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Directional Couplers and SWR detectors for 145MHz - 435MHz.
Cheaply, dependently and easily made.

pa0nhc.  Ver. E4; 2003 10 03. / corr.20110306.   V5 : 20130509.  20150217
Site located at : www.pa0nhc.nl

 

  Possible uses.

1 Attenuator in conjunction with a standard dummy load, to measure the spectrum of a transmitter, using a receiver or spectrum scope.
2 Weak, influence-free coupling of a signal source onto a signal line.
3 Return loss measurements, using microwatts from a signal generator and a receiver, up to 50W transmitters.
4 Measurements on receiver sensitivity and transmitter sideband noise of a fully operational full duplex repeater.
4 Power- and SWR measurements, connected to a DC metering circuit.
5 Background monitoring and automatic adjustment of power output and load match.
A/B port 25W 2,5W 0,25W

145MHz:

(V) (V) (V)
A fwd. 1,91 0,520 0,070
B fwd. 1,90 0,52 0,065
A refl. 0,039 0,0038 0,0001

435MHz:

     
A fwd. 5,28 1,475 0,33
B refl. 0,300 0,0485 0,0043
A refl. 0,361 0,057 0,005

    Performance.
Tests on finished couplers showed nearly NO reflected power at 25W input, when terminated with a good dummy load.

Tests on the directional coupler with HF-output clearly showed differences in reflection damping between different types of low power 50/51 ohms terminating resistors and dummy loads. Good terminators showed over 20dB return loss.

Such a directional coupler with HF-output can be used to measure the match of filters. Conveniently using only microwatt-power, from a simple signal generator.
A high power transmitter is not needed
(see "band pass filters for 145MHz").

It also showed to be ideal in use for sensitivity measurements on our Rotterdam repeaters, which were working in full duplex mode on a dummy load or a antenna. The attenuation (coupling loss) from repeater-transmitter to the signal generator was -20dB to -30dB, so the signal generator could not be damaged, and could not cause IMD. Real sensitivity measurement of a fully operational repeater, without the use of an expensive measuring set, then is easy.

Note: all PCB's are double sided copper clad, dielectric constant=4.35 , quality class fr3 or fr4 1.5mm
(Bungard from Conrad electronics). 
The back plane is all copper. At several places it must be solder connected to the top plane with rivets.

The dimensions are designed to fit the rugged, small, HF-tight TEKO steel sheet housings ( www.barendh.nl ).

The resolution of the masks is 200 DPcm.
Drawings are accurate, with outer dimensions stated.
Use the PDFs for easy printing at size 100%.

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 The output of the measuring lines is on 435MHz three times higher than in 145MHz, and 15 times higher than on 29MHz.
Meter scales should be separately calibrated per frequency band.

For 28MHz, the coupling lines could be made longer to increase output.
For 1.2GHz, the coupling lines should be made (1296/435=) three times shorter.

The isolation distance between the transmissionline and the coupling lines must NOT be changed.

Print masks on a inkjet with :
max. resolution, max. contrast, max. saturation and in color.

 


Directional couplers with HF-output.
For measuring return losses using very low power.
Shown are PCB's for different connectors and casings.

BNC receptacles are of screw-type with PTFE dielectricum. Casings used are from from TEKO. N receptacles are of flat flange-type with PTFE dielectricum. The flanges of the recepticles are directly soldered onto the bottom copper (back plane) of the PCB, OR soldered to the side connections of the PCB.

IMPORTANT:
The little holes must be filled with rivets or pieces of thick wire, and be soldered on both sides of the PCB
These are interconnections between mass planes on both sides of the PCB.

The "50 ohms" terminating resistors comprises each of three SMD resistors 150 ohms in parallel.
They are stacked soldered for the lowest self inductance and good heath transfer. 

 



    
     Versions with connectors at the sides:

HF currenst should flow through the backplane of the PCB, NOT through the metal screening of the box.
Leave the PCB free from the box.

The PCB mass planes must ONLY be soldered to the nuts or thread of the connectors.
These mass connections to be soldered are marked with a "ground" sign.
The signal tracks (RF and DC) are soldered directly under the center-pins of connectors.

 

PCB 48x44mm, BNC receptacles on sides.
Download this mask as PDF.

 

PCB 78mmx44mm, BNC receptacles on sides.
Download this mask as PDF.

 


 


PCB 48x44mm, with BNC receptacles on two sides,
and two N receptacles screwed to the outside of the back.
Per N receptacle, use two short screws with their heads inside.
Further use two longer screws with the heads outside.

Then place inside on each some nuts as spacer.
Fix the PCB to the long screws with a nut and a teeth washer.
Then install the BNC's and solder the PCB to all receptacles.
Download this mask as PDF.

 

Universal PCB 48x44mm, BNC receptacles on all sides,
or N-receptacles on the on the back.

Download this mask as PDF.

 


Directional couplers with DC-output.
For use in SWR/power meters,
PCB's of different types for different connectors and casings:

Read also the instructions above about mass connection.

BNC receptacles are of screw-type. Casings from TEKO.
N receptacles are of flat flange-type, with the flange directly soldered onto the bank plane of the PCB, and then screwed into the back plane of the box.
Solder the back plane of the PCB on marked places to the component side via the rivets.
The "50 ohms" terminating resistors comprises each of 3 SMD resistors 150 ohms in parallel. 
The PCB tracks are soldered directly under the center-pins of all BNC's.
Solder the  the mass-connections of the PCB to the nuts of the BNC receptacles, not to the box itself.

