[Elecraft] Palstar WM150L SWR Meter

[Pete Goudreau]

John Grebenkemper, KI6WX wrote:

> 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.

Actually one of the things that draws me toward building the TM over 
any of the other options is the accurate method of calculating power 
and VSWR and then displaying them both in a linear fashion.

The meter work I was thinking of below was that of the actual 
fabrication using a drawing program and getting the arc radii and 
angular extensions correct.  Not sure how to do that easily.  I 
suppose scanning one in, verifying dimensions, editing, and printing 
to heavy photo card stock would do the trick.  Will be interesting to 
try.

> 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.

That's exactly what I had in mind.  I've little interest in high 
power work figuring the Moto AN762 amp at 140W PEP is just fine for 
my use.  That and it'll be fun to build and package with a homemade 
diode switching circuit.

> There is a circuit board available from FAR, but I have heard of
> problems with its quality (its definitely not an Elecraft quality
> board).

Thanks, found them and the board.  Seems a bit pricey if the quality 
isn't all that good but what the heck, I'm sure it can be hacked and 
modified easily.

> 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 figured that might be a possibility but what the heck, the less the 
changes, the less the potential problems I figure.  Nothing wrong 
with the Excalibur opamps, they'll do fine I'm sure.

> 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.

Yep, good point.  I'll take the linear design over the processor just 
for that reason.  And it'll be easier to keep running for a much 
longer time, IMHO.

Thanks very much,
Pete, AD5HD

> -John KI6WX

> ----- Original Message -----
> From: "Pete Goudreau" <goudpj@mac.com>

> Tom Hammond N=D8SS 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.