Dummy Load
CONSTRUCTION
The 2x 100 ohm resistors can be 3x 150 ohm, or even just a single 47 ohm,
but the more leads in parallel, the lower the inductance and 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 and 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 and 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 and 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 not so good, and wire wound ones MUST NOT
be used!
In the case of internal BNC plug mounting, heat conductive paste can
be used to increase dissipation/ratings.
MATHS
Accuracy can be improved a bit with a better diode, and by allowing for
the 0.7V drop in calculations. For very low powers (nW and mW), bias the
diode with a negative voltage
(-9V and 10M ohm), and 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 and use the same V2/100 formula).
With many rigs and 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
and meter power confusions sets in!
73 De John, G8MNY @ GB7CIP
|
