Hi, I want to make some clarification here. The yagi's driven element has the J shape and has a feed point impedance of around 150 ohms in free space. (The regular folded dipole structure is employed for a wide bandwidth (as in analogue TV receiving antenna) and has a feed point impedance around 300 ohms) Here in this antenna impedance matching (tweaking for low SWR) to a 50-75 ohm coaxial cable can be done by trimming a bit off the free end. That's why the free end of it is shorter compared to the first director. The explanation for the working of the yagi-uda antenna itself needs clarification too. The dipole (in free space) alone has a feed point impedance of around 75 ohm +j43 ohm and has a directivity gain of 2.15 dBi. In the form of a yagi uda directional antenna the feed point impedance of the dipole (driven element) gets lowered to around 35 ohm or even less depending on the number of parasitic elements (reflector and the directors) you are going to mount on the boom to achieve more directivity gain in a particular direction. This is because the dipole current is getting shared among the parasitic elements by mutual coupling. The spacing between the driven element and the reflector is worked out to around 0.2 lambda; the spacing between the driven element and the first director is around 0.15 lambda. It may be varied for a specific Front to Back ratio and cancellation of the side lobes. The signal from the dipole tends to propagate in all directions and particularly in both forward and reverse directions in the plane of the yagi antenna. The reflector (usually being 5% larger in size) and the typical spacing result in interference with the signal emanating from the active driven element. Part of the reflected signal after a small delay constructively interferes with the signal on the driven element and moves forward. The current on the director element too constructively interferes with the signal from the driven element and all move forward as if guided along the boom. With each half cycle the wavelets alternate the polarity but keep moving forward as if a light beam (after all light itself is the visible part of the EM spectrum) is being directed by a parabolic reflector and condenser lenses etc. The centre of the boom being at isoelectric potential can have the centre of a folded dipole or the centre of the driven element used in this antenna and all the parasitic elements' centre anchored to it. Any small metal fragments scattered in between the elements unconnected to each other and to any of the elements or the boom would be invisible to the RF energy propagating in the forward direction. So, the letters having copper in them would not affect the performance. This 2.4 GHz Wi-Fi yagi uda antenna must have produced the S11 pattern as found in the literature that came along; I don't know why you didn't get it right. Moreover there's no ground plane or the need for a ground plane as such. A dipole is a balanced device. The coaxial cable (intrinsically unbalanced device) has both the hot (centre conductor) and the cold (outer shield braid) terminals that are connected to the dipole at the center. The half lambda dipole has maximum current in the middle and maximum voltage at the ends; the current and the voltage are 90 degrees out of phase. Because of maximum current and minimum voltage at the center the impedance is low, ie, around 75 + j43 ohm. The RF power fed to the feed point of the dipole is dissipated in the (virtual resistor) Radiation Resistance; yes at resonance the inductive reactance and the capacitive reactance are getting cancelled in the otherwise series resonant half lambda dipole antenna elements. The voltage at the ends maybe around 2500-4000 ohm or more depending on many factors like height of the feed point above the ground, nearby RF absorbing, scattering or reflecting structures. The EFHWA antenna is fed with a 49:1 UnUn for this reason. De VU2RZA
I tend to think WA5VJB added the lettering just to make a point (not exactly sure what that is; but, I can guess.) Anyway, I tend to think... that like you said... that lettering is in between the directors... and in the near field of the antenna... so at least the phasing effects of the directors will make the copper lettering's presence immaterial. (Maybe IMASI guy can grind them off, and see some significant difference?)
