Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: vertiacal antennas: do radials or counterpoise radiate?  (Read 4699 times)
AC7CW
Member

Posts: 975




Ignore
« Reply #15 on: October 01, 2017, 12:22:53 PM »

Sloping roofs present a good opportunity to build a vertical with sloping ground plane wires.
Logged

Novice 1958, 20WPM Extra now... (and get off my lawn)
RFRY
Member

Posts: 479


WWW

Ignore
« Reply #16 on: October 01, 2017, 05:14:09 PM »

Thanks to W5DXP and W9IQ for the good comments about my recent posts in this thread.
Logged
WB6BYU
Member

Posts: 17066




Ignore
« Reply #17 on: October 01, 2017, 05:31:02 PM »

Quote from: W9IQ

It is also interesting to consider that in free space the radials do not change the gain nor the Rr (radiation resistance) of the antenna. They simply make the antenna easier to feed.



How are you feeding it to measure the input impedance if you don't connect the radials?
What did you connect to the other side of the feedpoint?
Logged
W9IQ
Member

Posts: 1706




Ignore
« Reply #18 on: October 01, 2017, 06:21:53 PM »

The radiation resistance does not need to be physically measured. It can be calculated as a function of the Aem (maximum effective aperture) or the As (scattering aperture), for example. More commonly, most antennas are modeled as a collection of short dipoles. Since the radiation resistance of any short dipole is well understood and described, the PADL (Phase, Amplitude, Direction, [3D] Location) of the accelerating charges in the antenna under study determines the aggregate Rr of the short dipoles.

The ability to feed an antenna at its Rr is another matter. The radials in this case facilitate that goal.

- Glenn W9IQ

Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
YL3GND
Member

Posts: 55




Ignore
« Reply #19 on: October 03, 2017, 01:46:34 AM »

The in-phase current entering each pair of horizontal, co-linear radials at their common point flows in opposite physical directions, so their net far-field radiation effectively is zero (the two fields are 180° out of phase in the far field).

..exactly the opposite is true for a dipole antenna. The non-radiating currents in the transmission line are 180o out of phase and when one on them makes a 90o turn to the left and the other makes a 90o turn to the right at the antenna feedpoint, they become in phase radiating antenna currents.


RFRY made it correct. I am sorry, but it seems to me, that You, W5DXP, are a bit wrong again.. Try to draw a dipole and currents - at first try it will looks like You are saying, but then move current in time by 45 or 90 deg from "first try picture", and You will see - we can't say that current itself has became in phase by just diverting wires.
Logged
W5DXP
Member

Posts: 4193


WWW

Ignore
« Reply #20 on: October 03, 2017, 04:31:31 AM »

... it seems to me, that You, W5DXP, are a bit wrong again.

Just quoting a convention that has been around for a long time.

Logged
VK6HP
Member

Posts: 154




Ignore
« Reply #21 on: October 03, 2017, 05:51:38 AM »

Although I'm not sure exactly what YL3GND is concerned about, I wondered if he is confused by phasor conventions and the implicit suppression of the time variable term. But I'm not sure...having doodled on my notepad the elements of the diagram that Cecil posted above.  But maybe appreciating that the instantaneous phase differences remain constant helps.
Logged
G8HQP
Member

Posts: 596




Ignore
« Reply #22 on: October 03, 2017, 06:51:19 AM »

Quote from: YL3GND
You will see - we can't say that current itself has became in phase by just diverting wires.
Yes we can. In the feeder the currents are antiphase. In the dipole antenna they are in phase, and so produce in phase radiation. Draw a diagram if this confuses you.
Logged
RFRY
Member

Posts: 479


WWW

Ignore
« Reply #23 on: October 03, 2017, 07:21:00 AM »

To embellish a bit on the clip from Kraus, posted above by W5DXP:

Note that the currents at the dipole feedpoint have opposite polarity at every instant of time, as must be true to prevent radiation from the transmission line.  In other words, they are always 180° out of phase, and of equal amplitude.

After their connections to the arms of a dipole, those equal, out-of-phase currents travel along those arms in opposite physical directions in space.  Those changing r-f currents produce e-m radiation.

Due to driving those arms 180° out of phase, and as a result of the physical orientation of the dipole conductors, currents along both arms always travels in the same physical direction at every instant of time, and their e-m waves reinforce each other in the far field.



Logged
RFRY
Member

Posts: 479


WWW

Ignore
« Reply #24 on: October 03, 2017, 07:50:28 AM »

Missed the time limit for edits, but...

» The currents on both sides of a dipole antenna travel in the same physical direction in space, and their radiation reinforces each other in the far field.

» The currents on each half of a linear, elevated radial of a Ground Plane antenna travel in opposite physical directions along that conductor, and radiation from each half cancels each other in the far field.
Logged
KD7RDZI2
Member

Posts: 224




Ignore
« Reply #25 on: October 05, 2017, 12:16:54 PM »

Missed the time limit for edits, but...

» The currents on both sides of a dipole antenna travel in the same physical direction in space, and their radiation reinforces each other in the far field.

» The currents on each half of a linear, elevated radial of a Ground Plane antenna travel in opposite physical directions along that conductor, and radiation from each half cancels each other in the far field.

Thank you all for the comments, I have a much better understanding now! I might be very wrong but it seems to me one could make an antenna that radiates very little in the far field without the use of inductors or resistors. Suppose you take 4 radials, 2 connected at the center in opposite directions (like two radials connected to the inner of the coax) and the other two in opposite directions like ordinary radials. Would SWR be still 1:1?
Logged
N3OX
Member

Posts: 8910


WWW

Ignore
« Reply #26 on: October 06, 2017, 06:18:08 AM »

Suppose you take 4 radials, 2 connected at the center in opposite directions (like two radials connected to the inner of the coax) and the other two in opposite directions like ordinary radials. Would SWR be still 1:1?

