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G8MNY  > TECH     13.12.24 08:19z 187 Lines 10015 Bytes #171 (0) @ WW
BID : 21790_GB7CIP
Read: GUEST
Subj: QRO 1kW HF Metered Dummy Load
Path: SR1BSZ<IW0QNL<IZ3LSV<I0OJJ<GB7CIP
Sent: 241213/0815Z @:GB7CIP.#32.GBR.EURO #:21790 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                    (Updated May 24)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

This is a peak reading metered QRO dummy load, & with a low level RF monitor.

SCHEMATIC
                      ________         _________
RF o)ÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÄ´Peak    ÃÄÄÄÄ>o´Range CalÃÂÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄ¿
in  ³    ³    ÚÄÁÄÄ¿ ³Detector³Range ³ÃÄÄÄÄÄÄÄÄÄ´³ ÚÄÄÁÄÄ¿ ÚÄÄÄÄÁÄÄÄÄÄ¿ ÚÄÄÁÄÄ¿
    ³ ÚÄÄÁÄÄ¿ ³1kW ³ ÀÄÄÄÄÂÄÄÄÙ SW   À´ Speed upÃÙ ³Meter³ ³   Meter  ³ ³Meter³
RF o)Ä´Atten³ ³Load³      ³           ÀÄÄÄÄÄÄÄÄÄÙ  ³Damp ³ ³Protection³ ÀÄÄÂÄÄÙ
mon ³ ÀÄÄÂÄÄÙ ÀÄÂÄÄÙ      ³                        ÀÄÄÂÄÄÙ ÀÄÄÄÄÂÄÄÄÄÄÙ    ³
    ÀÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÙ

LAYOUT
It uses a commercial tube 1kW 52ohm coaxial long 5cm dia x 30cm load, mounted
in a perforated steel panelled screwed together metal box 43 x 13 x 13cm.

      Ú - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ¿ -40dB RF
      ³                                                           75³__Monitor
      ³-----¿      RF HOT                           Earth clamp   Ã-|__| BNC
     _³     |   |~|___Clamp                                  ===----³Ä³U EARTH
    ³M³ PEP |   |S³°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°  ² ºAtten
    ³E³     |   |U³            15mm copper tube       spacer| pin ± º__
    ³T³METER|   |PÀÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ/_/_³   ³COAX
    ³E³     |   |PÚßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßß ~\~³___³SO239
    ³R³ CAL |   |O³<tube                                    |    ßÜ º
    ³_³     |   |R³°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°° ßÜ ºdiodes
     _³ CCT.|   |T|~~~            52ohm tube load            ===--³-³U EARTH
    ³ ³-----Ù    ~                                                C1³
    ³_³_³:                                                          ³
Range À - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Ù
Switch

The input socket is bolted through a 2cm wide U shaped copper earth strip, that
is clamped on to the outer of the dummy load with a capacitor type clamp. The
socket centre is soldered to the monitor attenuator R, diode probe stack, & a
4mm banana plug pin that engages into a banana socket soldered to the 15mm dia.
copper centre tube. This tube forms the centre of the "5cm dia coax" load to
make it low radiating. It is supported inside the load at the pin end with a
card spacer to hold it central for assembly.

The copper tube's far end is split & flatted out to make 2 long tags that are
bolted to the "RF hot clamp". This clamp has an insulated paxalin (copper less
PCB) sheet support from the case bottom with a metal right angle strip.

CIRCUIT                               - - - - - - - - - - -
                                     | ÚÄR5ÄÄRV1Ä¿ CAL CCT |
            Peak Detector           1kW³   C2    ³  BOARD  |
                              Range oÄÄÁÄÄÄ´ÃÄÄÄÄ´         |
     Ú´>ÃÄ´>ÃÄ´>ÃÄ´>ÃÄ´>ÃÄ´>ÃÂÄÄoÄÄ> |           ³  Meter  |
Input³        D1-6           ³       | ÚÄR6ÄÄRV2Ä´  Damper |
  o)ÄÁÂÄÄÄ¿                  ³      oÄÄ´   C3    ÃÄÄÄÄÂÄÄÄÄÄÄÄÄÄÂÄÄÂÄÄÄÄ¿
239³  R1  ³                  ³     100WÀÄÄÄ´ÃÄÄÄÄ´   +³    |   _³_ ³   +³
   ³  ³   R4                 ³       |           ³   ===   |   \_/ ³C6 _³_
   ³  R2  ³ DUMMY        C1 ===     oÄÄÂÄR7ÄÄRV3Ä´  C5³    | D7 ³ === /   \
  o)ÄÄ´   ³ LOAD             ³     10W ³   C4    ³    ³    | D8_³_ ³ ³Meter³
BNC³  R3  ³                  ³       | ÀÄÄÄ´ÃÄÄÄÄÙ   RV4   |   \_/ ³  \___/
   ³  ³   ³                  ³       |                ³    |    ³  ³    ³100uA
 RFÀÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÁÄÄÄÄÙ
Mon Attenuator                        - - - - - - - - - - -      Meter
                                       Cal & Speed up          Protection

COMPONENTS
No   Resistors     Capacitors      Diodes        Preset RV       Misc
1   * 3K9 3W        0u33 400V   BYW29 200V 35nS     1M      Meter 100uA 1kê
2   * 3K3 2W      @ 23n  1kV         "             330K     BNC chassis mount
3   * 75R         @ 68n  400V        "             100K     SO239 chassis mount
4   * 52R 1kW     @ 150n 400V        "              10K     Switch 1 pole 3 way
5     2M2 0.25W   @ 10u  10V         "                      Perf Boxing panels
6    820K 0.25W   $ 1nF  25V         "                      Nuts & bolts
7    220K                        $ 1N4001                   4mm Plug & Socket
8                                $ 1N4001                   15mm í Copper tube

* = non inductive type,   @ = for my meter ballistics,  $ = mounted on meter

RF MONITOR
This give about -40dB output (-46dB terminated) for scopes or analysers, & uses
just 3 Rs that are laid down close to the metal panel to reduce extra RF
pickup. The output impedance is 75R. It should be noted that the peak reading
diodes will cause some even harmonics to appear on suddenly increasing signal
levels as the peak dummy load RF actually gets slightly clipped.