Thanx to you and IMSAI Guy. My understanding of antenna is very weak. When I see something that doesn't make sense, I try to find out why rather than to assume the designer made a mistake. His drawing reminds me of two ways to feed a dipole. @6:00 One with a grounded center, the driven element connected to the boom. I believe this was an unbalanced feed. The other had the driven element isolated from the would be a balanced feed. It seems the Mosley TA33 and TA33 Classic is where I saw these configurations (30 years ago). One used a gamma match and the other used a t-match. Do I understand this correctly? I also notice the reflector is connected to the driven element, where the directors are not. It seems this connection is important. Why? I read the Richard Feynman book QED and one chapter described diffraction gratings. I think this helped me understand the co-linear antenna from the perspective of the diffraction grating. By cancelling every other 1/2 wavelength it would only provide, what you called constructive interference. So when I see the fold on the left of the lower drawing @6:00, I see this to cancel the 1/2 destructive wave. I would be interested to hear from a knowledgeable person about the correlation between the refraction grating and the co-linear antenna.
Confirmed. It is a typical Yagi-Uda antenna and method to feed the antenna is very common , including the short transmission line and connection to the reflektor. I expected almost that element length measurement should make confusion in this video. If assuming correct calibration, tuning could be somewhat improved. A possibility is that this antenna is a copy of a copy design. Different PCB types and PCB thickness then original intended design can then cause minor detuning. A Smith-chart hade been informative both about antenna tuning and calibration.
WA5VJB (Kent Britain) is an RF guru particularly when it comes to microwave antenna design. I spoke with him about his antenna designs maybe 15 to 20 years ago. The "strange" feed mechanism is by design: it offers a balun function, impedance match, and makes it a little more broadband, at the expense of a slightly off-bore direction, which is insignificant for this relatively low gain. Of note is that you must remember that the PCB substrate is not benign here: it has a significant effect on the antenna's performance in terms of impedance matching and size due to the relative permittivity of the dielelctric. I've had lots of success with his many Yagi designs over ht past couple of decades, from VHF to SHF using his designs: they are the most reproducible Yagi antennas I've ever built. Basically, he _really_ knows his onions: I'd be very careful about second guessing his expertise in this area!
I found this video (and its comments) to be very informative and I thank you for it... any mistakes or incomplete explanations be damned. The video started a fine conversation and the comments have been filling in the gaps. This is all good :)
The ends are voltage (E field) anti-nodes and current (B field) nodes. The center is an E node and B anti-node. The total distance across is half of a wavelength and not a full wavelength. As a first approximation, the reflector acts like a mirror and places an image of the dipole (and directors but with 1/r^2 intensity reduction) behind itself. This mirror changes the charge sign and the distance delays the phase, however.
Active element looks like end fed antenna. Also coax feeder may transmit/receive and shape of feeder may impact antenna's parameters if PCB path do not balance line enough.
You keep drawing the driven element as a full wave antenna, IT IS A HALF WAVE! The plots on the data sheet were done with a Rhode & Schwarz FSL analzyer. Letters are only 1% of a wavelength wide. Way to small to affect an EM wave. Make the center conductor of your coax shorter and Return Loss / SWR will look better. Kent WA5VJB
Thank you for such a unique episode Feeding the Dipole in such a way remind me of "SUPER TROMBONE Antenna" and Half Folded Dipole , However the Half Folded Dipole is easy to understand but it i still struggle with Super Trombone Antenna Hopefully your tell us more about such a weird feeding for a Dipole and more about the old "Super Trombone Antenna" and why it's extinct
It's really easy to get PCB boards built. It would be interesting to go though the design process for this type of antenna. As I recall, there are charts that give the spacing and length of the elements, but I'm not sure how much the FR-4 dielectric will influence these numbers. Of course if it was influencing the design, a lot of it could be milled out from the board...
FR4 has a significant effect, it is not benign, but mostly in terms of the Er reducing antenna size rather than losses. The FR4 Er is fundamentally built into the design.
There is not the one and only dimensioning for Yagis. You can optimize it for best match, maximum gain, maximum front to back ratio. And the dimensioning is always a little different.
Another issue is that the PCB materials epsilon_r is not exactly the same in each charge. That might be a reason that the matching is not exactly as it is promised.