No, it definitely wouldn't be 1:1 because the impedance of that antenna wouldn't be 50 ohms. In fact, it's around 5 ohms.

It does have the pattern that is maximum halfway in between each radial like in RFRY's simulation:



Again we get back to radiation resistance. What's the voltage developed at your feedpoint for a given drive current at the feedpoint, and how does that depend on the arrangement of currents on the antenna? For a dipole it's around 73 ohms in free space at resonance. For your proposed antenna at its resonance, the ratio of the voltage to the current is about 5 ohms. For three fed against three to get a six lobed pattern it's a little under half an ohm.

Here's the pattern of the six wire version:


When you set up some currents on some wires, you'll get some radiation. If you had wires with no losses you could get nearly ANYTHING to radiate all the power you apply to it, theoretically.

But the more cancellation of radiation you have because of the current phases and physical arrangement of wires, the lower the real part of the impedance will be, because it's difficult to couple electrical energy at the feedpoint to electromagnetic waves propagating out from the antenna. You need a lot more current for a given amount of wave energy leaving the antenna, so the "resistance" that consumes power from your feedline and emits it as radiation is really low.

If you have an antenna that's JUST crossed "radials" fed against each other so that there's a lot of radiation cancellation, and it's made of REAL wires, what's going to happen is that you're going to divide the power between the very low radiation resistance and the loss resistance in the antenna. For example, my three-against three only radiates about half the power applied to it when built of 3mm diameter aluminum. It's like a magloop in that sense (which has extreme cancellation). It can radiate well but you have to make it very low loss.

Another thing that you'll see comparing the four lobed to the six lobed pattern is that the nulls on the six lobe pattern for the three-against-three antenna are much shallower. This because radiating vertically polarized radiation from the eight inch vertical wire I have connecting the crossed radials in my simulation is, in a sense, getting "easier" compared to radiating horizontally polarized radiation from the crossed horizontal wires with all that cancellation. The omnidirectional vertically polarized radiation from that short wire is filling in the hex lobe pattern:



If we keep adding radials, and especially if we get the top radial wires in line with the bottom, we'll have eventually built an eight inch vertical dipole with two giant capacitance hats. The radiation will be almost all vertical, the radiation resistance will be tiny, the antenna won't be very efficient because of wire losses, but the "radial" radiation will mostly go away compared to the vertical radiation.

And if you think about a BIG vertical, well the current on the vertical radiator is going to be the path of least resistance for radiating RF energy, so you get very little radiation from the radials and a lot of radiation from the vertical radiator. And because the combined radiation resistance of that arrangement is a lot higher than the loss resistance in the wires, it's a very efficient antenna.

Small antennas like magnetic loops (which have the most radiation cancellation of any transmitting antenna in common ham use) and short, loaded verticals (http://n3ox.net/projects/n3oxflex/) can be made to radiate very efficiently, but you have to carefully control all the losses so that the loss resistance is much lower than the very low radiation resistance. For magloops this often means keeping the loss resistance down in the tens of milliohms.

For big HF antennas you don't have to worry so much, because the radiation resistances are high. That just means it doesn't take very high currents on the antenna's conductors to launch 100W or 1500W of radiated electromagnetic waves. And just like any other set of electrical conductors, because the currents are relatively low, you don't get a lot of power lost to heat in the conductors.

But you wouldn't necessarily get *low* far field radiation from the sort of crossed-radial antenna you first proposed. Five ohms is a very manageable radiation resistance that can be efficiently matched to. With tubing construction, the losses would be quite low. The idea that "radials don't radiate in the far field" is RELATIVE to the easy radiation from the vertical radiator. There's a fraction of the current on each radial and there's a lot of cancellation, so only a little of the power you apply at the feedpoint gets radiated in the multilobe radial pattern. But cancellation doesn't mean destruction of energy, so if you take away the vertical part, you can radiate a lot more.

If you wanted a four-lobed horizontally polarized pattern for some reason, you could use the antenna you proposed, though you'd need an extremely good balun to keep the feedline from becoming part of the antenna, possibly an impractically good one.

And I can't really imagine a reason why you'd want that pattern unless you wanted the nulls, so you'd have to be really careful with the feedpoint and not put in a vertical connection like I did that ruins the null depth (though that was mostly a modeling convenience).
« Last Edit: October 06, 2017, 06:36:17 AM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
Member

Posts: 8910


WWW

Ignore
« Reply #27 on: October 06, 2017, 06:52:59 AM »

Missed the time limit for edits, but...

» The currents on each half of a linear, elevated radial of a Ground Plane antenna travel in opposite physical directions along that conductor, and radiation from each half cancels each other in the far field.

If you manage to feed that antenna in isolation, which I've done here with in-phase current sources, a midplane capacitance hat, and Q=600 inductors to tune out the capacitance, you get a couple ohm radiation resistance in each source and cone-shaped patterns around each wire:



Radiation efficiency is around 70% with Q=600 coils and 10mm aluminum conductors.

Just want to draw the distinction between the fact that it's the radiation cancellation of the radials *relative* to the vertical radiating element that means that there's no important radiation from the radials. If you feed isolated quarter-wave wires in this weird way against a structure that has even more radiation cancellation you could radiate quite a bit of power. No idea why you'd WANT to Cheesy
« Last Edit: October 06, 2017, 06:55:40 AM by N3OX » Logged

73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!