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Re: [HCDX] Dxers Unlimited's weeken edition script for June 30 - July 1 2007
Radio Habana Cuba
Dxers Unlimited
Dxers Unlimited's weekend edition for 30 June -1 July 2007
By Arnie Coro
radio amateur CO2KK
Hi amigos radioaficionados ! Welcome to the weekend edition of Dxers
Unlimited coming to you from Havana. I am Arnie Coro, radio amateur
CO2KK, and here is
item one: Solar activity moving up, after many days of a spotless
Sun... Item two: Ever heard about the ELECTROMAGNETIC ground plane
antenna... ? The fancy name of this low profile antenna comes from its
similarity with magnetic antennas. The EMGP, or Electro Magnetic Ground
Plane antenna is now in use by several AM broadcast band stations
operating between 1200 and 1700 kilohertz with excellent results. Among
the advantages of the EMGP antenna design, is that the mast or tower
required for its installation is of much lower height than the one used
with the typical one quarter wavelength vertical monopole antenna system
used by AM broadcast stations for local or semi-local coverage. For old
timers , the EMGP antenna is also known by another name... the inverted
L of the first years of radio, but according to engineers that have done
advanced research using the NEC or numerical electrical code based
software and the method of moments to calculate what really happens with
those antennas, the present day EMGP designs are much more efficient
than the classic Inverted L that broadcast radio stations used before
tall steel towers became
popular .
More about the EMGP antenna, and how radio amateurs that has very small
areas available for installing antennas may benefit from its unique
design features, later as Dxers Unlimited's weekend edition continues...
I am Arnie Coro here in the sunny capital of Cuba, where the summer
holidays are now in full swing...
.....
Si amigos, most of the world's more than two million amateur radio
operators are very short of space for installing HF antennas, required
to operate on the bands between
80 and 10 meters, and of course very , very few of us are able to
install adequate
antennas especially for the 160 and 80 meters band... That's why when I
heard from some interesting antenna modeling work that has led to a very
short in height vertical antenna system, I decided to gather as much
information as possible, and then proceed to do some practical testing
of the concept... After several months of intermittent work on the
topic, I was able to arrive to certain conclusions that today are on the
air here at Dxers Unlimited's weekend edition... Conclusion number one:
It came after testing both a 6 meters and a 10 meters band EMGP antenna
under almost ideal conditions... Conclusion number one: When properly
built and provided with a very well built ground system, the EMGP
antenna's efficiency is very close to a full height quarter, wave
vertical monopole antenna... By replacing the EMGP, with a full size
quarter wave vertical monopole located right at the center of the ground
system, comparative field strength measurements were made , allowing me
to tabulate the results, and results showed that the efficiency of the
EMGP antennas was very high, almost as high as the efficiency obtained
with the classic vertical quarter wave systems.
Conclusion number two: If need to install a low profile , single band
antenna within a very small area... the EMGP is an excellent choice.
Conclusion number three: The EMGP does require careful adjustment of its
matching system in order to obtain good results... in other words, you
will have to spend some time with antenna measuring instruments doing
field work, so that the antenna will be properly set up for the
operating frequency...
Now, here is the information about the EMGP I am sure that you are
waiting for:
First of all, the vertical portion of the antenna is only one twelfth of
a wavelength long at the operating frequency.... To put that into real
world figures... a 20 meters band
EMGP antenna ... operating at a wavelength of 21 decimal two meters,
will only need to be 1.7 meters high above the ground plane !!! And the
horizontal portion of the
antenna is only one sixth of a wavelength or 3.53 meters long... The
actual radiating element of the antenna has a total length of 5.23
meters, which is very near to the overall length of a quarter wave
antenna system at 14.15 megaHertz that can be considered for all
practical purposes the center of the 20 meters ham band, ...
At many locations, installing a vertical antenna that is a bit more than
five meters high above the roof is quite difficult or impossible
according to zoning restrictions or buildings rules and regulations...
but a very small plastic mast, less than 2 meters high can be easily
disguised as a sanitary system vent pipe... By the way, the EMGP antenna
is usually made of copper wire, so it is much less noticeable than an
antenna made using an aluminum or copper pipe or rod...
In a few seconds , after a short break, I will be providing you with
actual measurements to build EMGP antennas for several bands...
.......
You are listening to the weekend edition of Dxers Unlimited amigos, and
yes you can send your radio hobby related questions directly to me ,
send mail to arnie@xxxxxx , again arnie@xxxxxx, or VIA AIR MAIL to Arnie
Coro, Radio Havana Cuba, Havana, Cuba... Now let's review the basic
information about the EMGP, or electromagnetic ground plane antenna
system...
Repeating the basics...