Are those "wrong looking" dimensions of the dipol and the directors not for impedance matching? An ideal dipol doesn't have 50 Ohm. Also the folded radiating element is not making any balun thingi so the coax shield doesn't act as part of the antenna?
The dipole doesn't appear to have the same centerline as the first reflector ... it seems to be shifted to one side a bit. (Can't be certain as I used measurements from my screen and the angles are usually oblique, but it certainly appears that way in multiple views.) I wonder what *that* would do to the pattern. (I'm not an RF guy and have no expertise there.)
Why is the dipole part of the yagi shortened? ... well... WA5VJB's design is using... AFAICT... a "gamma match" (the folded part)... and that bottom leg (the parallel part) adds reactance. One can sometimes reduce/compensate for that, by shortening the antenna length. Seems like that is what WA5VJB did. It's hard to get a gamma match just right.
It will be interesting to see the difference when the letters are ground off. I expect a big difference. I also expect the manufacturer used metal letters, but not at the designer's request.
If unsure. Do take a base line measurement with open/short/load standards and test the antenna again. If still unsure, recalibrate the measurement with same standards then base line test with just the load, to expect -50dB or better before testing the DUT. Hope this helps.
As I recall, Kent Britain is a RF EE by career, with amateur radio as a hobby side. He was actively involved with the North Texas Microwave Society (NTMS) for may years. Why not reach out to him by email? IMO, a simple Yagi constructed on a dielectric like G10-FR4 is a compromise antenna, requiring empirical experimentation to optimize it.
RobotZer0 - It is strange that the voltage signal at the center is 0. But it is the current flow that counts. See the Wikipedia article on Dipole Antenna with an animated diagram. Although it is still puzzling because we would think there should be a voltage signal across the 50ohm load due to the current flow through the resistor. Hmm. 🤔
Boy did you cause trouble with this video 😮 ! I always try to read the comments before I say anything and I did. Now I will just say Hello 😊 Your 5-9 in Connecticut. 73 Leo. K. LOL
Ai caramba! what a terrible description of the workings of a Yagi antenna. I had to double check the posting date what with the dodgy lobe maps to check it wasn't another April Fool video! The reflector is in no way a ground plane. Electrons are not sent forward by the antenna. The copper lettering is not a problem, each letter being a tiny fraction of the wave lengths involved here and also not connected to each other or indeed anything else. Others have already commented on the fed system but I dont think anyone mentioned that the two tracks that connect the coax solder points to the driven element are probably a eighth wave matching section, this would help matching and provide additional balun function. 73 de G0AFV
It is. Without it, you'd need a balun and matching transformer of some sort, as the parasitic elements would take a dipole driven element impedance much much lower than 50 ohms. The folded element affords significantly increased impedance, as well as acting as a balun. Side bonus is that it increases bandwidth somewhat, making it more reproducible, and discarding any need for tuning.
1. Reflectors don't, in general, need to be grounded. Like directors, they couple just fine floating. Usually all elements are grounded on lower frequency Yagi-Uda antennas to prevent static build up. 2. The driven element here is an end fed half wave with an approximately 1/4 wave impedance transformer to produce the high impedance feed. 3. The length of the parasitic elements go according to the Yagi formulae, because that's all that controls their resonance, and thus, phase shift. (reflectors lead and directors lag so that the traveling wave in the desired direction passes each element in phase with what that element is re-radiating. The driven element need only be resonant, and in combination with the drive impedance, is whatever length this requires, not necessarily in the Yagi sequence (it comes closer to that with the typical gamma match used at lower frequencies). 4. If your hand was anywhere near the antenna when you took your readings, such as holding it by a corner of the board, or by the COAX at the edge of the board, then your measurements are worthless. 5. Don't confuse voltage with field. There is,, for instance, E field near the center of the element, it's just parallel to the element rather than radial.
thanks. on #2, does that end up at 50ohm? and once inside the reflectors and directors the impedance is lowered right? can a folded dipole (300 ohms) end up at 50 ohms if all is adjusted to each other. or is a balun always needed.
