With the focus on SWR and the constant back and forth i was having a difficult time understanding what would be the most important to have good antenna. This really helped me as a new Ham.
easy way to think of it. Reactance is the opposition to an Alternating current. Resistance is the opposition to all current. Inductive and capacitance reactance oppose each other, and by changing the inductance and capacitance you can get them to cancel out ( low net reactance) . When Capacitance and inductive reactance have equal values (cancel), resonance is achieved. So there is no opposition to the AC signal and more of the energy may be radiated. It might be good sometime to talk about what impedance matching really means.
Great explanation. When reactance is cancelled out, any mismatch on the antenna system will be purely resistive. This will be manifest in elevated SWR as a 50 ohm impedance match is not being presented to the transceiver. I think that's a concept Joe and I will have to tackle in a future video.
50ohm for coax was selected as a compromise in between best power handling (30 ohms ) and lowest loss (75 ohms) for air dielectric coax that was used in the 1940's. it drove a standard. Polyethylene filled (delelectic) coax has is 50ohms impedance for the ratios of diameters are (shield to center conductor) are about 3.6. so 50 ohms is a good choice.
Tough subject to tackle. At resonance we only have resistance to worry about and working with and transforming resistance is easy. Throw reactance in to the equation and it is not simple. I have seen amplifiers that will work into a 2:1 SWR, however the actual impedance can be 25 ohms or 100 ohms with no reactance or it can be some resistance with inductive reactance or some resistance with capacitive reactance . Almost an unlimited set of possibilities and still have 2:1 SWR. However the tuning of the amp changes drastically depending on the R and X. Sometimes the amp is not capable of matching some of these loads due to the combination of resistance and reactance. If radiation resistance is 50 and coax is 50 we have a perfect match if and only if the reactance is Zero. However if the resistance is 50 and we have reactance we will not have a 1:1 SWR and we must tune out the reactance with opposite kind to both achieve resonance and obtain that 1:1 SWR. If we have 100 ohms and no reactance we can transform the 100 to 50 easily with a transformer. If there is reactance it’s not quite so easy.
I am a Technician and trying to understand and adjust a 40 m inverted V dipole. The coax to my radio is about 150’ long. My best SWR is 1.65 at 7.075 at the radio. If I measure (NanoVNA) with a 25’ length of coax, the SWR is 1.97. Hmmmm. So, do I measure the SWR as close to the feed point as possible? I’ll be working through my just-arrived Antenna Book, but I’d love a video on improving the performance of dipoles. I mean, what should you look for or what can you do to make the antenna better? Great stuff. Thanks!! P.S. I bought a 2m dipole from KB9VBR. 👍
Yes, you should make your SWR measurements near the feed point of the antenna. With dipoles, this can be a challenge, but even through 25 feet of cable, you will get a more accurate measurement than with 150 feet. What is happening is that due to the impedance mismatch (impedance mismatches between the antenna and the transceiver are characterized as your Standing Wave Ratio), the coax is absorbing the SWR through dielectric heating. If your coax was long enough, the resistance in the cable would give you a perfect 1:1 match. DX Engineering recently produced an excellent video on SWR and how feed line losses affects it: ua-cam.com/video/L1_NLEpsW90/v-deo.html
I assume you are referring to the withstand voltage printed on the capacitor. That might well be 2 kV. I am not 100% sure, but as far as I know such a capacitor is used mainly to reduce reactance in the upper bands where the antenna as such might otherwise have inductive reactance as it is too long. Any expert may correct me if I am wrong - I am new to the ham radio hobby.
Very interesting indeed, not fully understood to be honest, and I have no idea what my ants reactance or impedance etc is, but I know it works extremely well, with qso's to the other side of the world everyday. If I see my swr is not through the roof and iam working long haul dx iam happy. Not sure what's happening with my 40m vertical on 30m, swr is really high, but with a tuner it works fantastic when it really shouldn't I don't think. I would hesitate to say, regardless of many things, if it works for you, just leave it hi.
