If there
is such a thing as the perfect antenna, then why does every issue of every ham
magazine have an antenna article? Fortunately, for the crystal radio nut,
most ham antennas are less than optimum for general listening use, and require
special care and feeding due to the need to have the transmitter and antenna
somehow matched to near perfection and for the antenna and its associated parts
robust enough to handle power without arcing and sparking while also keeping
the transmitted signal out of the shack and also out of the neighbor's tv set.
Crystal radio antennas have some special
requirements:
a. large
frequency range
b. large
signal gathering capability
c. efficient
in getting and then transferring the received signal to the set.
Your basic problem is normally just getting enough
wire in the air to have a resonant circuit. For 99 percent of us, the
answer is: you can't. Even the shortest resonant antenna for the
top of the AM broadcast band, a quarter wavelength Marconi antenna, would have to be about 137 feet long; the
bottom of band calls for about 425 feet. Unless you are
one of the fortunate few to be able to erect a bona fide antenna farm
everything you do from this point on will be a compromise of one sort of
another. This means you have to start thinking about your antenna system
rather than just what you hang up outside the house.
First, the antenna itself: I recommend you use
stranded copper wire, at least 20 gage. Try to get at least 50 feet of it
"out the window" and up in the air as high and as far from the house
as possible. My experience is that anything shorter gets real hard to
tune, and gives away a whole bunch of signal to boot. Longer is better,
higher is better. Use plastic strips or ceramic separators or whatever
you can get to ensure the antenna wire is not touching anything that could be
grounded, including the tree you may be using for a support. I usually
just use stranded wire with a tough plastic coating, wrapping it over tree
limbs as I go, and connect it at the far end with heavy nylon fishing
line. Do not go over or under power lines. If you are coming
in through the bottom of a window, as many of us do, be sure to use
insulated wire for the final lead in. If you have to put it inside, such
as up in the attic, make sure you are not hanging it under a metal roof or
ridge vent, otherwise it could very well shield your wire from the signals you
are trying to get. High straight antennas are probably best, but do the
best you can, and don't be afraid to bend it in order to get more wire out
there. Contrary to popular wisdom, you can tune a bent antenna. If
you like to try out different stuff, put an alligator clip on the end in the
shack to make quick connections to your latest xtal wonder rig. If you
replace your antenna, or otherwise have room, a second antenna can come in
handy, if for nothing more than comparison purposes. If your main antenna
is mostly long and horizontal, a shorter, and more vertical antenna might give
some pleasant results, particularly with distant stations and higher
frequencies. Since you will probably be using what is considered a random
wire antenna, connected at one end to the set, it may be of some value to know
that the horizontal part of the antenna "points" in the direction of
best reception off the free end.
Next, get a good ground. This is the other
half of your antenna, and a good earth ground is essential. By
earth ground I mean a good electrical connection to the earth. (I once had a
student who tried just laying the ground wire on the ground - didn't work) I
know of several hams who invest more time, effort and money in their ground
than any other part of their antenna system. Mine is currently a tight
connection to a close by cold water pipe. If you have plastic pipes, this
doesn't work. I have also used pipes hammered into the ground, the
ground rod the telephone company provides where their service comes into the
house, and even the screw holding on the cover of electrical plug cover plates
in a pinch (this is usually electrical ground, and may even lead electrically
to the earth somewhere - works better for some locations than others). I
haven't resorted to burying assorted lengths of wire in the ground yet, but
many dedicated types do. If you are located in the desert or on a granite
mountain, and good earth grounds are hard to come by, you need to read up on
dipole antennas, ground radial systems and counterpoises; good
luck. If you can not make a short connection to a good ground, and your
ground lead is long enough to act as an antenna itself, you might want to try
this trick: use a piece of coaxial cable between the set and the ground
connection, using the inner core for the connection; connect the shield to the
core at both ends with a capacitor, say, about 0.01 uF or so, to shield the
ground lead from rf. I haven't tried this myself, but it makes sense, I
think.
Whatever you have done up to this point, your antenna
is (1) too long or too short (usually the latter), (2) is not resonant at
the frequencies of interest, and (3) will not efficiently transfer whatever
signals you get to your set. Guaranteed. Despite all these
deficiencies, it will probably work pretty well, at least on the stronger
stations. If you are happy at this point, declare victory and quit.
