|A power tap retrieves a sample
of the output of an RF power amplifier. The for the driving transmitter
used software compares the transmitter output spectrum with the power
amplifier output spectrum, and eliminates the differences in both
spectra. The distortion generated by the power amplifier is then
eliminated. A purer amplifier power output is the result. The amplifier
output spectrum is then (nearly) equal to the driving transmitter output
This Power Tap does not contain an internal attenuator,
ensuring a relative high value of RF voltage sample, with the
best interference insensitivity. The attenuation by this Power Tap is
nearly 35dB (3000x less power, 1/2W @ 1.5 kW), loaded with an internal
power tap 50 Ohm load, plus an external 50 Ohms (attenuator) network.
My circuit ensures good match at both
connecting coax ends. Resulting
in high and very constant output voltage at the end of the connecting coax
(+/- 0.3 dB @ 0.2MHz - 50 MHz), and the best possible Pure Signal
As the Power Tap output (1.3Vrms @
100W and 5Vrms @ 1600W) will mostly be to strong, an external attenuator is needed in
the TRX pure signal input. See the attenuator table and the
modification for a Hermes Lite2 at the bottom of this page
Cheap BNC plugs sometimes have bad
contact resistances between grounding contacts. To achieve minimal common mode
current problems, use only high
quality plugs and jacks with high temperature resistive PTFE
insulation, and good (double screened)
I recommend a box
with small width / height dimensions,
internal RF path lengths short as possible.
used a tinned steel box as housing for the power tap, in order to be able to solder the connectors
onto the outside of it.
Solder all seems at the inside of the box, at
about 2cm intervals. Use a 120W solder iron at
400C. As the closing lid cannot be soldered
at the inside, be sure that the solder flows there in-between the box outer
surface and the inner surface of the lid. See sketch.
aluminum die cast box could also be used. Be then sure, that the grounded
parts of the N- and BNC connectors make good electrical contact only
with the outside of the box. Tap thread into the box
for the screws to achieve good ground contact.
Construction in a tin plated steel sheet box.
The hole diameter for the receptacles depends on
the connector version.
The hole diameter for the (left hand) N-male plug :
The diameter of the hole in the box =
the diameter of the hole in the inside
of the N-connecor nut (about 11 mm).
diameter for the (right hand) N-female receptacle :
The diameter of the hole in the box =
the diameter of the PTFE insulation in the flange
(about 11 mm).
diameter for the BNC-female receptacle is about 10 mm.
The coil :
The coil is a current transformer. More turns = less output
current (and less RFvoltage over the totally 25 Ohms load).
Wind 27 turns side-by-side, in one layer, on a FairRite
core, or Amidon FT82-61 ring core.
Only use mix #61 ferrite.
Wind 1.2mm outer dia. insulated hookup wire 27
times through the ferrite core hole.
The output load resistor is 50 Ohms. For powers up
to 2.5 kW, a 1W low inductance resistor should do.
Or use 2x100 Ohms 1/2W low
inductance in parallel, or 2x 56 + 2x47 ohms 1/4
W low inductance in parallel.
TWIST both coil ends firmly.
Solder both wire
ends DIRECTLY onto the BNC inside end. Solder one wire to its center pin, and the
other wire to the outer grounding
If the phase of the output signal needs to
then exchange the wire posistions at the BNC.
The ferrite coil is only MAGNETICALLY coupled
with the CENTER
The piece of coax screening solely functions as a static screen
between the the coupling coil and the current carrying coax center.
The screening of the coax is only at ist
beginning connected to the inside of the
male plug, and at its end insulated, is there not
connected to ground.
NO RF current runs
therefore in the coax screening.
Assuming that the RF current in the
main power channel flows at the center conductor from source to load, the
return current from load to source can run only back through the
inside-surface of the box, as the screen of the coax is
interrupted. Its lenght can be of influence to the results.
To keep the return-path-length at the inside
surface of the box short, a small box with small widths is preferred.
plugs, coax and coil:
Use good quality connectors with PTFE
insulation. Put a plug on the soldered receptacle to keep the center
1. Solder the nut of the N-male
plug to the outside of the box, centered to its hole in the box.
Solder the N-female receptacle to the outside of the
box, with its flange centered to
its hole in the box. Solder the BNC-female receptacle to the outside of the
2. Pre-install a (RG214) coax piece into the remaining plug
body of the N-male, and roughly cut it to the width of the box.
3. Shift this coax piece through the hole in the box, then temporarily tighten
the plug, and cut the coax at exact length.
4. Then unscrew the plug.
5. a. Remove 5mm at the end of its screening.
b. Remove 3mm at the end of its inner insulation.
6. Shift the coax piece through the hole in the box,
and shift the ferrite coil over the coax.
If the coil fits to loosely over the coax, then wind some insulation tape
over the coax, until the coil fits snugly and centered over the coax.
7. FIRMLY tighten the N-plug body onto its soldered nut. It may
not loosen during later usage.
8. Solder the coax center wire to the pin of the female N-bus.
REM :This end of the coax screening must nowhere be connected.
9. Twist the coil ends, trim them and solder them directly to the BNC pin and
11. After testing, fix the coil to the coax with glue, to prevent changes
12. Solder the lid.
|The completed "Power Tap" is here connected to the
antenna (output) bus of my PA.
As no RF radiation problems existed caused by common mode
currents, no Common Mode Chokes needed to be installed over the coaxes.
|Power tap measuring setup.
The (Nano) VNA was first calibrated including 1.5m
REM : Y-scale ranges below are sometimes very small.
are very flat.
Results < 50MHz :
Coupling Main to BNC-out.
REM : ZOOMED, only
1dB Y-scale range !
Flat response :
0.2 - 50 MHz : -34.7 +/- 0.3 dB.
Ref = 21MHz :
30 MHz : + 0.034 dB.
50 MHz : + 0.304 dB.
|Response < 1MHz Main to BNC-out
(Ref = 21MHz )
Y-scale range :
REM : Below 100 kHz characteristic of the NanoVNA
itself also falls.
0.20 MHz : - 0.304 dB.
0.65 MHz : + 0,015 dB.
|Red : Main channel
Blue : BNC-output.
Y-scale range 15dB.
|Main channel loss.
Max 0.25 dB including 1.5m RG58.
REM : ZOOMED-IN, only
2 dB Y-scale range !
|Blue : BNC-out
coupling showing the
VHF resonance in the coupling coil at 121.5 MHz.
Useable output frequency range :
100kHz - 75 MHz +/- 1dB.
Red : Return
loss of the main
channel with 1.5 m RG58 coax connected.