automatic (direct reading) SWR Meter
 

Hi
I'm looking for Direct reading SWR meter (schematics, pcb, solution,
ideas - anything)

Help please
MAc

PS - got QST 97/2

automatic (direct reading) SWR Meter
 

Mac,

You will probably get more responses if you describe what you want in
more detail: frequency of operation, inline or test instrument, self
contained or PC based, remote head, analog or digital display, cost,
commercial or homebrew etc.

I suggest that you take a look at current and back issues of the ARRL
handbook and read the articles @ http://www.arrl.org/tis/tismenu.html

Regards - Roger


MAc wrote:
Hi
I'm looking for Direct reading SWR meter (schematics, pcb, solution,
ideas - anything)

Help please
MAc

PS - got QST 97/2

automatic (direct reading) SWR Meter
 

automatic (direct reading) SWR Meter
 

I haven't seen a lot of examples out there, but the project seems very
simple; perhaps trivially simple.

First, you build a directional coupler -- there are a number of designs out
there, but none are especially difficult. Then you rectify the voltage and
perhaps amplify it. Feed the two voltages into a microcontroller that does
the math and operates the display (digital of course). Analog isn't going
to be so popular because you not only need a meter (generally harder to get
and expensive these days), but you will need some custom meter face, and
what it needs to look like won't be all that obvious.

You can get 10 bits on a microcontroller A/D pretty easily and cheaply, more
bits will generally cost more, sometimes a lot more. While you don't need
terrific resolution for SWR, you will if you want to measure SWR over a wide
range of power levels. A lot of micros have 5 or 10 A/D's, though, so
perhaps you could get cute and avoid the need for something expensive.

Not many micros have D/As, but many have PWM outputs that can be filtered
and would be plenty enough resolution for an analog meter. The cool part of
that is that you could have your retro instrument, and you could adjust the
output to match the meter face you have.

Of course, if you aren't afraid of a little work, a graphical LCD could be
made to display a meter face .. after all, it's only ones and zeroes g
Some of these color LCDs from cell phones (pricey) are all the rage among
some PIC hobbyists these days.

...

"MAc" wrote in message
...
napisal(a):

You will probably get more responses if you describe what you want in
more detail: frequency of operation, inline or test instrument, self
contained or PC based, remote head, analog or digital display, cost,
commercial or homebrew etc.


;-) ANY ideas. ANY schematics, Any The topic is "direct reading" :-)

mac

the best would be:
analogue,
single meter,
1,8-30
of course homebrew - it's r.r.a.h group

automatic (direct reading) SWR Meter
 

You will probably get more responses if you describe what you want in
more detail: frequency of operation, inline or test instrument, self
contained or PC based, remote head, analog or digital display, cost,
commercial or homebrew etc.
;-) ANY ideas. ANY schematics, Any The topic is "direct reading" :-)

I haven't seen a lot of examples out there, but the project seems very
simple; perhaps trivially simple.

First, you build a directional coupler -- there are a number of designs out
there, but none are especially difficult. Then you rectify the voltage and
perhaps amplify it. Feed the two voltages into a microcontroller that does
the math and operates the display (digital of course). Analog isn't going
to be so popular because you not only need a meter (generally harder to get
and expensive these days), but you will need some custom meter face, and
what it needs to look like won't be all that obvious.

Another possibility along these lines would be a little more analog-ish.

Analog Devices makes a line of neat little "log amp" ICs. Feed them a
sample of RF on one pin, and you get out a DC voltage which is
proportional to the log of the RF voltage. They have a wide frequency
range and if I recall correctly the Vout=log(Vin) relationship holds
up pretty well over several decades of power level.

Build two direction couplers, as suggested above, but without the
rectifier diodes. The ARRL handbook has instructions for building
directional couplers for HF out of copper pipe. For a physically
smaller design (for lower power) google around for "stripline
directional coupler", "monimatch", and similar terms, or go to the
Boat Anchors Archive and hunt around for the manual for an old
Heathkit SWR meter.

