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Re: [HCDX] Dxers Unlimited script for the weekend edition June 16-17
Dxers Unlimited
Dxers Unlimited weekend special edition for June 16-17 2007
By Arnie Coro
radio amateur CO2KK
Hi amigos radioaficionados around the world ! This is the weekend
edition of your favorite listener oriented radio hobby program. A
special edition of the program devoted to TEN of the most frequently
asked questions about compact high Q magnetic loop antennas...I am
Arnie Coro, radio amateur CO2KK, and as always it is my pleasure to
share with you some time on the air and on the web .
Today I'll not be able to provide our exclusive and not copyrighted HF
plus low band VHF propagation update and forecast because I will be out
of town when you are listening to this program
Now here is our technical topics section totally devoted today to
MAGNETIC LOOP ANTENNAS, for a very good reason... when properly built,
and I want to underscore those two words, properly built, well, when
magnetic loops are constructed the way they should, there is no doubt
that they are a great and sometimes the only solution for those that can
not install any other antenna types due to space restrictions... Some
radio amateurs living in apartment buildings have magnetic loops that
can be nicely disguised as artwork or even elevated in front gardens
disguised as bird feeders !!!
Please remember that today I'll not be providing our HF plus low band
VHF propagation update and forecast,because of my travel schedule.
Stay tuned for more radio hobby related information
coming to you from Havana.
Our e-mail address is arnie@xxxxxx, and VIA AIR MAIL you may request our
QSL card by sending a postcard with your signal report and comments
about the program to Arnie Coro, Radio Havana Cuba, Havana , Cuba.
.....
This is a special edition of Radio Havana Cuba's radio hobby program..
the name of the show is Dxers Unlimited and YES, we do attempt to cover
all the more than 80 ways you and I can enjoy this wonderful hobby, from
sending fast scan low resolution TV on the 10 meters amateur band to
picking up more FM stations for your log via meteor scatter
propagation... from talking to another radio amateur across town with
your 2 meter band FM handie talkie, to picking up signals from deep
space probes!!!
ANTENNAS... ANTENNAS... yes they are one of the favorite topics of Dxers
Unlimited's listeners, according to the correspondence that I receive here..
Today our technical topics section,as I said earlier, will be devoted
to the magnetic loops, and some frequently asked questions about them...
Let's start with a definition !!!
1. What is a Magnetic Loop?
A Magnetic loop also known as an STL or small transmitting loop is
nothing else than a large coil with a capacitor added to bring the
system to resonance. It is unlike a large resonant loop (a more common
antenna type) in that the loop size has to be smaller than 1/4
wavelength and does not resonate on its own, it does need an external
capacitor in order to make the inductor capacitor circuit to resonate on
a specific frequency.Again, see the difference, the loop must be less
than 1/4 of a wavelength, and it needs the ""help"" of a capacitor to
resonate to the operating frequency. The loop is brought to resonance by
attaching a capacitor to the two open ends of the loop. The loop is
tuned by changing the value of the capacitor. An increase in
capacitance will bring the resonance of the loop lower in frequency, a
decrease in capacitance will make the loop resonate higher. Now here is
question number two
2. How well does it a magnetic loop perform compared to other antennas?
A Magnetic Loop's performance seems to be dependent on the care and
quality of its construction, placement above ground, and other factors.
A generalization of performance would place it a bit below that of a
dipole, and in some instances exceeding the performance of a dipole. It
usually will far surpass the performance of most mobile vertical
antennas, and other compromise configurations Question number 3
frequently asked about magnetic loops
3. Why some people build magnetic loops as squares instead of circles or
octagons? Aren't circles more efficient?
Yes, circles are more efficient, by about 10% as compared to a square
loop! But they are harder to make when your working with one or two inch
copper. I would rather just build the loop a little bigger to gain back
the loss. For instance, according to loop modeling software, it only
takes a extra 50 centimeters extra length or for that matter making the
loop circumference of an extra 12 millimeters more diameter of pipe to
gain back the lost efficiency. Also, squares are easier to make and have
less solder joints than octagons. Now the fourth question about magnetic
loops here at this special edition of Dxers Unlimited..
4. Is copper better than aluminum for making magnetic loops?
Absolutely and for the most part. It has over a 20% difference in
efficiency according to some modeling software. More efficiency equals
more performance. It's also a lot easier to solder, and can be found at
your local hardware store. However, aluminum is far lighter.
In a few seconds we will continue with this special edition of Dxers
Unlimited devoted to magnetic loop antennas... You can send your radio
hobby related questions to arnie@xxxxxx, or VIA AIR MAIL to Arnie Coro,
Radio Havana Cuba, Havana , Cuba
......
NOW question number 5 about magnetic loops
5. How important is loop circumference? How important is the size of the
conductor diameter?
Loop circumference relates directly to efficiency and performance The
larger the circumference of the loop, the better your loop will work --
however, if you exceed 1/4 wavelength, things seem to get mysterious.
