300ohm ladderline dipole antenna fed with ladderline... - Page 1
Forum section: electronics / rf-microwave

author: Beamin
author_rank: Super Contributor
timestamp: August 22, 2018, 12:44:15 am
content: When I was younger I built CB antennas with that brown 300 ohm ladderline wire. What I could never figure out was the instructions for building them gave a specific length for the dipole you would tie the ends together and make a break in the middle to attach your feed line. How does the radio/signal/RF current "know" where the antenna starts and the feedline ends if its made from the same wire? I had two of these perpendicular to each other so I could transmit east west and north south. One had a longer feed line but both got the same SWR and signal strength. The radio somehow knew when the antenna startedMagic?

author: tautech
author_rank: Super Contributor
timestamp: August 22, 2018, 01:00:07 am
content: See here:https://m0ukd.com/calculators/slim-jim-and-j-pole-calculator/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2018, 01:47:57 am
content: Splitting one lead of a transmission line and feeding that, unbalances the field on the line.If you initially assume the unsplit side has 0V (just opposite from the split), and the driving line has +/-V at the junction, then the average (common mode) voltage on each half of the split line is (0 + V) / 2 = V/2, and (0 + (-V)) / 2 = -V/2.An ideal transmission line is differential only, but real ones have common mode, which couples into space.At this point, it's effectively like driving a regular, solid wire (or, ribbon, the same size of the twin lead, really) dipole, with V/2 and -V/2 at the feedpoint.Note that the effective voltage is half. We applied a full voltage but get half. Also, the current divides between the two wires of the twin lead just the same. As a result, the folded dipole has four times the (feedpoint) impedance of a simple dipole. It's a 2:1 transformer. Which means it's a good match to 300 ohm twinlead, which is very handy!There are other effects due to the impedance (and velocity) of the transmission line itself (it's not quite an ideal folded dipole, as such), but that's a basic rundown of it.Tim

author: Beamin
author_rank: Super Contributor
timestamp: August 22, 2018, 10:15:44 pm
content: Quote from: T3sl4co1l on August 22, 2018, 01:47:57 amCan you recommend a term I can Wikipedia or google so I can better understand your post? I know you have just answered my question but it is about 1 step above my head, its close though! I understand how I am altering it when I cut the wire and add the feedline but are you saying it's the physical property of the two conductor next to each other that give the 300 ohms and I have altered that at that point? To me it seems like it would act like two antennas: one being my antenna I cut and the other the feed line. May be it was that the dipole is exactly the right length so it radiates the signal while the feed line is not because its too short to act as an antenna. I think the feed line was shorter. So a wire becomes a radiator when it hits that magic length? Rf is hard but for some reason its more interesting that other aspects of electronics like dim bulbs. I also seem to know a disproportional amount more about it too. How can you understand phase and using pin diodes as RF switches but not how ohms law effects light bulbs!?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2018, 10:40:37 pm
content: Never ask me for references, I read random articles and infer the rest...The feedline interacts with the nearby EM field, certainly -- but because the wires are close together, they experience the same induced voltage/current. The transmitter connects through a balun, rejecting in-phase voltage/current. The feedline is also perpendicular, so its polarization is opposite that of the dipole element.This is key: the field only sees the net total current or voltage of the wires in a small region. The field is large and poofy -- on the order of a wavelength -- while the wires are closely spaced.That's why we can make the distinction between the field "inside" a transmission line, the differential mode; and the field "outside" it, the common mode.The folded dipole is a differential to common mode converter, arranged in such a way that it radiates as a dipole antenna.Remember that electric field, or magnetic field, doesn't simplybe;work is required to generate a field. This work corresponds to the current flowing through, and the voltage present on, the elements.While there is no physical connection to the tips of the dipole elements, current nonetheless flows into them. This current is called the displacement current. The current is drawn by all points along the element, so the current in the wire is also not constant along the element.Likewise, the magnetic field generated by that current, is concentrated towards the center (near the feedline).It just so happens that, the fields are large enough, correctly spaced, and correctly phased (because remember, everything is moving at the speed of light, as well as oscillating at the radio frequency), to couple the electrical signals into free (propagating) EM waves in a certain direction (namely, the radiation pattern of the antenna).Tim

author: Beamin
author_rank: Super Contributor
timestamp: August 25, 2018, 02:15:17 pm
content: Quote from: T3sl4co1l on August 22, 2018, 10:40:37 pmBut couldn't you say that the conductors in the antenna are close together and make the same effect? they are the same wire. the ends of the antenna were tied together so it was a loop not like a dipole that goes off from the center. I had no specific balun for this just a crude 75 to 300 ohm tv converter, which I uess is a balun? I'm surprised it worked at all.Heres the ascii drawing of it (=====;-;====) where the feed line is put on the bottom conductor of the center ;-; the - is the uncut top wire and the , of the ; is the break where the feedline comes in. The ._. indicates a non break in the wire. ( is where the ends are tied together.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2018, 02:36:15 pm
content: What do you mean, "same effect"?It's all the same wire, certainly, but at what time? Signals propagate along the wire, referenced to each other (differential mode) and to space (common mode). It's because everything is in motion, that the antenna radiates at all (which is, well, "duh", more or less).Yes, a 75-300 adapter will be a balun. Should be okay bandwidth too (TV VHF was below FM BCB, and UHF is well above).Tim

