IZ3LSV

G8MNY  > TECH     22.03.13 00:00l 86 Lines 4030 Bytes #999 (0) @ WW
BID : 15861_GB7CIP
Read: GUEST
Subj: RF Directional Coupler
Path: IZ3LSV<IV3YXW<I3XTY<I0OJJ<GB7CIP
Sent: 130321/1309Z @:GB7CIP.#32.GBR.EU #:15861 [Caterham Surrey GBR] $:15861_GB
From: G8MNY@GB7CIP.#32.GBR.EU
To  : TECH@WW

By G8MNY                                       (Updated Nov 12)
(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 the regulator's
spectrum analyser etc. without turning the Tx off. It is not generally used by
other RF users!
            ÚÄÄÄÄÄÄÄÄ¿
            ³Spectrum³
            ³Analyser³
            ÀÄÄÄÂÄÄÄÄÙ
 Aerial        / '.
 System   To Ant   To Tx
   \³/     ÚÄÁÄÄÄÄÄÄÄÁÄ¿   ÚÄÄÄ¿
    ÀÄÄÄÄÄÄ´DirectionalÃÄÄÄ´Tx ³
           ³  Coupler  ³   ÀÄÄÄÙ
           ÀÄÄÄÄÄÄÄÄÄÄÄÙ
CIRCUIT
This is quite a straightforward 3 line directional coupler as used in some VHF
SWR bridges, but here all the lines are very accurate 50ê transmission lines.

          To Aerial                         Loss
            BNC                              dB
          ÚÄ(o)ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿            -17 ´                      _.-
 Aerial   ³  ÀÜÜÜÜÜÜÜÜÜÜÜÜÜÜ-50´   Tx       -22 ´                  _.-'
N Female o)ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ(o N Male    -28 ´              _.-'
          ³  _ÜÜÜÜÜÜÜÜÜÜÜÜÜÜ_  ³            -34 ´          _.-'
          Ã50               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. Some
spectrum analysers have calibration offset parameters that can allow for this.

HOMEMADE
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 match, it needed solder added to them to get a deep
SWR null on a good load. For this single pick up line design a 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 50ê at the frequency of interest the spectrum analyser result is
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 RF
mixing faults are internal & external to the Tx. So nearby corroded metal
joints or other nearby transmitter PAs acting as mixers etc, can be identified.
So it is very useful when tracking down the real cause of QRM.

³ LocalÞ                   ³                          ³
³    TxÞ                   ³   Local                  ³   Local
³      Þ                   ³    TxÞ   Nearby          ³    TxÞ   Nearby
³      Þ   Nearby          ³      Þ    ÞTx            ³      Þ    ÞTx
³      Þ    Tx             ³      Þ    Þ              ³ Mix  Þ    Þ   Mix
³      Þ    Þ              ³      Þ    Þ              ³  ³   Þ    Þ    ³
³ Mix  Þ    Þ   Mix        ³      Þ    Þ              ³  ³   Þ    Þ    ³
³  ³   Þ    Þ    ³         ³ Mix  Þ    Þ   Mix        ³  ³   Þ    Þ    ³
ÀÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄ     ÀÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄ     ÀÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄ
   Tx Port Spectrum        Aerial Port Spectrum       Aerial Port Spectrum
                           Showing Local Tx Mixing    Showing External Mixing
                           (mix less than Tx Port)    (mix more than Tx Port)


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


Read previous mail | Read next mail