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Author Topic: Looking for schematics for direct SWR analog meter  (Read 2190 times)
N5KBP
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Posts: 295




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« on: August 14, 2007, 08:32:18 PM »

I am in need of plans or schematic for a direct reading analog swr meter. I've googled etc and have come up empty.

Marty
N5KBP
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KD6ANF
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Posts: 31




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« Reply #1 on: August 14, 2007, 09:27:53 PM »

Hi Marty

Using Google or Blackle, the following text string was entered:

"schematics for direct SWR analog meter"

This is one of the hits displayed:

http://ludens.cl/Electron/swr/swr.html

"Wideband SWR meter"

This circuit is very similar to one in the 1990 ARRL handbook, page 34-9, titled "QRP SWR Indicater".

Hope this may be of interest!

Best regards

george potter   KD6ANF
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KA5N
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Posts: 4380




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« Reply #2 on: August 15, 2007, 03:02:38 AM »

Google SWR meters, SWR meter schematics, etc. and get thousands of hits.  Go to the MFJ website and download manuals for all thier meters (not to copy but for reference) same with Heathkit on the BAMA website.  Get an ARRL Handbook or ARRL Antenna Handbook.  
The analog SWR meter is one of the simplest devices around.  If you don't get information off the internet then your search methods are at fault.  Often you have to use several variations of the item being searched to get the hits you need.
Allen
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WA3SKN
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Posts: 5555




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« Reply #3 on: August 15, 2007, 04:02:36 AM »

There were two basic types... one using a tranmission line to induce voltage, the other using a toroid to do the same.  The ARRL has both plans, along with changes needed to do eith HF or VHF.
If the other links don't pan out, try them!

-Mike.
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N5KBP
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Posts: 295




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« Reply #4 on: August 15, 2007, 08:04:02 AM »

Apparently I need to be a little more specific about "direct reading". I'm not looking for the generic set/read or dual meter. I can find them everywhere. I need an analog CALCULATING meter. Ie no set, switch and read. Just look at the single meter and read the swr. I want to build one that uses a couple of lm3914s to light up a bargraph.
Sorry for the confusion

Any help would be appreciated.
Marty
N5KBP
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KE3WD
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Posts: 5689




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« Reply #5 on: August 15, 2007, 10:08:22 AM »

Then try googling "digital SWR meter plans"?


I think the keyword would be digital, maybe active peak reading, things like that.  


.
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KA5N
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Posts: 4380




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« Reply #6 on: August 15, 2007, 10:58:04 AM »

A google for calculating SWR meter gave

http://www.qrp4u.de/docs/en/swr_meter/index.htm
which uses two chips and an analog D'arsonval meter.
If you want led bar display you would have to convert to voltage to run the decoders.
Allen
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N5KBP
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Posts: 295




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« Reply #7 on: August 15, 2007, 01:18:04 PM »

Thanks Allen, That will give me a good start.

73
Marty
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WB6BYU
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Posts: 13573




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« Reply #8 on: August 15, 2007, 09:12:24 PM »

The LM3914 is far easier to do this with than any analog meter circuit -
it really is almost trivial!

Steal an SWR bridge circuit from somewhere else - you need a voltage
for FORWARD and one for REVERSE out of it.  The ones with a ferrite
toroid are frequency independent, while those with a stripline circuit
will have more response at high frequencies than at low ones, but
either will work.

Connect the FORWARD voltage to the REF input on the LM3914.  
Connect the REFLECTED voltage to the INPUT pin.   Since the REF pin
has something like 25K of resistance across it for the comparators, you
need a similar resistor across the REFLECTED line so they are seeing the
same load.

If the maximum output from your SWR bridge is considerably less than
the maximum input to the LM3914 you can use a dual opamp (like a
LM358 that will accept zero volts as an input) and amplify both channels -
that will give you better sensitivity.

Also apply a tiny bit of forward bias to the REF pin to keep prevent a
random display of segments when there is no drive.

There was such a circuit in QST some years ago, and I developed one
myself just from the LM3914 datasheet and applications notes.  Those
should be available on the National Semiconductor web site.  That will
also show you how to cascade two chips.  It also shows some other
options that may be useful to reduce current draw for battery operation.

You may find that with the linear response you don't have as much
resolution as you might like at the lower SWR values.  In that case apply
the FORWARD voltage to a resistive divider to reduce the voltage on the
REF pin.  Basically the number of bars displayed depends on the ratio
of the INPUT voltage to the REFERENCE voltage.  In this arrangement the
ratio gives you an indication of SWR regardless of the actual voltage
levels applied.  I got such a circuit to work over at least a 20 : 1 range
of powers, and probably more.  Should be able to make something
that will work from 2 to 100 watts, allowing tuneup at low power then
operation at full power without having to readjust the meter.
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N5KBP
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Posts: 295




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« Reply #9 on: August 16, 2007, 01:56:07 PM »

Dale, That is exactly what I am looking at doing. I was hoping someone had done it before and obviously you have. I have the application notes for the lm3914 and will try and find the article from QST.

Thanks
Marty
N5KBP
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WB6BYU
Member

Posts: 13573




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« Reply #10 on: August 17, 2007, 06:51:31 AM »

It appeared in QST for February 1997.  A summary (including the schematic)
was in the Technical Topics column of the RSGB journal RadCom in May
1997, and is reprinted in the third volume of the Technical Topics
Scrapbook (1995 - 1999), available from the ARRL and other radio bookstores.

Let me know if you have any questions - my email is my callsign at
A double R L dot net.
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N5KBP
Member

Posts: 295




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« Reply #11 on: August 17, 2007, 08:39:01 AM »

Found exactly what I was looking for at

http://www.discovercircuits.com/

You need to check out this site. It is an excellent reference for the homebrewer.

They have a link to an EDN article from 1999 that is perfect for my application.

Thanks for all the help guys. It got me going in the right direction.
73
Marty
N5KBP
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