NanoVNA RG6 Test Results

digitalanalog

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I am soon going to be doing a test on an antenna, so I decided to buy a new piece of coax to use for testing the antenna, I got a 25' - RG6 - 75ohm piece at Home DePot for $11.00 out the door.

I calibrated the VNA, and set to 30-1200Mhz, set phase 1 to Resistance.
I bought 2 SMA to F adapters to connect the coax to the VNA.

I uncoiled the new coax and worked it for a while to get it to lay in a big "U" shape on the ground.

The results were very confusing to me, I admit I don't understand a whole bunch about coax, but these
numbers don't look right, with a reading as low as 24 ohms and as high as 109 ohm. How can this
be coming from One piece of coax, The saw tooth affect of ups and down just tells me this coax is
Junk, Am I right?

30 mhz. - 52.9 ohm 310 mhz - 29.3 ohm 590 mhz - 34.9 ohm 910 mhz - 55.1 ohm
40 mhz - 100 ohm 320 mhz - 59.9 ohm 600 mhz - 29.1 ohm 920 mhz - 97.3 ohm
50 mhz - 49.8 ohm 330 mhz - 38.1 ohm 610 mhz - 55.4 ohm 930 mhz - 55.7 ohm
60 mhz - 58.7 ohm 340 mhz - 39.6 ohm 620 mhz - 25.3 ohm 940 mhz - 55.3 ohm
70 mhz - 74.4 ohm 350 mhz - 72.9 ohm 630 mhz - 38.8 ohm 950 mhz - 68.9 ohm
80 mhz - 38.2 ohm 360 mhz - 37.2 ohm 640 mhz - 38.7 ohm 960 mhz - 39.5 ohm
90 mhz - 69.2 ohm 370 mhz - 73.2 ohm 650 mhz - 24.2 ohm 970 mhz - 60.6 ohm
100mhz - 41.0 ohm 380 mhz - 56.2 ohm 660 mhz - 51.7 ohm 980 mhz - 42.8 ohm
110mhz - 38.2 ohm 390 mhz - 49.1 ohm 670 mhz - 25.4 ohm 990 mhz - 39.5 ohm
120mhz - 63.2 ohm 400 mhz - 106 ohm 680 mhz - 34.9 ohm 1000 mhz - 51.1 ohm
130mhz - 29.8 ohm 410 mhz - 49.5 ohm 690 mhz - 38.7 ohm 1010 mhz - 31.5 ohm
140mhz - 45.1 ohm 420 mhz - 65.2 ohm 700 mhz - 25.4 ohm 1020 mhz - 43.1 ohm
150mhz - 37.4 ohm 430 mhz - 100 ohm 710 mhz - 55.6 ohm 1030 mhz - 39.1 ohm
160mhz - 27.2 ohm 440 mhz - 50 ohm 720 mhz - 31.8 ohm 1040 mhz - 33 ohm
170mhz - 51.8 ohm 450 mhz - 97.7 ohm 730 mhz - 35.9 ohm 1050 mhz - 51.0 ohm
180mhz - 24.4 ohm 460 mhz - 64.5 ohm 740 mhz - 53.7 ohm 1060 mhz - 27.6 ohm
190mhz - 38.9 ohm 470 mhz - 52.2 ohm 750 mhz - 30.8 ohm 1070 mhz - 34.5 ohm
200mhz - 32.6 ohm 480 mhz - 103 ohm 760 mhz - 66.2 ohm 1080 mhz - 35.7 ohm
210mhz - 24.8 ohm 490 mhz - 44.2 ohm 770 mhz - 40.5 ohm 1090 mhz - 33.2 ohm
220mhz - 49 ohm 500 mhz - 63.4 ohm 780 mhz - 43.8 ohm 1100 mhz - 44.3 ohm
230mhz - 24.8 ohm 510 mhz - 65.5 ohm 790 mhz - 76.9 ohm 1110 mhz - 31.1 ohm
240mhz - 33.5 ohm 520 mhz - 35.1 ohm 800 mhz - 42.7 ohm 1120 mhz - 37.6 ohm
250mhz - 38.7 ohm 530 mhz - 73.8 ohm 810 mhz - 65.3 ohm 1130 mhz - 38.4 ohm
260mhz - 23.9 ohm 540 mhz - 41.8 ohm 820 mhz - 72.5 ohm 1140 mhz - 29.1 ohm
270mhz - 53.7 ohm 550 mhz - 37.5 ohm 830 mhz - 49.4 ohm 1150 mhz - 47.9 ohm
280mhz - 27.0 ohm 560 mhz - 63.1 ohm 840 mhz - 109 ohm 1160 mhz - 37.6 ohm
290mhz - 38.3 ohm 570 mhz - 27.2 ohm 850 mhz - 63.5 ohm 1170 mhz - 44.2 ohm
300mhz - 45.4 ohm 580 mhz - 52.9 ohm 860 mhz - 67.5 ohm 1180 mhz - 44.9 ohm
870 mhz - 94.5 ohm
880 mhz - 53.2 ohm
890 mhz - 93. ohm
900 mhz - 66 ohm
 

prcguy

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Your using a 50 ohm instrument to measure 75 ohm coax, its acting normally and the coax is acting like an impedance transformer at different frequencies like its supposed to do. If you were using a 75 ohm instrument it should show about 75 ohms at all frequencies if both ends are terminated in 75 ohms.
 

