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] 2 Next   Go Down
  Print  
Author Topic: vertiacal antennas: do radials or counterpoise radiate?  (Read 4285 times)
KD7RDZI2
Member

Posts: 213




Ignore
« on: September 30, 2017, 11:18:46 AM »

Suppose the simple case of a vertical antenna with 4 radials like a ground plane.
Logged
W5DXP
Member

Posts: 4187


WWW

Ignore
« Reply #1 on: September 30, 2017, 12:04:45 PM »

Suppose the simple case of a vertical antenna with 4 radials like a ground plane.

The following is true for elevated radials.

Let's say you have a 1/4WL radial running north and an identical one running south tied together at the feedpoint. The currents in those two symmetrical radials are 180 degrees out of phase with each other and therefore the radiation from those two radials tends to cancel (destructive interference). If you have one 1/4WL radial, it will radiate like 1/2 of a dipole.

In the case of 4 symmetrical radials, each pair that are 180 degrees apart will undergo cancelled radiation.
Logged
KD7RDZI2
Member

Posts: 213




Ignore
« Reply #2 on: September 30, 2017, 12:52:10 PM »

Now think at an ordinary 2 meter/70cm handheld transceiver. Any stock antenna is without radials. The chassis, the ear buds and ultimately the user himself should radiate as well ! I hope I am wrong ! Am I ?
Logged
K4SAV
Member

Posts: 2383




Ignore
« Reply #3 on: September 30, 2017, 02:41:10 PM »

Now think at an ordinary 2 meter/70cm handheld transceiver. Any stock antenna is without radials. The chassis, the ear buds and ultimately the user himself should radiate as well ! I hope I am wrong ! Am I ?

No you are correct.

Another tid bit of information:  The fields from symmetrical radials cancel in the far field but there is lots of radiation in the near field.

Jerry, K4SAV
Logged
WB6BYU
Member

Posts: 17040




Ignore
« Reply #4 on: September 30, 2017, 02:45:53 PM »

Quote from: W5DXP

...The currents in those two symmetrical radials are 180 degrees out of phase with each other and therefore the radiation from those two radials tends to cancel...



That assumes that the radials are horizontal, of course.  If the (identical) radials slope
downwards, the horizontally polarized radiation cancels in the far field, but the vertically
polarized component
is in phase for all of the radials, and will contribute to the net radiation.



Quote from: KD7RDZI2

Now think at an ordinary 2 meter/70cm handheld transceiver. Any stock antenna is without radials. The chassis, the ear buds and ultimately the user himself should radiate as well ! I hope I am wrong ! Am I ?




Radio Shack used to advertise the "Exclusive Range Boost Circuitry" in their CB walkie-talkies.
It consisted of two strips of metal down the sides of the case to make contact with the user's
hand, thereby turning the user into a counterpoise.

At UHF, the radio chassis is often large enough (at least with the older radios) to act as a
low impedance ground plane.    It works OK on 2m, but adding a quarter wave wire radial
handing down from the ground side of the antenna jack can improve your signal considerably.
(I've measured up to 10 dB.)

How much RF you get on an attached wire (such as headphones or a speaker mic) depends
on the effective RF length of the wire (that is, the RF impedance looking into it.)  A 3' lead
is close to half a wavelength, which is a high impedance, so you probably won't have much
current on it.  But a 19" or 58" (or thereabouts) would be a low impedance, and it would
tend to have more RF current on it.
Logged
K6AER
Member

Posts: 4663




Ignore
« Reply #5 on: September 30, 2017, 06:30:45 PM »

Suppose the simple case of a vertical antenna with 4 radials like a ground plane.

The following is true for elevated radials.

Let's say you have a 1/4WL radial running north and an identical one running south tied together at the feedpoint. The currents in those two symmetrical radials are 180 degrees out of phase with each other and therefore the radiation from those two radials tends to cancel (destructive interference). If you have one 1/4WL radial, it will radiate like 1/2 of a dipole.

In the case of 4 symmetrical radials, each pair that are 180 degrees apart will undergo cancelled radiation.

The 1/4 wavelength radials connected to the feed point base of a 1/4 wave vertical will exhibit opposite phase from the vertical radiator. Just like the two halves of a dipole. The voltage/current on the radials will be the same as the vertical radiator divided by the number of radials. This is why with many radials you can get away with much smaller wire.

