CONTENTS Top1. Which Filter? |
Which Filter?Transmitters For the amateur VHF/UHF bands, you can make very effective filters using ordinary coaxial cable. These will generate sharp, deep notches which can be placed on the amateur bands. Problem: FM Broadcast Breakthrough
Problem: UHF TV Breakthrough
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CONTENTS Top1. Which Filter? |
VHF and UHF Transmitter Harmonic FiltersThese filters connect to the output of your transmitter and notch out all the major harmonics. There are four open-circuit quarter-wave stubs, to notch out the 2nd, 3rd, 4th and 5th harmonics. The 5th-harmonic stub S4 is optional. Leaving it out improves the VSWR from about 1.5 to a near-perfect 50-ohm match.
Construction:
Power handling: Constructed in RG58 with inductors as shown, these
filters will handle up to about 100W. Performance of 144MHz filter - note the deep rejection notches at the harmonic frequencies of 288, 431, 576, 720 and 864MHz.
Reference: 'In Practice', RadCom (RSGB), March 1997. |
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CONTENTS Top1. Which Filter? |
FM Broadcast Lowpass Filter (144MHz reject)This amazingly simple filter has low loss (<1dB) and a flat passband in the FM broadcast band up to 110MHz, and then rolls off to give a deep notch at 144MHz. The filter is designed for a 75-ohm FM antenna system.
Note that the coaxial stub is open-circuit. Suitable inductors would be TOKO p/n NE547SN-100126 (blue), available from BEC Distribution (BEC p/n 427126 - BEC sell to UK and world-wide). Performance: Reference: 'Stub Filters Revisited' by John Regnault, G4SWX, RadCom (RSGB), November 1994. |
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CONTENTS Top1. Which Filter? |
UHF TV Bandpass Filter (432MHz reject)This is a real "brick wall" filter to pass only the European TV band! It has low loss (<1dB) from 470MHz upwards, but 30dB notches at 432MHz and 1.3GHz. The highpass response also rolls off to give excellent rejection at 144MHz and below. The filter is designed for a 75-ohm TV antenna system.
Note that this coaxial stub is short-circuited and grounded only at the bottom. The rest of the stub must be kept away from ground. Use a long, narrow metal box with the input and output sockets both at the same end, and the stub suspended 'in air' along the middle of the box and grounded only at the far end. Suitable 30nH inductors would be 2 turns of 1.5mm wire (UK 16SWG, USA 14AWG) of 4mm (5/32in) inside diameter, turns spaced 1 wire diameter and with 3mm (1/8in) leads. Wind each coil on a 3mm (1/8in) drill to allow for the turns to spring open, insert the larger drill to check the inside diameter, and then bend the leads out straight while the coil is still supported on the drill. Alternatively, use TOKO inductor No. NE547SN-100122 (orange), available from BEC Distribution (BEC No. 427122 - BEC sell to UK and world-wide). Performance:
Reference: 'Stub Filters Revisited' by John Regnault, G4SWX, Radio Communication (RSGB), November 1994. |
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CONTENTS Top1. Which Filter? |
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Updated
1 March, 2014
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