Smith Chart, OSL and Calibration AA-650

[NightThumper]

Recently upgrade my antenna analyzer to a RigExpert AA-650. Have a couple of questions regarding calibration and the Smith Chart using a OSL calibration kit.

First my understanding is that every time you change frequency technically you need to re-calibrate.

Secondly if the electrical length changes so do the plots on the Smith Chart. Which would mean that any variance in manufacturing of the connectors be they of the SO-239 type, Sma or N type results in Pico/Nano changes in the electrical length.

Attached are three pictures showing my Smith Chart plots...open end at the RigExpert, open end 1.5 feet of coax and open end 17 feet of coax. Help me understand what I’m seeing on the three plots.

Now onto calibration questions.
1. For the best accuracy would I need to calibrate by disconnecting the antenna, connecting my OSL (open, short, load) at the end of the coax nearest the antenna feed point?
2. My understanding is that calibrating at the analyzer output point is not where I want the calibration to take place. Would a short piece of coax be best?
3. Obviously this isn’t a military/space/precision setup...only HF/6 meters and UHF/VHF.
4. My take is that the Pico/Nano seconds of change in the electrical length is probably non-consequential when it comes to the frequencies/bands involved.

I’m sure I’ll have other questions along the way.

No I’m not designing antennas just seeking to expand my limited understanding.

Thanks in advance
Brad
KE0XS
South of Pittsburgh

The first picture is using a 17 foot piece of coiled coax.
The second picture is nothing connected to the analyzer
The third picture is using a 1.5 foot pice of coax

 

[DainBramage]

...

First my understanding is that every time you change frequency technically you need to re-calibrate.

Secondly if the electrical length changes so do the plots on the Smith Chart. Which would mean that any variance in manufacturing of the connectors be they of the SO-239 type, Sma or N type results in Pico/Nano changes in the electrical length. ...

The aa650 does not require any calibration, however if you want to do the OSL routine, it is best done with the actual coax you will be using and "OSL"ed at the disconnected end.

Yes, any UHF connector (or an N connector @ the higher freqs) will be seen as an "impedance lump". The effect, while measureable, will be practically non-
existent at HF Amateur freqs.

 

[The DB]

1. For the best accuracy would I need to calibrate by disconnecting the antenna, connecting my OSL (open, short, load) at the end of the coax nearest the antenna feed point?

I'm not sure on this specific analyzer. However, mine from another company has a general calibration, which allows you to plug the analyzer directly into the antenna and get good data.

However, if I want to factor out the effects of coax in between the analyzer and antenna I have to create a new calibration file that includes the coax. This file covers a specific frequency range.

2. My understanding is that calibrating at the analyzer output point is not where I want the calibration to take place. Would a short piece of coax be best?

If you calibrate out the coax every time it will be more accurate. If you are measuring HF and limit yourself to something like a 6 inch length of coax, that will also be close enough.

#3 isn't a question...

4. My take is that the Pico/Nano seconds of change in the electrical length is probably non-consequential when it comes to the frequencies/bands involved.

What the device does is pulse a frequency, and waits for an SWR reflection to come back. It doesn't matter what frequency is used because they all travel at the same speed down the feed line.

Some devices will then ask for a velocity factor to adjust the results as all feed lines have a velocity factor, although not all devices do this, so you might have to do that yourself.

From here most will give a raw measurement length, while others will factor in the frequency used to give a number of electrical lengths down the feed line.

Here is a video of someone using an oscilloscope to do this visually.




The DB