Coaxial Stub Filters

A range of easy-to-build RFI filters
by John Regnault, G4SWX

 

CONTENTS

Top

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)

End, Back

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 filters using ordinary coaxial cable. These will generate sharp, deep notches which can be placed on the amateur bands.

Problem: 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 your transmitter may need an additional 2nd-harmonic notch filter.
  • 144MHz transmitter: your strong 144MHz signal may cause breakthrough, so your neighbour's FM receiver may need an FM broadcast 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.

Problem: UHF TV Breakthrough

  • 50MHz transmitter: UHF TV breakthrough is very unlikely
  • 144MHz transmitter: Your 4th, 5th and 6th harmonics fall in or near the European TV band. Harmonic levels that comply with national regulations may still be enough to cause breakthrough, so your transmitter may need an additional harmonic notch filter.
    Also, your neighbour's UHF TV receiver may need a TV bandpass filter to protect it against overload from your strong fundamental signal.
  • 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.
  

CONTENTS

Top

1. Which Filter?
2. VHF and UHF Transmitter Filters
3. FM Broadcast Lowpass Filter (144MHz reject)
4. UHF TV Bandpass Filter (432MHz reject)

End, Back

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

Component 50MHz 70MHz 144MHz
Cable length in RG58/RG213 (mm)
S1: 0.25 wl at 3f 327 238 115
S2: 0.25 wl at 4f 247 179 86
S3: 0.25 wl at 2f 489 355 172
S4: 0.25 wl at 5f (optional) 196 142 68
L1
(enCu = enameled copper wire)
160nH - 10 turns 0.5mm enCu on T37-12 toroid
or 4 turns 1.5mm enCu close-wound on 9.5mm dia. former
110nH - 8 turns 0.5mm enCu on T37-12 toroid
or 3 turns 1.5mm enCu close-wound on 9.5mm dia. former
55nH - 3.5 turns 1.0mm Cu on 5.0mm former, spaced wire diameter

432MHz

Component Cable lengths in RG58 (mm)
S1: 0.25 wl at 3f 38
S2: 0.25 wl at 4f 29
S3: 0.25 wl at 2f 57
S4: 0.25 wl at 5f (optional) 23
T1: 0.265 wl at f 121

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.

  

CONTENTS

Top

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)

End, Back

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.

  

CONTENTS

Top

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)

End, Back

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.

CONTENTS

Top

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)

Back to GM3SEK's Amateur Radio Technical Notebook

Updated 1 March, 2014
Page (c) 1997-2013 IFWtech. Contents (c) 1994-1997 RSGB