If, however, you have a free hand left with which to flog yourself while you
tune with the other, or really want to go after them, read on.
The first quick fix you can make is to effectively
add additional length to your antenna by putting a large coil of wire between
the antenna and the set. Wind 50 to 100 feet of 20 to 24 gage wire around
a core between 2 and 5 inches in diameter, and tap it about every 10 turns or
so. Attach one end of this coil to the antenna. Use an alligator
clip to connect the xtal set to various taps until you get the loudest
signal. While this doesn't add much to the antenna in the way of length,
it can make the antenna closer to resonance with the frequency you are trying
to dig out.
Quick fix number two, which can be done after or in
lieu of number one, is to add some capacitance between the antenna and the
set. This seems to work best if your antenna is too long for the
frequency of interest. Radio Shack uses this technique in some of
their Bunch-in-One electronic project kits, using a fixed 10 pF capacitor for a
short antenna and a 100 pF capacitor for long antennas. Short refers to
antennas 50 feet or less in length. I prefer to use a variable capacitor
if I go this route. Putting the capacitor in series with the antenna this
way lowers the inherent capacitance of the antenna. One of the more
popular designs a number of years back put the variable capacitor in
between the xtal set and the ground connection. If it is the same size as
the main tuning capacitor in the rig, you can even gang them and make tuning a
bit less tiresome, with little or no adverse effect on results. This
actually works very well. If you are using a set that has the detector or
the antenna attached directly to the top of the tank circuit, and don't want to
make taps in the tank coil for better impedance matching, this fix might
be(I should say is) essential, and is certainly worth trying. The problem
with connecting the antenna to the top of the tank circuit coil is complex, to
say the least. I observed that the first effect is to lower the resonant
frequency of the tank circuit drastically, even with a very short (a few feet)
antenna. If you're going to try to get away with this, and don't have a
variable capacitor to spare, you can make a gimmick capacitor by twisting a
piece of wire connected to the set around the bitter end of the antenna.
I did it another way, using some 18 gage twin lead wire; turned out that the
capacitance of the twin lead was about 1.2 pF per inch, and the circuit resonance
from the antenna was not affected if you used enough twin lead (one side
connected to the set, the other to the antenna) to give you about 1 pF per foot
of antenna.
Quick fix number three, which is an excellent idea
even if you don't do the first two, is to connect the antenna to a tap on the
tuning coil. This gives a better match between the impedance of the
antenna and the tuned circuit of the rig. Techtronics does this with
their nice little crystal set, providing two places to connect the antenna
(really three, but they don't mention the third tap, for some reason. It
is between the other two.) Probably the absolute worst place, in my
opinion, to have the antenna connected is at the top of the set tuning
coil along with the connection to the detector, yet that is what
several web page offerings would have you do. The tap you use will have
to be determined experimentally, but I find that low on the coil close to
ground helps.
Quick fix number four, which I recommend with fix
number one: Use an antenna coupling coil . Wind
the coupling coil on approximately the same diameter core as the main tuning
coil, using about one turn for every six in the main tuning coil, and
then put it in line (on the same axis) with the main tuning coil, with the
ability to move them apart. You will get better selectivity, and
can sharpen it up a bit more by moving the two coils apart - what the trade
calls loose coupling. Oh yeah, the antenna and ground are connected
only to the coupling coil, although you might try connecting the ground to the
set as well. The "one for six" rule is just a rule of
thumb. If you don't mind experimenting a bit, pick the number of turns on
the coupling coil that gives you the best coverage across the band,
particularly if you are going to use your set without a tuner. It will
still be a compromise, and the best number of turns will depend on the length
of your antenna and your location, but will give better results. Of
course, you can dispense with the coupling coil if you are inductively coupling
the antenna coil directly to the set's tank coil as in the basic tuner below.
If you have
tried one or more of the tricks above, and are happy, fine. If not,
I don't blame you, so it is time to get serious about tuning the antenna and
coupling it to the set for optimum sensitivity and selectivity. In other
words, it is time to talk antenna tuning and coupling methods, subtitled
"how I went from 5 to 50 stations , and then to 98, heard on my xtal
set".
