G8MNY  > TECH     31.03.25 09:15z 93 Lines 4280 Bytes #100 (0) @ WW
BID : 29243_GB7CIP
Subj: RF Directional Coupler
Path: ED1ZAC<ED1ZAC<GB7CIP
Sent: 250331/0909Z @:GB7CIP.#32.GBR.EURO #:29243 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                       (Updated Jul 19)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
This is an item used for VHF broadcast transmitter monitoring. It provides a
low level calibrated RF sniff point in the aerial coax for Ofcom (UK regulator)
spectrum analyser etc. without turning the Tx off. It is not generally used by
other RF users!
             ÚÄÄÄÄÄÄÄÄ¿
             ³Spectrum³
             ³Analyser³
             ÀÄÄÄÂÄÄÄÄÙ
 Aerial        .ú'ú.
 System    To Ant  To Tx
   \³/     ÚÄÁÄÄÄÄÄÄÄÁÄ¿   ÚÄÄÄ¿
    ÀÄÄÄÄÄÄ´DirectionalÃÄÄÄ´Tx ³
           ³  Coupler  ³   ÀÄÄÄÙ
           ÀÄÄÄÄÄÄÄÄÄÄÄÙ
CIRCUIT
This is quite a straight forward 3 line directional coupler as used in some VHF
SWR bridges, but here all the lines are very accurate 50ohm transmission lines.

          To Aerial                         Loss
            BNC                              dB
          ÚÄ(o)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿            -16 ´                      _.-
 Aerial   ³  ÀÜÜÜÜÜÜÜÜÜÜÜÜÜÜ50R´   Tx       -22 ´                  _.-'
N Female o)ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ(o N Male    -28 ´              _.-'
          ³  .ÜÜÜÜÜÜÜÜÜÜÜÜÜÜ.  ³            -34 ´          _.-'
          Ã50R              ³  ³            -40 ´      _.-'
          ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ(o)ÄÙ            -46 ´  _.-'
                           BNC              -52 ´-'
                          To Tx             -58 ÅÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂÄÄÄÂ
                                               12   25  50 100 200 400 800MHz

Such couplers do not have a flat response, they typically have a 6dB per Octave
rise until they are near a resonance or some other losses affect them. Ofcom
said some spectrum analysers have calibration offset parameters that can allow
for this? (I have never seen one)

A HOMEMADE ONE
Simpler one I made up was from PCB..

         To Aerial     To Tx
            BNC         BNC
          ÚÄ(o)ÄÄÄÄÄÄÄÄÄ(o)Ä¿
 Aerial   ³  ÞÜÜÜÜÜÜÜÜÜÜÜÝ  ³   Tx
N Female o)ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ(o N Male
          ³                 ³
          ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
The 'N' connectors were bolted to 1" x1" PCB plates, the whole double sided PCB
box soldered up around them & the double sided PCB lines inside that. The 50ohm
double sided PCB lines were salvaged from an old VHF SWR detector bridge. But
to make a really good accurate match, it needed solder added to them to get a
deep SWR null >30dB, on a good load. For this single pick up line design a 50R
BNC terminator plug is used in the unused port.

IN USE
There are accuracy limits to all test gear, here if the transmission system is
not 50ohm at the frequency of interest & the spectrum analyser results can be
meaningless! This is generally the case off frequency on a resonant aerial.

Actual power measurement is possible provided all the patch lead losses are
known & the match is good.

However the directional coupler does have the ability to show up which of any
RF mixing signals are internal & external to the Tx! So nearby corroded metal
joints, or other nearby transmitter PAs acting as a mixer etc, can easily be
identified as "QRM not in the local Tx". So it is very useful when tracking
down the real cause of QRM.

³ Local²                  ³                         ³
³    Tx²                  ³   Local                 ³   Local
³      ²                  ³    Tx²                  ³    Tx²
³      ²                  ³      ²   Other          ³      ²   Other
³      ²   Other          ³      ²    ÞTx           ³      ²    ÞTx
³      ²    Tx            ³      ²    Þ             ³ Mix  ²    Þ   Mix
³      ²    Þ             ³      ²    Þ             ³  ³   ²    Þ    ³
³ Mix  ²    Þ   Mix       ³      ²    Þ             ³  ³   ²    Þ    ³
³  ³   ²    Þ    ³        ³ Mix  ²    Þ   Mix       ³  ³   ²    Þ    ³
ÀÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄ    ÀÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄ    ÀÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄ
 Tx Port Spectrum          Aerial Port Spectrum       Aerial Port Spectrum
                          Showing Local Tx Mixing    Showing External Mixing
                         (mix smaller than Tx Port) (mix bigger than Tx Port)


See my tech bul on "FM Stereo Radio Principles", & "A Versatile Pulse Tester"


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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