Antenna Modeling Uploads

[bob85]

i don't know of a way to use a current probe to see if a sigma style antenna has in phase radiation from the sleeve eddie,
how would a simple meter separate transmissionline mode currents flowing in one direction from antenna mode currents flowing the opposite direction?

 

[The DB]

How about some comments from others on your results if you've experimented with a coaxial choke of some type.

OK, lets see if this helps.

I begin tuning with my analyzer connected directly to the antenna. I then create a custom calibration file which effectively nullifies the effects of the coax. The measurements on the antenna with the analyzer directly connected should be very close to the measurements through the coax and the custom calibration file.

Sometimes they are not as close as I would like. When that happens I add an air choke at the antenna feedpoint, which typically resolves the issue, although not always.

When the air choke at the feedpoint doesn't resolve the tuning issue I move the choke 1/8 wavelength down the coax, and again 1/4 wavelength down the coax from the antennas feedpoint. Whichever of the three measurements taken with chokes at the various points in the feedline is the closest to the measurements from the analyzer plugged directly into the antenna determines the final placement of said choke.

I will say most antennas have no need for a choke as far as tuning is concerned. Other antennas definately need a choke, and if they don't their tuning is greatly affected.

Coax length affecting the impedance where the common mode currents are is a fairly new idea to me. However, I can see where once an antenna has already been tuned, tuning the feedline to be naturally resistant to CMC's on a band (or bands) of use would be a benefit. I am hoping that doing such would potentially do away with the need of a choke to begin with.

Note the red text, I am not in any way saying that an antenna should be tuned solely or primarily by varying the length of the feedline. I am suggesting that after the antenna has been tuned there may be a benefit to varying the length of the feedline for other purposes, such as minimizing CMC's.

Also, none of this addresses the potential issue of the antenna being electrically connected to a mast (electrically grounded or otherwise) and the effects that mast may have on the antenna.

I have not always done things this way. Much of the why behind what I do I learned from reading this very forum, and I am still learning.


The DB

 

[Marconi]

i don't know of a way to use a current probe to see if a sigma style antenna has in phase radiation from the sleeve eddie,
how would a simple meter separate transmissionline mode currents flowing in one direction from antenna mode currents flowing the opposite direction?

Good point Bob. I rectum we'll never know for sure.

Hey Henry how are you coming on your CST project?

 

[Marconi]

OK, lets see if this helps.

I begin tuning with my analyzer connected directly to the antenna. I then create a custom calibration file which effectively nullifies the effects of the coax. The measurements on the antenna with the analyzer directly connected should be very close to the measurements through the coax and the custom calibration file.

Sometimes they are not as close as I would like. When that happens I add an air choke at the antenna feedpoint, which typically resolves the issue, although not always.

When the air choke at the feedpoint doesn't resolve the tuning issue I move the choke 1/8 wavelength down the coax, and again 1/4 wavelength down the coax from the antennas feedpoint. Whichever of the three measurements taken with chokes at the various points in the feedline is the closest to the measurements from the analyzer plugged directly into the antenna determines the final placement of said choke.

I will say most antennas have no need for a choke as far as tuning is concerned. Other antennas definately need a choke, and if they don't their tuning is greatly affected.

Coax length affecting the impedance where the common mode currents are is a fairly new idea to me. However, I can see where once an antenna has already been tuned, tuning the feedline to be naturally resistant to CMC's on a band (or bands) of use would be a benefit. I am hoping that doing such would potentially do away with the need of a choke to begin with.

Note the red text, I am not in any way saying that an antenna should be tuned solely or primarily by varying the length of the feedline. I am suggesting that after the antenna has been tuned there may be a benefit to varying the length of the feedline for other purposes, such as minimizing CMC's.

Also, none of this addresses the potential issue of the antenna being electrically connected to a mast (electrically grounded or otherwise) and the effects that mast may have on the antenna.

I have not always done things this way. Much of the why behind what I do I learned from reading this very forum, and I am still learning.


The DB

DB, I'm curious. Is this file you create something connected with the software used with your analyzer?

If not, how do you create such a file and what is used to do it?

 

[The DB]

DB, I'm curious. Is this file you create something connected with the software used with your analyzer?

If not, how do you create such a file and what is used to do it?

The software for my analyzer (AIM4170C) creates a standard calibration file for anything as plugged directly into the SO-239 plug. To make this file the analyzer asks for a short circuit load, an open circuit load, and a pure resistive load of some amount. These loads came with the analyzer for this purpose.

I can also create a custom calibration file using these same loads (or their electrical equivalents if I am using a balun for example) and calibrate out the effects of pretty much anything. The analyzer uses this file to effectively cancel out whatever was between the analyser itself and the point the test loads were attached so anything else that is attached to that point is treated like it is plugged directly into the unit (or very close to it).

Because of this I can compare results as measured from both sides of the feed line with only minimal feedline effects coming into play. If the readings are noticeably different it is from one of two causes. One is an open or short in the feedline where one shouldn't be. These will drastically interfere with the readings effectively creating a test load condition no matter what is happening elsewhere. The other is other currents on the feedline that were not present when I made the custom calibration file.

This capability has functions beyond antenna tuning as well.


The DB

 

[Marconi]

The software for my analyzer (AIM4170C) creates a standard calibration file for anything as plugged directly into the SO-239 plug. To make this file the analyzer asks for a short circuit load, an open circuit load, and a pure resistive load of some amount. These loads came with the analyzer for this purpose.

I can also create a custom calibration file using these same loads (or their electrical equivalents if I am using a balun for example) and calibrate out the effects of pretty much anything. The analyzer uses this file to effectively cancel out whatever was between the analyser itself and the point the test loads were attached so anything else that is attached to that point is treated like it is plugged directly into the unit (or very close to it).

Because of this I can compare results as measured from both sides of the feed line with only minimal feedline effects coming into play. If the readings are noticeably different it is from one of two causes. One is an open or short in the feedline where one shouldn't be. These will drastically interfere with the readings effectively creating a test load condition no matter what is happening elsewhere. The other is other currents on the feedline that were not present when I made the custom calibration file.

This capability has functions beyond antenna tuning as well.


The DB

Thanks of the info, but I don't understand it. I was clearly interested in how it might be done if not using your software. I figured I would not understand much using your analyser.

I'm not sure, but I think Eznec will allow one to do something similar with the load, but I don't know how to use those features either.

 

[The DB]

It is a feature that most analyzers I am aware of do not have, and it definitely has its uses.

I think once you used it once or twice you would catch on, it isn't a hard analyzer to use, but you do need a computer. If I find myself in your neck of the woods one day I'll bring it by and see what you think.


The DB

 

[Marconi]

It is a feature that most analyzers I am aware of do not have, and it definitely has its uses.

I think once you used it once or twice you would catch on, it isn't a hard analyzer to use, but you do need a computer. If I find myself in your neck of the woods one day I'll bring it by and see what you think.


The DB

That would be great, I'll PM you my phone #.