Thank you for trying to explain reactance and resonance. It is my understanding that while it is true that net reactance is close to zero when an antenna (or system) is resonant, zero net reactance does NOT mean that the antenna system is resonant. It could, but not necessarily. Is that statement incorrect? Also, an antenna doesn't have to be resonant to radiate efficiently.....case in point, a 5/8th wave mobile vertical 2m whip.
You are correct, and I can think of several non resonant antennas that exhibit close to zero reactance. When we simplify concepts, we achieve a greater level of understanding, only to have edge cases mess up the whole system.
Have watched this a couple times and have passed the Extra exam, but still not on the air... 07:11: In this example then, would you want to insert a capacitor to offset the excess 10 ohm inductive reactance and bring net reactance to zero? If so, is there a formula to estimate the size capacitor required? Or what is an acceptably low reactance? Earlier you seem to dismiss the concept of trying to reduce SWR to as near 1:1 as possible. This is intriguing because everyone else seems to state the opposite and vendors have inserted limited SWR matching circuits in some radios. Can you clarify? Sorry to be dense but I'm clearly missing a concept or three. Really appreciate your channel! 73, John KK7JBZ
You’re not being dense. You asked some good questions. There are formulas that calculate the impedance of a capacitor or an inductor at a particular frequency. So, for any specific frequency there’s an inductance and capacitance that cancel out each other. For an inductor, the impedance in Ohms equals 2*Pi*frequency*L where the frequency is in Hertz and the inductance, L is in Henries. For a capacitor, the impedance is the inverse of that: impedance in Ohms = 1/(2*Pi*frequency*C) where again frequency is in Hertz and C is the capacitance measured in Farads. Because we use frequencies in the millions (sometimes billions) we end up with inductance and capacitance that are more easily expressed using prefixes such as milli, micro, and pico.
Is Return Loss in dB important for dialing in an antenna? It seems to coincide with SWR to some extent. So what I sense is that there is low SWR for the Wrong Reasons AND low SWR for the Right Reasons... I think that topic is one that you both should further explore.... with examples.... Art W1SWL
Return loss is just another way to quantify SWR. The lower your SWR, the higher your return loss value (in -dB) will be. But to your point, there are right and wrong for an SWR value to be present. If your antenna is resonant any SWR will be purely resistive, that is, not a pure 50 ohm match. If the resistance is within the tolerances of the transmitter, you can operate without using a transformer to get a perfect match (usually 2:1 or less). And that's the point I was trying to make with the Sporty Forty. I could have used to my tuner to 'fix' the match, but opted not to add another level of inefficiency into my antenna system. As you can see, we can get deep into the weeds really quick when discussing resonance, resistance, and reactance. Entire books have been written on the subject with more math than you can shake a stick at. Maybe Joe and I can break this stuff down more in future videos.
I have real dipoles in the real world. Zero reactance is fine in theory but how many people achieve it? I don't. If you do tune an aerial exactly to resonance we move up and down in frequency so it's no longer resonant. I go for a low SWR because we have to work with what we've got, height above ground, brickwork, trees and buildings affect SWR and resonance and impedance. My dipoles for 20, 17, 15 and 10m range from 43 Ω to 75 Ω with varying SWR readings and reactance values.. Do I worry? No! I'll still say tune for minimum SWR as the tx is happy and I get out actually resonant or not. I had a guy say we must know the impedance and reactance to tune a ¼λ vertical. I asked him four times knowing these values what would he do to tune the aerial and finally said he wouldn't do anything because he didn't need to. How he can say that I don't know because I asked him what frequency his aerial worked at and got no answer. He repeated it's 36 Ω and that's all. I don't believe he had either an aerial or VNA. He's using a textbook value and so all he knows is it's 36 Ω. I said we move up and down in frequency so it's only resonant on one frequency so would he never move from it's point of resonance. He didn't answer. I said getting a low SWR is good enough and he again criticised me. He finally got round to insults saying I'm clueless and showing up my G callsign. Which means almost all 3 million amateurs Worldwide who tune aerials this way must also be clueless according to him. G4GHB.
With the focus on SWR and the constant back and forth i was having a difficult time understanding what would be the most important to have good antenna. This really helped me as a new Ham.