Take the directionally-coupler-sampled RF signals (forward and
reverse) and run them into a pair of the ADC log-amps.

The two DC voltages which come out will be proportional to the
logarithms of the forward and reverse voltages. If I remember my math
correctly, this means that the difference of the two is proportional
to the logarithm of the ratio of the forward and reverse voltages,
and would thus be a pretty nice indication of the return loss in dB.
Zero voltage difference would mean zero return loss (infinitely high
SWR), and the voltage difference would increase with the return loss
(lower SWR). The upper limit on the voltage difference will probably
be set by the directionality limit of the directional couplers.

Take this voltage difference, run it through a voltage-to-current
converter (a.k.a. a series resistor) and into an analog meter, and
*bingo* you've got a direct-reading SWR meter. It'll read at the high
end of the scale for a 1:1 SWR and will not deflect if the SWR is
infinitely high.

It might need to be recalibrated for a proper full-scale deflection at
1:1 on any given frequency, to adjust for the directionality limits of
the couplers.

Take a look at the "DEM ABPM" (Down East Microwave All Band Power
Meter) at www.downeastmicrowave.com - it's a professionally kitted
version of a homebrew product, which uses one of the AD logamps for
frequencies from ultrasonic up through UHF, and a less-sensitive-but-
higher-frequency Linear Technologies part for frequencies from VHF
through microwave. Readout is through a common DC-to-LED-bargraph IC.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

automatic (direct reading) SWR Meter
 

Dave Platt napisa³(a):

Build two direction couplers, as suggested above, but without the
rectifier diodes. [cut...]

Many many thanks. I'll go this way.

Best
73
MAc
sp9mrn

automatic (direct reading) SWR Meter
 

xpyttl napisa³(a):
Feed the two voltages into a microcontroller that does
the math and operates the display (digital of course). Analog isn't going
to be so popular because you not only need a meter (generally harder to get
and expensive these days), but you will need some custom meter face, and
what it needs to look like won't be all that obvious.

I was thinkink about it, but i want to do it "as much analogue as
possible" ;-)

Thank you for response

MAc
sp9mrn

automatic (direct reading) SWR Meter
 

MAc wrote:

I was thinkink about it, but i want to do it "as much analogue as
possible" ;-)

You'll get some good ideas from "The Tandem Match - An Accurate
Directional Wattmeter" by John Grebenkemper, KA3BLO, in Jan. 1987 QST.
See also corrections to the article in Jan. 1988 Technical Correspondence.

Roy Lewallen, W7EL

automatic (direct reading) SWR Meter
 

Roy Lewallen napisał(a):

You'll get some good ideas from "The Tandem Match - An Accurate
Directional Wattmeter" by John Grebenkemper, KA3BLO, in Jan. 1987 QST.
See also corrections to the article in Jan. 1988 Technical Correspondence.


Thank You, Roy - I did't know that article. Im just reading it and
trying to understand :-)

73
MAc
sp9mrn

automatic (direct reading) SWR Meter
 

If you're going to be doing the math anyway, analog or digital, and
providing some sort of calibration mechanism, you don't need to get too
weird about the directional coupler. For HF anyway, almost any old thing
will work. It sort of seems a little like black magic. Just stick a couple
chunks of wire next to a section of coax with the braid removed and all of a
sudden a directional coupler. I think the toriod based couplers are a
little more reproduceable, though.

Somebody, maybe even AD, has an RF power chip, too. Not sure of the
specifics of it. Might not be too useful for an analog circuit. A lot of
the cool chips these days are little bitty thing with lots of legs that you
can't hardly see.

...

"MAc" wrote in message
...
Dave Platt napisa³(a):

Build two direction couplers, as suggested above, but without the
rectifier diodes. [cut...]

Many many thanks. I'll go this way.

Best
73
MAc
sp9mrn