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G8MNY > TECH 02.01.03 22:46l 49 Lines 1952 Bytes #999 (0) @ WW
BID : 36056_GB7CIP
Read: GUEST OE7FMI
Subj: QRP Power Meter & Dummy Load
Path: DB0MRW<DB0RGB<OK0PPL<RZ6HXA<SP7MGD<GB7YKS<GB7YFS<GB7COV<GB7KHW<GB7CIP
Sent: 030102/0939Z @:GB7CIP.#32.GBR.EU #:36056 [Caterham] $:36056_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 -
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. For 10W you can use 2 220R
& 2 180R 2.5W all paralleled up.
Build this lot into a PL259 plug, 'N', or a BNC, keeping the leads as small as
possible, & you will have a an accurate metered load good for 1296MHz. 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 cap recommended.
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 but accuracy may be
poor if everything is not ideal !
Note for sign wave carriers the RMS power is equal to the Voltage squared over
100. The peak instantaneous power (NOT PEP) is Voltage squared over 50.
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.
73 De John, G8MNY @ GB7CIP
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