Here is a very interesting and well-written article from Electronic Design magazine on a topic that we should all be very familiar with.
Darryl, KF6DI
What's The Difference Between A Dipole And A Ground Plane Antenna?
Jul. 16, 2013 Lou Frenzel | Electronic Design
This article shows how a dipole and and ground plane antenna are similar but also different.
Every wireless device needs an antenna. This conductive mechanical device is the transducer that converts the radio frequency (RF) power to be transmitted into the electric and magnetic fields that make up a radio wave. It also converts the received radio wave back into an electrical signal. An almost infinite number of configurations is possible for antennas. However, most are derived from two basic types: dipole and ground plane antennas.
Table Of Contents
. Antenna Fundamentals
. The Dipole Antenna
. The Ground Plane Antenna
. Summary
. References
Antenna Fundamentals
A radio wave comprises an electric field perpendicular to a magnetic field. Both are perpendicular to the direction of propagation (Fig. 1). An antenna creates this electromagnetic field. The signal to be radiated by the antenna is developed in a transmitter and then sent to the antenna by way of a transmission line, usually coax cable.
1. The lines are the magnetic and electric lines of force that travel together and support one another as they move outward from the antenna.
The signal voltage develops an electric field around the antenna elements. The current flow in the antenna produces a magnetic field. The electric and magnetic fields combine and regenerate one another according to Maxwell's famous equations, and the combined wave is launched from the antenna to travel through space. At the receiving site, the electromagnetic wave induces a voltage in the antenna, which converts the electromagnetic wave back into an electrical signal that can be further processed.
A primary consideration in the orientation of any antenna is polarization, which refers to the orientation of the electric (E) field with the earth. It is also the orientation of the antenna elements with respect to the earth. A vertically mounted antenna, perpendicular to the earth, radiates a vertically polarized wave. So, a horizontally positioned antenna radiates a horizontally polarized wave.
Polarization also can be circular. Special configurations such as helical or spiral-wound antennas can launch the wave into rotation, creating a circularly polarized wave. The antenna may induce either a right-hand or left-hand rotation.
Ideally, the antennas at both the transmitting and receiving sites should have the same polarization. At frequencies below about 30 MHz, the wave is usually reflected, refracted, rotated, or otherwise modified by the atmosphere, earth, or other objects. Therefore, polarization matching at the two sites is not critical. At VHF, UHF, and microwave frequencies, polarization must be the same to ensure maximum signal transfer. And, note that antennas exhibit reciprocity-that is, they work equally well for transmitting or receiving.
The Dipole Antenna
The dipole antenna is a half-wavelength structure made of wire, tubing, printed-circuit board (PCB) copper, or some other conductive material. It is divided into two equal quarter wavelengths and fed with a transmission line (Fig. 2).
2. The patterns show the distribution and strength of the voltage (V) and current (I) on a dipole antenna.
The lines show the distribution of electric and magnetic fields. One wavelength (?) is:
? = 984/fMHz
where wavelength is in feet and the frequency is in MHz. One half wavelength is:
?/2 = 492/fMHz
The actual length is usually shortened depending on the size of the antenna conductors. A better approximation to electrical length is:
?/2 = 492 K/fMHz
K is a factor relating conductor diameter to length. It is 0.95 for wire antennas at a frequency of 30 MHz or less. Or:
?/2 = 468/fMHz
The length in inches is:
?/2 = 5904 K/fMHz
The K value is smaller for larger diameter elements. For half-inch diameter tubing, K is 0.945. A tubing dipole for 165 MHz would have a length of:
?/2 = 5904(0.945)/165 = 33.81 inches
or two 16.9-inch segments.
The length is important because the antenna is a resonant device. For maximum radiation efficiency, it must be tuned to the operating frequency. However, the antenna does operate well enough over a narrow range of frequency just like a resonant filter.
The antenna bandwidth is a function of its structure. It's usually defined as the range where the antenna standing wave ratio (SWR) is less than 2:1. SWR is determined by the amount of reflected signal from the antenna back down the transmission line feeding the antenna (see "Standing Wave Ratio," below). It is a function the antenna impedance with relationship to transmission line impedance.
The actual impedance of the antenna at its center feedpoint varies with frequency and antenna height. At resonance and one half wavelength above ground, the antenna impedance is approximately 73 ? resistive. Off resonance, the antenna impedance will include either an inductive or capacitive reactance component.
