IZ3LSV

G8MNY  > TECH     12.11.12 14:42l 85 Lines 3943 Bytes #999 (0) @ WW
BID : 672_GB7CIP
Read: GUEST
Subj: RF Directional Coupler
Path: IZ3LSV<IR1UAW<IW2OHX<OE6XPE<UA6ADV<GB7CIP
Sent: 121112/0028Z @:GB7CIP.#32.GBR.EU #:672 [Caterham Surrey GBR] $:672_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To  : TECH@WW

By G8MNY                                       (New Aug 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¿
    ÀÄÄÄÄÄ´DirectionalÃÄÄ´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, I needed to add solder to them to get
a deep SWR null on a good load. For the single pick up line a BNC terminator
plug is used in the unused port of course.

IN USE
There are accuracy limits to all test gear, here if the transmission system is
not 50ê at the frequency of interest the results are meaningless, & this is
generally the case on a resonant aerial off frequency! Actual power measurement
is possible provided all the patch lead losses are known & the match is good.

The directional coupler does have the ability to show up RF mixing faults that
are external to the Tx. Like nearby corroded metal joints or other nearby
transmitter PAs acting as mixers etc. So it is very useful when tracking down
QRM.

³ LocalÞTx                 ³                          ³
³      Þ                   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
                            (less than Tx Port)        (more than Tx Port)


Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP


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