A properly employed antenna tuner will do a lot of
"heavy lifting" for you. First, it can make your antenna
resonant at the frequency of interest, and bring up the signal level at the
same time. Second, it will screen out a lot of unwanted stations,
including those out of band ghosts. Third, it will transfer the signal
efficiently to the main tuning and tank circuit, and do so such that you can
vary the selectivity of your set as a bonus. This seems like a set of
tall orders, but it isn't nearly as hard as it may seem. I will discuss
mostly sets using air core solenoid tuning coils (that is, toilet paper
rolls wrapped with a single layer of wire or the like), since that's
what I usually use and have the most experience with. Before we start,
let me say that an antenna tuner is not just another stage of passband
filtering, such as the Miller 595 crystal set used. A key to a passband
tuner design is when you see a two or more section variable capacitor, with one
section in the gang working with one coil and the other section working with
the second coil. Also let me add that an antenna tuner is essential for
serious work.
Basic tuner: Wind a
coil on a core the same diameter as your xtal set's main tuning
coil. You may want to add about 30 to 50% more turns, but it should be at
least as many turns as are on the main tuning coil. Put a tap at least
every 10 turns (every 5 is better). Put the two coil axes in line, and
connect the end of the antenna coil that is closest to the xtal set to
ground. Attach the antenna to taps on the antenna coil using an alligator
clip until you get the best signal. No direct connection to the xtal set
is needed. You should find that you can separate the two coils by a
distance of one coil diameter or more and get a good signal; in fact, moving
them closer together may even make things worse. The closer the antenna
is tapped to ground on the coil, the higher the frequency. You can
leave the unused antenna coil turns "dangling", as it were, or attach
the antenna to the other end and short the unused turns with the tap - both
ways are acceptable. You should be able to notice a difference in signal
strength of a station by moving just one or two taps along the coil. If
you find that you never need to use more than half the coil for the best signal,
even at the bottom of the AM band, don't worry; you may need them
later. In this setup, you are essentially making the antenna appear the
correct length, so you are in effect adding length to compensate for the short
antenna. If you can't inductively couple the tuner to the set's tuning
coil, you can attach your earth grond to the chassis ground of the set, and
attach the ground end of the antenna coil to a suitable tap on the set's tank
coil.
The Teenie Tuner: This is a
variant of the basic tuner, using a coil wrapped around a ferrite core.
Here's how to make one: Get a ferrite core of some sort, either a rod or
a bar. Wrap some typing paper over it and glue the paper into a sleeve -
put a spacer between the paper sleeve and the core while winding the coil so
that the core will slide easily in and out of the coil after it is wound.
Over the paper sleeve, wind a coil of some 50 to 90 turns of magnet wire in a
single layer; no taps needed. Glue or tape the coil onto the
sleeve. Now, mount the sleeve only on a support, such as a scrap of wood,
or in line with the set's coil. Connect one end of the tuner coil to the
antenna, the other to ground, and vary the tuning by sliding the ferrite rod in
and out of the coil. You should get a range of inductance of about 10:1
this way, effectively lengthening your antenna as he rod is pushed further into
the coil. As with the basic tuner, you can set the tuning coil alongside
the main coil, in line with it, or connect it's ground end to the antenna tap
of the set. Just for grins, affix the teenie tuner to the back of your am
superhet and see the improvement in your reception.
Series tuner: Here we
add a variable capacitor, the same size as the one in your xtal set, in series
with the antenna coil used in the basic tuner. Traditionally, you put it
between the end of the antenna coil and the ground connection. Now you
have two tuner adjustments to make (not counting distance of the antenna coil
from the main tuning coil). For the first time, I recommend you get the
set tuned to a moderately strong station, then put the tuner capacitor in the
mid position, and then find a tap which allows you to hear the station.
Then you can use the capacitor to bring in the station more sharply. You
will probably find you need to use more turns of the coil than you did with the
basic tuner, since the capacitor "shortens" the antenna.