The ideal transmission line is a balanced conductor pair with a 75-? impedance. Coax cable with a 75-? characteristic impedance (Zo) can also be used. Coax cable with a 50-? characteristic impedance can also be used since it is a good match to the antenna if it is less than one half wavelength above ground.
Coax is an unbalanced line since RF current will flow on the outside of the coax shield, creating some undesirable induced RF in nearby devices, although the antenna will work well enough. The best feed method is to use a balun at the feedpoint with coax cable. A balun is a transformer-like device that converts balanced signals to unbalanced signals or vice versa.
The dipole may be mounted horizontally or vertically depending on the desired polarization. The feed line ideally should run perpendicular to the radiating elements to avoid distorting the radiation pattern, so the dipole is most often oriented horizontally.
An antenna's radiation pattern depends on its structure and its mounting. The physical radiation is three dimensional but is usually represented by both horizontal and vertical radiation patterns.
The horizontal radiation pattern of the dipole is a figure 8 (Fig. 3). Maximum signal occurs broadside to the antenna. Figure 4 shows the vertical radiation pattern. They are ideal patterns that are easily distorted by the ground and any nearby objects.
3. This is the horizontal (E field) radiation pattern of a half-wave dipole antenna. The antenna elements are located along the 270° to 90° line.
4. This is the vertical radiation pattern showing elevation of signal from a horizontal dipole one half wavelength above the ground.
An antenna's gain is related to the directionality of the radiation pattern. The gain is usually expressed in dB with respect so some reference such as an isotropic antenna, which is a point source of RF energy that radiates in all directions. Think of a point source of light illuminating the inside of an expanding sphere. An isotropic antenna has a gain of 1 or 0 dB.
If the antenna shapes or focuses the radiation pattern and makes it more directional, it has gain over an isotropic antenna. A dipole has a gain of 2.16 dBi over an isotropic source. In some cases, the antenna gain is expressed with relationship to a dipole reference in dBd.
The Ground Plane Antenna
A ground plane antenna is essentially one half of a dipole mounted vertically. The term monopole is also used to describe this antenna. The earth ground below the antenna, a conducting surface a least ?/4 in radius or a pattern of ?/4 conductors called radials, makes up the other half of the antenna (Fig. 5).
5. Ground plane monopole and Marconi antennas are one quarter wavelength high. The ground or radials are the other half of the antenna.
If the antenna is connected to a good earth ground, it is called a Marconi antenna. The ground structure serves as the other ?/4 half of the antenna. If the ground plane is adequately sized and conductive, the performance of the ground plane is equivalent to a vertically mounted dipole.
The length of a quarter-wave vertical is:
?/4 = 246 K/fMHz
The K factor is smaller than 0.95 for verticals, which are usually made with wider tubing.
The feedpoint impedance is one half that of a dipole or about 36 ?. The actual figure depends upon height above the ground. Like the dipole, the ground plane is resonant and typically has a reactive component to its primary impedance. The most common transmission line is 50-? coax, as it is a relatively good match to the antenna impedance with an SWR below 2:1.
The ground plane antenna is omnidirectional. The horizontal radiation pattern is a circle where the antenna radiates equally well in all directions. Figure 6 shows the vertical radiation pattern. Compared to the dipole's vertical radiation pattern, the ground plane has a lower angle of radiation, giving it a longer-range propagation advantage at frequencies below about 50 MHz.
Summary
Virtually all other antennas in common usage are variations of the dipole or ground plane antennas. For example, a Yagi-Uda antenna adds parasitic elements like a director and/or reflector to a dipole to increase its gain and directionality. Multiple dipoles can be stacked vertically or arranged in various arrays that greatly increases gain. UHF bowtie TV antennas and PCB meanderline antennas used in some wireless devices are dipole variations. Patch (microstrip) and slot antennas used at microwave frequencies are also dipole derivatives.
In addition, two or more ground plane vertical antennas can be arranged to produce a more directional antenna with gain. For example, a directional AM radio station uses two or more towers to direct a strong signal in one direction while suppressing it in another.
References
American Radio Relay League, The ARRL Antenna Book, 1997-98.
American Radio Relay League, The ARRL Handbook for Radio Communications, 2003.
Bruno Delorme, Antennas and Site Engineering for Mobile Radio Networks, Artech House, 2013.
William Orr, Radio Handbook, 22nd edition, Howard W. Sams & Co. Inc., 1981.
