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Using a AA-55 to Null coax.

Metalhead

Member
Jan 15, 2020
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I'm making 1/2 wave jumpers with my Rig Expert 55 for 27mhz. My question is, I'm using a tee and a dummy load on top. Do I calibrate the 55 with just the tee on it and nothing else or do I calibrate it with the tee and the dummy load?
 

groundwire

Sr. Member
Jul 19, 2014
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There is no calibrating the unit at all to make coax stubs. You set the impedance in the menu and use the coax stub feature to cut to resonance. It will tell you to short or open the end as you go. You dont need the whole t and dummy load thing.
 

brandon7861

Well-Known Member
Nov 28, 2018
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There is no calibrating the unit at all to make coax stubs. You set the impedance in the menu and use the coax stub feature to cut to resonance. It will tell you to short or open the end as you go. You dont need the whole t and dummy load thing.
I used to use a tee and dummy load on my RigExpert AA-170. it was an older one without the stub feature (b+w screen). The reason for the tee and dummy load was so that when the coax was 1/2 wavelength, that side of the tee would see an infinite impedance and the RigExpert would only see the 50ohm dummy load and read 1:1 match. Any other length will be a reactance in parallel with the dummy load and change the SWR. It is a good method if you are careful and stop an inch early to account for the leg of the tee. These impedance analyzers are not very accurate at high impedances, which is why I figured measuring the stub at 50Ω was ideal. It saves a lot of messing around trying to short the end between trimmings.

Edit:
Perhaps these new RigExperts have a built-in 50Ω load that is switched in when that impedance is selected. I don't know, but the idea of having to short it when it asks tells me they cheated and are doing it with math and sketchy high-Z/low-Z readings. With the poor accuracy at high impedances (like half-wave open-ended cables), I cannot imagine why they would design it to measure stubs any other way. I am curious if anyone knows how they did it... Is there an internal load to parallel with the stub or did they cut that corner?
 
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brandon7861

Well-Known Member
Nov 28, 2018
161
182
73
36
I'm making 1/2 wave jumpers with my Rig Expert 55 for 27mhz. My question is, I'm using a tee and a dummy load on top. Do I calibrate the 55 with just the tee on it and nothing else or do I calibrate it with the tee and the dummy load?
My older rigexpert didn't have a calibration option (unless using the computer and applying the calibration to the data after the fact). You should be good to go as is, but if you were to calibrate (if that model allows it), then definitely calibrate without the tee.
 
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groundwire

Sr. Member
Jul 19, 2014
876
1,213
153
I used to use a tee and dummy load on my RigExpert AA-170. it was an older one without the stub feature (b+w screen). The reason for the tee and dummy load was so that when the coax was 1/2 wavelength, that side of the tee would see an infinite impedance and the RigExpert would only see the 50ohm dummy load and read 1:1 match. Any other length will be a reactance in parallel with the dummy load and change the SWR. It is a good method if you are careful and stop an inch early to account for the leg of the tee. These impedance analyzers are not very accurate at high impedances, which is why I figured measuring the stub at 50Ω was ideal. It saves a lot of messing around trying to short the end between trimmings.

Edit:
Perhaps these new RigExperts have a built-in 50Ω load that is switched in when that impedance is selected. I don't know, but the idea of having to short it when it asks tells me they cheated and are doing it with math and sketchy high-Z/low-Z readings. With the poor accuracy at high impedances (like half-wave open-ended cables), I cannot imagine why they would design it to measure stubs any other way. I am curious if anyone knows how they did it... Is there an internal load to parallel with the stub or did they cut that corner?
You can do it open end non shorted. Either way when i made all my transmission lines they came out absolutely perfect.
 

brandon7861

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Nov 28, 2018
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You can do it open end non shorted. Either way when i made all my transmission lines they came out absolutely perfect.
I never short mine, always open-ended when I make them. I started watching the above video and I am surprised he is judging the quality of those analyzers based on a velocity factor on a website that isn't even from the manufacturer. With all that test equipment, why not fire up that oscilloscope, toss a TDR pulse down the coax, and measure the actual velocity factor. I don't think I can give that video the hour and a half it requires (darned live streams lol).