@@IMSAIGuy Yes, being inside the Yagi does affect the impedance (part of why they're tricky to design. The gamma match on the typical HF or VHF Yagi embodies two independent adjustments (trombone capacitor and tap point), which is pretty flexible. Yes, the matching section, if properly designed, means that you don't need the non-1:1 transformer aspect of a voltage balun. But it is still useful to have a current balun to keep RF off of the outside of the shield. For one thing, it's an extra impedance attached to only one side, so it "unbalances" things and changes the feed point impedance, but it also distorts the pattern, always reducing gain. A few ferrite beads around the COAX near the antenna should work, but you would need to research what mix might be appropriate: the usual 43 or 31 mixes may not do what is needed at 2.4 GHz. It will probably also help to keep the COAX perpendicular to the elements for a few wavelengths, and if using multiple beads, spread them out a bit over this part of the run. It also doesn't hurt to have a bead at the radio end of the COAX, but I suspect that you're not talking about enough power to be concerned about RF burns.
See VU2RZA explanation below. (By the way, he has a large antenna farm; but no yagi antennas?!) Anyhow the folded element in the yagi is called a "gamma impedance match" (which is 1/2 of a T match) ... and the gamma match is used to match the impedance of the antenna to the transmission line... and the balanced load (the yagi antenna) to the unbalanced transmission line (the coax cable)... without the need of a balun transformer. The coax connects to the gamma match.
Sometimes, when there is a HF yagi up... you can see the gamma match as a rectangular section, parallel to one element of the dipole section. There is an adjustable shorting bar connecting the dipole element to the parallel leg of the gamma match (You can see a fixed version of that shorting bar, in WA5VJB's design... its the U at the end.) Anyway, on the big HF yagi antennas with an adjustable shorting bar... you can have endless hours of fun moving it to try and get the best match.
@@IMSAIGuy The way I've understood his designs based on this is that it's an unbalanced folded dipole, so effectively a J pole driven element. It offers balun, matching and wider bandwidth all in one clever design. Two further benefits: his designs are very reproducible and usually don't need tuning. If you tried feeding it with coax just using a dipole without a balun (1) you'd radiate on the coax outer too [I note you didn't mention this in your treatise] and (2) the impedance would be far too low [due to the parasitic element loading] for 50 ohm coax so you'd need a transformer. This J pole design acts as the integrated transformer and balun, with the added benefit of increasing bandwidth thus making it more tolerant to minor dimension aberrations and more easily reproducible. In short, WA5VJB knows his onions: if you weren't aware, he's an absolute guru in VHF & microwave antenna design.
FYI WA5VJB may be a radio ham, but he is one of only a handful of people I consider to be an absolute guru when it comes to VHF & microwave antenna design. I'd imagine he's retired now, I haven't seen him for about 15 years, but then I'm in the UK, but on the handful of occasions I have met him, he knows WTF he's talking about. But what would I know, I only do wireless and mixed signal design for aerospace.
Наука
Hi, I want to make some clarification here.
The yagi's driven element has the J shape and has a feed point impedance of around 150 ohms in free space. (The regular folded dipole structure is employed for a wide bandwidth (as in analogue TV receiving antenna) and has a feed point impedance around 300 ohms) Here in this antenna impedance matching (tweaking for low SWR) to a 50-75 ohm coaxial cable can be done by trimming a bit off the free end. That's why the free end of it is shorter compared to the first director.
The explanation for the working of the yagi-uda antenna itself needs clarification too.
The dipole (in free space) alone has a feed point impedance of around 75 ohm +j43 ohm and has a directivity gain of 2.15 dBi.
In the form of a yagi uda directional antenna the feed point impedance of the dipole (driven element) gets lowered to around 35 ohm or even less depending on the number of parasitic elements (reflector and the directors) you are going to mount on the boom to achieve more directivity gain in a particular direction. This is because the dipole current is getting shared among the parasitic elements by mutual coupling.