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 25, 2018, 02:46:15 pm
content: Like you say it yourself, a folded dipole is nothing but a loop antenna; a loop can have various shapes - it remains a loop - but, depending on the shape, the impedance changes; a folded dipole has an impedance of ~300 Ohms, while a quad- or delta-loop has around 100 Ohm; an open dipole ranges at about 75 Ohm.If you talk about the feedline, you have to keep in mind, that only in ideal symmetry the feedline doesn't radiate the signal; you can get quite close to an ideal feedline with a common mode choke - note that the 1:4 Balun (which is in this context actually a little misleading word) matching the different impedances doesn't do you the necessary symmetry for a non-radiating feedline - you always need both - a voltage balun for the impedance matching and a current balun for the common mode rejection on the feedline

author: Beamin
author_rank: Super Contributor
timestamp: August 25, 2018, 02:49:38 pm
content: Effect being mode. So is this antenna even a dipole since it forms a physical loop out of the copper or just a flattened loop? Or is it that it acts as a dipole because the loop is 1/2" tall and 15' wide? with the height dimension looking like zero to the signal because of the ratio of HxL. But if that's the case why bother to tie the ends together? I wish I still had this to measure I think I found the instructions on a news group, forums where you had to wait a day for a reply.I'm sure I will be reading something next week and go AHHA this is what they were explaining to me on the forum.

author: Beamin
author_rank: Super Contributor
timestamp: August 25, 2018, 02:52:04 pm
content: Quote from: HB9EVI on August 25, 2018, 02:46:15 pmSo this is why newer TV stuff was 75 ohm and older was 300 ohm when they only had VHF it better matches the physical antenna size? Read your post after I clicked post on my last comment.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 25, 2018, 03:08:41 pm
content: As far as I know, the 300 Ohm system impedance was because of the wide usage of folded dipoles - which as we know have 300 ohms impedances as well. Additionally it was a balanced system like the dipole is a balanced antenna - so this system perfectly matched the common antenna type on the receivers side.Note also that on the receivers side the mechanical antenna size doesn't matter that much, like impedance matching doesn't matter so much either - important is a good signal to noise ration - only that matters on a receiving system.Todays usage of ladderline, in hamradio for example is because of the better attenuation properties compared to a coax like the venerable RG58

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 25, 2018, 03:27:20 pm
content: No, they aren't the same. They are complementary in a manner, though; and this observation:Quote from: Beamin on August 25, 2018, 02:49:38 pmis the basis of a transformation between the two.A resonant loop radiates perpendicular to the plane the elements are in (i.e., along the loop axis); a dipole radiates perpendicular to the elements' axis (i.e., in a "belt" around center).(Resonance matters, because a small (non-resonant) loop has its patternin the planeof the loop.)The difference is that the dipole, within the plane of its radiation pattern, is omnidirectional around that plane (the radiation pattern is symmetric, a torus), while the resonant loop also looks like a dipole along one direction (parallel to the feedline), having the null only in the third cardinal direction (perpendicular to the other two axes).Tim

author: Beamin
author_rank: Super Contributor
timestamp: August 26, 2018, 01:30:31 pm
content: Quote from: T3sl4co1l on August 25, 2018, 03:27:20 pmI'm going to have to draw these out. The actual orientation of the 300 wire in my antenna say laying it flat stapled to the floor of the attic vs somehow making it stand up 1/2 inch high(total pain in the ass) doesn't matter does it? It just forms a Taurus around it and all that matters is which way the ends point? What happens if the feedline runs along the antenna part? I know mine did before it got to the attic door then it went down.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 26, 2018, 02:12:20 pm
content: Quote from: HB9EVI on August 25, 2018, 02:46:15 pmA folded dipole has an impedance of 300 ohms, pretty much, (within reason), independent of the spacing of the two conductors that make up its construction.For that reason, you can use 300 Ohm ribbon shorted at each end, with a break in one side to connect the feeder.The feeder doesn't have any way of knowing what it is feeding, it could be a 300 Ohm resistor, or a 75 to 300 Ohms balun, as far as it is concerned.A 300 Ohm ribbon folded dipole mounted in the clear, fed by a 300 Ohm ribbon feeder gives you a good chance of the feeder being quite close to ideal symmetry.One problem is that the "TV" ribbon commonly available today is physically far less robust than the old stuff was.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: August 26, 2018, 07:00:49 pm
content: Sure, but symmetry is only given in a fully balanced system; as soon as we connect our commonly unbalanced RF devices to symmetric antennas, we have very well an issue with common mode currents on the feedline, no matter if balanced twinleads or coax

author: vk6zgo
author_rank: Super Contributor
timestamp: August 28, 2018, 12:14:00 am
content: Quote from: HB9EVI on August 26, 2018, 07:00:49 pmIndeed, but that is a special case.Conversion between balanced to unbalanced was quite commonly done in AM Broadcasting, (usually at both ends) to obtain the benefits of balanced feeders.Most such work is now done with coax, so it is pretty much unbalanced all the way.