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Author Topic: Homebrew Wattmeters  (Read 9219 times)
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WU2D
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CW is just a narrower version of AM


« on: February 22, 2014, 05:40:33 PM »

Anybody out there ever build a Wattmeter or SWR bridge? I wanted to try one as a built-n for a homebrew tuner. I collected a few schematics and found a reasonable sized aluminum 5 x 2.5 x 3 box which could be mounted inside right at the TX port. There are several popular circuits, all bridges which use one or two torroidal core coils which are slipped over a section of transmission line. The basic schematic that I used is attached.


* Wattmeter 4.jpg (8.62 KB, 259x194 - viewed 500 times.)
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WU2D
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« Reply #1 on: February 22, 2014, 07:28:27 PM »

This is a good article on bridge couplers. http://www.na0tc.org/Homebrew%20SWR%20Meter.pdf

The style that I tried was a Breune. This is a simple circuit but one that is easy to mess up. After a couple of fails, I figured out how to build it so it nulled properly. The core was a Type 62 but a type 43 would work as well or better. The winding was a bifilar twist of 20 turns of # 24 with a crossed over centertap in conventional transmission line transformer fashion. I thought that this would be a better balanced and wider bandwidth approach than a conventional 40T with center tap on the core. The coax was 5 inches of LMR-240 with a shield split at 2/3 and the core on the TX side. I used ordinary 1N34 diodes to 1000 pF filter feedthrus. Both shield ends of the coax were grounded. The split with exposed center connected to the nulling trimmer which connected to the center tap. From the center tap to ground I put a 1mH choke, a 2700 Ohm resistor and two 180 pF capacitors with very short leads splayed symmetrically to two ground lugs. The neat construction and symmetry gave an excellent null on reverse energy.     
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Pete, WA2CWA
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« Reply #2 on: February 22, 2014, 09:01:22 PM »

I've built a number of SWR bridges (HF and VHF) over the years and have always used this basic circuit as a starting point.

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« Reply #3 on: February 23, 2014, 08:37:40 AM »

The line sampler type that you showed is pretty easy to build and it works well for SWR. Those type of line couplers are frequency sensitive and are hard to use for calibrated power measurement over several bands. The transmission line transformer types do a good job on both SWR and power over the whole HF band if you use the right components and actually very good from 160-20M with junkbox components.

The diodes are another subject. I used a pair of 1N34's matched for forward voltage within 10 mV and these drive the meters. Schottky's like BAT-85's would be better. Detectors operate in square law over a fairly wide range, then cross over to linear. That is what causes power meter scales to compress at the high end. The square law region can be extended by using a lower load impedance, but then you have no voltage to drive the meter. So the ideal solution is to use a virtual ground like the inverting terminal of an OP-AMP and use the current through the feedback loop to develop the voltage. 
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« Reply #4 on: February 26, 2014, 09:31:56 PM »

Mike,
     I have built several of the Stockton type SWR/Power meters and they work well for me. I found plans for them in Doug DeMaw's  ARRL publication "W1FB's Design Notebook." This book is out of print , but I could send you a copy of the circuit and details  if you like. Far Circuits has the boards also and it makes a fun and very useful swr/wattmeter. Good luck with your project.---Marty, KK4RF---
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« Reply #5 on: February 26, 2014, 10:09:58 PM »

I have become very suspicious of meters for RF power. I always had to double check with an RF sniffer bulb because many reading were bogus. The bulb doesn.t lie ever. In the end I decided just to use the bulb. Often wondered about using it with a light intensity meter or a photocell and meter with the sniffer bulb. Meters in general do not perform well around large RF fields. If you don't believe that put your digital voltmeter near your linear next time you are on the air.
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« Reply #6 on: February 26, 2014, 11:22:39 PM »

Just as experiment I built the QRP RF wattmeter from ARRL's Solid State Design (1977).  It's another Bruene variant (on pg 150) using either 50 or 100 ua meter, 1N34's, and single current transformer wound on T68-2.  Used some old Johnson 11puff air trimmers I had laying around in voltage divider and it seems to null in both directions without any problem although I'm stretching trimmers to down around minimum C values.  Is plenty sensitive between 5-10 watts (using it to peak my 7 watt 5763 rig) but still need calibrating.  Was thinking about using scope to roughly calibrate it into 50 ohms--should be close enough for Govt work <hihi>.  Built-in a switchable 10 watt dummy load for tune-up and eventually will have built-in low-power pick-up for my REA Mod Monitor.  I've got nice Oak Hills QRP meter and am using it with this set-up but want to put it back for use with my other (lower power) QRP rigs and not dedicate it to the AM PW transmitter.

