cherokee tm-3000 swr readings seem way off.

[romakdaddy]

i got this meter from a friend a long time ago and always thought it was accurate till another friend got a brand new astatic 700.i checked the swr in my mobile with the new wilson 5000,matcoe 225 plus and a uniden pro 510 providing the signal.antenna is on the roof and my meter shows 1.1 swr barefooted,4 on low power,3 on medium and 3 on high.astatic shows 1 barefooted,1.1 on low and medium and 1.5 on high....seems my meter needs to be calibrated but can the astatic be trusted if its a better meter i may just get a new astatic700 myself.
discuss

 

[Beetle]

Did you take the Astatic out of the line and put the Cherokee exactly where the Astatic was? If not, try that.

 

[mackmobile43]

I use the same cherokee meter and it has been a decent meter which is close to all the other meters I operate(bird43p,ratshack and heathkit).

I'm not sure of the quality of the astatic meter mentioned but I'm sure if you recalibrate the cherokee at each power level you'll see a lower vswr and if you are sure you amp is ok then there's really no reason to believe what your meter shows you while checking the radio's vswr.

 

[romakdaddy]

Did you take the Astatic out of the line and put the Cherokee exactly where the Astatic was? If not, try that.

yes we tested them in the same place,there arent any radio techs locally,i wonder if i should find someone to send it to to have it calibrated.ive read good reports on this meter and they can still be had under the redman label and the MFJ grandmaster looks identical also.

 

[Beetle]

An actual laboratory "calibration" could easily run you a couple hundred dollars, which could probably be better used in buying a new meter.

Checking the meter against somebody's bench meter won't necessarily do you any good: how do you know the bench meter's within spec? If it doesn't have a current valid calibration label, it quite likely isn't.

 

[sp5it]

Calibration is simple.
All you need is a few non inductive resistors with proper power rating.
Connect radio, SWR meter and 100ohm resistor instead of antenna. Meter should show SWR 2:1
75ohm should give 1.5:1
50ohm 1:1
If not you can calibrate your meter with pots inside (if any).
Mike

 

[romakdaddy]

Calibration is simple.
All you need is a few non inductive resistors with proper power rating.
Connect radio, SWR meter and 100ohm resistor instead of antenna. Meter should show SWR 2:1
75ohm should give 1.5:1
50ohm 1:1
If not you can calibrate your meter with pots inside (if any).
Mike

not trying to be a smart A$$ but arent you describing a dummy load?

 

[sp5it]

Yes, but different resistance than 50ohm.
Mike

 

[Beetle]

In order to make it a true "calibration", however, those resistors themselves have to be calibrated. If they are the standard, they have to be traceable to the National Institute of Standards and Technology (formerly the National Bureau of Standards). That's for the USA. Other countries have their own Standards Laboratories.

The word "calibrated" doesn't mean "adjusted". All a basic calibration procedure does is compare (literal meaning) a measuring device against a standard having known characteristics and a significantly higher degree of accuracy and precision. You have to document the temperature and humidity (with calibrated thermometer and hygrometer!)...it's a complex process but it ensures that the meter or other device fully complies with the specifications that the manufacturer provides.

As far as those resistors goes, you'd need to know the true resistance, the temperature, how much current was flowing through them and how much that increased the temperature of the resistor and how that affected the final resistance...

 

[sp5it]

Lol. To kill a fly are you using a cannon?
Mike
p.s. 99% of chicken band and wannabe HF meters are showing wrong SWR readings.
Usually I'm going to the shop with set of three dummy loads. 50, 75 and 100 ohm. In most cases none of meters from the shelf are even in 10% error margin.
That's why I built VNA. Now I can really see what antenna can do.
M.