6. This is the vertical radiation pattern of a ground plane antenna over a so-called perfect ground.
Standing Wave Ratio
Standing waves are voltage and current distribution patterns along a transmission line. If the characteristic impedance (Zo) of the line matches the generator (transmitter) output impedance and the antenna load, the voltage and current along the line is constant. With impedance matched, maximum power transfer occurs.
If the antenna load is not matched to the line impedance, not all of the transmitted power is absorbed by the load. Any power not absorbed by the antenna is reflected back down the line, interfering with the forward signal and producing variations of current and voltage along the line. These variations are the standing waves.
The measure of this mismatch is the standing wave ratio (SWR). SWR is usually expressed as a ratio of maximum and minimum values of forward and reverse current or voltage values along the line:
SWR = Imax/Imin = Vmax/Vmin
Another simpler way to express SWR is as the ratio of the transmission line characteristic impedance (Zo) to the antenna impedance (R):
SWR = Zo/R or R/Zo
depending on which impedance is greater.
The ideal SWR is 1:1. An SWR of 2 to 1 indicates a reflected power of 10%, meaning that 90% of the transmitted power gets to the antenna. An SWR of 2:1 is generally regarded as the maximum allowable for the most efficient system operation.
To all,
The Fiesta Parade is coming up on August 2nd . I need happy willing
volunteers who can work a radio location. Each ham participant will get a
ticket to the Horseman's Rendezvous BBQ held at the Carriage Museum just
after the parade. Please get back to me if you are interested. It is a
very fun event and a great BBQ afterwards. I know many of you have already
replied back to me, but for the sake of my bad short term memory, please
send it back in email form as well.
Thank you
73
Rick Whitaker
Cell: 451-3436 leave a message
rickwhtr(a)cox.net
The Fiesta Parade is coming up on August 2nd , We have
supported this event for more years than I can recall. If you are
interested in participating then please send me an email. You will receive a
ticket to the Fiesta Parade Horseman's Rendezvous Bar-B-Que after the parade
at the Carriage Museum. This is coming up quickly so please respond soon.
73
Rick Whitaker
SBARC VP of Operations
rickwhtr(a)cox.net
7/16/2013
To: SBARC ListServer
From: Bill Talanian
Subject: New Vessel Tracking system
As a public service to the boating enthusiasts and those interested
with shipping within the Channel Islands SBARC has installed two AIS
sites. The first Automatic Identification System (AIS) was
contributed by the estate of the late Robert Block, K6LX. This unit
is installed at our downtown Mesa site. The second unit was provided
by SBARC and installed today at Santa Ynez Peak. The exciting screen
view will draw interest to the many off-shore activities along our
coastline. The down-link from Santa Ynez Peak is provided by the
SBARC 5.8 GHz microwave system with connectivity through the Mesa
site thence to Gibraltar Peak. Thanks go to the SBARC technical team
and contributors for making it all happen.
When viewing the AIS site please note the Source:
SBARC-SBA is our downtown site and SBARC-SYP is the Santa Ynez Peak site.
<http://www.marinetraffic.com/ais/default.aspx?centerx=-119.55¢ery=34.10&>
Fellow HAMs and SBARC members
It is with sadness that I pass along the news that Norm Salzman, KA6UHP, a
long time member of SBARC and active ARES team member passed away at the
age of 99 years.
Sam Salzman (965-8800) reported his passing to James, AJ4VP in Ventura for
our Club Station Manager, Shackmaster Dave (K6HWN) notification.
Those of us who worked with Norm will remember him as a great friend and
good company to have with us on many ARES exercises. May he rest in Peace,
He will be missed!
73,
de AL - WA6VNN
SBARC
73
de Dave - K6HWN "Shackmaster"
73
de James Adriansen AJ4VP
Ventura, CA
--
------------------------------
CONFIDENTIALITY NOTICE: This is a transmission from the Santa Barbara
Unified School District and may contain privileged and confidential
information. It, and any attachments, are intended only for the
addressee(s) . If you are not the intended recipient, you are hereby
notified that any review, dissemination, distribution or duplication of
this communication and its attachments is strictly prohibited by applicable
state and federal law. If you are not the intended recipient, please
contact the sender by reply email and destroy all copies of the original
message and attachments.
