Mensaje originado el: 13/08/10 a las: 23:40 z



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 indictance & better the SWR. For 10W you can use 2 220R 
& 2 180R 2.5W all parralled 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
distroy the diodes, so a series safety resistor of a 1K between the meter &
the cap recomended.

Accurracy can be improved a bit with a better diode, & by alowing 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 instantanous 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
dissapation/ratings.

73 De John, G8MNY @ GB7CIP