digitalanalog

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Good point, I never thought about that.
So that means that anyone doing testing with a NanoVNA can only use 50 ohm coax to test an antenna if they want the test results to be acturate? Is that safe to say.
 

prcguy

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For professional (Agilent, HP, etc,) Vector Network Analyzers you can buy them in 75 ohm versions or use 50 to 75 ohm adapters and the cal process will normalize the instrument as a 75 ohm system so you can measure 75 ohm stuff properly. I had a 50 ohm system at my last job and purchased a 75 ohm system for some specific satellite TV projects.

I don't know if you can use the same adapters on the cheaper units and if they will cal properly. A few quick tests will tell you if it will work like cal everything and put two 75 ohm loads on a 75 ohm T adapter and see if the instrument reads 2.0:1 VSWR, etc. You would have to do that at a low frequency so the internal lengths of the adapter doesn't skew the measurement.


Good point, I never thought about that.
So that means that anyone doing testing with a NanoVNA can only use 50 ohm coax to test an antenna if they want the test results to be acturate? Is that safe to say.
 

prcguy

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Here are a few "minimal loss pads" which are resistive matching devices for going between 50 and 75 ohm things. They have 5.7dB loss which must be accounted for in the measurement or calibrated out. If the instrument can't cal it properly then the loss will skew any VSWR measurements because it will make a terrible match look much better. You would need to perform not only a through loss cal but a short/open/load cal so that any future VSWR or impedance measurements are accurate on the 75 ohm side of the minimal loss pads.

I usually have a pair of N to BNC, N to F, SMA to F and BNC to BNC minimal loss pads but I haven't done any 75 ohm measurements in the last few years and they are scattered around or loaned to somebody. Probably loaned my friend Bill. Bill, if you read this, give me back my stuff!

pads.JPG
 

digitalanalog

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the antenna has a 300-75ohm matching transformer, will my test results be accurate using the 50 ohm coax?
or will the transformer become an issue as well? I just what the test results to be accurate and not false for any reason.
 

prcguy

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A TV transformer is a 4:1 ratio to go from 75 ohms to 300 ohms. Some have capacitors and other parts inside so I don't know how married they are to 75 ohm coax. If its just a transformer then using 50 ohm coax would make it 200 ohms but I would have to make some measurements to see of that all holds true.

For your antenna, if its designed to be terminated in 300 ohms and it probably is, then terminating in 200 ohms would be a slight mismatch.

the antenna has a 300-75ohm matching transformer, will my test results be accurate using the 50 ohm coax?
or will the transformer become an issue as well? I just what the test results to be accurate and not false for any reason.
 

Ubbe

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Connect the VNA to the antenna using as short as possible 50 ohm coax and measure the antennas impedance over it's frequency range. That will tell how good match it will have to a 300/75 ohm transformer.

If you have a 330 ohm resistor, that are not frequency dependent, you can connect that to the 300 ohm side of the balun and then VNA measure the 75 ohm side for impedance over the frequency ranges it is suppose to operate at.

You can connect two baluns back to back, their 300 ohm side connected together, and measure two port with their 75 ohm side and check impedance and trannmission loss in the balun at different frequencies.

/Ubbe
 

digitalanalog

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I checked the two transformer mounting screws (where the 75/300 transformer would mount) with 2 seperate multi meters 1 digital and 1 analog, I get no reading what so ever, meaning there is no short and that is good, but how can i see what the ohms are at that transformer mounting location are?, Maybe a 75/300 ohm transformer is not what I need.

So question is, how do I check the antenna at the feed point to verify the ohm's? Is this possible, I have good meters not 9.00 dollar store ones so it's not the meter(s).
 

prcguy

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You would need a Vector Network Analyzer.

I checked the two transformer mounting screws (where the 75/300 transformer would mount) with 2 seperate multi meters 1 digital and 1 analog, I get no reading what so ever, meaning there is no short and that is good, but how can i see what the ohms are at that transformer mounting location are?, Maybe a 75/300 ohm transformer is not what I need.

So question is, how do I check the antenna at the feed point to verify the ohm's? Is this possible, I have good meters not 9.00 dollar store ones so it's not the meter(s).
 
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