Joe is a natural teacher! Wow what a simple explanation for calculating impedance.
easy way to think of it. Reactance is the opposition to an Alternating current. Resistance is the opposition to all current. Inductive and capacitance reactance oppose each other, and by changing the inductance and capacitance you can get them to cancel out ( low net reactance) . When Capacitance and inductive reactance have equal values (cancel), resonance is achieved. So there is no opposition to the AC signal and more of the energy may be radiated. It might be good sometime to talk about what impedance matching really means.
Well done
Great explanation. When reactance is cancelled out, any mismatch on the antenna system will be purely resistive. This will be manifest in elevated SWR as a 50 ohm impedance match is not being presented to the transceiver. I think that's a concept Joe and I will have to tackle in a future video.
Thanks!
Thank you for your support, it is greatly appreciated.
50ohm for coax was selected as a compromise in between best power handling (30 ohms ) and lowest loss (75 ohms) for air dielectric coax that was used in the 1940's. it drove a standard. Polyethylene filled (delelectic) coax has is 50ohms impedance for the ratios of diameters are (shield to center conductor) are about 3.6. so 50 ohms is a good choice.
Tough subject to tackle. At resonance we only have resistance to worry about and working with and transforming resistance is easy. Throw reactance in to the equation and it is not simple. I have seen amplifiers that will work into a 2:1 SWR, however the actual impedance can be 25 ohms or 100 ohms with no reactance or it can be some resistance with inductive reactance or some resistance with capacitive reactance . Almost an unlimited set of possibilities and still have 2:1 SWR. However the tuning of the amp changes drastically depending on the R and X. Sometimes the amp is not capable of matching some of these loads due to the combination of resistance and reactance. If radiation resistance is 50 and coax is 50 we have a perfect match if and only if the reactance is Zero. However if the resistance is 50 and we have reactance we will not have a 1:1 SWR and we must tune out the reactance with opposite kind to both achieve resonance and obtain that 1:1 SWR. If we have 100 ohms and no reactance we can transform the 100 to 50 easily with a transformer. If there is reactance it’s not quite so easy.
I am a Technician and trying to understand and adjust a 40 m inverted V dipole. The coax to my radio is about 150’ long. My best SWR is 1.65 at 7.075 at the radio. If I measure (NanoVNA) with a 25’ length of coax, the SWR is 1.97.
Hmmmm.
So, do I measure the SWR as close to the feed point as possible?
I’ll be working through my just-arrived Antenna Book, but I’d love a video on improving the performance of dipoles. I mean, what should you look for or what can you do to make the antenna better?
Great stuff. Thanks!!
P.S. I bought a 2m dipole from KB9VBR. 👍
Yes, you should make your SWR measurements near the feed point of the antenna. With dipoles, this can be a challenge, but even through 25 feet of cable, you will get a more accurate measurement than with 150 feet. What is happening is that due to the impedance mismatch (impedance mismatches between the antenna and the transceiver are characterized as your Standing Wave Ratio), the coax is absorbing the SWR through dielectric heating. If your coax was long enough, the resistance in the cable would give you a perfect 1:1 match.
DX Engineering recently produced an excellent video on SWR and how feed line losses affects it: ua-cam.com/video/L1_NLEpsW90/v-deo.html
Would the same principal be applied for a 9:1 unun in a sloper config?
What is the purpose of a 2KV capacitor on the inside of an antenna? What is its purpose?
Capacitors are usually measured in Farads. Do you mean a resistor?
I assume you are referring to the withstand voltage printed on the capacitor. That might well be 2 kV. I am not 100% sure, but as far as I know such a capacitor is used mainly to reduce reactance in the upper bands where the antenna as such might otherwise have inductive reactance as it is too long.
Any expert may correct me if I am wrong - I am new to the ham radio hobby.
great video thanks
Very interesting indeed, not fully understood to be honest, and I have no idea what my ants reactance or impedance etc is, but I know it works extremely well, with qso's to the other side of the world everyday.
If I see my swr is not through the roof and iam working long haul dx iam happy.
Not sure what's happening with my 40m vertical on 30m, swr is really high, but with a tuner it works fantastic when it really shouldn't I don't think. I would hesitate to say, regardless of many things, if it works for you, just leave it hi.
