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Modify that AM antenna
by
Bruce Carter
Open any radio
with an AM section and you are sure to find a grayish black rectangular
or round bar of metal, with wire wond around it on one end. This
is the AM antenna. If you are not satisfied with your AM reception,
you are probably wondering "is there anything I can do to improve
it?" Well, yes you can.
The
Ferrite Bar
Your solution
might be as easy as installing a larger ferrite rod. Ferrite is
the grayish-black metal portion of the antenna. You have several
things working for you:
- Usually,
the ferrite rod is secured by wax or some other to soft material
to its mountings and to the coil. It should be fairly easy to
loosen it and slide it out. Be careful not to break any of the
wires on the coil, however! If you do you are in for a hard
time finding a replacement. Sometimes the coil is not wound
on a cardboard form, and is wound directly on the rod. If so,
you are out of luck.
- Cost usually
dictates that they use as small a ferrite rod or bar as possible.
There is usually plenty of room for a larger one inside the
case. If not, or if there are components that interfere, don't
try to replace it. Consider an add-on
amplifier.
- If you
are like me, you usually save parts from scrapped radios. I
have a couple of dozen ferrite bars and rods. They come in standard
diameters an areas, so it is usually easy to find one that will
slide into the coil. Don't be afraid to make a rectangular coil
round or vice versa. It will work just fine. A good rule of
thumb for ferrite - the longer the better. Also, the larger
the cross section, the better. But you don't have control over
that.
Fortunately,
the size of the ferrite rod does not have a lot to do with the
tuning of the radio. Even if you have to compromise on a station
near the center of the band, there will still be very worthwhile
overall improvement. Especially if you are only after that one
station!
To re-tune
the radio:
- Pick weak
station near the low end of the band. Slide the ferrite through
the coil and find the position that gives the clearest reception.
- Trace
the connections on the coil and find the one that goes to the
tuning capacitor (or varactor antenna tuning stage).
- Tune to
a weak station on the high end of the band.
- If there
is a trimming capacitor (there usually is), adjust it for best
reception.
- Go back
and forth a couple of times, re-adjusting the low and high parts
of the dial - particularly if you made a big change in ferrite
size.
- DO NOT
touch the ferrite on the high part of the band, or the trimming
capacitor on the low part of the band!
- When you
are satisfied you have the best adjustment, use wax or rubber
cement to secure the coil to the ferrite, and the ferrite to
its mounting.
External
Antenna Input
Can't do
anything more with the ferrite bar? No problem, you are just beginning!
You need to add an external antenna input to your radio. I will
leave it to you to decide whether you want a screw terminal or
a connector, I personally prefer one of those cable TV type connectors,
but you may use just about any type of connector, even one of
those RCA audio types.
Take 3 to
4 turns (no more) of 22 or 24 AWG wire, and wrap it around the
end of the ferrite bar opposite the antenna coil. It should be
about the same distance from the end as the coil is from the opposite
end. Connect one side of the wire to receiver ground, and the
other to the center conductor of the connector. Attach a ground
wire to the sheild side of the connector. That is all there is
to it!
These
schematics are fairly typical of AM radios, and show what you
had before and after installing the external input:
Before modification
After modification Now - what
do you hook up to that external antenna input?
Long
Wire Antenna
By far, the
simplest type of antenna to add is a "long wire" antenna. If you
are good at this, you might be thinking "quarter wave whip" by
now. When I was still in high school in Midland, Texas, I actually
hooked up a quarter wave whip antenna to my AM radio. I think
the length was 464 feet, I am pretty sure it was 22 guage. I had
it stretched down a culvert that ran in back of my house. The
results were impressive - the normally barely receivable KRLD
1080 from Dallas was clear and static free! Of course, I could
not leave my wire stretched out down there - vandals would have
quickly destroyed my antenna. I had to settle for about 160 feet,
wrapped all around the fences of my house. The results were not
nearly as good, but worth the effort.