Reverse the diodes if negative DC output is wanted.
The RF-filters in the DC-outputs comprises of SMD-components:
     at the diode 100pF to mass, r=150 ohms in series, and 10nF to mass at the output.
X= The output connection to BNC receptacles, or to feed-through-Cs and external screened wires.

PCB 48x44mm, BNC receptacles on sides.
Download this mask as PDF.

 

PCB 48x44mm, with BNC receptacles on two sides for DC output,
and two N receptacles for RF screwed to the outside of the back.
Per N receptacle, use two short screws with their heads inside.
Further use two longer screws with heads outside,
and place inside each some nuts as spacer.
Fix the PCB to the long screws with a nut and a teeth washer
Then install the BNC's and solder the PCB to all receptacles.

Download this mask as PDF.

 

Universal PCB 48x44mm, BNC receptacles on all sides, or N- receptacles on the back.
Download this mask as PDF.

 


History.

Several years ago, i needed an SWR-meter/power-meter for use on 145MHz. As money was short, a cheap one ("Made in Italy") for 27MHz was purchased, as specifications stated is was "designed" for up to 150MHz.

Well, it was rubbish of cause. In the back, soldered on two SO239 receptacles, was a SINGLE sided PCB with some sharply zigzagging tracks and loosely connected wires, using the back plane of the housing at 5mm distance as a "ground plane". This could never result in a proper impedance of the measuring lines, even on 27MHz. The SWR  (reflection damping) of this PCB itself was bad on 2mtrs, and power indication was a laugh.

In an old ARRL handbook was a picture of a PCB for use in a SWR-meter. Antenna books described how to make measuring lines from coax or using Toro ides. I tried some of these, but none of them was working properly on VHF, and descriptions did not say what the influence was of the coupling-wires, or the PCB-material on the properties of the whole thing. But i did found details about impedance of PCB-tracks, and decided to try to design my own directional coupler PCB.

    Important for construction:
The width of the tracks and the dielectric constant of the PCB material determine the characteristic impedance of the tracks. Therefore, only use PCB with the specified dielectric constant. I was surprised to see that good quality hard-paper PCB (Conrad Electronics) had also the correct dielectric constant. But better use glass-fiber PCB as this is stronger.

The standard RF source- and load impedance of VHF/UHF communication equipment is 50 ohms. The power-line between the input and output connectors of a directional coupler therefore MUST have also that impedance of (exactly) 50 ohms.

The impedance of the measuring lines could have any practical value, as long as one side of each measuring line is terminated with the characteristic impedance of that measuring line.

The back plane of the PCB serves as mass plane and screen. Very short mass connections between receptacles and back plane are a MUST.

If the coupler is to be used as a directional coupler with HF-output from the measuring lines into 50 ohms loads, the measuring lines MUST have an characteristic impedance of 50 ohms as well.

It then is very easy to terminate those measuring lines correctly with resistive 50 ohms loads, made from 3x 150 ohms SMD resistors in parallel. If 90 degr. corners in the tracks are changed to 2x 45 degr; the unit can be used on 450MHz and even higher frequencies, as those corners do not reflect RF-waves back to the source anymore. This makes it easy to construct good performing instruments.

The output of a measuring line (sensitivity) depends on the wavelength of the signal, the length of the coupling line, and the spacing between the coupling line and the power line. 3x higher frequency means 3x higher output from the same configuration. If one PCB should be used for two different frequency bands, the sensitivity should be calibrated for each frequency band separately by trim potentiometers, if one scale is to be used on the meters. For higher frequencies (23cms) the coupling lines should be made shorter. For a low power range, a separate scale should be made to compensate for detector diode non-linearity.

The diodes should have low knee voltage, and suitable for high frequencies. Resistors and condensers are SMD types. Resistors should be soldered on top or beside each other, making good thermal contact to the PCB-tracks. Maximal power handling is determined by the maximum allowable power dissipation of the terminating SMD-resistors.

The coupling between the power line and measuring lines can be adjusted by varying the distance between them. This distance is critical, and care should be taken when photographing and etching the PCB. A coupling attenuation of less than 20dB will influence the SWR of the power line.


Pictures of prototypes:

Directional coupler with HF-output, -30,5dB @ 145MHz, -20dB @ 435MHz.
This unit is very useful for weak coupling of HF-sources, and return loss measurements with very low, to high power.
The housing is a small TEKO box.
Directional coupler with detectors and DC-output.
This unit can be used as power meter and SWR meter for 28MHz to 435MHz.

 

Directional coupler PCB with detectors and DC-output.
Replacing the very poor detector circuit in a cheap citizens band "power/SWR meter".

It now can be used to measure on 28MHz, 145MHz and 435MHz.
Sensitivity of the SWR-metering is: 145MHz/5W, 435MHz/1W.
Full Scale Deflection for power measurements was adjusted to 10W.
Read remarks below.

As shown here, a simple but accurate PCB works, with receptacles soldered on the mass side (back), screwed to the housing.
"Forward" and "Reflected" DC outputs are connected to separate meters.
A switch is used to calibrate the "Forward" meter for full scale deflection on 145MHz or 435MHz when used as a power meter.
When used as an SWR meter, the double potentiometer adjusts the forward meter for Full Scale Deflection.

To prevent HF radiation, a better solution is to put the PCB in a dedicated, RF-tight box.
The DC-outputs should be RF filtered before they are connected to the outside if this box, and to the meters.