Thank you for trying to explain reactance and resonance. It is my understanding that while it is true that net reactance is close to zero when an antenna (or system) is resonant, zero net reactance does NOT mean that the antenna system is resonant. It could, but not necessarily. Is that statement incorrect? Also, an antenna doesn't have to be resonant to radiate efficiently.....case in point, a 5/8th wave mobile vertical 2m whip.
You are correct, and I can think of several non resonant antennas that exhibit close to zero reactance. When we simplify concepts, we achieve a greater level of understanding, only to have edge cases mess up the whole system.
Have watched this a couple times and have passed the Extra exam, but still not on the air...
07:11: In this example then, would you want to insert a capacitor to offset the excess 10 ohm inductive reactance and bring net reactance to zero? If so, is there a formula to estimate the size capacitor required?
Or what is an acceptably low reactance?
Earlier you seem to dismiss the concept of trying to reduce SWR to as near 1:1 as possible. This is intriguing because everyone else seems to state the opposite and vendors have inserted limited SWR matching circuits in some radios. Can you clarify?
Sorry to be dense but I'm clearly missing a concept or three.
Really appreciate your channel!
73,
John KK7JBZ
You’re not being dense. You asked some good questions. There are formulas that calculate the impedance of a capacitor or an inductor at a particular frequency. So, for any specific frequency there’s an inductance and capacitance that cancel out each other. For an inductor, the impedance in Ohms equals 2*Pi*frequency*L where the frequency is in Hertz and the inductance, L is in Henries.
For a capacitor, the impedance is the inverse of that: impedance in Ohms = 1/(2*Pi*frequency*C) where again frequency is in Hertz and C is the capacitance measured in Farads.
Because we use frequencies in the millions (sometimes billions) we end up with inductance and capacitance that are more easily expressed using prefixes such as milli, micro, and pico.
Is Return Loss in dB important for dialing in an antenna? It seems to coincide with SWR to some extent. So what I sense is that there is low SWR for the Wrong Reasons AND low SWR for the Right Reasons... I think that topic is one that you both should further explore.... with examples.... Art W1SWL
Return loss is just another way to quantify SWR. The lower your SWR, the higher your return loss value (in -dB) will be. But to your point, there are right and wrong for an SWR value to be present. If your antenna is resonant any SWR will be purely resistive, that is, not a pure 50 ohm match. If the resistance is within the tolerances of the transmitter, you can operate without using a transformer to get a perfect match (usually 2:1 or less). And that's the point I was trying to make with the Sporty Forty. I could have used to my tuner to 'fix' the match, but opted not to add another level of inefficiency into my antenna system.
As you can see, we can get deep into the weeds really quick when discussing resonance, resistance, and reactance. Entire books have been written on the subject with more math than you can shake a stick at. Maybe Joe and I can break this stuff down more in future videos.
The sound was really bad and with graph or any illustration it will be more informative
I have real dipoles in the real world.
Zero reactance is fine in theory but how many people achieve it? I don't. If you do tune an aerial exactly to resonance we move up and down in frequency so it's no longer resonant. I go for a low SWR because we have to work with what we've got, height above ground, brickwork, trees and buildings affect SWR and resonance and impedance.
My dipoles for 20, 17, 15 and 10m range from 43 Ω to 75 Ω with varying SWR readings and reactance values.. Do I worry? No!
I'll still say tune for minimum SWR as the tx is happy and I get out actually resonant or not.
I had a guy say we must know the impedance and reactance to tune a ¼λ vertical.
I asked him four times knowing these values what would he do to tune the aerial and finally said he wouldn't do anything because he didn't need to. How he can say that I don't know because I asked him what frequency his aerial worked at and got no answer. He repeated it's 36 Ω and that's all. I don't believe he had either an aerial or VNA. He's using a textbook value and so all he knows is it's 36 Ω.
I said we move up and down in frequency so it's only resonant on one frequency so would he never move from it's point of resonance. He didn't answer.
I said getting a low SWR is good enough and he again criticised me.
He finally got round to insults saying I'm clueless and showing up my G callsign. Which means almost all 3 million amateurs Worldwide who tune aerials this way must also be clueless according to him.
G4GHB.