The spacing between the driven element and the reflector is worked out to around 0.2 lambda; the spacing between the driven element and the first director is around 0.15 lambda. It may be varied for a specific Front to Back ratio and cancellation of the side lobes.
The signal from the dipole tends to propagate in all directions and particularly in both forward and reverse directions in the plane of the yagi antenna. The reflector (usually being 5% larger in size) and the typical spacing result in interference with the signal emanating from the active driven element. Part of the reflected signal after a small delay constructively interferes with the signal on the driven element and moves forward. The current on the director element too constructively interferes with the signal from the driven element and all move forward as if guided along the boom. With each half cycle the wavelets alternate the polarity but keep moving forward as if a light beam (after all light itself is the visible part of the EM spectrum) is being directed by a parabolic reflector and condenser lenses etc.
The centre of the boom being at isoelectric potential can have the centre of a folded dipole or the centre of the driven element used in this antenna and all the parasitic elements' centre anchored to it.
Any small metal fragments scattered in between the elements unconnected to each other and to any of the elements or the boom would be invisible to the RF energy propagating in the forward direction. So, the letters having copper in them would not affect the performance.
This 2.4 GHz Wi-Fi yagi uda antenna must have produced the S11 pattern as found in the literature that came along; I don't know why you didn't get it right.
Moreover there's no ground plane or the need for a ground plane as such.
A dipole is a balanced device. The coaxial cable (intrinsically unbalanced device) has both the hot (centre conductor) and the cold (outer shield braid) terminals that are connected to the dipole at the center.
The half lambda dipole has maximum current in the middle and maximum voltage at the ends; the current and the voltage are 90 degrees out of phase. Because of maximum current and minimum voltage at the center the impedance is low, ie, around 75 + j43 ohm. The RF power fed to the feed point of the dipole is dissipated in the (virtual resistor) Radiation Resistance; yes at resonance the inductive reactance and the capacitive reactance are getting cancelled in the otherwise series resonant half lambda dipole antenna elements.
The voltage at the ends maybe around 2500-4000 ohm or more depending on many factors like height of the feed point above the ground, nearby RF absorbing, scattering or reflecting structures.
The EFHWA antenna is fed with a 49:1 UnUn for this reason.
De VU2RZA
I tend to think WA5VJB added the lettering just to make a point (not exactly sure what that is; but, I can guess.) Anyway, I tend to think... that like you said... that lettering is in between the directors... and in the near field of the antenna... so at least the phasing effects of the directors will make the copper lettering's presence immaterial. (Maybe IMASI guy can grind them off, and see some significant difference?)
Thanx to you and IMSAI Guy. My understanding of antenna is very weak. When I see something that doesn't make sense, I try to find out why rather than to assume the designer made a mistake.
His drawing reminds me of two ways to feed a dipole. @6:00
One with a grounded center, the driven element connected to the boom. I believe this was an unbalanced feed.
The other had the driven element isolated from the would be a balanced feed.
It seems the Mosley TA33 and TA33 Classic is where I saw these configurations (30 years ago). One used a gamma match and the other used a t-match. Do I understand this correctly?
I also notice the reflector is connected to the driven element, where the directors are not. It seems this connection is important. Why?
I read the Richard Feynman book QED and one chapter described diffraction gratings. I think this helped me understand the co-linear antenna from the perspective of the diffraction grating. By cancelling every other 1/2 wavelength it would only provide, what you called constructive interference. So when I see the fold on the left of the lower drawing @6:00, I see this to cancel the 1/2 destructive wave.
I would be interested to hear from a knowledgeable person about the correlation between the refraction grating and the co-linear antenna.
Confirmed. It is a typical Yagi-Uda antenna and method to feed the antenna is very common , including the short transmission line and connection to the reflektor. I expected almost that element length measurement should make confusion in this video.
If assuming correct calibration, tuning could be somewhat improved. A possibility is that this antenna is a copy of a copy design. Different PCB types and PCB thickness then original intended design can then cause minor detuning. A Smith-chart hade been informative both about antenna tuning and calibration.