To the OP, put the tape measure away and trust your RigExpert and your current plan. It will work great.
 

Metalhead

Member
Jan 15, 2020
7
2
13
61
I never short mine, always open-ended when I make them. I started watching the above video and I am surprised he is judging the quality of those analyzers based on a velocity factor on a website that isn't even from the manufacturer. With all that test equipment, why not fire up that oscilloscope, toss a TDR pulse down the coax, and measure the actual velocity factor. I don't think I can give that video the hour and a half it requires (darned live streams lol).

To the OP, put the tape measure away and trust your RigExpert and your current plan. It will work great.
I just wanted people to see what I talking about using the tee and dummy load. So I posted that video. The guy who made the video didn't clarify if he calibrated the AA-55 and if he did at what point.
 
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brandon7861

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Nov 28, 2018
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Actually it's for calibrating your "reference plane" (which I believe would change if I used a tee) and I have the calibration loads you mentioned.
That is part of it, especially when the analyzer does not offer port extensions (which are for moving that reference plane without needing to recalibrate). However, you do want to calibrate without the tee. When the coax on one side of the tee is 1/2 wavelength, the path through the tee to the dummy load will be a continuous 50Ω because both the tee and the dummy load are 50Ω. Therefore, calibrating at the port is just fine. You are not taking an impedance measurement, you are taking an SWR measurement which is irrespective of coax length (unless there are major losses or the coax acts as an antenna). Calibrating with the tee would be equally fine, except if you forget you did that and tried using the analyzer after the tee is removed, there will be an error. Calibrating without the tee (in this case at least) will not affect your measurement, but calibrating with the tee will affect future measurements of feed lines connected to an antenna. Therefore, it makes more sense to calibrate without the tee and, using the tee and dummy load, cut the cable using the SWR measurement only. Besides, as @groundwire said, you need an accurate calibration kit to calibrate properly so just leave it alone unless you have a $300 cal kit.

Edit: you can make a cal kit that will likely work fine on 11m for a few bucks, but it will be wrong at higher frequencies, especially VHF and up. Better to just leave it alone.
 

Metalhead

Member
Jan 15, 2020
7
2
13
61
That is part of it, especially when the analyzer does not offer port extensions (which are for moving that reference plane without needing to recalibrate). However, you do want to calibrate without the tee. When the coax on one side of the tee is 1/2 wavelength, the path through the tee to the dummy load will be a continuous 50Ω because both the tee and the dummy load are 50Ω. Therefore, calibrating at the port is just fine. You are not taking an impedance measurement, you are taking an SWR measurement which is irrespective of coax length (unless there are major losses or the coax acts as an antenna). Calibrating with the tee would be equally fine, except if you forget you did that and tried using the analyzer after the tee is removed, there will be an error. Calibrating without the tee (in this case at least) will not affect your measurement, but calibrating with the tee will affect future measurements of feed lines connected to an antenna. Therefore, it makes more sense to calibrate without the tee and, using the tee and dummy load, cut the cable using the SWR measurement only. Besides, as @groundwire said, you need an accurate calibration kit to calibrate properly so just leave it alone unless you have a $300 cal kit.

Edit: you can make a cal kit that will likely work fine on 11m for a few bucks, but it will be wrong at higher frequencies, especially VHF and up. Better to just leave it alone.
Thanks for the reply. I do have the calibration ends. It's only a shorted, open and 50 ohm end. That's all you need. To be plan what I'm looking for is to get the resonance as close to "0" as I can. I have seen calibrating it at the meter and at the tee with and without the dummy load get you different readings. I'm just unsure as to which one is correct. I've lost a good portion of my mind at this point.
 
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brandon7861

Well-Known Member
Nov 28, 2018
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Thanks for the reply. I do have the calibration ends. It's only a shorted, open and 50 ohm end. That's all you need.
In that case, feel free to calibrate with the tee. It will account for the capacitance of the coax port that is not used during the cal process and should allow you to cut to 1/2 wavelength without considering the extra inch the tee adds to the coax. Just remember to recalibrate without the tee before you make antenna measurements.
 
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