The more radials the higher the vertical capacitance is to the ground plane and the lower the vertical E-Plane radiation will be.

Although: I have a 1/4 wave wire vertical in the trees for 40/80 Meters. I run high power on those bands at night and the 14 gage vertical wire has never gotten hot at the base from current, About 5.5  amps. The wire is just soldered to the center of the coax feed. Because my radial system is in the grass, I have 30+ radials to make up for earth losses. Bandwidth is good and my signals are as good as many others on any given night.  The dipole in the trees is still quieter for receive.




« Last Edit: September 30, 2017, 06:49:15 PM by K6AER » Logged
KC4ZGP
Member

Posts: 1637




Ignore
« Reply #6 on: September 30, 2017, 07:22:40 PM »


Of course they radiate. Why would one half of an antenna not radiate?

Kraus





Logged
W5DXP
Member

Posts: 4187


WWW

Ignore
« Reply #7 on: September 30, 2017, 08:28:22 PM »

Why would one half of an antenna not radiate?

Essentially the same reason that a transmission line or top hat doesn't radiate in the far field - destructive interference. Horizontal, symmetrical radials are like a top hat turned upside down.
Logged
YL3GND
Member

Posts: 55




Ignore
« Reply #8 on: October 01, 2017, 01:41:54 AM »


Let's say you have a 1/4WL radial running north and an identical one running south tied together at the feedpoint. The currents in those two symmetrical radials are 180 degrees out of phase with each other and therefore the radiation from those two radials tends to cancel (destructive interference).

How on earth currents in radials could be out of phase with each other ?  Roll Eyes Its the same current flowing from one point and splitting, so it is in phase.
Logged
RFRY
Member

Posts: 479


WWW

Ignore
« Reply #9 on: October 01, 2017, 04:11:40 AM »

... How on earth currents in radials could be out of phase with each other ?  Roll Eyes Its the same current flowing from one point and splitting, so it is in phase.

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).   See the graphic below...

« Last Edit: October 01, 2017, 04:31:25 AM by RFRY » Logged
W9IQ
Member

Posts: 1705




Ignore
« Reply #10 on: October 01, 2017, 05:54:31 AM »

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.

- Glenn W9IQ
« Last Edit: October 01, 2017, 05:57:10 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
Member

Posts: 1705




Ignore
« Reply #11 on: October 01, 2017, 06:59:14 AM »

I suppose in my above post I should say no meaningful difference in gain or Rr. With radials Rr is ~22 ohms and gain is ~1.25 dBi compared to ~25 ohms and ~1.82 dBi without radials.

- Glenn W9IQ
Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W5DXP
Member

Posts: 4187


WWW

Ignore
« Reply #12 on: October 01, 2017, 09:28:36 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).

Very nice graphics, Richard, and in addition to that fact of physics, 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.

Each of the two directions of current flow in a wire is given a different mathematical sign, by convention. DC current flowing toward the load was given a "+" sign and flowing away from the load was given a "-" sign. That convention carried over into AC with one direction given a "+" sign and the opposite direction given a "-" sign. That difference in physical direction amounts to a mathematical difference of 180o in the phase of the two signals.
« Last Edit: October 01, 2017, 09:45:52 AM by W5DXP » Logged
RFRY
Member

Posts: 479


WWW

Ignore
« Reply #13 on: October 01, 2017, 11:37:03 AM »

We should all give a tip of the hat to the late Dr. George H. Brown of RCA Laboratories, Princeton, NJ, who invented the ground plane antenna in 1937.

In his autobiography and part of which I was   Recollections of a Research Engineer, he states that 2 horizontal radials spaced at 180° intervals worked as well for the ground plane as 4 horizontal radials spaced at 90° intervals.  

When the 2-radial form of the antenna was presented to the RCA sales organization, they doubted that the ground plane would produce a uniform radiation pattern over the service area.  So to quiet them, the ground plane was marketed with 4 radials, and he wrote "When the antenna became really popular, we did not dare confess to our ruse."
« Last Edit: October 01, 2017, 11:39:08 AM by RFRY » Logged
W9IQ
Member

Posts: 1705




Ignore
« Reply #14 on: October 01, 2017, 12:18:10 PM »

That's an interesting story. I had not heard that previously. Thanks for sharing it.

- Glenn W9IQ
Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
Pages: [1] 2 Next   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!