Ultimate signal strength may not show much improvement, but it should help
selectivity. This setup is a favorite of the "pros". As
before, the antenna and main tuning coils are in line. You should have to
make fewer tap changes with a series tuner compared to the basic tuner, since
the capacitor does some of the tuning for you.
Parallel tuner: Use the
same coil and capacitor as with the series tuner, except this time the
capacitor is connected to both ends of the coil. One end of the coil, the
one closest to the set, is connected to ground, and the antenna is connected to
the coil tap that gives the best signal; incidentally, I have seen a few
circuits where the antenna just connects to the other end of the coil from the
ground - I don't recommend this, even though it works fairly well, sometimes,
maybe, I think. Adjust the capacitor as needed to peak the
signal. This setup is often used when the antenna is
"very" short. My recommendation to add turns to the
antenna coil will sometimes let you use series tuning across the whole
band. If your two coils are the same length, I think you will find that
the series arrangement works best at the high end of the band, and the parallel
arrangement is needed at the bottom; the longer your antenna, the lower in
frequency you can use a series tuner. As with the series tuner, a
parallel tuner gives you more selectivity than does the basic tuner.
A broadband tuner: This is
not a classic design, to my knowledge, and I stumbled on it by accident, but it
does a lot of heavy lifting, and helped me well over the single digits in
stations heard. I thought I was copying faithfully a design out of an
ARRL handbook for dealing with random length antennas, but ended up with
something a bit different, and won't try to analyze how it works, but here it
is: Don't sweat the coil/capacitor specs; what you have in the series or
parallel tuner will work fine. However, the connections shown going to
receiver antenna and ground are actually connected to that coupling coil
I mentioned up in quick fix number four. The tuner's orientation with
respect to the set is not a factor, since the signal is transferred via the
coupling coil. It will generally be closer to the main tuning coil than
described in the three tuners above. I like this tuner arrangement
for casual listening, as I don't have to play with it much over the band.
Find the settings that work best for you. It is a particularly good setup
to use when you are finding out about a new xtal set, and don't want to have to
mess with the tuner every hundred kHz or so, as you do with a more selective
tuner, or don't want to "tailor" a tuner just for the new rig.
This setup works well when you can't inductively couple the antenna
tuner to the set. It also works without the coupling coil by connecting
the end of the antenna coil to a low tap on the main tuning coil of the set.
All of the above tuning methods can be accomplished using a simple
coil and capacitor arrangement, which I made up according to the schematic
below:
A selective tuner: Richard O'Neill used this tuner
effectively in the 1999 DX contest (and everybody used it in the
2000 contest), and it is perhaps best known in connection with the Tuggle
Circuit, which is described in detail in a couple of fine (and inexpensive)
publications of the The
National Radio Club / DX Audio Service. Essentially, it
is the same as a parallel tuner, except, you use a ganged
capacitor, such as a dual 365 pF capacitor (some are still around), or even a
"superhet" variable capacitor with an antenna and an oscillator
section. One section (the larger one for a superhet capacitor) is in
parallel with the antenna coil, and the other is between the parallel circuit
and ground. I have tried this and it seems to work fine, and is very
sharp. As before, try attaching the antenna to taps on the coil for best
results; I find attaching it to the top of the coil works
fine. Several popular crystal sets have used same double
capacitor arrangement. If you don't use ganged capacitors, tuning this
baby can be really tricky, since the two (make that three) capacitors interact
a lot. When hooking this up to a ganged dual capacitor, the antenna and
the top of the antenna coil are hooked to one stator. The bottom of the
coil goes to the rotor (frame), and the ground to the second stator. Note that
with decent Q coils, you will experience some hand capacitance effects. I
simply used some 9/32 " aluminum tubing on the 1/4 " capacitor shaft,
then used about 2 inches of 1/4 " dowel rod as an extension, and put the
tuning knob on the dowel; worked fine. Sometimes, when tuning the top of
the band (higher frequencies), I can spread the tuner out a bit by making it a
series tuner. All I have to do is disconnect the first stator from the
top of the coil. Yeah, this means the ground is not attached to the rotor
as in a proper series tuner, but it works fine, particularly if you have
already done the hand capacitance elimination step.