73's Geo 
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« Reply #7 on: February 27, 2014, 12:05:46 AM »

I like the Millen Stripline "reflectometer."

Very simple and effective.

The second file is the one I am building into my new LCL tuner:

Phil - AC0OB

* Millen 92200[1].pdf (2032.69 KB - downloaded 225 times.)
* FWD RFL Power Meter.pdf (32.48 KB - downloaded 195 times.)
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« Reply #8 on: February 27, 2014, 01:32:41 AM »

Not sure how accurate the stripline type is, but it's hard to destroy, easy to make, and works with a variety of signal levels.
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« Reply #9 on: February 27, 2014, 11:35:26 AM »

The stripline approach is really good for upper HF and on up. It is tough to get much directivity/sensitivity in the lower HF and MF bands.

But I think Pete meant the basic design of it is a great start. The coupling method to the line would need to changed for MF and Low HF but after that it's a place to start. Directivity feedback is missing though.
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« Reply #10 on: February 27, 2014, 11:41:01 AM »

P=I2R is pretty easy if you know the actual impedance....

I like RF ammeters best.

73DG
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Tom WA3KLR
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« Reply #11 on: February 27, 2014, 08:47:28 PM »

The ARRL "monimatch", and what I call the CB bridges are, as I found, not accurate for the SWR reading; they read quite low (too good).  They do null around 50 Ohms so are ok for tuning up an antenna tuner, but a real SWR of 3:1 reads much lower on these bridges.

I spent as much time as I could a few years ago fooling around with trying to improve one, and the next step was to re-build one with a strip about 3 feet long so that the two end SO-239’s were far apart to eliminate coupling between the two port areas as part of the investigation of why they didn't work well, but I had to quit.  So if you always want to see a good SWR reading, use these meters!

What is accurate for SWR readings?  I found 3 types.  These all agreed, every else is junk:
1.   Bird 43 directional wattmeter and graph
2.   Toroid-based SWR meters
3.   MFJ-259B.

Take the challenge,  if you have one of these above three meters, compare it against your monimatch or stripline CB/Radio Shack type bridge looking at some 3:1 or 4:1 SWR load.
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« Reply #12 on: February 27, 2014, 11:04:09 PM »

Makes sense about the CB wattmeters. I have a RS one with that same coupler. The reason it's good for CB is that so many CB techs and mobile users are totally nuts about the match being perfect, and once it is 1:1 with the proper calibration knob setting, they will increase the CAL control and tweek the antenna or tuning system til it is as low as possible. At some point the reading showing 'better than reality' is compensated for by the practice.
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« Reply #13 on: February 28, 2014, 07:13:25 AM »

When I was Johnny Novice, I was given two precious instruments, the Lafayette SWR meter as described (like the RS coupler) with a tiny meter and a Lafayette WAVEMETER. The wavemeter was a little instrument with a whip antenna, a from 1.5 to 100 MHz with a small calibrated dial and an even tinier meter. With these, I could prevent the dreaded OO report or Pink Ticket.

Now the wavemeter was a device used as a field strength meter but more importantly, to hunt or reveal harmonics.

Has anyone built an inline calibrated in dBW version of this? With a built-in coupler, 50 Ohm load and off the coupled port - an attenuator to actually measure harmonics like a simple spectrum analyzer? A simple schottky diode detector provides accurate square law over at least 40 dB.
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« Reply #14 on: February 28, 2014, 08:03:55 AM »

I was given one of the CN-101L Daiwa's by a friend. Frankly, it's a piece of junk and little better than than a CB meter. Commercial offerings tend to be either junk or expensive. Of course, you gets what you pay for.

This seems to be the best thing going however:

http://www.telepostinc.com/lp100.html

Calibration traceable to a NIST standard.

FEELEEP
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« Reply #15 on: February 28, 2014, 10:33:15 AM »

I have these file photos of the transmission line trough from my 1971 Archer SWR bridge before it was removed; the heart of most small reflectometers:


* archer_trough4.JPG (125.28 KB, 640x480 - viewed 424 times.)

* archer_trough3.jpg (127.46 KB, 640x480 - viewed 468 times.)
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
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