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Pure Signal Power Tap for Hermes Lite 2. pa0nhc UPDATE 20210514.
(Copyright) The use, copy and modification of all info on this site is only permitted for non-commercial purposes and thereby explicitly mentioning my radio amateur call sign "PA0NHC" as the original writer / designer / photographer / publisher.

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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 spectrum. 

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 correction. 

As the Power Tap output (1.3Vrms @ 100W and 5Vrms @ 1600W) will mostly be to strong, an external attenuator is needed in (or at) 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) coax.

 I recommend a box  with small width / height dimensions, to keep internal RF path lengths short as possible. 

I 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.

An 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.
     Drilling holes.
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).

The hole 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).

The hole 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 OrderNr. 5961000601 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
(see photo).
If the phase of the output signal needs to be changed,
then exchange the wire posistions at the BNC.


The ferrite coil is only MAGNETICALLY coupled with the CENTER conductor of the coax.

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. 

Installing the plugs, coax  and coil:
Use good quality connectors with PTFE insulation. Put a plug on the soldered receptacle to keep the center pin/bus centered.

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 box.
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 ground surface 
(see photos).
11. After testing, fix the coil to the coax with glue, to prevent changes of performance. 
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 RG58 cable.

REM : Y-scale ranges below are sometimes very small.

Frequency characteristics 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 : 6dB.

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 return loss.
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.

 


Attenuator details for use with the Hermes Lite2 SDR TRX.
As at the moment Pure Signal in the by me used SDRconsole does not work, figures below are calculated.

The TRX PCB and the LPF PCB in the HL2 are interconnected by a 2-row IDC connection strip ("STR" in the schematic).
A HL2 Pure Signal input SMA or BNC jack (including 10cm thin coax) is installed at the HL2 rear panel in hole "RF3".
I installed the attenuator inside the Hermes Lite2 ("HL2") box, connected to the RX input at STR pins 3+4 and 6.  (see schematic).
Solder the coax screen to  STR pins 3 and 4, and the coax center to the connection between attenuator resistors R1 and R2.
The HL2 RX input saturation level is about 280mVrms. A Pure Signal sample input of about 200mVrms peak seems to be adequate.

REM : In may 2021 SDRconsole still had no pure signal possebility.

The HL2 Rx inputZ at "STR" = 50 Ohm.
Zstr // R3 = 43,4 Ohm.

According to measurements by DL1YCF can be calculated, that the saturation level of Rxinput @STR is 282 mVrms. 
200mVrms peak input should therefore be adequate. 

Calculated values for R2 and RX input voltages for about 200mV peak at STR pin6 :

Ptx
[Wrms]
Utap
[Vrms]
Ptap
[mWrms]
R2
[Ohm]
Att
[X]
Att
[dB]
Urx-in
(@STR)
[mVrms]
100 1.257 31.6 220 6.1 16 206
400 2.514 126 470 11,8 21.5 213
800 3.555 253 680 16.7 24.5 213
1600 5.028 506 1000 24 27.5 210

The power tap ferrite core :
The advised Mix#61 core material is optimal for magnetically coupled circuits (transformers) up t
25 MHz. It has very low losses up to very high temperatures (> 200C) and a constant
permeability (Ui).

Wind all turns if the coil in one layer, side-by-side, on a FairRite OrderNr. 5961000601 core, or Amidon FT82-61 ring core, 27 times through the hole, with 1.2mm outer dia. insulated hookup wire. 

 


Part of Hermes LIte2 schematic with "RF3 Power Tap input", with extra internal attenuator R1/R2/R3.
For actual R2 values see table above.