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Author Topic: 11 meter SWR meter on 2 meter?  (Read 2808 times)
KJ4RWH
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« on: March 18, 2010, 04:07:18 PM »

I'm new to Amateur Radio (Tech ticket, 4 weeks old) and have a nice Courier "Porta-Lab 500" swr/power meter from my CB days. Can I use it to accurately measure swr for my 2 meter equipment? I've already discovered it doesn't read FM power (meter kicks on key-up and returns to zero)
What can you recommend for a good balance in price/value?
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W2AEW
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« Reply #1 on: March 18, 2010, 04:20:55 PM »

I think you'll find that the CB SWR meter won't well (or at all) on 144MHz.  In fact, if it was going to work, it would read power just fine on FM.  The fact that it doesn't tells you right away that it is not going to work for SWR either on 2m.

There are a lot of VHF/UHF SWR meters out there.  You don't have to buy new.  You can look for a decent used meter on the classified here, or on www.qth.com or on www.qrz.com.  Or, even eBay if you dare.

Name brands will generally be decent meters:  Daiwa, Kenwood, Yaesu, Diamond, Autek, etc.  Even MFJ SWR meters should be fine.  There are some nice Heathkits out there too, but with these you are subject to the skill of the original builder.
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WB6BYU
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« Reply #2 on: March 18, 2010, 04:45:00 PM »

I use CB-style SWR meters all the time on 2m.  They aren't horribly accurate, of course, but
for most purposes the ones I've are quite adequate.  It does depend on how the meter is
built, however.

Most CB-style meters use two conductors in parallel with the feedline, etched onto a printed
circuit board.  They certainly show you "higher" or "lower" SWR, which is usually sufficient for
tuning an antenna.  I wouldn't rely on it to tell accurately whether the SWR was 1.8 : 1 or
2.2 : 1, but they may not have that accuracy on the intended frequency, either.

One thing that you'll find is that the meters take MUCH less power to drive on 2m than on
HF.  I use them with hand-helds, up to about 5 watts max, and have to reduce the
sensitivity way down.  (I've driven them to full scale with just a signal generator, but if
you are just reaching full scale the indicated SWR will read low.)  If you ran higher power
through the unit and it no longer indicates power, you may have blown the diode, which
should be a simple repair.

They work fine on FM signals (except that any calibrated power markings are way off)
but if your set includes a "modulation" meter it won't read anything.

They certainly aren't precision pieces of test equipment, but will do the job for tuning
an antenna for minimum SWR or checking coax for opens and shorts.

If you want to check the performance of the unit on 2m, first put on a good dummy
load and see how close it reads to 1 : 1 SWR.  Then find a second dummy load and
connect them to both sides of a T connector and put that on the output - it should
read 2 : 1 SWR.  You can also reverse the antenna and transmitter ports and use
the meter backwards to see if you get the same readings.  If so, it works well enough.
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K9FON
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« Reply #3 on: March 18, 2010, 05:04:12 PM »

Id look for a cheap 2 meter SWR meter or invest in an MFJ analyser. The MFJ analysers are worth thier weight in gold. I have worn out my MFJ 249 to a point that the on/of switch no longer works.
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WB2WIK
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« Reply #4 on: March 18, 2010, 08:37:25 PM »

Some "CB" SWR bridges work fine on 2 meters; some don't.

My old "Midland" SWR bridge (sold for CB use in the 1960s) was actually quite accurate on 2m, and for $9.95 was a good deal.  Some aren't that good.
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K1JHS
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« Reply #5 on: March 18, 2010, 11:36:28 PM »

>>>>K9FON>>>Id look for a cheap 2 meter SWR meter or invest in an MFJ analyser. The MFJ analysers are worth thier weight in gold. I have worn out my MFJ 249 to a point that the on/of switch no longer works<<<

Has your MFG 249 been upgraded to a 259? I read somewhere there was a kit to do that but have been unable to find out any more. Thanks..... John
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VK1OD
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« Reply #6 on: March 19, 2010, 03:07:56 AM »

You know, a key issue at a time when you are learning is whether you have confidence in your instrument.

If the instrument is specified to work over a particular range, and if it is a quality instrument, you can trust it to a certain extent.

With knowledge and experience, you might well be competent to decide whether your CB SWR meter is suited to 2m.

In my article VSWR measurement I list six measurements that you can perform to validate the instrument. Some, but not all, of these have been mentioned in the thread so far.

Owen

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WA3SKN
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« Reply #7 on: March 19, 2010, 06:37:19 AM »

There were two main types of VSWR meters, the bridged design using resistors or combination resistors and capacitors, and the later toroid transformer designs.
The bridged design became inaccurate up at 2 meters because of stray capacitances, while the ferrite designs were more restrictive on their frequency coverage due to the ferrite material involved.  Either could be converted to 2 meter use at the expense of HF coverage.
But since it would be easier to just get a meter already designed for 2 meters (they are fairly cheap), I would not recommend the conversion, except as a learning experience.
73s.

-Mike.
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WB6BYU
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« Reply #8 on: March 19, 2010, 09:09:28 AM »

Quote from: WA3SKN
There were two main types of VSWR meters, the bridged design using resistors or combination resistors and capacitors, and the later toroid transformer designs.


Actually, most inexpensive CB meters are of a third type having conductors parallel to the
center conductor of the coax built on a printed circuit board.  This has the advantage that
you can leave it inline and transmit through it at full power (unlike the resistor bridges).
The circuit is simpler than the toroid transformer design, especially since the critical
dimensions are built onto the PCB rather than depending on the windings of the toroid.
The toroid has the advantage that the output is relatively independent of frequency:
this allows the power calibration to hold reasonably well over much of the HF range, but
this isn't needed in a meter designed for CB use.

