Hi John, I am having difficulty seeing why your J200 reactance value is the ideal and why self resonance should be avoided. If the goal is to reduce common mode, would not the higher the Z the better the attenuation? If that is the case (and I can see no mathematical reason why this should not be so) then the high R of self resonance should be the goal. And if +J200 works, why would -J200 not perform as well? A -J200 choke could be realized by haveing self resonance BELOW the operating frequency. If I model a 40M half wave antenna in EZNEC and include a 3rd wire perpendicular to the horizontal wires to model the outside shield of a 36 foot piece of coax and then observe the current on the 3rd wire I see the following currents in the segment of wire #3 closest to the center fed dipole.: no choke current: 0.9A +J200 choke 0.20A -J200 choke 0.19A 200 Ohm choke 0.16A 2K Ohm choke 0.02 So I see that an inductive choke has the same efficacy as a capacitive choke. A resistive 200 Ohm choke is better than either reacive choke and a 2K resistive choke is about 10X better at reducing common mode than any of the others. 2K and even 4K or so is easily achieved in an air wound choke at self resonance. The EZNEC model is extremely simple and as I change segmentation it converges very nicely. So I do not see any reason to expect the model is not accurately representing a real world antenna system. Can you explain the math whereby you derived an optimum choke is +J200? What if the coax length was such that its shield reactance were -J200? That then would be the equivalence of series resonance and the current would be back to the highest value of 0.9A.
The formula to calculate the inductance needed for XL = 200 Ohm is wrong (19:31). It should be L [uH] = 200 / 2*Pi*f[MHz], derived from XL = 2*Pi*f*L LB8X
It would be great if someone could roll all of these calculators into one, so that we didn't have to jump to various websites. Seems like there's a need for this information amongst amateurs and some professional radio people. That way we don't have to work things backwards, and watch the video over and over to catch the web links.
Respectfully, 1.) SWR is NOT the determinate nor does it have any direct relationship to balun heating. Balun heating is primarily I^2*R heating in conductors and dissipative losses in cores and dielectrics. Balun heat has no direct or consistent relationship to SWR or mismatch. A system can have 100:1 SWR and nearly perfect energy transfer...and no noticeable heat. On the other hand a system can have 1:1 SWR and convert most of the RF energy to heat. 2.) The ideal impedance of a balun, or the minimum acceptable impedance is complicated. The ideal or even the minimum impedance is dependent on the entire system. Some antenna systems require no balun at all in a balanced to unbalanced junction and have almost perfect balance. In some systems even thousands of ohms is not enough. In some systems a balun or common mode choke can degrade performance. People do not like to hear it, but the real answer is system dependent. I doubt it is ever 200 ohms in any system. I don't know why anyone would say some precise target impedance, rather than a typical range for a system or a specific application.. 3.) A "square" form factor is only near ideal in a low reactance low impedance system. As operating impedance or inductor reactance increases optimal form factor becomes longer and smaller diameter. This is because shunt capacitance from turn-to-turn or end-to-end starts to increase circulating current in the inductor. In high impedance chokes and RF inductors, the optimal form factor tends towards being many times longer than diameter. The common range is from 1:1 L/D form to greater than 4:1 L/D.
Hi John,
I am having difficulty seeing why your J200 reactance value is the ideal and why self resonance should be avoided. If the goal is to reduce common mode, would not the higher the Z the better the attenuation? If that is the case (and I can see no mathematical reason why this should not be so) then the high R of self resonance should be the goal. And if +J200 works, why would -J200 not perform as well? A -J200 choke could be realized by haveing self resonance BELOW the operating frequency.
If I model a 40M half wave antenna in EZNEC and include a 3rd wire perpendicular to the horizontal wires to model the outside shield of a 36 foot piece of coax and then observe the current on the 3rd wire I see the following currents in the segment of wire #3 closest to the center fed dipole.:
no choke current: 0.9A
+J200 choke 0.20A
-J200 choke 0.19A
200 Ohm choke 0.16A
2K Ohm choke 0.02
So I see that an inductive choke has the same efficacy as a capacitive choke. A resistive 200 Ohm choke is better than either reacive choke and a 2K resistive choke is about 10X better at reducing common mode than any of the others. 2K and even 4K or so is easily achieved in an air wound choke at self resonance. The EZNEC model is extremely simple and as I change segmentation it converges very nicely. So I do not see any reason to expect the model is not accurately representing a real world antenna system.
Can you explain the math whereby you derived an optimum choke is +J200? What if the coax length was such that its shield reactance were -J200? That then would be the equivalence of series resonance and the current would be back to the highest value of 0.9A.
The formula to calculate the inductance needed for XL = 200 Ohm is wrong (19:31).
It should be L [uH] = 200 / 2*Pi*f[MHz], derived from XL = 2*Pi*f*L
LB8X
It would be great if someone could roll all of these calculators into one, so that we didn't have to jump to various websites. Seems like there's a need for this information amongst amateurs and some professional radio people. That way we don't have to work things backwards, and watch the video over and over to catch the web links.
Respectfully,
1.) SWR is NOT the determinate nor does it have any direct relationship to balun heating. Balun heating is primarily I^2*R heating in conductors and dissipative losses in cores and dielectrics. Balun heat has no direct or consistent relationship to SWR or mismatch. A system can have 100:1 SWR and nearly perfect energy transfer...and no noticeable heat. On the other hand a system can have 1:1 SWR and convert most of the RF energy to heat.
2.) The ideal impedance of a balun, or the minimum acceptable impedance is complicated. The ideal or even the minimum impedance is dependent on the entire system.
Some antenna systems require no balun at all in a balanced to unbalanced junction and have almost perfect balance.
In some systems even thousands of ohms is not enough. In some systems a balun or common mode choke can degrade performance.
People do not like to hear it, but the real answer is system dependent. I doubt it is ever 200 ohms in any system. I don't know why anyone would say some precise target impedance, rather than a typical range for a system or a specific application..
3.) A "square" form factor is only near ideal in a low reactance low impedance system. As operating impedance or inductor reactance increases optimal form factor becomes longer and smaller diameter. This is because shunt capacitance from turn-to-turn or end-to-end starts to increase circulating current in the inductor.
In high impedance chokes and RF inductors, the optimal form factor tends towards being many times longer than diameter. The common range is from 1:1 L/D form to greater than 4:1 L/D.