• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

why antenna impedances are what they are...

Status
Not open for further replies.

unclejed613

Well-Known Member
Most Helpful Member
i was asked whether paralleling 300 ohm antenna elements would yield a lower impedance. the short answer is it would raise the impedance. i wont go into as much detail as my original answer, but suffice it to say that a 300 ohm antenna element is made of two shorted 1/4l stubs, and a 50 or 75 ohm antenna element is made of two open 1/4l stubs, paralleling shorted stubs RAISES the impedance towards infinity, and paralleling open stubs LOWERS the impedance toward zero.

the first drawing shows how the voltage and current are seen on open and shorted quarter wave stubs.

the second shows the voltage and current distribution on an open ended dipole. the fact that the current is high and the voltage is low at the feedpoint means LOW impedance.

the third shows a folded dipole and the voltage and current distributions on it, the voltage is high, and the current low at the feedpoint, so this is a high impedance.

paralleling quarter wave stubs only makes them more efficient at what they do, so paralleling two open dipoles would bring the impedance closer to zero. paralleling folded dipoles would bring the impedance closer to infinity.
 

Attachments

user_88

Member
tuning a dipole ....

Given the current and voltage graphic configurations for a dipole antenna .....
Would it be possible to use an unconnected incandescent light bulb ... say the bulb was on a pole or similar extension for convenience ..... to run along the dipole leg and locate the minimum current point? ...

If the dipole were cut 'long', towards the end of the dipole leg, would there be a current minimum, and then an increase in current, as you progressed along the length of wire?

Maybe a fluorescent bulb would show some sort of RF current activity in the vicinity of a current maximum, if not an incandescent bulb.
 
Last edited:

unclejed613

Well-Known Member
Most Helpful Member
i don't know if modern editions of the ARRL handbook show using lamps like that to measure current and voltage in antenna, but some of the older ones did. the incandescent bulb method used a loop of wire parallel to the antenna to pick up current readings, and a neon bulb to sense voltage. a few years ago when i was in the Army, i was on a field exercise, and was working with a track crew setting up their radio. they weren't receiving anything, and so thought they might not be transmitting. i pulled a neon bulb out of one of their intercom boxes, held it up next to the antenna cable and keyed up the transmitter. the bulb lit up very bright. they were up on the air, but had a problem with the receiver antenna relay. i ran a separate antenna to the receiver and they were back in business. nobody, not even the guys from the radio shop had ever seen that trick before (the NCO of the track crew had me demonstrate that again when the radio tech got there...)...


as for determining whether an antenna is too long, there would be a voltage minimum near (but not at) the feedpoint if the antenna is too long for an open ended dipole, and it would be a current minimum for a folded dipole. for an antenna that's too short, you would never reach the minimum current or voltage, since it would be higher than normal beginning at the feedpoint and rising from there. antenna impedance is a complex quantity, consisting of capacitive and inductive reactances that should be equal values at resonance an open ended dipole is equivalent to a series resonant circuit, and a folded dipole is equivalent to a parallel resonant circuit, antenna analyzers measure the balance between capacitive and inductive reactances at the frequency of interest, and can tell you instantly whether the antenna is short or long (as well as the impedance and SWR)
 
Last edited:

unclejed613

Well-Known Member
Most Helpful Member
to clarify a bit, the reference for the current and voltage curves is set at the ends of the antenna, not at the feedpoint. if the antenna is an open dipole, the current is zero at the end of the antenna, and the voltage is maximum, and the relationship between current and voltage working back to the feedpoint is a function of distance from the open end. same holds true for a folded dipole, except current is max at the end and voltage is zero. a full wave dipole (each leg 1/2l) would have (ideally) the same voltage and current at the feedpoint as at the ends because the phase relationship is now 180 degrees instead of 90 degrees, the "standing waves" are "reflected" from the ends, not from the feedpoint.
 
Last edited:

unclejed613

Well-Known Member
Most Helpful Member
to clarify a bit, the reference for the current and voltage curves is set at the ends of the antenna, not at the feedpoint. if the antenna is an open dipole, the current is zero at the end of the antenna, and the voltage is maximum, and the relationship between current and voltage working back to the feedpoint is a function of distance from the open end. same holds true for a folded dipole, except current is max at the end and voltage is zero. a full wave dipole (each leg 1/2l) would have (ideally) the same voltage and current at the feedpoint as at the ends because the phase relationship is now 180 degrees instead of 90 degrees
 

Attachments

user_88

Member
Thanks .... useful to know ....

For amateur radio ... most dipoles are 20 ... 30 .....40 ....ft. or more high ....
.... It would be interesting to attach a small neon light to one of the ends though.
... It should light up every time the code key is pressed.
 

