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Re: [HCDX] Dxers Unlimited's script weekend edition March 3-4 -2007
Radio Havana Cuba
Dxers Unlimited
Dxers Unlimited’s weekend edition for March 3-4 2007
By Arnie Coro
Radio amateur CO2KK
Hi amigos radioaficionados… here is now on the air and on the web the
weekend edition of your favorite radio hobby program, the one and only
that covers each and every aspect of this wonderful way of enjoying
every single minute of our spare time : RADIO…
From the tense moments lived during an amateur full time weekend
contest, to the much more relaxed but also challenging AM medium wave
contest that has as its number one contest rule that you must use a
single active device in your radio… yes amigos, there are many ways of
enjoying radio, and homebrewing antennas is one of the most interesting
of them… So today at the request of many listeners, I am going to
provide a complete, very detailed explanation about the EMGP antenna,
the so called Electro-Magnetic Ground Plane antenna that has made
possible for radio amateurs living in very tight quarters to get on the
air on one HF band, yes, at least on one HF band, and that can be a big
difference from just having access to the VHF and UHF bands …
So if you have a tape or MP3 recorder at hand, be ready to copy the
complete , detailed analysis about the EMGP antenna, or you can read
about it at some of the mailing lists and websites that usually carry
Dxers Unlimited’s scripts , and there is yet another option, you can
drop me an e-mail to arnie@xxxxxx, and I can send you this script for
you to read and re-read as many times as you wish…
Si amigos, yes my friends, oui mes amis…Dxers Unlimited is YOUR radio
hobby program , with an immediate feedback path for your questions … As
I like to say YOU HAVE QUESTIONS…. And your amigo Arnie has the answers”
something that has made many of you laugh a lot for obvious reasons…
Stay tuned , as Dxers Unlimited’s weekend edition continues in few
seconds with the feature about the EMGP antenna…
I am Arnie Coro in Havana…
…….
So, ready to copy ?
Here is the explanation about the so called EMGP antenna, that in my
humble opinion is an excellent option for those like you and me that
don't own the area of real state that we will obviously wish to have.
You start with the best possible ground system that can be installed
at your location... Yes, try to test both radials laying on the ground,
buried at a small depth or if possible, the fantastically efficient
ELEVATED RADIALS…
At my home, the experimental EMGP antenna for the 20 meters band is
located at the center of a group of 16 copper wire radials that are
5.5 meters long each, that simply lay on the rooftop, kept in place
by a fastener at the end of the wire. They are all soldered to a copper
ring to which the shield of the coaxial cable feeder is connected.
It took quite some time to assemble this circular pattern of copper
wires, but I spent no money on them, as they came from a recycled
power transformer’s primary winding, that I had the patience to
carefully unwind…
Tests have been done with 1, 4, 8 and 16 radials, and the difference
between 1 and 4 is very noticeable, between 4 and 8 you can notice
it, and between 8 and 16 it is barely noticeable... and this gives
you a "real life picture" of what happens in actual practice ( as
you know well by listening to Dxers Unlimited, I am a very "practical
engineer by all standards" not a theory minded person at all.
The actual CO2KK experimental EMGP 20 meters antenna consists of an
aluminium mast of 25 millimeters ( one inch ) diameter, supported at
the bottom on a plastic polyethylene insulator... This mast is exactly
1.5 meters high at this moment ( but I have tested heights of this mast
between 50 centimeters and 2.5 meters ).
From the top of the mast, a copper wire ( number 12 gauge minimum
diameter) extends horizontally, that is parallel to the ground
for a distance of ( at first ) 3.7 meters. This wire is supported
from a PVC thick wall pipe that acts as an insulated mast...
So for you to have the picture, when you see the antenna from
the street, you see a shining aluminum mast on one side and the
white PVC water pipe mast on the other, and the wire extending
between the two masts.
If you calculate the 1/4 wave resonant length for 14.15 mHz,
with the formula 71.25 divided by the frequency of operation, this will
yield a total length for the 1/4 wave vertical antenna of 5.035 meters.
Now for some required theory : 1.5 meters is very near 33 percent
of the total length of the 1/4 wave vertical ....
And the ANTENNA CURRENT is highest precisely in that first segment of
the vertical monopole radiating element ...
FOLLOW ME ???
So , your EMGP does most of the actual radiation from the 1.5 mast
section, but you do need the "extra wire" to achieve resonance.
Depending on the ground , the sorrounding objects etc, the length
of the wire must be"trimmed" to achieve minimum SWR at the center
frequency that you want.
I matched this antenna for 14.060 , so that it will be optimized
for the CW QRP operating frequency and also for the 14.070 PSK hot spot
of 20 meters.
The antenna impedance at the base is , as expected , less than 50 ohms
and this makes it necessary to use a simple L network matching system,
made with a coil and a variable capacitor.
Then I realized that I could do away with the coil and the hard to
find variable air spaced capacitor... and here is how I did it...
which is indeed a very clever solution.
Instead of making the top wire 3.5 meters long, I made it longer, until
the total length of the antenna ( summing up the 1.5 meters vertical
aluminium tube and the wire ) was approximately 1/3 of a wavelength
long...
