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G8MNY > TECH 07.02.08 23:01l 166 Lines 8114 Bytes #999 (0) @ WW
BID : 45962_GB7CIP
Read: GUEST
Subj: QRO 1kW HF Metered Dummy Load
Path: IZ3LSV<IW2OHX<OE6XPE<DB0RES<ON4HU<ON0BEL<GB7CIP
Sent: 080207/0024Z @:GB7CIP.#32.GBR.EU #:45962 [Caterham] $:45962_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Oct 07)
(8 Bit ASCII Graphics use code page 437 or 850)
This is a peak reading meter, QRO dummy load, & with a low level RF take off.
It uses a commercial 52ohm 1kW coaxial 5cm dia x 30cm long load, mounted in a
perforated screwed together panelled steel metal box 43 x 13 x 13cm.
LAYOUT
Ú - - - - - - - - - - - - - - - - - - - - - - - - - - - - ¿ -40dB RF
³ ³__ Sniff
³-----¿ RF HOT Earth clamp -|__| BNC
_³ | |~|___Clamp ===---³
³M³ PEP | |S³°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°° ² º
³E³ | |U³ 15mm copper tube spacer! pin±_º__
³T³METER| |PÀÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜ/_³³ ³COAX
³E³ | |PÚßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßßß ³³___³SO239 I/P
³R³ CAL | |O³<tube ! diodesº
³_³ | |R³°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°° ß º
_³ CCT.| |T|~~~ 52ohm load ===-ß-³
³ ³-----Ù ~ ß ³
³_³_³: ³
RANGE À - - - - - - - - - - - - - - - - - - - - - - - - - - - - Ù
SWITCH
The input socket is bolted through a 2cm wide U shaped copper strip, that is
clamped on to the outer of the dummy load. The socket centre is soldered to the
pick off attenuator R, diode probe stack, & also a 4mm banana plug pin that
engages into a banana socket soldered to the 15mm dia centre tube. This tube
forms the centre of 5cm dia coax to make the low radiating load. It is
supported inside the load with a card spacer to hold it central for assembly.
The tubes' 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 BOARD |
Peak Detector 1kW³ C2 ³ |
Range oÄÄÁÄÄÄ´ÃÄÄÄÄ´ |
Ú´>ÃÄ´>ÃÄ´>ÃÄ´>ÃÄ´>ÃÄÂÄoÄÄ> | ³ Meter |
Input ³ D1-5 ³ | ÚÄR6ÄÄRV2Ä´ Damper |
o)ÄÁÂÄÄÄ¿ ³ oÄÄ´ C3 ÃÄÄÄÄÄÄÄÂÄÄÄÄÄÂÄÄÄÄÄÄÄ¿
³ R1 ³ ³ 100WÀÄÄÄ´ÃÄÄÄÄ´ +³ ³ | +³
³ ³ R4 ³ | ³ === _³_ | _³_
³ R2 ³ DUMMY C1 === oÄÄÂÄR7ÄÄRV3Ä´ C5³ /_\'| / \
o)ÄÄ´ ³ LOAD ³ 10W ³ C4 ³ ³ D6³ | ³Meter³
RF ³ R3 ³ ³ | ÀÄÄÄ´ÃÄÄÄÄÙ RV4 ³ | \___/
Sniff³ ³ ³ ³ | ³ ³ | ³
ÀÄÄÁÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÁÄÄÄÄÄÄÄÙ
Attenuator - - - - - - - - - - - - - - -
Cal & Speed up Protection
COMPONENTS
No Resistors Capacitors Diodes Variables Meter
1 * 3k9 3W 0u33 630V BYW29 200V 35nS 1M 100uA 1k
2 * 3k3 2W @ 23n 1kV " 330k
3 * 75R @ 68n 400V " 100k
4 * 52R 1kW @ 150n 400V " 10k
5 2M2 0.25W @ 10u 10V "
6 820k 0.25W 3V Zener (across meter)
7 220k
* = non inductive type @ = for my meter ballistics!
RF TAKE OFF
This uses just 3 Rs to give about -40dB output for scopes or analysers, & they
should be layed down close to the metal panel to reduce extra RF pickup. The
output impedance is 75ê. 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 slighty clipped.
PEAK READING
The 5 fast recovery tab type diodes, have there tabs sawn off (cutting I found
damaged them). Other diode types easily blow up in this passive peak reading
application! They are soldered in series & have individual heat shrink over the
tap remains so there in no chance of shorting out.
ÚÄÄÄÄ¿ all sleeved
³ ³ x5 ÚÄÄÄÄ¿ÚÄÄÄÄ¿ÚÄÄÄÄ¿
ÃÄÄÄÄ´ gives ³____³³____³³____³
³____³ | | | | | |
| | / ³~~~~³³~~~~³ \
ÀÄÄÄÄÙÀÄÄÄÄÙ
C1 is mounted on a tag around the input socket so there are next to no lead
inductance to give false readings. There can be very high pulse current into C1
on applying sudden RF, as the source Z is actually 25 ohms & there could be
>300V peak that is 12A peak current on a 1kW sudden RF pulse! The meter load is
only 100uA max, so there is no real loss due to the load. C1 has to store the
peak value until the nest speech peak not just the RF peak, hence it is very
large value for an RF detector.
The long DC lead away crom C1 to the meter switch is glued to the base plate to
kept it away from the dummy load R4, so there is little RF pick up & no chance
of it getting burnt.
The resultant DC + diode losses is then exactly the AC peak value.
2
V
POWER = -----
100 (for 50ohm)
Leads to & from the cal circuit matrix board are also kept close to the front
case as possable to reduce RF pick up from the load.
CALIBRATION
From the above the 3 meter scales can be calibrated from 1st principles. The
meter scale 0-100% needs to be drawn in Watts e.g. a square law scale ...
%FSD VOLTS WATTS VOLTS WATTS VOLTS WATTS
0 0 0 0 0 0 0
5 1.7 .03 5 .3 17 3
7 2.2 .05 7 .5 22 5
10 3.2 .1 10 1 32 10
14 4.5 .2 14 2 45 20
17 5.5 .3 17 3 55 30
23 7.1 .5 23 5 71 50
32 10 1 32 10 100 100
39 12.2 1.5 39 15 123 150
45 14.1 2 45 20 141 200
50 15.8 2.5 50 25 158 250
54 17.3 3 54 30 173 300
63 20 4 63 40 200 400
71 22.4 5 71 50 224 500
78 24.5 6 78 60 245 600
84 26.6 7 84 70 265 700
89 28 8 98 80 281 800
94 30 9 94 90 300 900
100 32 10 100 100 316 1000
CAL RV3 CAL RV2 CAL RV1
The diode loss of about 1-2V 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 proved 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 & diode detectors all start to go wrong & can't be trusted!
If the RF has a high harmonic content (eg, 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!
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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