As long as you have gone this far, why not try connecting the
detector to the top of the detector coil instead of to a tap. Works
pretty well with a crystal earplug, and cuts out the shortwave intruders.
With magnetic phones, using an impedance matching transformer per Ben Tongue's
page is rally the thing to do. However folks, with the tuner you already
have an additional filtering stage in operation, and can sacrifice a bit of
selectivity with the high detector tap; anything to get rig of the intruders in
a pinch, particularly in the early evening when they can take over your set.
While I mentioned putting the two coils on the same
axis, you can also place them side by side, as Mike Tuggle does with his
Lyonodyne #17 (getting 2500 mile contacts), using an antenna tuning coil wound
with Litz wire on a ferrite rod; just make sure they are parallel for
maximum coupling. You can vary the coupling by either separating them
from each other or by rotating one of them from the parallel position, as is
done with variocouplers. Minimum coupling occurs for a given distance
when the axes of the two coils are perpendicular.
For you people using toroids in your xtal set,
I have only to offer what you see in the "quick fixes".
My tuners essentially rely on loose coupling to the main coil, and I really
haven't played with the other type tuning coils enough. Al Klase uses toroids in his
very fine crystal set, and you should look at his page for coupling to
that. Also look at his work on XTAL Sets 102 for ideas.
In your quest for sensitivity and selectivity, you
may now be ready to try a wave trap, sometimes known as a QRM coil, to get rid
of offending, overbearing bandmaster stations. The premise is simple;
trap the strong signal and it won't bury the weaker ones. While I can
separate two local stations that are 40 kHz apart, with one of them at least 3
dB louder than the other one, I often can't eliminate the louder station
entirely, and can often hear it in other parts of the dial except when I am
between stations. Then there is the signal from the secret Radio Habana
transmitting site, which pokes through at odd times. Crystal radios tend
to go with the signal that is the loudest, hanging on until something better
comes along. A decent wave trap just might be the answer.
. One basic design is around a high resistance coil, 32 gage wire
or so, in parallel with a variable capacitor, and a 15 turn link around the
coil which connects antenna to set (or wherever you want to try it. The
idea is that the resonant LC circuit not only is set into motion by the
offending signal but also dissipates it in the high resistance windings of the
coil. The link lets the other signals bypass the trap with little
loss. Here is a link to a nice short page on wave traps. Medium wave wave trap.
These traps, with no coupling coil, work best for out of band intruders when
used as in-line traps in the antenna line, or when inductively coupled to your
set's tank circuit. I think the one I use is better for in-line use, and
you can find a description, picture, and discussion of it and trapping in
general here . I
realize that a wave trap might not be considered technically a part of the
antenna system, but as long as it is in front of the main tuning circuit,
I will consider it so. Wave traps can be used as either rejectors, to
block an offending station, or as acceptors, which can be sometimes used
to boost a desired station or, more commonly, to add another stage of filtering
to increase overall selectivity.
Final warning! When you do anything
to the antenna system, you can expect it to affect some other aspect of the
crystal radio's tuning. When you are ready to start making adjustments,
have a comfortable chair, a tall drink, maybe something to make notes with, and
the time to be patient. And oh yeah, I didn't even mention the
possibility of using different antennas, different grounds, loop antennas, no
ground at all.... and did I mention lightning arrestors?
If you noodle
around on the web a bit, you will find some excellent antenna articles. I
am just trying to get you started. As I said before, every
antenna is a compromise - just try to get a setup that works for you, and then
you can get back to building and testing your own crystal set creations.
A word or two about small loop antennas, portable
antennas and the like: You will find at times references, plans and
testimonials in praise of the small loop antennae. If you live near
strong stations, they may work for you. Where I live, they are
almost useless, and do require a ground. I personally believe that
if you are going after the distant stations with a loop you will be very
disappointed. These are specialty antennas and have limited value - but
where they do work, they get high marks. As an aside, I suspect that if
you can't get at least 30 feet of honest antenna out the window, you might try
a large diameter loop antenna instead. If you are new to this business,
start at the top with an honest antenna (please try for at least 50 feet out
the window), and then, when you start getting experience, and have a set you
know works, try what you like. Trying to hook up a first crystal set with
a small loop is an invitation to frustration.