A 1/3 of a wavelength long antenna is 120 electrical degrees long !
And it has an impedance slight higher than 50 ohms and shows
a reactance of an inductive nature, so it is very easily matched
to the 50 ohms coaxial cable by connecting a series capacitor between
the antenna and the coaxial cable center conductor. I find the optimum
value of the capacitor first with a variable air spaced capacitor, that
gives a 1 to 1 SWR... Then take the capacitor to the RLC bridge and
measure its capacitance...
Final step: make a high voltage capacitor from a length of coaxial cable
and put it in place of the air spaced variable. You connect the braid of
the length of coaxial cable acting as the series capacitor to the center
of the coaxial downlead, and the center of the coax acting
as a capacitor to the base of the antenna...
As you can guess, you can extrapolate the design data for other bands...
I tested the "concept of the 1/3 wavelength EMGP" on the 10 and 6 meters
bands, and then decided to try it out on 2 meters because it would be a
very good antenna for 2 meters FM work via repeaters when hurricane
winds are blowing, as the actual "height" of the antenna is only was
kept at 20 centimeters and the top "wire" was also made of the same 1/2
inch copper tubing...
A friend has built an EMGP for 40 meters , and he found out that it
works very well for DX ( low angle radiation ) but is no good at all
for Near Vertical Incidence Skywave contacts. It does work, of course,
at short distances via groundwave, but it is a very poor antenna to talk
to other Cuban stations at distances between 50 and 500 miles for which
a high take off angle is required...
I hope that this "non graphic explanation is OK for you...
As I have no image scanner at hand here , my hand drawings of the
prototype can't be sent, but I am sure that you can draw them from
the description.
Do realize that this low profile antenna is no "kids stuff at all " as
now an American antenna manufacturing company is selling a version of
this system to AM broadcast stations , claiming 95 percent efficiency as
compared to a 1/4 wave vertical monopole radiator , the typical AM
station antenna.
The only difference between my "low cost " design, and the "Kinstar"
antenna is that they use 4 antennas of this type connected in parallel
and fed via a much more complex matching network.
I have tested the "concept" with just two antennas, and it does require
twice the "real state" and also twice the area covered by the ground
radials ( but on 20, 10, 6 and 2 meters ) it really doesn't make
a measureable difference...
The EMGP is a very attractive antenna design for operating on the 160,
80 and 40 meter bands from locations that have very limited space
available, and let me add that a friend of mine came up with the idea
that a 40 meters band EMGP could be disguised as a clothes hanging line,
and he even added a few pieces of clothing hanging from the antenna to
give it a much more credible look to his neighbors…
As a matter of fact he said: Arnie, the EMGP clothes hanging line
antenna works nicely on the 40 meters band , making the metal radiating
mast 2.5 meters or about 8 feet high. The actual plastic insulated
antenna wire goes from the top of the aluminum mast to the top of the
plastic PVC thick wall pipe that is at the other end.. Then he explained
that a Dacron rope was added at a height of about a bit less than six
feet, to serve as the fake clothes hanging line… The top plastic covered
wire can also serve to place coat hangers with clothes to make it look
still more like the real thing !!!
As you may guess, this has proven to be an excellent stealth antenna
system, because the coaxial cable feedline is nowhere to be seen, as it
goes to the feedpoint right close to the rooftop, or in the case of a
backyard installation, the coaxial is buried using a PVC pipe that is
put into a trench that is latter covered by soil and grass…
I must stress that this is in theory at least a single band antenna, but
my friend is in the process of doing some experiments, one of them
consisting on using the 40 meters EMGP antenna on the 30 meters amateur
band, something that he says is easily done with the help of my
universal PI-L networks antenna tuner that he built several years ago…
Again, if you want to read the script of this special edition of Dxers
Unlimited, just drop me an e-mail to arnie@xxxxxx, and I will be very
happy to help you understand the way this very interesting stealth
antenna works, so that you can homebrew your own and start operating on
the HF , the short wave amateur bands…
And last but not least…I must say that my prototype 20 meters EMGP
antenna works very well as a short wave broadcast bands monitoring
skywire… delivering excellent signals on the bands between 9.4 and 21.8
megaHertz too…
And now amigos, as always, at the end of the show, when I am here in
Havana next to my solar telescope, here is Arnie Coro’s , Dxers
Unlimited’s HF plus low band VHF propagation update and forecast… The
spring equinox is just around the corner now, and I am already seen some
equinoctial propagation conditions at this moment… be ready for what
well may be an outstanding AM broadcast band and Tropical Bands
equinoctial DX season if solar activity continues to be as low as it is
right now… Solar flux around 75 units and rather steady, almost no
chances of sporadic E short skip openings, and terminator line
propagation is going to be better and better as we approach the spring
equinox… Don’t forget to send me your signal reports and comments about
the program as well as any radio hobby related questions you may have to
arnie@xxxxxx or VIA AIR MAIL to Arnie Coro, Radio Havana Cuba, Havana, Cuba
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THE INFORMATION IN THIS ARTICLE IS FREE. It may be copied, distributed
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