ANTENNA VERTICAL PART = 1/12 of a wavelength at the operating frequency
ANTENNA HORIZONTAL PART= 1/6 wavelength at the operating frequency
(you will need to trim carefully to adjust for resonance at the desired
part of the band)
TYPICAL ANTENNA BUILDING MATERIAL FOR A SHORT WAVE ANTENNA WITHIN
THE FREQUENCY RANGE BETWEEN 7 AND 29 MEGAHERTS: 4mm diameter copper wire
ANTENNA DOES REQUIRE ... a very well-built GROUNDING SYSTEM,
consisting of no less than 12 radials of 1/4 wavelength each. The best
approach is to use a ground matt close to the antenna feed point with a
radius of about 0.1 wavelength, and to that add no less than 30 radials
made from 1.5 to 2 mm copper wire with a length of 1/4 of a wavelength
at the operating frequency.
ANTENNA FEEDPOINT impedance will depend very much on how effective
the ground system is, but WILL ALWAYS BE BELOW 12 OHMS OR LESS. So you
MUST take that low impedance into consideration, so that the matching to
the 50 ohm coaxial cable will be properly made, hence achieving maximum
signal transfer from the feed line to the antenna while transmitting,
and from the antenna to the feed line when in the receive mode.
EMGA DIMENSIONS FOR THE 40 and,20 meters bands:
ALL LENGTHS IN THE METRIC SYSTEM
40 meters:
Vertical part= 3.55 meters
Horizontal part= 7.10 meters (must be adjusted to resonance,
something that you can do with a loosely coupled grid-dip meter, or with
an antenna analyzer if you can have access to this wonderful instrument).
SEE... a 40 meter antenna that has ALMOST the same performance as a
full size 1/4 wavelength vertical, and YET IS ONLY 3.5 meters high!!!
The EMGP data for the 20 meter band follows:
20 meters EMGP built with 4 mm copper wire
VERTICAL PART= 1.75 meters
HORIZONTAL PART = 3.5 meters
THE ABOVE TWO SYSTEMS ARE THE ONES I HAVE EXPERIMENTED WITH ... You can
calculate the data for the other bands by using the following general
formulas ( and don't forget that the HORIZONTAL PART must be trimmed for
resonance... and that the GAMMA MATCH will also require careful
adjustment...
THIS IS NOT A CUT AND INSTALL ANTENNA... THIS ONE DOES REQUIRE CAREFUL
ADJUSTMENTS... BUT, this antenna has an extremely LOW PROFILE that will
make many radio amateurs be able to operate at least on one band from
locations that make it difficult or impossible to install other antenna
systems. For example, a 40 meter band EMGP may be installed disguised as
a clothes line support, with one of the poles longer than the other. You
can use nylon or Dacron lines to tie the wire to the other pole!
The HORIZONTAL PART of the antenna MUST BE TRIMMED in order to assure
low SWR at the operating frequency. You can use this antenna with an
antenna tuner of course, but it will work without a tuner when properly
adjusted. Operating bandwidth won't be sensational, but if you know what
part of the band you do want to operate more frequently, then the EMGP
can be adjusted for minimum SWR there.
OK ... here once again are the formulas:
VERTICAL PART = 1/12 of a wavelength
HORIZONTAL PART= 1/6 of a wavelength
ANTENNA IMPEDANCE at the base is estimated to be no higher than
10, maybe 12 ohms, SO... the GROUND SYSTEM IS ESSENTIAL for the antenna
to operate.
There best for approach to matching an EMGP antenna to a coaxial line
feeder, is by means of a classic impedance transformation network, the L
network, that is calculated to match the 50 ohms impedance of the
coaxial cable, or in the case you use 75 ohms cable , then this will be
the impedance to match at the cable end... The L network must match the
low impedance of the EMGP antenna that should be typically around 12 to
15 ohms, although in actual practice, depending on the ground system
this impedance may be lower... Anyway, this antenna system has proven to
be an attractive and easy to implement option when the space available
is very limited, and also you need to include a low profile into the
antenna's specifications . My tests with the 20 meters band version have
shown that it also makes a nice receiving antenna for short wave
listening on the 16, 19, 22 and 25 meters international short wave
broadcast bands, by just connecting it to my short wave receiver ,
without the use of an antenna tuner.
I hope that this antenna design will help many of you, Dxers Unlimited
listeners around the world that keep telling me about your difficulties
for installing external antennas... and YES, there is a lot of
difference in the performance of a rooftop or garden installed antenna,
and one inside your home...
And now, just before going QRT here, ready to copy ?... Our exclusive
Arnie Coro's Dxers Unlimited's HF plus low band VHF propagation update
and forecast...Sporadic E openings happening now on an everyday basis...
over North America, the Caribbean, Northern Africa and Europe, with some
phenomenal 2 meters E skip contacts reported from western Europe this
week. And, more to come, amigos as this Sporadic E season has taken a
turn for the better during the last week of June... Solar flux at very
low levels, a small coronal hole located at a geo effective position are
combining to produce rather poor HF propagation conditions, typical of
sunspot minimum years, although late in the evening the 20 and 30 meters
amateur bands are opening up from the Caribbean to the Asia-Pacific
region ..Send your signal reports and comments to arnie@xxxxxx, or VIA
AIR MAIL to Arnie Coro, Radio Havana Cuba, Havana , Cuba, they do help
me make a better program for you amigos !!!
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