WA5VJB (Kent Britain) is an RF guru particularly when it comes to microwave antenna design.
I spoke with him about his antenna designs maybe 15 to 20 years ago. The "strange" feed mechanism is by design: it offers a balun function, impedance match, and makes it a little more broadband, at the expense of a slightly off-bore direction, which is insignificant for this relatively low gain.
Of note is that you must remember that the PCB substrate is not benign here: it has a significant effect on the antenna's performance in terms of impedance matching and size due to the relative permittivity of the dielelctric.
I've had lots of success with his many Yagi designs over ht past couple of decades, from VHF to SHF using his designs: they are the most reproducible Yagi antennas I've ever built.
Basically, he _really_ knows his onions: I'd be very careful about second guessing his expertise in this area!
I found this video (and its comments) to be very informative and I thank you for it... any mistakes or incomplete explanations be damned. The video started a fine conversation and the comments have been filling in the gaps. This is all good :)
That was a good explanation of Yagi antennas and an interesting video. Thank you.
The ends are voltage (E field) anti-nodes and current (B field) nodes. The center is an E node and B anti-node. The total distance across is half of a wavelength and not a full wavelength. As a first approximation, the reflector acts like a mirror and places an image of the dipole (and directors but with 1/r^2 intensity reduction) behind itself. This mirror changes the charge sign and the distance delays the phase, however.
Active element looks like end fed antenna. Also coax feeder may transmit/receive and shape of feeder may impact antenna's parameters if PCB path do not balance line enough.
You keep drawing the driven element as a full wave antenna, IT IS A HALF WAVE! The plots on the data sheet were done with a Rhode & Schwarz FSL analzyer. Letters are only 1% of a wavelength wide. Way to small to affect an EM wave. Make the center conductor of your coax shorter and Return Loss / SWR will look better. Kent WA5VJB
Thank you for such a unique episode
Feeding the Dipole in such a way remind me of "SUPER TROMBONE Antenna" and Half Folded Dipole , However the Half Folded Dipole is easy to understand but it i still struggle with Super Trombone Antenna
Hopefully your tell us more about such a weird feeding for a Dipole and more about the old "Super Trombone Antenna" and why it's extinct
It would be interesting to see a Smith chart for this antenna. 73
It's really easy to get PCB boards built. It would be interesting to go though the design process for this type of antenna. As I recall, there are charts that give the spacing and length of the elements, but I'm not sure how much the FR-4 dielectric will influence these numbers. Of course if it was influencing the design, a lot of it could be milled out from the board...
FR4 has a significant effect, it is not benign, but mostly in terms of the Er reducing antenna size rather than losses.
The FR4 Er is fundamentally built into the design.
I recognize style of the plot that seller provides. The plot was made with Tektronix instrument.
Antenna theory: "the gift that keeps on giving". Must see TV is back. Thank you for the gray matter transfer.
There is not the one and only dimensioning for Yagis. You can optimize it for best match, maximum gain, maximum front to back ratio. And the dimensioning is always a little different.
@2:30 Nope not going to say it...
2:45 Nope not going to say it either…
I was holding it together well until he increased the size of the central "lobe"
I didn't even get to RF emissions
It's clearly a uh... frog.... drawn french style
@@IMSAIGuy I laughed way harder than I should have at this! 😀
Another issue is that the PCB materials epsilon_r is not exactly the same in each charge. That might be a reason that the matching is not exactly as it is promised.
Are those "wrong looking" dimensions of the dipol and the directors not for impedance matching? An ideal dipol doesn't have 50 Ohm. Also the folded radiating element is not making any balun thingi so the coax shield doesn't act as part of the antenna?
The dipole doesn't appear to have the same centerline as the first reflector ... it seems to be shifted to one side a bit. (Can't be certain as I used measurements from my screen and the angles are usually oblique, but it certainly appears that way in multiple views.) I wonder what *that* would do to the pattern. (I'm not an RF guy and have no expertise there.)
