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G8MNY > TECH 21.07.03 11:58l 54 Lines 2471 Bytes #999 (0) @ WW
BID : 6538_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: QRP Power Meter & Dummy Load
Path: DB0FHN<DB0RGB<OK0PPL<DB0RES<ON0AR<7M3TJZ<IK1ZNW<IK6IHL<SV1AAW<LZ0FBB<
GB7CRV<GB7CIP
Sent: 030721/0841Z @:GB7CIP.#32.GBR.EU #:6538 [Caterham] $:6538_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY
Low Power Loads
_
PLUG / \ !\ ! 1N4148 2
OR =====-----------! >!------------o + V
COAX \_/ ! ! !/ ! ! \ RMS Power = ---
! 100 100 ----- Volt 100
! OHM OHM ----- 1nF Meter
! ! ! ! /
------------------------------o -
CONSTRUCTION
The 2 100 Ohms can be 3 150 Ohms, or even just a 47 Ohm, 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 MUST NOT be used!
In the case on BNC 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 voltage 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 sign wave carriers (with low harmonics!) the RMS power is equal to the
voltage squared over 100. The peak instantaneous power (NOT PEP, what the US
like to quote on AF amps) 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 inder FM mode or the rigs ALC is slow, higher peaks will be seen
in SSB.
/QSL
73 De John, G8MNY @ GB7CIP
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