Some notes
about long wire antennas:
- Use the
ground terminal to connect your receiver to a good earth ground.
- Do not
attempt to ground the far end of the antenna.
- Larger
guage wire is better, 16 guage will be more robust and hold
its form better. Whatever supports you use, you will probably
be putting the wire horizontal.
- Unless
your long wire antenna is going to be very long, you probably
should consider a coupling
circuit to load it properly. The coupling link also contains
a fantastic construction article for long-wire antennas.
- There
is no real advantage in placing the wire vertically, some sources
I have read say this will actually increase fading.
- It may
be desirable to use coax transmission line out to a remote antenna.
The bane of my early DX'ing was my mother, whose addiction to
soap operas jammed my AM reception with TV generated horizontal
output whine. Oh - if only I had known the coax trick back then!
- Unknown
to me at the time, I actually had created attenuation for myself
by wrapping that wire around the fence. I had created a loop
antenna, and laid it on its side, where it had a sharp null
for ALL stations! I would have been better off with a 50 foot
staight wire, tied from one corner of the house to the farthest
fence post.
- In areas
of frequent lightning, laying out several hundred feet of wire
can create a dangerous lightning rod! Either disconnect your
antenna when not in use and preferably throw the end out the
window (string tied around it makes a convenient way to retrieve
it), or invest in lightning surge protection!
Loop
Antennas
In
areas where space is cramped, such as apartments, the best solution
is probably a loop antenna.
First, a
little background theory.
Take a closer look at the ferrite bar antenna. The coil is actually
a loop antenna! If you remove the ferrite, the radio will still
be able to receive stations, but only strong local ones. If you
carefully observe, you will notice with the ferrite inserted,
the radio is most sensitive to stations broadside to the ferrite.
With the ferrite removed, it is most sensitive to stations on
the axis of the coil. What is going on, here? Why the ferrite,
why does it make the antenna better, why does the direction of
the antenna change?
If
you have been around for a while, you might remember a time before
ferrite bar antennas were invented. If you disassembled a radio
decades ago, even as late at the 60's, you would probably find
a flat loop of wire inside, glued to the wooden or fiber back
panel of the radio. The chassis of the radio is metal, but the
housing and particularly the panel against which the antenna was
mounted were non-conductive, wood or plastic (in later years).
These materials would not sheild or de-tune that loop antenna.
Radios were most sensitive to stations that were broadside to
the plane of the loop. If you think about your little coil inside
your radio, you can see that if it were wound flat instead of
stacked, it would operate the same way the loop antennas of the
past worked.
So far, so
good. The rules haven't changed, loops work the same way they
always did. By now, you have guessed that the reason your ferrite
bar antenna doesn't work very well is the size of the loop.
This is true, the larger the cross sectional area of the loop,
the more sensitive it is, and the more stations the radio will
receive. I learned this first hand with my first table radio,
which happened to have a very nice loop antenna attached to a
fiber panel in the back. But - I also had a somewhat larger loop
from an older radio that I had junked. When I attached it in place
of the smaller loop, reception increased dramatically! That loop
was soon permanently attached to the back of the radio, sandwiched
between two pieces of cardboard that had an attractive piece of
art on them (so it wouldn't look too bad). It was not long before
I located a professional loop, manufactured decades before, mounted
in a precision fiber case, with a nice rotary pedastal. It was
only about 12 inches by 10 inches, however. Even larger loops
are much better!
OK,
how does this relate back to that little tiny loop inside your
radio? Where does the ferrite come in?
First of
all, the larger the cross sectional area of the loop, the more
the gain, and the better the reception. conversely, the smaller
the loop, the worse the reception. But - not just any loop will
work. A loop must be constructed with an inductance that will
resonate with the tuning capacitor inside the radio, and the resonant
frequency range must correspond with the AM band of frequencies.