I've got at least 4 of the various CB meters that I've picked up over the years of
different brands (though two of them appear to be identical internally.)  All use the
parallel conductors, and all work reasonably well on 2m for tuning antennas for
minimum SWR.
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WB2WIK
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« Reply #9 on: March 19, 2010, 09:18:36 AM »

>RE: 11 meter SWR meter on 2 meter?       Reply
by WB6BYU on March 19, 2010    Mail this to a friend!

Actually, most inexpensive CB meters are of a third type having conductors parallel to the
center conductor of the coax built on a printed circuit board.<

::Then, there's a fourth type which is as you describe, but no printed circuit board at all!  The really old Midlands, etc from the 1960s used a small diameter copper tubing as the bridge's "center conductor" and two plated copper wires on opposite sides of that as the pickup lines, with detector diodes and terminations at opposite ends of those lines.  The wires were supported by simple phenolic spacers hanging onto the copper tubing to maintain their spacing.  Simple as hell.

Very frequency sensitive, yes: You could not change bands without recalibrating the "set" point; however this design works from HF through VHF and even UHF.  My old 1965-era Midland "$9.95 SWR meters" are actually pretty accurate at 222 MHz.  

They fall off in sensitivity a LOT at the lower HF spectrum and might take 100W or more to "full scale" on 80 meters.  At 27 MHz, 5W is all it takes; at VHF, even less!
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K1BXI
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« Reply #10 on: March 19, 2010, 10:00:32 AM »

"They fall off in sensitivity a LOT at the lower HF spectrum and might take 100W or more to "full scale" on 80 meters. At 27 MHz, 5W is all it takes; at VHF, even less!"

Since this is the elmers forum there may be some that would like to know why they are more sensitive as one goes higher in frequency. It puzzled me many years ago why my VSWR bridge was more sensitive at 10 meters than 80 meters, the opposite of my S-40B receiver.

At some point I figured it had to do with the physical wave length compared to the pick up length, or is it the sensitivity of the diodes? Or maybe both.

John
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W8JI
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« Reply #11 on: March 19, 2010, 10:33:23 AM »

There are a variety of ways to build an SWR bridge. Most of them are frequency restricted for a variety of reasons.

Personally, having designed a few dozen types over the years, I would never trust a 27 MHz bridge on two meters unless it was tested and verified. Because it seems to work does not mean it will work.

The three basic types are a Wheatstone bridge (wrongly called a resistor bridge), a transmission line coupler, and a lumped component coupler.

Most small analyzers are Wheatstone bridges or forms or Wheatstone bridges. This is by far the least frequency sensitive style when it uses all resistors. Low power resistor styles can go from dc to upper UHF with a good layout, but they are generally not suitable for transmitting through (although they can be made to transmit through if you give up frequency range).

Most CB meters are transmission line types. They have a small open transmission line and one or two sample lines that are terminated transmission lines. They change sensitivity because the transmission line couples tighter and tighter to the main line as it gets long and shorter in fractions of a wavelength as frequency is varied. When the lines are tiny fraction of wave long, they pick up very little energy from the through line. When they start to be a large fraction of a wave long, they can pick up a very large percentage of transmitter power. They are full of problems like calibration variation with frequency, sensitivity changes, and through-put SWR errors. They are really very poor wideband bridges, but they are good on a spot range. Motorola and others used them commercially, as did many ham manufacturers in the early days.

The most reliable type capable of power has lumped components. They use a capacitive divider to ground and a magnetic pickup (current transformer), resistor divider to ground and a magnetic pickup, or a transformer divider to ground and magnetic current pickup. This can be built with a variety of designs, but they have limited bandwidth also because of the bandwidth of the transformer and reactances of resistors or capacitors inside.

The Bird 43 is either the second type, or the last type, depending on the slug. Higher frequencies use a transmission line loop in the slug, while lower frequencies use a ferrite core wound with wire. The line through the meter is a transmission line (but then so is the line through a regular lumped unit in many cases).

Something built for 1-30 MHz is rarely good on 144, although there are lucky cases. Usually they will either not read SWR right, or they will have a terrible throughput SWR.

73 Tom
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VK1OD
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Posts: 1697




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« Reply #12 on: March 19, 2010, 01:36:42 PM »

"They fall off in sensitivity a LOT at the lower HF spectrum and might take 100W or more to "full scale" on 80 meters. At 27 MHz, 5W is all it takes; at VHF, even less!"

Since this is the elmers forum there may be some that would like to know why they are more sensitive as one goes higher in frequency. It puzzled me many years ago why my VSWR bridge was more sensitive at 10 meters than 80 meters, the opposite of my S-40B receiver.

At some point I figured it had to do with the physical wave length compared to the pick up length, or is it the sensitivity of the diodes? Or maybe both.


You are talking about an SWR meter of the so called Monimatch style.

Dealing with the diodes first, they rarely determine the upper frequency limit of such a coupler. The diodes are non linear devices, and depending on the diode V/I operating range, they may introduce calibration errors outside of their intended use, but this is more likely to be an issue lower in frequency.

When the coupler is very short wrt wavelength, the RF voltage fed to the detectors (diodes) depends on the reactance of the coupling capacitance between the main line and coupled line, and the reactance of the mutual inductance between the main line and coupled line. The capacitance and inductance are relatively constant, therefore the reactance is proportional to frequency, and RF voltage is proportional to frequency.

This equivalent lumped analysis is reasonably valid when the coupler is very short wrt wavelength, not so as the length is increased... but then the coupler isn't as good then for the same reasons.

Owen
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