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LW1DSE > TECH 09.07.12 19:21l 266 Lines 14315 Bytes #999 (0) @ WW
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Fabricating Impedance Transformers
for Receiving Antennas
John Bryant
May 2001
With the proliferation of local neighborhood noise sources and the
growing popularity of wire antennas in configurations other than the inverted
L, coaxial cable has become the antenna lead-in of choice for most radio
hobbyists. Since the impedance of most commonly available coax is either 50
or 72 ohms and since many wire antennas exhibit impedance of 400 to 1000 ohms
or more at the feed point, directly connecting the coax to a wire antenna
invites very significant signal losses due to the impedance mismatch. Given
this situation in the listening hobbies, it has been a mystery to me why
impedance transformers and baluns for receiving antennas haven't been more
commonly available on the retail market. Further, the few transformers that
are available are offered at around $60.00. While this may be a fair price,
considering labor, profit and retail mark-up, the parts cost for a good
weather-tight balun or impedance transformer is well under $10, paying retail
for the parts! If you own a soldering iron and can make even a semi-reliable
connection, you really ought to consider "rolling your own" baluns and
transformers.
Selecting Components.
--------------------
You need to obtain some connectors, a weather-tight box, and the
guts of the transformer. For the wire connectors, I have come to use the type
of binding posts that also accept a banana plug in the top of the post. That
way I have several choices of connection methods at the antenna. These parts
are all available at Radio Shack. I have fallen in love with one special form
of banana plug, though, that is only available from professional parts houses
(Mouser #17HR549, #17HR550). The shaft of the male plug has a stiff wire
running down it and it makes a VERY reliable connection by actually plowing a
shallow furrow in the metal of the female banana socket. The connector for
the coax is a normal chassis-mount coax connector of your particular flavor.
More and more hobbyists seem to be switching to BNC connectors for their ease
of use and better weather characteristics, though only professional-class
receivers yet use this type.
Some DXers swear by all-metal boxes for this application. If you
fall into this camp, the cast aluminum Hammond boxes are my choice. I buy
mine through Antique Electronics Supply (http://www.tubesandmore.com/). Those
who use metal boxes are usually attempting to maintain the system as RF
sanitary as possible, preventing stray signal pick-up by grounding the box,
usually to the coax shield. However, the majority of DXers I know, and most
manufacturers, use cast plastic enclosures. They reason that the stray signal
pick-up is miniscule, compared to the size of the antenna, and let the ease
of using the plastic boxes, and their significantly lower cost, make that
decision. Personally, if I'm in one of my rare obsessive/compulsive phases, I
use metal; if I'm "normal," I use plastic. I've never been able to
distinguish between the two designs in actual use.
All of the impedance transformers with which I am familiar are based
on some sort of ferrite core with windings around it. Several well-known East
Coast DXers favor manufactured transformers from Mini-Circuits. and just wire
this tiny transformer in the appropriate box and add connectors. The very
small 9-to-1 Mini-Circuit transformers are perfect for converting the 450
ohms of beverages and many other wire antennas to 50 ohm coax. I used these
small units for several years, before swearing off them entirely. DXing on
the prairies North America generally exposes wire antennas to a good bit of
static electricity. Both Bill Bowers and I had several random failures of the
Mini Circuits units, probably due to the hair-small wire used to wind the
transformer. The main problem was that the failures were often partial,
making us think - for several nights running - that conditions were really
bad. What a waste! After the third such failure, I returned to the
tried-and-true "roll your own" techniques based on relatively larger ferrite
toroids. These techniques were originally taught to me over a decade ago by
Nick Hall-Patch, Technical Editor of IRCA and published in a co-authored
article in Fine Tuning's Proceedings 1988.
Once the decision is made to roll your own using ferrite toroids,
there are just two decisions left: the size of toroid and the specific
ferrite mix to be used.
In North America, at least, most of us use the toroids from Amidon.
These may be purchased over internet directly from the manufacturers
representative at (http://www.bytemark.com/amidon/). I've used three sizes of
cores over the years. The smallest that I have used is a 1/2" diameter toroid
(Amidon's FT-50) which looks like a half-eaten Life Saver candy. To get the
proper turn count through the donut hole, you must use very fine magnet wire
and a large needle. I found the whole operation overly fussy and I was also
concerned about static electricity burning the hair-thin wires. I saw no
advantage in using cores this small and I don't recommend them now. The
largest toroids that I've used measure 1.4 inches in diameter (FT-140.) These
work great, but they are a little heavy and expensive for my taste. I
recommend the middle size, 1.14 inch diameter donuts which are large enough
to handle easily and I use 30 gage insulated wire from Radio Shack
( # 278-501, 502 or 503) for the windings. This wire is small enough to make
a neat close-wound coil of the proper turns-count on the toroid and yet the
wire is large enough and stiff enough to be easy to handle.
The Amidon web page has a great deal of technical data to assist you
in selecting the proper ferrite material. However, all of that stuff is
rather arcane and will take you a while to wade through. What it comes down
to is this: if you want to work from .5MHz to 30 MHZ, select "Type 43"
material. This material also gives "reasonable" performance on LW. If you are
certain that your interests are limited to LW, MW and Tropical Bands (.2 to
15 MHz), then "Type 75" ferrite is what you need.
If you've followed me this far through this technical thicket,
it's probably time to take a break. Lets hunker down in the shade and let me
scratch out an illustration to show you where we are headed:
CALCULATING THE TURNS COUNT:
---------------------------
Lets calculate the turns count for impedance matching a beverage
antenna with an impedance of 450 ohms to 50 ohm coax. Because this is a
step-down transformer, the primary (attached to the antenna) will be the
larger winding and we'll deal with that first.