The
following formulas relate the number of turns of the coil and
the cross sectional area, and will give the loop the correct inductance:
|
N
|
AREA
|
SIDE
|
DIAG
|
|
1
|
58709.3
|
242.3
|
342.6
|
|
2
|
14677.3
|
121.2
|
171.3
|
|
3
|
6523.3
|
80.8
|
114.2
|
|
4
|
3669.3
|
60.6
|
85.7
|
|
5
|
2348.4
|
48.5
|
68.5
|
|
6
|
1630.8
|
40.4
|
57.1
|
|
7
|
1198.1
|
34.6
|
48.9
|
|
8
|
917.3
|
30.3
|
42.8
|
|
9
|
724.8
|
26.9
|
38.1
|
|
10
|
587.1
|
24.2
|
34.3
|
|
11
|
485.2
|
22.0
|
31.1
|
|
12
|
407.7
|
20.2
|
28.6
|
|
13
|
347.4
|
18.6
|
26.4
|
|
14
|
299.5
|
17.3
|
24.5
|
|
15
|
260.9
|
16.2
|
22.8
|
|
16
|
229.3
|
15.1
|
21.4
|
|
17
|
203.1
|
14.3
|
20.2
|
|
18
|
181.2
|
13.5
|
19.0
|
|
19
|
162.6
|
12.8
|
18.0
|
|
20
|
146.8
|
12.1
|
17.1
|
|
21
|
133.1
|
11.5
|
16.3
|
|
22
|
121.3
|
11.0
|
15.6
|
|
23
|
111.0
|
10.5
|
14.9
|
|
24
|
101.9
|
10.1
|
14.3
|
|
25
|
93.9
|
9.7
|
13.7
|
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For your convenience, I have done the calculations
for loop antennas from 1 turn to 25 turns, and placed
them in this table.
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The integer
number of turns is listed in the first column. There is
no absolute requirement for the number of turns to be an integer,
you could tap off one side of the loop for the start, and the
opposite side of the loop for the end, just use the formula.
The
ferrite bar
- revisited for a moment
The
ferrite acts to concentrate magnetic flux in the center
of the coil, and therefore acts to increase inductance.
The loop is offset to one side to limit the inductance -
there is more in the center of the bar or rod, less near
the end. The value of inductance has more to do with the
number of turns of wire on the ferrite, and by the fact
that there is ferrite present, than it does the amount of
ferrite in the rod. This is why the trick above (putting
a larger ferrite rod in the core) can work. Unless there
is a large difference in the size of the ferrite, the inductance
will close to the same, or adjustable to the same range.
You might have observed that the larger the ferrite rod,
the closer to the end you have to place the coil.
Because
the ferrite, not the loop coil, is the primary object concentrating
flux from the received station, the ferrite bar antenna
is most sensitive to stations broadside to the ferrite bar,
not the loop. The longer the ferrite bar, the more flux
it can couple into the loop.
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The second
column is the area of the loop in square inches. Use this column
if you are making a loop rectangular instead of square.
If you are making a square loop, the third column is the
length of each side.
The fourth column is used if you are going to construct the
loop of two crossbars (see below).
Notice that
as the number of turns increases, the cross sectional area decreases,
but the effect tapers off with increasing number of turns. To
get down to one square inch, you need 242 turns! To get to the
cross sectional area of the little loop inside your radio, you
would need as much as 750 turns of wire! If you examine that little
loop inside your radio, you can quickly tell there is nothing
close to 750 turns of wire. 40 or 50 is more like it. Aha, another
reason for lousy reception without the ferrite - not only is the
cross sectional area small, but there are not enough turns, and
therefore not enough inductance to properly resonate with the
tuning capacitor. Therefore - there is a need to increase the
inductance of the loop so it will resonate with the tuning capacitor.
That is the reason for the ferrite.
Constructing
a loop antenna
There is
a trade-off that needs to be made: fixed vs. rotatable. If there
is only one station or city you are interested in receiving, you
may be able to construct a loop on a wall facing that direction.
