 -60 340 -50 380 -40 400 -30 415 -20 420 -10 427 -3 435 -1.5 (0.05 dB flat from 427 to 443 MHz) 443 -3 450 -10 460 -25 470 -30 480 -40 500 -50 535 -60 560 -70 580 -75
This low loss bandpass filter is very small can handle 40 watt CW continuously and can find many applications where low loss at 435 MHz and high rejection outside the 70 cm band is required.
If you find one of this filter in your junk box do not throw this piece of cake out of the window.
73" de
i8CVS Domenico