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G8MNY > TECH 07.12.03 13:23l 58 Lines 2636 Bytes #999 (0) @ WW
BID : 29270_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: QRP Power Meter & Dummy Load
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<VE3FJB<IK6PYS<IS0HHA<IK1ZNW<GB7CRV<
GB7CIP
Sent: 031207/0937Z @:GB7CIP.#32.GBR.EU #:29270 [Caterham] $:29270_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
_
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 a an accurate metered load good for 1296MHz.
For 10W you can use 2 220R & 2 180R 2.5W all paralleled up, wired around the
back of a of plug, or on PCB. For more than 10W use 2 diodes in series. 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.
Another building approach is to build it on a PCB ground plane around the
socket, or Coax point, or even screen it & put it in a tin, then the 1nF can be
a feed through.
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!
Note for sine wave carriers (with low harmonics!) the RMS power is equal to the
voltage squared over 100. The peak instantaneous power (NOT PEP, but what the
US quote Audio amplifiers Watts in!) is Voltage squared over 50.
With an oscilloscope to measure the peak DC voltage, the true SSB PEP can be
measured, read the peaks under mormal modulation (eg the heigest value & use
the same V2/100 formula).
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 rigs ALC is slow, 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 confusions
sets in!
/QSL
73 De John, G8MNY @ GB7CIP
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