Santa Barbara Unified School District - "Every child, every chance, every
day"
Not only is the reaction to this [Executive Order] wildly over the top in some corners, but the Executive Order itself is nothing more than a restatement of policy that has been in place in decades and grants no authority to the President or the Cabinet that they don't already have under existing law.
The Defense Production Act has been in effect since the Truman Administration, and authorizes the President to direct private business to allocate resources to national defense as needed in a time of national emergency. Since the end of the Cold War, if not before, the Act has been used primarily to use DOD contracting practices to direct investment in new technologies that would be used for defense purposes, however it still requires the Executive Branch to at least plan for the possibility of allocating resources for national defense in the event of a national emergency in much the same way that rationing was implemented during World War II. Now, fortunately, we have not faced a national emergency of that type in the 50-odd years that the DPA has been in effect but that doesn't mean that such an eventuality, brought about by either international crisis or natural disaster, should not be planned for. Despite that it was released as part of the "Friday Document Dumps" that have become all too common in official Washington, that appears to be all that this Executive Order is about.
73, Don, W6YN ---------- Original Message ----------
From: "Darryl Widman" <auditron(a)juno.com>
To: <B.Bascom(a)hotmail.com>, <sbarc-list(a)lists.netlojix.com>
Cc:
Subject: Re: [Sbarc-list] Obama Signs Executive Order to Allow Shut Down ofAll US Communications
Date: Fri, 12 Jul 2013 16:33:44 -0700
Hello Bob,
Thank you for keeping us all on our toes! I was wondering what RT meant. Now
we all know. Wikipedia says that RT means Russia Today.
Now I feel much better.
Darryl, KF6DI
----- Original Message -----
From: "Bob Bascom" <B.Bascom(a)hotmail.com>
To: <sbarc-list(a)lists.netlojix.com>
Sent: Friday, July 12, 2013 8:16 AM
Subject: Re: [Sbarc-list] Obama Signs Executive Order to Allow Shut Down of
All US Communications
Thank you for the news.
I'm wondering how the president's order may have changed the laws already
previously in effect, and how any new changes may possibly affect radio
amateur
operators or their stations.
The news source that started this hysterical uproar, RT, is the Russian
Times.
The date on the executive order
<http://www.whitehouse.gov/the-press-office/2012/07/06/executive-order-assig…>
is July 06, 2012.
Below are the results of brief research:
<http://gcn.com/articles/2010/06/28/n...lins-bill.aspx>
"Contrary to what's being widely and incorrectly reported in a number of
media
outlets, there is no kill switch in the Lieberman-Collins-Carper
cybersecurity
bill (S 3480) that would give the President of the United States new powers
to
shut down critical networks. But there is one already on the books in the
Telecommunications Act. of 1934.
The Lieberman-Collins bill just authorizes standard filtering like that done
by
Internet service providers every day, but in a nationally-coordinated
fashion.
The only kill switch appears to be in Section 706 of the Communications Act
of
1934 that already gives the President the power in a time of national
security
emergency to shut down or disrupt network traffic."
Here is the wording from Section 706 (c) of the Communications Act of 1934:
<http://www.fcc.gov/Reports/1934new.pdf>
"Upon proclamation by the President that there exists war or a threat of
war,
or a state of public peril or disaster or other national emergency, or in
order
to preserve the neutrality of the United States, the President, if he deems
it
necessary in the interest of national security or defense, may suspend or
amend, for such time as he may see fit, the rules and regulations applicable
to
any or all stations or devices capable of emitting electromagnetic
radiations
within the jurisdiction of the United States"
It's not like the shutdown of radio communications has no precedent in
history.
<http://transition.fcc.gov/omd/history/radio/quality.html>
In 1917, when the U.S. entered WWI, as a result of powers given to it by
the Radio Act of 1912 (a law motivated in part by the Titanic disaster),
the federal government shut down all private radio operations in the
United
States.
<http://earlyradiohistory.us/sec013.htm>
all amateur and commercial use of radio came to an abrupt halt on April
7,
1917 when, with the entrance of the United States into World War One,
most
private U.S. radio stations were ordered by the President to either shut
down or be taken over by the government, and for the duration of the war
it
became illegal for private U.S. citizens to even have an operational
radio
transmitter or receiver -- in fact, it was Treason to Possess Wireless
Stations according to one zealous city manager, reported on the front
page
of the April 23, 1917 San Jose Evening News.
