[Elecraft] Palstar WM150L SWR Meter

Thu May 2 02:22:01 2002

One of the goals of the original Tandem Match design was to have a linear
output for both SWR and power.  This was done because, in those days (20
years ago), there was no easy way to re-draw meter scales and a simple
digital voltmeter could be used to display both power and SWR.  The circuit
could be simplified if you are willing to take non-linear scales.

The directional coupler shown in the design provides a power range of 0.1
watts to 2 kW, but you need a watt or so of power to get a good reading of
the SWR.  If you wind the two transformers with 10 turns on the secondary,
you will decrease the power range by a factor of 10.

There is a circuit board available from FAR, but I have heard of problems
with its quality (its definitely not an Elecraft quality board).  You can
use almost any op amp you want as long as you provide the proper power to it
and it has a very high input impedance.  I could re-design the Tandem Match
today using a quad op amp and a PIC processor.  Interestingly, you probably
couldn't build that version 20 years from now whereas you can build the 20
year old design today.

-John
 KI6WX

----- Original Message -----
From: "Pete Goudreau" <[email protected]>

Tom Hammond N�SS wrote:

>>> I have an Autek WM-1 and it is a nice meter, but it's accuracy
>>> degenerates very quickly below 10W.

>> So it's not the same circuit design as the Tandem Match then. Makes
>> more sense then to build the TM, I guess.

> That, and the fact that the designer of the Tandem Match is a member
> of this reflector, John Grebenkemper, KI6WX.

Well, dang, how cool is that?

Just finished printing out the QRP Directional Wattmeter article by
Lewallen, W7EL, along with the datasheets of the 3046 and the 3146
(discontinued) to make sure it was still buildable -- it is.  I'm not
a big fan of the TI Excalibur opamps but they obviously worked fine
in the Tandem Match and I'm not disposed to change them.  Looks like
the biggest work is in generating meter faces for the unit.