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: [1]   Go Down
  Print  
Author Topic: Current through radials  (Read 515 times)
KE8BGJ
Member

Posts: 5




Ignore
« on: May 18, 2019, 04:30:48 PM »

Hello all.

I just put a 4-BTV vertical in the backyard, complete with many radials.  The question is:

Can someone give a brief overview of how current flows through the radials? Do signals come in through the air, travel down the antenna, down through the radials, absorb into the ground, and then when the polarity switches (because radio waves are AC signals right?), current comes back from the ground and back into the radials.... At some point the incoming signal has to get into the radio.

Sorry for the run-on sentence.  Thanks for your help!

Mike
KE8BGJ
Logged
AA4PB
Member

Posts: 14969




Ignore
« Reply #1 on: May 18, 2019, 04:53:08 PM »

The radio is connected between the antenna and the radials via the feed line.
Logged

Bob  AA4PB
Garrisonville, VA
KE8BGJ
Member

Posts: 5




Ignore
« Reply #2 on: May 18, 2019, 05:34:03 PM »

Thanks for the reply, but that doesn't really help me understand how current flows in and out of the radials (if at all).
Logged
KE6EE
Member

Posts: 2735




Ignore
« Reply #3 on: May 18, 2019, 06:19:04 PM »

https://www.eham.net/ehamforum/smf/index.php/topic,41578.0/wap2.html
Logged
KE8BGJ
Member

Posts: 5




Ignore
« Reply #4 on: May 18, 2019, 07:36:26 PM »

So basically the vertical part radiates, and the horizontal ground part doesn't and gets cancelled out by itself.  Does any of this have to due with inductive and capacitive reactance?

I do apologize but I only grasp part of the info in the provided links.  Maybe this is just too complex of a question for a newbie such as myself.

Thanks
Logged
KG5AHC
Member

Posts: 237




Ignore
« Reply #5 on: May 18, 2019, 08:39:04 PM »


In this reference, "Dan" explains that "the radials serve as a reservoir for charge". I like this view of a radial field. "You want the electrons  to happily slosh through the feedpoint at the frequency at which you're trying to drive them."

Just as AA4PB has pointed out.  the radio's receive oscillator circuit is physically inserted between the vertical element and the radials, and therefore, being osillated by the flow of electromagnetic energy on its way from the vertical to the radial field and back (at high frequency). The vertical and the radials are NOT directly connected to each other.

if you disconnect the feed line from the antenna, Using a multi-meter, i expect you would find that there is no electrical conductivity between the radials and the vertical radiator.

It is my understanding that radials in or on the ground do not have to be specific length relative to target frequency becase the ground itself will collect and release charge between/around the radials.
  
Regards,
Jeff KG5AHC
Logged
KG5AHC
Member

Posts: 237




Ignore
« Reply #6 on: May 18, 2019, 09:11:51 PM »

So basically the vertical part radiates, and the horizontal ground part doesn't and gets cancelled out by itself. 

More or less, it's like that... but at some distance away from the antenna, aka the far field.   when viewed from a distance, the RF escapes the antenna system from the vertical radiator because there is no opposite charge to cancel it.  but since the radials are carrying charges going in opposite directions, they do end up cancelling each other .. in the far field.

Logged
K0ZN
Member

Posts: 1849




Ignore
« Reply #7 on: May 18, 2019, 10:20:58 PM »

Think of it a little bit like this....  Think about a dipole with a feed line. The current in the TWO feedline wires (coax or twin lead both have two conductors) flow from the transmission line into EACH SIDE of the dipole, right.....and everybody is happy.   Now replace ONE side of the dipole with a radial system.....the currents still essentially flow as before into EACH SIDE of the antenna. The vertical section (more correctly called a monopole) is "THE TOP HALF" of the Vertical dipole and the BOTTOM HALF is now the radial system, but rather than just a single wire, you have (again a generalization!) the entire PLANET connected to that side of the feedline and the Earth has so much capacity that the current happily flows in it as it would a single wire half of a dipole. Some books refer to the ground radial system as providing an Image for the "bottom half" of the antenna.  

The very important thing to note about ground mounted verticals is that....as common sense would tell you.....plain old dirt is nowhere near as good a conductor of Radio Frequency energy (which is just AC current) as a copper or other metal wire/plate, etc. This is why a poor/very small radial system is lossy....it is just Ohm's law: dirt has more resistance than wire.....so you want a lot of wires for the current to flow in to reduce the ground SYSTEM resistance.   As a gross generalization, nobody in their right mind would fill a large PVC pipe with dirt and use it for one side of a dipole instead of a copper wire. A radial system is in the same situation, but with a much larger area.....the game is to make the energy flow in a low resistance piece of wire instead of dirt.

On receive, an antenna is simply operating in a reciprocal mode to transmit. The radio waves in the air flow over the antenna and generate currents in it that are taken to the receiver for detecting.

Again the foregoing is a VERY SIMPLISTIC, GENERALIZATION to help a new ham.....this is Not an IEEE paper for the "experts" on the site; no need to nick pick.  I am sure you will get some long, fairly technical comments to your question which may further help. I am just trying to answer your BASIC question.

You are to be commended for asking that question. There is a TON to learn in this game and your curiosity will be your friend in the long run.
You appear to have an interest in antennas; I would strongly recommend picking up a copy of the ARRL Antenna Book and putting into some book time; I suspect you will enjoy it. You can get an old one, even a 30 year old Cheap copy of the ARRL Antenna Book off of Ebay is still fine for basic theory. There are a lot of myths and mis-information floating around about antennas and antenna theory and a few blurbs off a website is not adequate. The Antenna Book will give you correct information and there are a lot of graphical examples that help understanding a lot. Antenna theory is not difficult but it is not intuitive in many cases. Your antenna is what connects your radio to the world; the more you know about how to maximize it the better your station will be. A $4,000 radio connected to a crappy antenna is still a crappy station! The more you know about antennas the more you will ultimately enjoy the hobby due to a stronger signal.