The reception pattern of the loop is a figure 8, and will have
sharp nulls for stations in the plane of the loop. It will be
equally sensitive to stations coming from the front or the back.
If you are lucky, you can find a wall facing the right direction.
In practice, even up to 45 degrees off won't matter much. Just
map out an area of the wall as large as an area in the chart above.
Take the area, and divide it by the height, and that will tell
you how wide it has to be. If you don't have enough area, just
increase the number of turns until you can make a loop fit.
To install
a loop antenna, get four short wooden dowels or PVC pipe pieces,
and notch them every 1/4 inch to hold the wire. Mount them vertically
to the wall (don't ask me how), and wrap 20 guage or larger copper
wire around them for the proper number of turns.
A rotatable
loop is going to be smaller and more turns, by necessity. It can
be constructed of almost any material, provided it is non-conductive.
The most common is wood or PVC pipe. Make it larger than the diagonal
measurement above. Make one end even longer for the rotational
mechanism. For large loop antennas with only a few turns of wire,
it is better to mount them flat, and leave at least 1/4 inch between
the turns of the wire.
When the
loop is complete, one end should be sticking out, and you can
use this to attach it to some sort of base or rotational mechanism.
A rotatable loop gives you the capability of nulling out stations
on the plane of the loop to hear more distant stations.
Connecting
to your radio
In some rare
instances, there are only two wires connecting to the ferrite
loop antenna. In these cases, you can simply substitute the loop
you just constructed, and adjust the tuning capacitor on a station
at the high end of the band for greatest sensitivity. The vast
majority of cases, however, are more complicated.
If your ferrite
loop antenna has three or more wires going to it, you are not
going to be able to directly substitute your loop antenna. The
extra wires are part of what is called a "converter" stage, that
takes advantage of the fact that the oscillator of the radio can
double as a mixer. This results in cost savings to the radio manufacturer.
If you want to experiment with the loop antenna by adding windings
to accomodate the converter, be my guest. It may be trial and
error to get the correct number of turns, a good starting place
is the ratio of turns on the original ferrite bar.
It is far
easier to use the external antenna input that you installed in
your radio, as described above. What you will be attempting to
do is construct a new tuned RF stage in front of the radio's antenna
stage. To do this, you need a scrap tuning capacitor from a discarded
radio. The loop described above can be used as the inductive element
to resonate with the capacitor and form the new stage. It is coupled
into the existing antenna through the small coil you installed
earlier. This system is not optimum, and filled with losses, but
the incredible increase in surface area of the loop will overcome
the losses and give a huge increase in sensitivity. Even a radio
that does not have any connection to the loop will experience
a huge increase in sensitivity when it is brought close to the
loop!
As a general
rule, I hook the inner wire from the loop to ground, and the outer
wire to the signal input. The tuning capacitor ground is hooked
to ground, and the variable section is hooked to the antenna.
If you used a tuning capacitor from a discarded radio, it will
have two sections, a big section and a small section. Use the
big section. There should be many plates, not just two or three,
or you have an FM tuning capacitor and need to find a AM. The
following schematic shows the connections for both longwire and
loop antennas to the external input you installed above:
The RF stage
you have just attached to the radio will not, unfortunately, track
the tuning of your radio. You will need to tune the radio first,
then tune the antenna. You might have to go back a forth a couple
of times for maximum gain. Sometimes the new antenna will overload
the radio because it has so much gain. You might have to de-tune
it slightly to receive the station you want.
If
this antenna is still not enough, you can always consider an add-on
RF amplifier.
This article relys heavily on information contained
in several articles from decades ago:
"Compact BCB DX Antennas"
F.J. Bauer Jr., Popular Electronics, January 1965, pages 75 and
76
"Low
Cost Loop Antenna Extends AM Radio Reception" Douglas Kohl, Popular
Electronics, July, 1978
"A BCB Loop Antenna for DX'ing" Norman Fallon, Popular Electronics,
March 1976, pages 51 to 53
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antennX 
Install larger ferrite rod;