SWR Meter Calibration (how-to?)

[5artist5]

Ok Here is my situation. I have 3 SWR meters and they all give me different readings on the same setup. Two of then are the single needle ones that you "calibrate" and then measure, the other one is a cross needle one. The cross needle one was 3x more expensive but does that mean that it is calibrated properly? I can believe that it is more accurate than the other two and that it has nicer parts inside, but I am at a point that I feel like i need to calibrate it myself and know that it is telling me the truth. (I spend a lot of time letting this annoy me lately)

So my question is how do i calibrate it?

I assume that I need a signal generator and some way of amplifying the signal to the wattage I am looking for? And that this will take care of the FWD side?

But then do i have a second singal generator amplified so i can precisely control the reflected side?

Any help on this would be great, or any point in the right direction....

 

[gary in ohio]

Those are not lab equipment so dont expect lab results.
If all three meters are about that same then your done.
How far off are they? People get way to hung up on SWR
wanting the perfect 1:1. Good SWR means nothing about antenna performance.
I have give your an antenna with a 1:1 SWR and that doesnt radiate at all.
If your the lowest you can get under 2:1 your good. For most mobile environments your SWR is going to swing ALL over the place as you drive. The antenna, vehicles passing near by with change it. The proper way to measure the SWR is be in an open area with nothing including you around it.

If you want to calibrate your meter you find a good Bird watt meter and verify against a know good meter.

 

[5artist5]

There is a big difference. One may read 1.6 and the other may read 7!
I wasn't looking to set any antenna matching records or anything but I also
don't want to damage the radios, not to mention at 7 i'm probably better off with no antenna at all!

 

[gary in ohio]

Well you also need to take into account the frequency that meter is for. I your using a CB meter on 2m your going to have issues. I suspect with a 1.7 and 7 you either have a bad unit or frequency mismatch.

 

[5artist5]

Well one meter is the MFJ-818 and the other is the Vanco SWR-1. The Vanco one is for sure the right scale for CBs and I think that the MFJ one is in the range since it says 1.8MHZ to 30MHZ and CBs are 27mhz i think. So yeah I have to think that there may be a bad unit or something way out of calibration or something...

It would seem that the MFJ one would be more dependant on the calibration though since it is actually 2 meters and needles and you depend on both meters being calibrated in order to have then cross at the right spot on the indicator face. While the Vanco one is actually showing the ratio of one signal to the other with one needle that you calibrate each time?

But I was reading in the MFJ manual that you can calibrate it by using a dummy load and a "power meter" that is known to be accurate. I am interested in trying this. So can i use a dummy load such as this? And where do I find a "power meter that in known to be accurate"?

 

[DaveInDenver]

A bridge SWR meter that you adjust with each use is not inherently any more or less accurate than a calibrated cross needle meter. I have the same SWR-1 and I have a MFJ-864. If pressed, I trust the MFJ on CB primarily because I compared both forward and reflected power to a Bird watt-meter on my ham at 2m and it was well within any visual error, being a small display. If it's the SWR-1 that telling you a SWR of 1:7.0 (not 1:1.7, right?), I would toss it out. Either the measurement was made wrong or it's dead (half the bridge could be shorted, for example). An antenna that's even sorta close to being the right band will be better than that. My CB antenna is a Larsen NMO-27 and with it fresh out of the box it read 1:3.0 and I cut about 4 inches off the whip to get it tuned. An SWR of 1:7.0 is essentially an open, I'm not even sure my MFJ reads beyond 1:5.0 VSWR.

But, yet, the MFJ calibration is to use a known good watt-meter and/or known value amplifier. It's not complex and certainly only going to be as good as the chain of calibrations and measurements. The whole idea of getting a perfect 1:1 VSWR is sorta silly. If you measure below about 1:2 and are getting good range, then chasing that last few percentage is really not worth the time. Unless you know without a doubt that your meter is accurate, the difference between 1:1.and 1:1.5 SWR is a loss of about 4% of incident power. Even at SWR of 1.2.0, you have reflected about 10% of the forward power. So assuming your measurement station is absolutely perfect (you are doing SWR in a RF chamber, right?), the moment you hit the real world the amount of reflected power changes. The antenna flops around, objects around you, a change in atmospheric conditions, connections and cable that corrode, wear or loosen, all that will change what your radio sees reflected.

 

[5artist5]

Ok, again, I am not looking to get 1:1 I will be very happy with 1.3:1 so long as i can trust that it really is that low and not over 3:1 like I have been seeing.

It is the MFJ meter that is telling me readings over 3.

Ok so if I were to get a Bird wattmeter with a 50H 2-30mhz 50W element.

I should be able to determine the fwd wattage of the radio correct?
And then compare it to what the meter is saying it is?

And then would I be able to plug the transmitter in to the antenna side and do the same for the reflected?

Or would I have to have the radio to the radio in on the swr meter then the watt meter then the dummy load and see how the reflected compares?

 

[DaveInDenver]

How old is the MFJ and did you get it new? If it reads 3.0:1 SWR, then you probably really have high SWR. But if it's an old, used MFJ, it's certainly possible that someone abused it (i.e. 50W on 5W range, etc.).

A Bird will read both forward and reflected power. So if the one you have can be trusted (again, assuming it has not been abused), then you compare the SWR meter to the Bird and see which is right.

If you are using a dummy load, then you read the forward power into the load and see if matches the known power of the amplifier. Connect the meter as normal, radio to transmitter jack, antenna to antenna jack. Most current radios are going to be very close to their spec, my Yeasu FT-8800 is within 1% of 50W. It should be a perfect 1:1 SWR, there will be essentially no reflected power from a dummy load, all the power will heat the load. If the meter shows any SWR other than 1:1, then the meter is wrong.

To adjust the reflected, reverse the antenna and transmitter connections. BE CAREFUL! When you do this, be aware of the maximum reflected power is NOT THE SAME AS THE FORWARD. In the case of my MFJ-864, the 0-300W forward scale allows 0-60W reflected. So if I am checking the forward scale, I will use 100W on 0-300W, adjust the forward needle to indicate exactly 100W. When adjust the reflected scale, I would use something like 20W transmitted and adjust the reflected needle to 20W. YOU MOST DEFINITELY DO NOT WANT TO USE 100W FORWARD AND REFLECTED!!!!

 

[5artist5]

The MFJ meter is brand new.

Ok DaveInDenver, This seems like the route I need to take in order t osleep better at night.

So I will start looking for a dummy load and a wattmeter.

What do you think of the meters this guy has?
Or do you have any other suggestions about where to get one?

 

[DaveInDenver]

The MFJ meter is brand new.

Ok DaveInDenver, This seems like the route I need to take in order t osleep better at night.

So I will start looking for a dummy load and a wattmeter.

What do you think of the meters this guy has?
Or do you have any other suggestions about where to get one?

A Bird 43 is the one, $200 seems like an OK price. A new one is about $300 before you get a slug. The full set-up with a 100W slug will run you $450 easily new.

But, really, if the MFJ is new, I would trust it to read correctly. Make sure the environment and installation is good. If you did a good job installing, make sure the area is free of objects (buildings, phone poles, wires). A wide open parking lot is a good idea, not inside a garage or in an alley, for example. MFJ makes quality stuff, like I say my MFJ-864 was certainly close enough for my needs compared to a Bird.