73,  K0ZN
« Last Edit: May 18, 2019, 10:34:37 PM by K0ZN » Logged
KE8BGJ
Member

Posts: 5




Ignore
« Reply #8 on: May 18, 2019, 10:45:24 PM »

Lots of great information.  I appreciate the replies.

K0ZN - Excellent explanation! I get it.  Thank you!
Logged
AH7I
Member

Posts: 109


WWW

Ignore
« Reply #9 on: May 19, 2019, 02:33:50 AM »

Hello all.

I just put a 4-BTV vertical in the backyard, complete with many radials.  The question is:

Can someone give a brief overview of how current flows through the radials? Do signals come in through the air, travel down the antenna, down through the radials, absorb into the ground, and then when the polarity switches (because radio waves are AC signals right?), current comes back from the ground and back into the radials.... At some point the incoming signal has to get into the radio.

Sorry for the run-on sentence.  Thanks for your help!

Mike
KE8BGJ

This may help frame things for you.

The radio wave exerts a force on charge in a conductor (your vertical). The charge accelerates. It now has some energy. In order to extract some of that energy we provide something for the charge to push against. That's where the radials come in. As conductors, they have charge to push against. The trunk lid on the police car serves the same purpose for the little vertical you see there.

Think water, water wheel, half a millstone, some barley, another half millstone attached to the earth. We extract energy to grind the grain so we can make our beer. Without that other half millstone to push against we'd get no work.

The vertical is your water wheel. Your radio receiver is the grain. The radials are the millstone attached to the earth.

It's a little more complicated with the radio but conceptually the same. In the antenna the direction of the force is alternating and the energy stored in the radials is returned. Analogy of someone on a swing. The resistance of dragging their feet lightly is the energy going into your receiver. You pushing on one side is radio wave pushing charge in the vertical. The energy stored in the field around the radial wires is the energy when the swing is high opposite to the side you are pushing on. 

Why is there current in the radials? Because when we push on the  charge in the radials, it can move. What happens with the current in the radials? It is a radial current so there is no net acceleration of charge and no radiation. There is a magnetic field set up around each wire (from the current in that wire). Energy stored in that field is returned to the wire like the energy when the swing is high on the opposite side you are pushing from.  With a dipole, in addition to stored energy you have energy from the the radio wave. There are two of you, each pushing from the other side of the swing.

Why does the radio wave exert a force on a charge and why does an accelerating charge make a radio wave? There is a good math model that describes what is observed. I can't answer, why we see it that way.


Distracting beer reference https://www.youtube.com/watch?v=tfLAK8f6wtI


Logged
RFRY
Member

Posts: 677


WWW

Ignore
« Reply #10 on: May 19, 2019, 03:41:01 AM »

... Can someone give a brief overview of how current flows through the radials?

For the case of a vertical monopole using a set of radial wires buried in the earth:

1. The transmitter generates r-f current, which flows through the transmission line to the feedpoint of the antenna system.
2. That current/charge acceleration on the vertical conductor of the monopole generates e-m fields ("radio waves") radiating away from the monopole.
3. The part of that radiation arriving at the surface of the earth produces r-f currents flowing on and just below the surface of the earth. 
4.  Those currents within 1/2-wavelength of the base of the monopole need to be collected from the earth and returned to the transmitter "ground" terminal, in order to form a complete path for r-f current to flow on the monopole, itself.
5.  The buried radial wires reduce the ohmic losses present in the earth near the base of the monopole, and minimize the I2R power loss during their conduction back to the transmitter "ground' terminal -- which maximizes the fields that will be radiated from that transmit/antenna system.

The monopole antenna systems of most AM broadcast stations use a set of 120 x 1/4-wave buried radials, and radiate more than 90% of the Z-matched r-f power available at their feedpoints.
Logged
KE8BGJ
Member

Posts: 5




Ignore
« Reply #11 on: May 19, 2019, 06:24:40 AM »

It is way more clear to me now!

I've searched a bit but haven't found much yet, on a video or animation that shows this process in action.  Any chance anyone may have a link to such a video?
Logged
W5DXP
Member

Posts: 4645


WWW

Ignore
« Reply #12 on: May 19, 2019, 09:11:52 AM »

I like this view of a radial field. "You want the electrons  to happily slosh through the feedpoint at the frequency at which you're trying to drive them."

To be technically correct, electrons do not "slosh through the feedpoint". At 10 MHz being driven by 1 amp of current, they oscillate back and forth for 1.2 picometers, essentially vibrating in place. 1.2 picometers is less than 1% of the width of a copper atom. Electrons do not flow (or slosh) at RF frequencies. 1.2 picometers is 1.2x10-12 meters. The electromagnetic fields and waves are what do the real moving at RF frequencies. All the way from the source, on the transmission line, on the antenna, and in the space around the transmitting and receiving antennas, it is the EM fields and waves that do the moving at the speed of light in the medium.

Think about it. We know that the RF energy travels at the velocity factor times the speed of light from the source to the receiving XMTR.  Electrons, having mass, cannot possibly do that. Anything traveling at the speed of light must necessarily have a rest mass of zero.
« Last Edit: May 19, 2019, 09:22:46 AM by W5DXP » Logged

My antenna says, "What makes me happy is when the tuner is adjusted for maximum available current through my radiation resistance!" 73, Cecil, w5dxp.com
Pages: [1]   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!