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Author Topic:  (Read 4587 times)

Posts: 1490

« on: December 11, 2002, 01:36:02 PM »

If I am ever to go after the million-mile-per-watt award, I need to know how I can verify my power output (input?) at that low level.

Does Bird make a slug that low for their wattmeter? Should I read my output power on an oscilloscope? Do I need to insert a milliammeter (and voltmeter) in the final circuit of my rig?

Maybe I will need to reduce the voltage to my tiny transistor final to reach milliwatt power levels, too?  (I am currently running an Argonaut 509 as my only rig.)
Thanks in advance.
73 es best rx de kt8k - Tim

Posts: 17483

« Reply #1 on: December 11, 2002, 03:49:55 PM »

Probably the easiest method is to run the rig at a
measurable output (perhaps 1 watt) and then use a
step attenuator in the output to reduce the level
actually going to the antenna.  (Of course this works
best if the attenuator is just in the transmit line,
and not there during receive.  My old 505 had some
extra RCA plugs on the back panel where you could
bring out the TX cable between the final amp and the
T/R relay - just remember to jumper the jacks when
not using the attenuator.)  Switched attenuators up to
60dB or more are available commercially, and are not
hard to build.  Just watch the power rating on the
resistors in the pad closest to the transmitter, where
the power levels will be highest.

You can also verify the output with a 'scope - I've put
a BNC "T" connector on the scope input and ran the
feedline through it.  But the attenuator approach
probably gives a cleaner signal - even though you can
turn the power down to zero on the Argonaut, the
spurious outputs don't reduce at the same rate as the
main carrier.  

Posts: 1490

« Reply #2 on: December 12, 2002, 01:00:13 PM »

That's a great idea - a step attenuator after the final, though I agree switching it in and out may be a challenge (especially not to spoil my Argonaut's wonderful QSK characteristics).  I've seen some pretty good step attenuators, and I could probably build one with a project box, the appropriate precision resistors, and some toggle switches.
I always wanted one for fox hunting anyway (it's so handy to be able to cut down the fox's signal when you're so close his signal swamps the S-meter, making it impossible to use a directional antenna).
Thanks for the suggestion.  
Anyone have another good alternative?
73 de kt8k - Tim

Posts: 17483

« Reply #3 on: December 16, 2002, 05:42:20 PM »

In the Argonaut, find the wire that runs from the
TX final to the antenna relay.  (These are on different
boards if I remember, so there should be a length of
RG-174 running between the board plug connectors.
Unsolder one end and run it to one of the unused RCA
jacks on the back panel.  Run another length of cable
from one of the other jacks back to the point where you
unsoldered it.  Put a jumper or the attenuator between
the two jacks, and it will have no effect on the
legendary QSK, while putting the attenuator in just
the transmit side.

While a good step attenuator can be used for either
QRPp or foxhunting, you may want different ones for
each application.  The resistor closest to the TX
will have to dissipate the most power, and since you
don't necessarily know which sections will be switched
in, it is probably best to wire all the attenuator
sections using 1- or 2-watt carbon or metal-film
resistors.  Construction and, especially, shielding,
are not as important at HF, and you probably only need
up to 30dB of attenuation (unless you want to go below

For foxhunting (especially on VHF) the shielding of
the attenuator is most important - not just between
sections, but good commercial units have a shield
down the middle of each section to isolate the two
sides of each switch.  But even the best commercial
shielded attenuators rarely give more than 60 or 70dB
of usable range - beyond that direct pickup through
the case of the receiver will be stronger than the
signal from the attenuator.  This is why most serious
hunters that I know use an "offset" (or "active")
attenuator - this is a frequency converter with a
variable injection level, capable of providing over
100dB of attenuation.  There are a number of designs
on the net.  Personally I like to use a double balanced
mixer since it reduces the response to paging
transmitters and other stray signals.

Posts: 59

« Reply #4 on: February 04, 2003, 01:15:01 PM »

The OHR WM-2 QRP wattmeter has 10W, 1W, and 100mW scales, and is quite accurate, so it makes it very easy to read my power output on my QRPp rigs.  I highly recommend it.  It's available as a kit or wired and tested.  They also turn up on the used market quite regularly.

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