Why is the dipole part of the yagi shortened? ... well... WA5VJB's design is using... AFAICT... a "gamma match" (the folded part)... and that bottom leg (the parallel part) adds reactance. One can sometimes reduce/compensate for that, by shortening the antenna length. Seems like that is what WA5VJB did. It's hard to get a gamma match just right.
It will be interesting to see the difference when the letters are ground off. I expect a big difference. I also expect the manufacturer used metal letters, but not at the designer's request.
"Antennas are weird."
Yep, they sure are.
Hello, my name is Yevhen, please tell me if I design a Yagi antenna in manna and make it on a pcb, will it work as designed?
If unsure. Do take a base line measurement with open/short/load standards and test the antenna again.
If still unsure, recalibrate the measurement with same standards then base line test with just the load, to expect -50dB or better before testing the DUT.
Hope this helps.
It is a very peculiar sort of a gamma match! :)
As I recall, Kent Britain is a RF EE by career, with amateur radio as a hobby side. He was actively involved with the North Texas Microwave Society (NTMS) for may years. Why not reach out to him by email?
IMO, a simple Yagi constructed on a dielectric like G10-FR4 is a compromise antenna, requiring empirical experimentation to optimize it.
Come for the yagi, stay for the erotic drawings!
5:14 Isn't the center suppose to be at max amplitude , if it is zero that means you have no signal? (when receiveing)
RobotZer0 - It is strange that the voltage signal at the center is 0. But it is the current flow that counts. See the Wikipedia article on Dipole Antenna with an animated diagram. Although it is still puzzling because we would think there should be a voltage signal across the 50ohm load due to the current flow through the resistor. Hmm. 🤔
@@robinbrowne5419 From that wikipedia article half wavelength antenna has maximum current at the feed point...
Boy did you cause trouble with this video 😮 ! I always try to read the comments before I say anything and I did. Now I will just say Hello 😊 Your 5-9 in Connecticut. 73 Leo. K. LOL
I stepped in it big time
Ai caramba! what a terrible description of the workings of a Yagi antenna. I had to double check the posting date what with the dodgy lobe maps to check it wasn't another April Fool video!
The reflector is in no way a ground plane.
Electrons are not sent forward by the antenna.
The copper lettering is not a problem, each letter being a tiny fraction of the wave lengths involved here and also not connected to each other or indeed anything else.
Others have already commented on the fed system but I dont think anyone mentioned that the two tracks that connect the coax solder points to the driven element are probably a eighth wave matching section, this would help matching and provide additional balun function.
73 de G0AFV
Dumb question. Shouldn't there be a "Balun" transformer between the cable and the antenna?
read the comments below, the odd 'dipole' I talk about is both a dipole but with a built in matching network.
That looks kind of like a sideways J pole for the radiator element.
It is.
Without it, you'd need a balun and matching transformer of some sort, as the parasitic elements would take a dipole driven element impedance much much lower than 50 ohms. The folded element affords significantly increased impedance, as well as acting as a balun. Side bonus is that it increases bandwidth somewhat, making it more reproducible, and discarding any need for tuning.
Thanks for the explanation.
1. Reflectors don't, in general, need to be grounded. Like directors, they couple just fine floating. Usually all elements are grounded on lower frequency Yagi-Uda antennas to prevent static build up.
2. The driven element here is an end fed half wave with an approximately 1/4 wave impedance transformer to produce the high impedance feed.
3. The length of the parasitic elements go according to the Yagi formulae, because that's all that controls their resonance, and thus, phase shift. (reflectors lead and directors lag so that the traveling wave in the desired direction passes each element in phase with what that element is re-radiating. The driven element need only be resonant, and in combination with the drive impedance, is whatever length this requires, not necessarily in the Yagi sequence (it comes closer to that with the typical gamma match used at lower frequencies).
