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G8MNY > TECH 24.11.11 12:32l 92 Lines 3755 Bytes #999 (0) @ WW
BID : 17995_GB7CIP
Read: GUEST
Subj: QRP Power Meter & Dummy Load
Path: IZ3LSV<IQ5KG<IW2OHX<IR2UBX<IK2XDE<DB0RES<ON0AR<UA6ADV<GB7CIP
Sent: 111123/2250Z @:GB7CIP.#32.GBR.EU #:17995 [Caterham] $:17995_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (Updated Feb 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
_
PLUG / \ 1N4148 2
OR =====ÄÄÂÄÄÄÄÂÄÄÄ´>ÃÄÄÂÄÄÄÄÄÄÄÄÄo + V
COAX \_/ ³ ³ ³ \ RMS Power = ÄÄÄ
³ 100 100 ³ Volt 100
³ OHM OHM === 1nF Meter
³ ³ ³ ³ /
ÀÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄo Ä
CONSTRUCTION
The 2x 100ê resistors can be 3x 150ê, or even just a single 47ê, but the more
leads in parallel the lower the inductance & better the SWR. Build this lot
into a PL259 plug for VHF, 'N', or a BNC for UHF, if you can keep the leads as
short as possible, you will have an accurate metered load good for 1296MHz.
ÚÄÂÂÂÄÂÄÄ¿
For 10W you can use ± ±³± ± _³_
2 220R & 2 180R 2.5W ± ±³± ± \_/
all paralleled up, ± ±³± ± ³
wired around the ÃÄÁÄÁÄŴô
back of a of plug, ³ ³ ÀÄÄ> To
or on PCB. ÚÙ-----À¿ Meter
For more than 10W use ³ ³ PL
2 diodes in series. ³_______³ 259
U
With higher powers shorting the DC will destroy the diodes, so a series safety
resistor of a 1k between the meter & the capacitor is recommended.
3x Â
Another building approach 150 ±
is to build it on a PCB ohm ±
ground plane around the ³
socket, or Coax point, ñ±±Ä@Ä´>ÃÄÂÄÄ>
or even screen it & put ³ ===
it in a tin, then the 1nF ± Á
can be a feed through. ±
Á
One last point all resistors must be carbon composite, accurate Carbon or Metal
oxide spiral cut are no so good, & wire wound ones MUST NOT be used!
In the case of internal BNC plug mounting, heat conductive past can be used to
increase dissipation/ratings.
MATHS
Accuracy can be improved a bit with a better diode, & by allowing for the 0.7V
drop in calculations. For very low powers (nW & mW), bias the diode with a
negative voltage (-9V & 10Mohm), & use this -ve standing voltage (eg. -0.4V) as
the 0 Watts starting point for the maths. In this way powers of just a few uW
can be detected, but accuracy may be poor if everything is not ideal!
The peak instantaneous power of the .-. -Peak Voltage
sine wave (What the PA has to do is / \
NOT the PEP, but what the US quote | | -RMS Voltage 0.707 peak
Audio amplifiers Watts in!) ³ ³
is Voltage squared over 50. ³ ³
With an oscilloscope to measure the peak DC voltage, the true SSB PEP can be
measured for good sine waves. Read the peaks under normal modulation (eg. the
highest value & use the same V2/100 formula).
Peak PEP Peak PEP
Volts Watts Volts Watts
2.8 0.08 28 8
3.3 0.1 33 10
4 0.16 40 16
5 0.25 50 25
5.6 0.32 56 32
7 0.5 71 50
8 0.64 80 64
10 1 100 100
14 2 141 200
16 2.5 160 250
20 4 200 400
With many rigs & PAs the PEP will be much the same for CW/FM as SSB, but where
the PSU dips under FM mode or the rig has a slow ALC, higher peaks will be seen
in SSB. In some valve PAs with droopy PSUs this can be as much as 2x the RF
carrier power seen on any meter, which is where PEP & meter power measurements
confusions sets in!
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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