It's a balance, try to make your magnetic loops as close to about 85-90%
of the wavelength in circumference, and that seems to be the magic
number. . NOW a very important question follows, number 6 most
frequently asked question about magnetic loops.
6. Do you really need to solder or weld all the joints on a loop?
According to almost all the literature I have seen on loops, all
connections must be soldered or welded. Please read on because I do not
believe it's that simple. It appears that mechanical connections create
resistance in the conductor which cause major losses in efficiency. It
is interesting to note that all the loop articles I have seen and read
do not discuss mechanically built loops, only welded versions. I myself
have built both mechanical and soldered loops and have not found major
variations in performance if the mechanical connections are well made. .
I have also built loops mechanically, tested them for some time, and
then soldered them together to see if there would be a measurable
change. As of yet, as long as the mechanical connections are well made,
there seems to be no measurable difference, especially if you keep the
magnetic loop antenna indoors,without exposing it to the weather, and
let me add that many radio amateurs have to use their magnetic loop
antennas inside their homes due to the infamous CCR restrictions,
especially at new urban developments.
7. Can one use regular air variables from an old transmatch for tuning?
Most loop literature would say that the wiper connections on commonly
found air variables are too lossy. Also voltage capacity is an issue.
Any capacitor used should be able to withstand high voltages in the
3-4KV range if you are running 50-100 watts. If you are using welded
stator or butterfly capacitors with a high enough voltage rating there
should be no problem or significant losses. However, high voltage stator
or butterfly capacitors with a large capacitive range usually cost about
the same as a nice used Jennings Vacuum Variable. I personally use the
vacuum variables. A cheap alternative to expensive high voltage
variables is the use of coax for monoband utility. Take a good piece of
coax, separate the braid from the conductor for a couple of inches and
attach each side to each open end of the loop. Use varying lengths of
coax to create different capacitance values for tuning. Trim the coax
till you reach your desired resonance. This works very well if you don't
mind being stuck on one band. You can also change the coaxial stubs when
you want to change frequency pretty easily. I have heard opinions put
forth that braid in coax is too lossy for good loop performance for
capacitors, but I have not found this to be the case at all.
8. How well does a loop made of coax work?
When using coax, either the ground braid or the center conductor can be
used. Unless you are using one inch hardline the diameter of the braid
is simply too small for high efficiency. Have I tried making a magnetic
loop out of the outer braid of RG-8U and other types of coax? Yes. Does
it work well? In my opinion the answer is no. One inch or even half inch
copper pipe will work much, much better. I would not recommend this
strategy at all since every experiment of this type yielded poorly
performing loops. Also, some opinions believe that the braid in coax is
too lossy because it is not one piece of metal but strands of copper
with minute gaps between the pieces introducing excessive losses. You
can certainly build a magnetic loop using half inch coaxial cable for
receive only applications, and such antenna works quite well according
to my experience, but don't try to work DX with it connecting your ham
radio transceiver to such a loop !
9. How do you compare copper or aluminum strap to pipe?
Modeling software claims a square strap is equal to one half the size of
full diameter pipe.
10. Does a loop have a gain pattern? What does it look like? Does it
matter?
A vertically positioned magnetic loop has a figure eight gain pattern
along the plane of the loop itself. In other words, think of two
doughnut shaped patterns radiating from the the vertical portion of each
vertical side. This pattern creates a null where the two donuts meet in
the center of the loop. So a loop with its vertical sides placed north
and south, will radiate best north and south. The same is true if you
rotate the loop east and west. Loops can be placed on rotors and the
loop can be turned for best signal strength or it can be used to null
out offensive interference. Magnetic Loops also radiate RF at all
angles, so they are well suited for both high and low angle reception
and transmission. This is advantageous for both high angle NVIS work,
and DX. Loops that are mounted horizontally radiate in an
omnidirectional pattern and do not share the benefit of a vertical
polarization. I would recommend vertical placement.NOW ... a very common
question ...
And now an extra question, that was added after the initial script was
written, in order to answer an e-mail message that came in just as I was
going to wrap up the program... So this is question number eleven about
magnetic loops
11. Why are commercial loops so expensive?
Magnetic Loops require high voltage capacitors for more than QRP power
level operation. These capacitors tend to be very expensive. A brand new
Jennings vacuum variable can cost at least five hundred dollars (they
are a lot cheaper used). High voltage stator and butterfly capacitors
also cost a lot of money. So the final product including the capacitor
can be quite expensive. Also, commercial manufacturers desire to build
the best possible loop, so they employ all welded construction on loop
joints. This also adds to the cost.
And now amigos, let me tell you once again that this show had to be
digitally recorded one day in advance of the day that it is going on the
air, because I had to travel to the provinces to attend a scientific
event. Hope to have you listening to the next edition of Dxers
Unlimited, and don't forget to send me your signal reports and comments
as well as any radio hobby related questions to arnie@xxxxxx, again
arnie@xxxxxx, and VIA AIR MAIL send a postcard or letter to Arnie Coro,
Radio Havana Cuba, Havana , Cuba...
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