4. If your hand was anywhere near the antenna when you took your readings, such as holding it by a corner of the board, or by the COAX at the edge of the board, then your measurements are worthless.
5. Don't confuse voltage with field. There is,, for instance, E field near the center of the element, it's just parallel to the element rather than radial.
thanks. on #2, does that end up at 50ohm? and once inside the reflectors and directors the impedance is lowered right? can a folded dipole (300 ohms) end up at 50 ohms if all is adjusted to each other. or is a balun always needed.
@@IMSAIGuy Yes, being inside the Yagi does affect the impedance (part of why they're tricky to design. The gamma match on the typical HF or VHF Yagi embodies two independent adjustments (trombone capacitor and tap point), which is pretty flexible.
Yes, the matching section, if properly designed, means that you don't need the non-1:1 transformer aspect of a voltage balun. But it is still useful to have a current balun to keep RF off of the outside of the shield. For one thing, it's an extra impedance attached to only one side, so it "unbalances" things and changes the feed point impedance, but it also distorts the pattern, always reducing gain. A few ferrite beads around the COAX near the antenna should work, but you would need to research what mix might be appropriate: the usual 43 or 31 mixes may not do what is needed at 2.4 GHz. It will probably also help to keep the COAX perpendicular to the elements for a few wavelengths, and if using multiple beads, spread them out a bit over this part of the run. It also doesn't hurt to have a bead at the radio end of the COAX, but I suspect that you're not talking about enough power to be concerned about RF burns.
See VU2RZA explanation below. (By the way, he has a large antenna farm; but no yagi antennas?!) Anyhow the folded element in the yagi is called a "gamma impedance match" (which is 1/2 of a T match) ... and the gamma match is used to match the impedance of the antenna to the transmission line... and the balanced load (the yagi antenna) to the unbalanced transmission line (the coax cable)... without the need of a balun transformer. The coax connects to the gamma match.
Sometimes, when there is a HF yagi up... you can see the gamma match as a rectangular section, parallel to one element of the dipole section. There is an adjustable shorting bar connecting the dipole element to the parallel leg of the gamma match (You can see a fixed version of that shorting bar, in WA5VJB's design... its the U at the end.) Anyway, on the big HF yagi antennas with an adjustable shorting bar... you can have endless hours of fun moving it to try and get the best match.
all the gamma matches I've seen don't go all the way out to the end. guess I need to model this someday
@@IMSAIGuy The way I've understood his designs based on this is that it's an unbalanced folded dipole, so effectively a J pole driven element. It offers balun, matching and wider bandwidth all in one clever design. Two further benefits: his designs are very reproducible and usually don't need tuning.
If you tried feeding it with coax just using a dipole without a balun (1) you'd radiate on the coax outer too [I note you didn't mention this in your treatise] and (2) the impedance would be far too low [due to the parasitic element loading] for 50 ohm coax so you'd need a transformer. This J pole design acts as the integrated transformer and balun, with the added benefit of increasing bandwidth thus making it more tolerant to minor dimension aberrations and more easily reproducible.
In short, WA5VJB knows his onions: if you weren't aware, he's an absolute guru in VHF & microwave antenna design.
@@willthecat3861 "you can have endless hours of fun moving it to try and get the best match"
I'll just bet you can! ;)
I knew there needed to be a balun somewhere. Excellent design!
Early in the video I saw the copper text and thought WTF?
Also seems to me a SMA could be integrated,..
all in all kinda amateur (PUN intended)
FYI WA5VJB may be a radio ham, but he is one of only a handful of people I consider to be an absolute guru when it comes to VHF & microwave antenna design. I'd imagine he's retired now, I haven't seen him for about 15 years, but then I'm in the UK, but on the handful of occasions I have met him, he knows WTF he's talking about. But what would I know, I only do wireless and mixed signal design for aerospace.
@@nezbrun872 my apology,
it was a nasty comment
What, 10 dB Gain ? Never !
dBi just the dipole is 2.15dBi
Why not?