PEAK READING
The 6 fast recovery tab type diodes, give a 1200V PIV 8A RMS 35nS detector.
They all have their tabs sawn off (tin snips cuts I found damaged them!) Other
slower diode types easily blow up in this peak reading application! The diodes
are soldered in series (1/2 about face & upside down) & have individual heat
shrink over the tab remains so there in no chance of shorting out.

ÚÄÄÄÄ¿            all sleeved
³ () ³  x6    ÚÄÄÄÄ¿ÚÄÄÄÄ¿ÚÄÄÄÄ¿  LOAD
ÃÄÄÄÄ´ gives  ³____³³____³³____³  /
³____³         |-+|-+|-+|-+|-+|-+|
 |  |       C1/  ³ÜÜÜܳ³ÜÜÜܳ³ÜÜÜܳ
                  ßßßß  ßßßß  ßßßß
           ÄÄ´<ÃÄ´<ÃÄ´<ÃÄ´<ÃÄ´<ÃÄ´<ÃÄÄ

C1 is mounted on a tag around the input socket so there is next to no lead
inductance to give false readings. There are very high pulse currents into C1
on applying sudden RF, as the source Z is actually 25ohm & there could be >300V
peak, that is 12A peak current on a 1kW sudden RF pulse! The meter load is only
100uA max (1/100,000) so there is no real loading loss. C1 has to store the
peak value until the next speech peak not just the RF peak, hence it is very
large value for a RF detector!

The long DC lead away from C1 to the meter switch is glued to the metal base
plate to kept away from the dummy load R4, so there is little stray RF pick up
& no chance of it getting burnt too.

The resultant DC + diode losses is then exactly the AC peak value.

              2         But sine wave                   2
          Vrms          crest power                  Vpk
 POWER = -----          is 2x RMS so...     POWER = ----
           R                                         100 (for 50ohm)

Leads to & from the calibration circuit matrix board are also kept close to the
front metal case as possible, to reduce RF pick up from the load.

CALIBRATION
From above, the 3 meter scales 0-10W, 0-100W & 0-1kW can be accurately
calibrated from 1st principles. The meter scale 0-100% needs to be drawn in
Watts. e.g. a square law scale...

%SCALE   VOLTS  WATTS      VOLTS  WATTS        VOLTS  WATTS  CORRECTED
  0        0      0          0      0            0      0        0
  5       17      3          5      0.3          1.7    0.03     0.1
  7       22      5          7      0.5          2.2    0.05     0.2
 10       32     10         10      1            3.2    0.1      0.3
 14       45     20         14      2            4.5    0.2      0.5
 17       55     30         17      3            5.5    0.3      0.6
 23       71     50         23      5            7.1    0.5      1
 32      100    100         32     10           10      1        2
 39      123    150         39     15           12.2    1.5      2.5
 45      141    200         45     20           14.1    2        3
 50      158    250         50     25           15.8    2.5      3.5
 54      173    300         54     30           17.3    3        4
 63      200    400         63     40           20      4        5
 71      224    500         71     50           22.4    5        6
 78      245    600         78     60           24.5    6        7
 84      265    700         84     70           26.6    7        8
 89      281    800         98     80           28      8        8.7
 94      300    900         94     90           30      9        9.5
100      316   1000        100    100           32     10       10
           CAL RV1           CAL RV2                CAL RV3

The diode loss of about 3V is almost insignificant except on the 10W scale &
the correction can be added in if you wanted to make that scale a special one.

A 1kW variac on mains with a DVM meter will confirm the calibration to 1000W,
as the RF power meter is quite flat from about 20Hz to 30MHz.

METER BALLISTICS for PEP measurement.
An attempt has been made in this circuit to use both meter damping to stop
overshoots with C5 & RV4 (terminate the meter movement) & also provide meter
speed up capacitors C2, C3 & C4 across the series calibration ratio Rs. My bul
on "Meter Damping & Speed Up" explains the principles in detail.

The capacitor values used are ONLY correct for my meter movement. Using a slow
RF step pulse of known square envelope (scope it), you can determine best
values for both of these, so there is no overshoot, but still a very fast rise
time.

CONCLUSION
The result here is a very accurate peak reading meter up to 30MHz. Above that
the coax line load & diode detectors all start to go wrong & can't be trusted!

If the RF has a high harmonic content (e.g. worse than -20dB) then there will
be quite an error in the power reading as the peak voltage will be wrong!

I have used the load for testing Ham HF amps up to 1kW & soak testing 531kHz AM
Tx at up to 800W PEP when it did get very hot!


See my tech buls "PEP Meter modification", "Meter Damping & Speed Up",
"AF 2 Tone Test Osc Design". "PA instability in ICOM IC735". "QRO Dummy Loads",
& "QRP Power Meter & Dummy Load".


Why Don't U send an interesting bul?

73 De John, G8MNY @ GB7CIP


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