Using factory made PCBs for pa0nhc 50 Ohms directional couplers.
The use, copy and modification of all info on this site is only permitted for non-commercial
thereby explicitly mentioning my radio amateur call sign "pa0nhc" as the original writer / designer / photographer / publisher.
These RF couplers are usable between 28 MHz and 435 MHz, and designed to have abt. -30dB @ 145 MHz output (30dB coupling loss) in respect to the main 50 Ohms thru-line. The value of their coupling loss rises 6dB / octave towards lower frequencies, and vice versa.
The 50 Ohms terminating resistors will dissipate 1W heath when 7,07Vrms is induced into them.
On 145 MHz the coupling loss is abt 30dB (1/1000
power). The maximal power input is 1kW.
On 435 MHz, the coupling loss is 3 times less : abt. 20dB (1/100 power).. The maximal power input is 100W.
On 29 MHz the coupling loss is 5 times higher (44dB). With 1kW input abt. 1.4Vrms is induced into the coupling lines.
REM : For 1kW input, i suggest to fill the tinned part of the through-line
with a layer of solder.
If such couplers should be used below 29 MHz, the coupling loss becomes very high. To compensate for this, the coupling lines could be made longer to decrease coupling loss to abt. 30dB at the frequency of interest. This will result in long lines (for instance 20cm @ 14 MHz), a long PCB, a long enclosure, and higher costs.
If such couplers should be used at frequencies higher than 435 MHz, the coupling space between the lines should be enlarged to increase coupling loss to abt. 30dB..
DC output values :
At the prototype DC output versions, loaded with 1Mohm or higher value, the output of the forward port at 145 MHz should be abt :
- abt. 0.1 Vdc output at abt. RF 1W input
- abt. 9Vdc output at abt. 800 W RF input
When the ports are loaded with lower value DC resistance, low values RF power will give lower readings, due to the diode characteristics and the voltage drop over R2/102. Readings will be less accurate at lower power levels.
REM : Best accuracy could be obtained, by
using an RFcoupler in combination with very accurate RMS RF detectors :
See : pa0nhc RF watt meter 20170808 v18
Here are six designs presented.
Four, especially shaped PCBs, will fit with 0.5mm free space inside RF-tight TEKO 371 and 372 1mm steel enclosures. Connections are four short length BNC busses (for max wall thickness 3.5 mm) with PTFE (Teflon (R)) insulation, nut mount, and each with two long ground lugs.
Installation in plastic enclosures is possible, if a little RF radiation is no problem.
The two PCBs without BNC busses are designed to be installed inside equipment, with coaxes soldered to the PCB. It is wise to wind a few windings of each coax through a Fair-Rite mix #31 core to bloch common mode currents.
The bottom copper of the PCBs is full mass plane. A number of carefully placed wide-hole-vias ensure good RF inter connections between top and bottom ground surfaces. The mass planes must only be connected to the mass of the BNC busses via two ground lugs at each BNC bus. If the current capacity of the through line track need to be improved, it can be filled with a thicker layer of solder tin. They intentionally are partly kept free from solder mask.
Each terminating resistor is one SMD resistor 50 Ohms 1% 1W size 2010, or are two 100 Ohm 1W size 2010 SMD resistors in parallel, soldered on top of each other (totally 50 Ohms 2W).
The detector diode is a Skottky UHF type, with very low threshold voltage, if loaded with 100 kOhms or higher value. If available, use a BAT62 (single diode, white stripe = k). A double diode version of it BAT62-03W can also be used (with one diode not connected). The PCB shows where the wide leg (anode) should be located. See picture ==> .
The enclosures are TEKO type 371 or type 372.
The BNC busses are short types (for max. 3.5mm wall thickness), with nut, lock ring, PTFE (Teflon (R)) insulation and each two long grounding lugs. For mechanical and electrical stability, their rim could be soldered directly onto the outside of the box.
R1/101 is 50 Ohms 1% 1W or two 100 Ohms
soldered on top of each other (parallel) size 2010.
DC output types (see schematic an parts table ==>).
D1 is a single diode BAT62, or (one half of) a double diode version BAT62-03W.
Observe the location of the Cathode on the PCB (k is connected to C1/101).
All other components size 1206.
Solder all components to the PCB, exept the diode..
Prevent static damage to semi conductors.
==> Connect the soldering iron and your body to a mass plane of the PCB.
Then quickly solder the diode as last. Use a soldering iron temperature of 260 C.
Before installing the BNC busses,
put the PCB in the enclosure first.
The PCB will be installed with the top copper surface against the underside of the BNC pins, and the groundlugs onto the top copper.
Before soldering a BNC bus, temporally install a BNC plug in it, to cool and center the pin.
After installing the BNC busses,
- solder the PCB traces (= TOP !) to the UNDER side of the BNC pins.
- solder both ground lugs of each BNC bus to their grounding surfaces on the top copper of the PCB.
- if high power should be measured, you could fill the blank parts of the RF input and RF output tracks with extra solder to lower their working temperature.
The RF couplers should show abt. 30 dB reflection damping at 145 MHz, when a good terminating resistor is connected to the main signal line "Out".
All my PCBs are of military quality, and contain component layout, solder masks and informative texts.
The drawings below show dimensions for installing these PCBs inside TEKO boxes.
Dimensions in millimeters.
Drill the BNC holes at a distance of 10mm from the bottom.
Small coupler with RF output.