Coaxial Stub Filters

CONTENTS

1. Which Filter?
2. VHF and UHF Transmitter Filters
3. FM Broadcast Lowpass Filter (144MHz reject)
4. UHF TV Bandpass Filter (rejects all amateur bands)

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Which Filter?

Transmitters
You should not need to use a narrowband filter after a transmitter. All the bandpass filtering should have been done at low level, in the exciter.  This means that the only significant spurious signals will be harmonics. For the amateur VHF/UHF bands, you can make very effective harmonic notch filters using ordinary coaxial cable.

FM Broadcast Breakthrough

• 50MHz transmitter: the second harmonic (100MHz) falls inside the FM band.  Harmonic levels that comply with national regulations may still be enough to cause breakthrough, so you may need an additional 2nd-harmonic notch filter.
• 144MHz transmitter: your strong 144MHz signal may cause breakthrough, so your neighbour's FM receiver may needs a lowpass filter which will pass the 88-108MHz FM signals but has a deep notch to reject your 144MHz signal.
• UHF transmitters: FM broadcast breakthrough is very unlikely.

UHF TV Breakthrough

• 50MHz transmitter: UHF TV breakthrough is very unlikely
• 144MHz transmitter: your neighbour's UHF TV receiver may need a TV bandpass filter with low loss above 470MHz (in Europe). Being a highpass design, this filter has excellent rejection on 144MHz and below.
• 432MHz transmitter: the second harmonic at 864MHz is in the European TV band, so you may need an effective 2nd-harmonic notch filter to eliminate this output. (This filter also has a deep notch at 1.3GHz - very useful in multi-multi contest situations!)
  Also your neighbour's UHF TV receiver may need a TV bandpass filter with low loss above 470MHz (in Europe) and a deep notch to reject your strong 432MHz signal. 
• 1.3GHz transmitter: UHF TV breakthrough is unlikely, but the TV bandpass filter also has a useful notch at 1.3GHz.

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VHF and UHF Transmitter Harmonic Filters

These 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.

50 - 144MHz

432MHz

Construction:

• Cut all stubs accurately to length. Lengths given are end-to-end of the inner insulation.
• Use very short connecting leads - not the way the circuits are drawn here! Lay out the filter in an "H" shape.
• Note that all stubs are open-circuit at the far end.

Power handling: Constructed in RG58 with inductors as shown, these filters will handle up to about 100W.
For higher power, use RG213 and make the inductors from heavier wire - but keep to the specified inductance value. RG213 stubs will handle at least 500W. For even higher power, make stubs from hardline.

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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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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UHF TV Bandpass Filter (432MHz reject)

This is a real "brick wall" filter for 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 at the bottom. The top 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 at the far end.

Suitable inductors would be TOKO p/n NE547SN-100122 (orange), available from BEC Distribution (BEC p/n 427122  - BEC sell to UK and world-wide).

Performance:

Reference: 'Stub Filters Revisited' by John Regnault, G4SWX, Radio Communication (RSGB), November 1994.