The first formula to use will give us the desired inductance of the
primary winding:
desired L of winding = XL/2γf
where:
L = Inductance in millihenries
XL= Reactance in ohms
f = Lowest frequency of operation in kHz
XL may be found by multiplying the impedance of the antenna to be
matched by a factor of 4. This XL would be 4 x 450 ohms or 1800 ohms. To
make things easy, lets use 500 kHz. as our lowest frequency of operation.
So, L of the primary winding = 1800/2 x3.1416 x 500 or .573 mH
Now that we know the inductance (L) needed for the primary winding, we can
apply the following formula to determine the number of turns needed for the
primary winding.
N = 1000 ϋ(L/AL)
In narrative, this formula should be read: Number of turns required
(N) is equal to 1000 times the square root of the Inductance (L) divided by
the constant AL.
The constant AL is determined from the Amidon technical literature
and takes into account the RF qualities and the size of a Type 43 toroid that
is 1.14 inches in diameter. The AL for the FT-114-43 is 603.
So, working the formula above,
N = 1000 ?.573/603 = 1000 x .030825 = 30.8 turns, use 31.
The turns count for the secondary winding (connected to the coax)
may be determined by the same method or by knowing that the impedance ratio
of a transformer is the turns ratio squared. We are trying to get from a 450
ohm antenna to a 50 ohm coax, so the impedance ratio is 9 to 1. The turns
ratio must then be 3 to 1... so, the secondary winding is 10 turns. Working
it out with the formula yields 10.2 turns.
By golly, we have waded clear through the technical thicket. Burn
any blood sucking leeches off your arms and legs, rest up in the shade for a
while and pop a cool one. You deserve it!
If you want to match one of the currently popular delta or pennant
antenna designs, determining the turns count follows the same process. The
impedance of pennants and flags is somewhat dependant on the size of wire
used and the height above ground. Most seem to fall in the range of 900 ohms
to 1000 ohms, so a 950/50 (or 19-to-1) step-down transformer would probably
work well for most of this type antenna. The same process as that above can
be used to determine the turns count. To save you the effort, here is a chart
of turns counts for both Type 43 and Type 75 Material
Turns Counts
Based on FT-114 Amidon Toroids
Antenna Antenna Ferrite Primary Secondary
Type Impedance Type Winding Winding
Beverage 450 ohms 43 31 10
Beverage 450 ohms 75 13 4
Delta 950 ohms 43 45 10
Delta 950 ohms 75 20 5
The windings should be placed on the toroid in a "close wound"
fashion, with the primary and secondary windings spaced as far apart as
possible on opposite sides of the donut.
IN USE
About all that we have left to discuss is various arrangements for
placing the transformer between your antenna and coax. Unfortunately, it is
at this point that we leave the world of science and begin to deal with
sorcery. It turns out that there are a lot of ways to hook these things up,
and most of the controversy centers on various grounding schemes. There are
two main circuits to discuss:
SINGLE WIRE ANTENNAS
There are three or four choices of circuit arrangements here and the
best one for your application probably should be determined by careful
experimentation. The "scientifically superior" arrangement is probably that
shown here as "Circuit A." The second side of each of the coils is connected
separately to ground. Signal current and static electricity flows from the
antenna through the larger coil directly to ground. Current between ground
and your receiver is induced into the smaller coil. The main argument against
Circuit A is that individual grounds must be separated by at least 12 to 15
feet to be electrically separate, or so I've read. Due to connection through
the ground itself, what is probably being accomplished, electrically, with
Circuit A is actually Circuit C. Circuit B is the one that Nick Hall-Patch
has used with excellent results for years. Grounding of the coax shield, if
any, is accomplished at some distance from the transformer.
Circuit C recognizes the problem of possible interaction between the
two "separate" grounds and simply connects the grounded side of both coils
and the coax braid to a single ground at the antenna feed point. This is the
circuit that I use when the feed point of the antenna is within three or four
feet of the ground and the grounding conditions are good. After writing this
article and having Nick pound his approach through my thick skull yet one
more time, I think I'll be switching my approach to Circuit B.
Circuit D is the arrangement that the "magnetic balun" manufacturers
normally use. The grounded sides of the coils are connected together and
hooked to the braid of the coax. The grounding that is so vitally necessary
for signal flow is accomplished through the coax braid and left totally to
the user. This is the circuit that I use for random wire antennas and I
ground the braid at least once between the antenna and the receiver. I will
confess, though, that I've used Circuit D with good success on occasion with
an ungrounded coax and an ungrounded receiver. I know that I'm taking a
chance with static electricity build-up, and I wonder whether all of the
signal energy is reaching my receiver; but there you are: my guilty little
secret.
LOOPS
Luckily, with loops like the KAZ delta, etc., things are fairly
straightforward. The larger coil of the transformer is connected directly
(in series) into the loop. One side of the smaller coil is connected to the
center conductor of the coax. The other side of the smaller coil may be
connected to the coax braid, may be grounded, or both. If the loop feed point
is within a foot or so of the ground, it is probably best to ground the
second side of the smaller coil. However, if you have a poor grounding
situation, or the feed point is in the air, the smaller loop ought to just be
connected to the coax braid.
CLOSING REMARKS
About the only issue left to mention is physically attaching the
transformer to the interior of the box. I'm rather sure that most
constructors will have a favorite method. Mine is to simply hot glue the
transformer securely to the plastic box. In some cases, I have even totally
encapsulated the transformer in hot glue. I have sometimes wondered if I
wasn't somehow affecting the magnetic qualities of the transformer with the
hot glue. Bill Bowers has run some quite sophisticated bench tests concerning
this and assures me that the qualities of the transformers are unaffected by
hot glue.
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