Return Loss sweep using IC7410, RL bridge, and RFPM1

This article demonstrates an automated Return Loss scan of an antenna using:

Automation facilitates:

  • measurement of a large number of data points;
  • improved accuracy by reducing the risk of recording errors; and
  • reducing the tedium of a measurement task.

Benchmark

The objective is to emulate what is achieved with a Spectrum Analyser with Tracking Generator, or Sweep Generator with logging power meter.

22 002

Above is a Return Loss scan of the antenna using a Spectrum Analyser with Tracking Generator and RLB. Essentially, this is the measurement to replicate with the simpler ham shack equipment.

Test configuration

Screenshot - 05_11_2015 , 15_38_48

Above is a block diagram of the test configuration.

The PC controls the IC-7410 using the built in USB CIV port. The RF output of the IC-7410 is attenuated to a level safe for the RLB. The the antenna system is connected to the DUT port of the RLB, and the other port connected to the RFPM1 power meter. The RFPM1's USB data logger is connected back to the PC to allow automated recording of the power level.

Automation

The test is controlled by a script on the PC.

@echo off
rem :more

rem RIGCTL parameters
set MODEL=367
set PORT=COM15
set SPEED=4800

rem scan loop parameters
set freq=7000000
set inc=10000
set steps=21

set RIGCTL="C:\Program Files\hamlib-w32-3.0\bin\rigctl"
set RIGCTLP=%RIGCTL% -m %MODEL% -r %PORT% -s %SPEED%

del capture.txt

rem main scan loop
for /L %%i in (0,1,%steps%) do call :measf %%i
%RIGCTLP% set_ptt 0"
goto l1

rem subroutine to make each measurement
:measf
%RIGCTLP% set_mode RTTY 300 set_freq %freq% set_ptt 1
rem sleep 1
rem measure
set meas=
set /p meas="Measure %freq%: "
rem set /a result=%RANDOM%
echo %freq%    %meas%
echo %freq%    %meas% >>capture.txt
set /a freq=%freq%+%inc%
exit /b

echo falling through

:l1
set meas=
set /p meas="Stop HID input then ENTER to quit:"
echo meas :%meas%
if not *%meas%==* goto l1

:end

Above is an example Windows bat file to orchestrate the measurement. The script sends configuration commands to the transmitter, and waits for the measurement result and writes it to a log file.

#!/bin/bash

#RIGCTL parameters
MODEL=367
PORT=/dev/ttyUSB0
SPEED=4800

#scan loop parameters
freq=7000000
inc=10000
steps=21

RIGCTL="/usr/bin/rigctl"
RIGCTLP="$RIGCTL -m $MODEL -r $PORT -s $SPEED"

#echo RIG: $RIGCTLP

>capture.txt

trap "break;" SIGINT

for ((i=$steps;i;i--)); do
  $RIGCTLP set_mode RTTY 300 set_freq $freq set_ptt 1
  read -p "Measure $freq: " obs
  if [ -z "$obs" ]; then
    break
    fi
  #timestamp=`date +"%Y%m%d%H%M%S"` 
  #echo -e "$timestamp\t$obs"
  echo -e "$freq\t$obs"
  echo -e "$freq\t$obs" >>capture.txt
  freq=$(($freq+$inc))
  done

#loop until the HID input is turned off
while true; do
  read -p "Stop HID input then ENTER to quit:" obs
  if [ -z "$obs" ]; then
    break
    fi
  done

trap - SIGINT
$RIGCTLP set_ptt 0

Above is an example *nix Bash script to orchestrate the measurement. The script sends configuration commands to the transmitter, and waits for the measurement result and writes it to a log file.

Execution

The measurement involves three phases:

  1. an ‘open' calibration scan with nothing attached to the RLB DUT port (0dB RL);
  2. a ‘measurement' scan with the DUT attached; and
  3. calculation and reporting of the difference between 1 and 2, RL, and from that, VSWR.

Results

Clip 053

Above is a plot of the calculated Return Loss and VSWR.

There are a couple of bumps in the graph which could be reduced by averaging several readings (say 10) in the script file, an exercise for the reader.

Conclusions

An automated Return Loss sweep using a CIV controlled transmitter, suitable power attenuator, and the RFPM1 with data logger is quite feasible.

References / Links