MikeMl

Well-Known Member
Most Helpful Member
i was asked whether paralleling 300 ohm antenna elements would yield a lower impedance.
I read through the entire thread and I don't understand what you mean by "paralleling" the elements. By paralleling, you are creating a driven array. The feedpoint impedance of the array could be higher or lower than 300 ohms. It depends on the spacing, and phasing of the elements.
 

Tesla23

Member
i was asked whether paralleling 300 ohm antenna elements would yield a lower impedance. the short answer is it would raise the impedance. i wont go into as much detail as my original answer, but suffice it to say that a 300 ohm antenna element is made of two shorted 1/4l stubs, and a 50 or 75 ohm antenna element is made of two open 1/4l stubs, paralleling shorted stubs RAISES the impedance towards infinity, and paralleling open stubs LOWERS the impedance toward zero.

the first drawing shows how the voltage and current are seen on open and shorted quarter wave stubs.

the second shows the voltage and current distribution on an open ended dipole. the fact that the current is high and the voltage is low at the feedpoint means LOW impedance.

the third shows a folded dipole and the voltage and current distributions on it, the voltage is high, and the current low at the feedpoint, so this is a high impedance.

paralleling quarter wave stubs only makes them more efficient at what they do, so paralleling two open dipoles would bring the impedance closer to zero. paralleling folded dipoles would bring the impedance closer to infinity.
I'm not really sure what you are saying. If you take a two resonant half-wave dipoles, impedance about 70 ohms each, if you mount them almost touching and parallel the terminals then you still see about 70 ohms. It will act like a fatter dipole.

In the case of paralleling folded dipoles you mean creating a dipole with two parallel elements instead of one, (a triple folded dipole) then this increases the impedance by a factor of 9. You can also get different impedance multiplications by using varying diameters for the two arms of a folded dipole, see here for example
 

unclejed613

Well-Known Member
Most Helpful Member
the original question i was asked was something along the lines of "if i connect 4 300 ohm elements in parallel, would that equal 75 ohms?". i was attempting to explain why that is not the case, as simply as possible. connecting elements together near-field (very small fractions of a wavelength apart) allows the elements to interact and it lowers the radiation resistance, but not neccesarily the impedance. lowering the radiation resistance of a folded dipole actually raises the impedance...... just trying to simplify a very complex subject and explain "why".

there are "hybrid" antennas that have characteristics of both the open and shorted dipoles, such as bazooka antennas and j-poles, and explaining why they work so well would probably take..... a week:eek:
 

Tesla23

Member
the original question i was asked was something along the lines of "if i connect 4 300 ohm elements in parallel, would that equal 75 ohms?". i was attempting to explain why that is not the case, as simply as possible. connecting elements together near-field (very small fractions of a wavelength apart) allows the elements to interact and it lowers the radiation resistance, but not neccesarily the impedance. lowering the radiation resistance of a folded dipole actually raises the impedance...... just trying to simplify a very complex subject and explain "why".

there are "hybrid" antennas that have characteristics of both the open and shorted dipoles, such as bazooka antennas and j-poles, and explaining why they work so well would probably take..... a week:eek:
What's missing from the circuit description is the mutual coupling between the antenna elements.

If the antennas are sufficiently far apart, or point in different directions to the extent that minimal current is induced in one by a current in the other, then you can reliably manipulate the impedances as simple circuit quantities.

If the antennas are close together so that a current flowing in one causes significant current to flow in the other, then you need to include the effect of mutuals - this is the case in folded dipoles. In this case you can't treat the impedances simply.
 
Last edited:

unclejed613

Well-Known Member
Most Helpful Member
when you don't have mutual coupling between the elements, then they're probably far enough apart to require a transmission line to couple them, which creates a whole other set of impedances and phase shifts to calculate. also, since antenna elements are of themselves transmission lines, they may not have mutual coupling, but they do interact because the signal from one will excite the other, and the signal will be re-radiated.
 