<http://ac6v.com/history.htm>
June 1940, the US invoked the Telecommunications Convention prohibiting
US
amateurs from contacting hams elsewhere; at the same time all portable
and
mobile operation below 56 MHz was banned (except the ARRL Field Day). At
the request of the ARRL, the ban was modified to allow the League's
Emergency Corps to continue work on the lower frequencies for training
and
drills. All licensees were required to send a set of fingerprints, a
photo,
and proof of citizenship to the FCC.
December 7, 1941, the US entered the war; hams were immediately ordered
to
go QRT.
After VJ Day in 1945, hams were given authorization to begin operating
again on the 2 1/2 meter band, on a shared basis with WERS. WERS was
terminated in mid-November. By the 15th of that month, the FCC released
bands at 10, 5, and 2 meters for amateur use. The post-war era of
amateur
radio had commenced.
On Thu, 11 Jul 2013 23:07:27 -0700, "Darryl Widman" <auditron(a)juno.com>
wrote:
>
>Subject: Obama Signs Executive Order to Allow Shut Down of All US
>Communications
>
>
>Obama Signs Executive Order to Allow Shut Down of All US Communications
>Posted on July 11, 2013 by V2A
>
>
>RT has reported that in a secretly unannounced move, President Barack Obama
>signed an executive order giving the Department of Homeland Security the
>ability to shut down all of the United States' communications systems upon
>his request. Barack Obama laid out a statement he titled "Assignment of
>National Security Emergency Preparedness Communications Function". No doubt
>a very confusing title. The reasoning behind his issuing of this executive
>order, is that he feels the government may one day need to access all of
>our telephones, computers, cable communications, etc., in the name of
>national security.
>"The Federal Government must have the ability to communicate at all times
>and under all circumstances to carry out its most critical and time
>sensitive mission."
>
>Post Continues on www.examiner.com
_______________________________________________
SBARC-list mailing list
SBARC-list(a)lists.netlojix.com
http://lists.netlojix.com/mailman/listinfo/sbarc-list
_______________________________________________
SBARC-list mailing list
SBARC-list(a)lists.netlojix.com
http://lists.netlojix.com/mailman/listinfo/sbarc-list
Subject: Obama Signs Executive Order to Allow Shut Down of All US Communications
Obama Signs Executive Order to Allow Shut Down of All US Communications
Posted on July 11, 2013 by V2A
RT has reported that in a secretly unannounced move, President Barack Obama signed an executive order giving the Department of Homeland Security the ability to shut down all of the United States’ communications systems upon his request. Barack Obama laid out a statement he titled “Assignment of National Security Emergency Preparedness Communications Function”. No doubt a very confusing title. The reasoning behind his issuing of this executive order, is that he feels the government may one day need to access all of our telephones, computers, cable communications, etc., in the name of national security.
“The Federal Government must have the ability to communicate at all times and under all circumstances to carry out its most critical and time sensitive mission.”
Post Continues on www.examiner.com
Not that I know of where they may be.
Bill T
At 12:40 PM 7/9/2013, WB6WCW(a)aol.com wrote:
>Hey There:
>
>3 of my swapmeet tables have disappeared. I believe they were in
>the SBARC Storage Trailer near the County Dump. Last used at the
>Santa Maria Father's Day Swapmeet.
>
>The 6 foot long tables fold in half in the middle so for traveling
>they are only 3 feet long.
>
>I need them for the Williams, Arizona, swapmeet on the 19th & 20th of July.
>
>Any information would really be appreciated.....I hate to buy 3 new
>ones........
>
>Tnx & 73
>
>Bill Gross W6FF
>(805) 968-4227
>(805) 722-0070 (Cell)
>====================================END===================================
>
>_______________________________________________
>SBARC-list mailing list
>SBARC-list(a)lists.netlojix.com
>http://lists.netlojix.com/mailman/listinfo/sbarc-list
Hey There:
3 of my swapmeet tables have disappeared. I believe they were in the
SBARC Storage Trailer near the County Dump. Last used at the Santa Maria
Father's Day Swapmeet.
The 6 foot long tables fold in half in the middle so for traveling they are
only 3 feet long.
I need them for the Williams, Arizona, swapmeet on the 19th & 20th of July.
Any information would really be appreciated.....I hate to buy 3 new
ones........
Tnx & 73
Bill Gross W6FF
(805) 968-4227
(805) 722-0070 (Cell)
====================================END===================================
