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Modify that AM antenna 

by Bruce Carter

Go to:
Install larger ferrite rod;
Fix external antenna;
Add a long wire;
Or a loop;
Construct it;
Connect it;
Add an amplifier
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:

  1. 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.
  2. Trace the connections on the coil and find the one that goes to the tuning capacitor (or varactor antenna tuning stage).
  3. Tune to a weak station on the high end of the band.
  4. If there is a trimming capacitor (there usually is), adjust it for best reception.
  5. 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.
  6. DO NOT touch the ferrite on the high part of the band, or the trimming capacitor on the low part of the band!
  7. 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:





































































































For your convenience, I have done the calculations for loop antennas from 1 turn to 25 turns, and placed them in this table.

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.

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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