Tesla23

Member
also, since antenna elements are of themselves transmission lines, they may not have mutual coupling, but they do interact because the signal from one will excite the other, and the signal will be re-radiated.
If feeding a signal into one antenna causes something to come out of the other then they have mutual coupling and the impedance of one will be affected by what you connect to the other. The only way this doesn't happen is if feeding a signal linto one causes nothing to happen at the other, then there is no mutual coupling and you may treat them as simple impedances.
 

unclejed613

Well-Known Member
Most Helpful Member
they ARE WIRED together. having a signal in one but not the other is impossible. they may not be electromagnetically coupled, but they share a common transmission line. they interact through their wiring. if they are elements with the same length, they will both be excited by the same signal, even though the received signal is being received by only one of them. if you have a horizontally polarized antenna and a vertically polarized antenna sharing the same feedpoint, and excite the vertically polarized element with a nearby transmitter, the horizontally polarized element will also have the same signal on it. it is being fed by the vertically polarized element through the common feedpoint and being resonant has the same voltage/current impressed on it. if these were truly lossless elements, their impedances would be equal, and the feedpoint's impedance would be the same. we're talking about resonant circuits here, when you put resonant circuits in series or parallel, they don't behave "according to plan", because you have currents or voltages whose combined magnitudes are exactly 180 degrees out of phase and the impedances appear to defy ohm's law (they actually don't, they just appear to). once you factor in the phase irregularities you will see why the impedances tend toward zero or infinity. it's actually because of real physical properties of wire and insulators that we get finite impedances of 50, 75, 300 and 600 ohms (the most common "standard" antenna impedances) a 75 ohm TV yagi that you buy ar radio shack, etc... is actually 75 ohms at only a small handful of frequencies. the rest of the time it's somewhere else between 25 and 100 ohms as you sweep through the VHF and UHF TV spectrum. 75 ohms is really an "average". same goes for a 300 ohm antenna which can dip as far as 100 ohms or peak as high as 1200 ohms. antenna elements do have a non-zero resistance as part of their impedance, and as far as the effects of this resistance, yes you will get a lower impedance from paralleling antenna elements that aren't magnetically mutually coupled, but this change in impedance isn't linear as in paralleling two resistors. there are factors in the calculation (especially at resonance) that are really difficult to wrap your mind around, like "j" which is a representation of the square root of a negative number (to further complicate things "j" usually has a sign of + or -).

if you really want to dive into antenna theory, i found this page from a course on antenna theory:
Antennas

it's actually quite simplified (but "j" does show it's face now and then, but not as often as on the wikipedia pages)
 

unclejed613

Well-Known Member
Most Helpful Member
If feeding a signal into one antenna causes something to come out of the other then they have mutual coupling and the impedance of one will be affected by what you connect to the other. The only way this doesn't happen is if feeding a signal linto one causes nothing to happen at the other, then there is no mutual coupling and you may treat them as simple impedances.

and that's the point. resonant transmission lines (antenna elements included) are not simple impedances. a signal received in an antenna sharing a common feedpoint or common transmission line with another antenna (even though they are not magnetically coupled) WILL be re-radiated by the other antenna, just because they do share a common feedpoint, and the voltages and currents present in one will be present in the other. if they share a common feed, it's impossible to have something happen in one without happening in the other.
 
Last edited:

Mikebits

Well-Known Member
Complicated subject. Seems many might be thinking in terms of parallel DC.



Impedance
 
Last edited:

Tesla23

Member
and that's the point. resonant transmission lines (antenna elements included) are not simple impedances. a signal received in an antenna sharing a common feedpoint or common transmission line with another antenna (even though they are not magnetically coupled) WILL be re-radiated by the other antenna, just because they do share a common feedpoint, and the voltages and currents present in one will be present in the other. if they share a common feed, it's impossible to have something happen in one without happening in the other.
There is nothing difficult or magical about antennas. If you have a single feedpoint, your circuit has only a single connection to the 'aether' and it may be treated as a simple circuit impedance. It cannot interact with any other part of your circuit as you have no antennas. Simply V = IZ (and in general Z = R + jX and will be a function of frequency)

If you have two antennas they generally interact, although some don't - for example perpendicular dipoles. Mind you if you plonk something in the nearfield that breaks the symmetry then you will cause them to interact. So with two antenna terminals, in general - using an impedance description:

V1 = Z11 * I1 + Z12 * I2

V2 = Z21 * I1 + Z22 * I2

so a current I1 flowing in antenna 1 can cause a voltage V2 to be induced on antenna 2, and vice versa. The elements Z12 and Z21 represent the mutual couplings. They are zero if the antennas don't interact.

Any circuit coupling through the feed network can be accounted for using simple circuit theory, it doesn't matter whether they are Ls, Cs, Rs or transmission lines.

So once you include the mutual couplings you can analyse the whole multiple antenna system using simple circuit theory.

Went looking for a reference to give you, see wikipedia article
scroll down to the bottom to
"Mutual impedance and interaction between antennas"

I have designed large arrays of antennas. If you need to accurately control the amplitude of the radiation from each antenna you need to include the effects of the element interactions. This is the basis of how the analysis is generally done.
 
Last edited:
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top