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Author Topic: SWR/WATT meter  (Read 11239 times)
KE5IUN
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Posts: 33




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« on: March 14, 2010, 02:35:04 PM »

I am building a Small Wonders Lab PSK20 and need a watt meter to make final adjustments to it. I have considered the OHR WM-2 and the Elecraft W1, however, I noticed that MFJ has one for QRP, model MFJ-813. I know that some MFJ products seem to have a bad reputation and was wondering if anyone knows anything about this meter. Its reasonably priced at 39.95. Any thoughts?
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AI4NS
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« Reply #1 on: March 14, 2010, 05:30:16 PM »

I have never had any problems with MFJ stuff. Understand there is a chance you need to tighten screws, etc. One thing to look at, is will you want to take the meter with you when you operate? With QRP, you need to maximize your signal, and that means minimizing losses. Simply repositioning your antenna can make a difference, but how will you know? I have the Elecraft W1, and it is a nice meter, but is more costly. I think you would be fine with the MFJ. You could also built a noise bridge to help you tune your antennas. They are small, easy to build. TenTec has one that is a nice kit, and there are others on the web. Look at QRP tuners as well. Dans kits and parts has a kit that is cheap, and works well.

Mike
AI4NS
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AD6KA
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« Reply #2 on: March 14, 2010, 06:34:46 PM »

I like using my Elecraft W1 here on the bench to make adjustments to QRP rigs because:

1) It is auto ranging with three different scales from 100mw to 140w.
100mw to 1.4w
1w to 14w
10 watts to 140 watts.
I have found it very accurate and responsive even in the 100mw to 1.4w range.

2) Since the power output and SWR readings are LED's, I find it easier to peak than looking at a meter needle. But that's just me.

The output power adjustments on the SWL PSK series rigs are quite simple, just tweaking two inductor cans. In a pinch you could even hook it up to a dummy load, then have a nearby receiver on tuned near that frequency. Disconnect the receiver's antenna, turn down the receivers gain and/or engage the antentuators. Then you can peak the PSK20 output either by the meter on the receiver or by listening to the volume of the audio tone. I've done that using
my PROII as the receiver and watching the signal come up on the display.
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AJ4MJ
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Posts: 48




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« Reply #3 on: March 30, 2010, 12:53:01 PM »

If you already have a "QRO" meter, don't overlook it.

I have the venerable MFJ-949E that I use for my 100 watt rigs.  The meter has two ranges - 30W and 300W.  On the 30W range, I can read down to about 500mW easily.  It also works just fine as a QRP tuner, though not exactly portable.

As far as I can tell it is accurate.  When I key down my Norcal into it, I get the expected readings for a given power supply voltage.
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K4FH
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« Reply #4 on: March 31, 2010, 07:43:13 AM »

Look at North GA QRP Club's SWR/WATT meter.  It can be built for either a 2w or 6w scale.
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STAYVERTICAL
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Posts: 875




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« Reply #5 on: April 14, 2010, 12:27:50 AM »

All the suggestions above are good, but consider making a small rf probe out of a 1N34 germanium diode, a 0.01uF capacitor and a 4.7Megohm resistor. Then read the voltage across the dummy load on any Digital Multimeter with a high input resistance (around 7 megohms), which is most of them.
Power = ( V * V) / Resistance of dummy load

The readings may not be very accurate below about 1 volt due to the germanium diodes small voltage drop, but after that they are pretty good.

For construction details try N5ESE or VK5JST and RF probe on google.

Works well for me.

73s
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KU3X
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Posts: 434




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« Reply #6 on: May 29, 2010, 05:40:08 PM »

If you run QRO and QRP, you may want to try one of these?
I use a Daiwa CN-101L. It's a cross needle meter. For QRO, the three ranges are 15, 150 and 1500 watts. The reflected meter's range is 5, 50 and 500 watts. This is an old QRP'r trick.
Hook the meter up backwards. Use the low power selection and you will be able to read 5 watts on down. Now you are using a scale the reads .5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5 and 5. Those are the readings on the scale.

Even if you hook the meter up in the normal fashion, the low power scale reads from 15 watts down to 1 watt or zero. I think it's a good low power / high power meter. I have two of them.
Barry
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W3JJH
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« Reply #7 on: May 30, 2010, 04:32:29 AM »

You get what you pay for--then you pay for what you've got!

My advice (which is worth every cent you're paying for it) would be to buy either the OHR or Elecraft meter if you can afford it.  I have an OHR.  It's very accurate, comparing favorably with HP and Agilent units in the lab at work.
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WB4CVZ
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« Reply #8 on: May 30, 2010, 07:02:01 PM »

I like an old NoGaWattmeter. Its pretty cheap and easy to assemble. If you are just looking for a maximum forward power reading, just use an old adjustable SWR meter. Just adjust the sensitivity to accommodate for the power level. It will not give you a reading of the actual power out, but it will work as a relative reading.
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KE5IUN
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Posts: 33




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« Reply #9 on: May 31, 2010, 04:58:31 AM »

Thanks for the replies guys, I went with the OHR meter. I built and calibrated it, though the meter out of the box would not zero so I'm gonna have to purchase another meter movement. Have not finished the PSK 20 yet but will post how it worked when done. To many workin in the yard projects but almost done with them :-)

Bill,
KE5IUN
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KD7RDZI2
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Posts: 229




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« Reply #10 on: June 13, 2010, 05:13:24 PM »

I had used the rfprobe (kit) offered by Hendricks
http://www.qrpkits.com/rfprobe.html.
I found it also worked precisely as a wattmeter up to 50 watts. You need a cheap voltmeter and make an easy calculation written in the manual. With that, I was able to calibrate and repair a brand new Comet wattmeter other than optimizing my qrp. Overall it is a very useful and simple device that works well as a wattmeter too!
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KB1NLW
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Posts: 69




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« Reply #11 on: August 01, 2010, 02:32:35 PM »

As an alternate.  Build a PSKMeter (~$48).  It has a power readout plus it  automatically adjust your computer audio output for optimum signal. 

I also use an Elecraft T1 auto tuner with my PSK 20 & PSK 40.
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