Dave's Homemade Loop Antennas
Welcome!
Welcome of my homemade loop antenna or loop aerial pages. All the loops described here were
designed and built by myself. The features are not real different from loop
to loop. These loops are made to enhance radio reception in the broadcast, or medium wave (MW)
band range. Some of the loops are just one coil, some have tuning capacitors
and some have an extra coupling coil. Read on and discover the fantasy of
loop antennas. Or fetish in my case.
A Word About Loop Antenna Design
All these loops involve a resonant tuned circuit made up by a multi-turn loop
coil and a variable capacitor. If you are using one of my designs shown on
these pages, you are all set to go ahead. But if you are designing your own
loop antenna, please read on.
As just mentioned, a resonant circuit is made with the coil and variable capacitor.
The old standard configuration to tune the medium wave band is a 240µH coil
and a 365pF variable capacitor. These numbers are fine when using a small diameter
coil, but are grossly inaccurate with a loop antenna coil.
This inaccuracy is due to a relatively large amount of self capacitance (also called distributed
capacitance) built in to large diameter wire coils. Increasing the wire spacing
doesn't do a lot to reduce this extra capacitance.
The tuning range is determined by the ratio of minimum capacitance to the maximum
capacitance. As an example, the ratio of a 365-14pF variable capacitor is 26. But
if you have 20pF of self capacitance of the coil, then the effective value of
the capacitor is 385-34pF or a ratio of just over 11. (This difference isn't
as bad as it sounds as all coils have self capacitance and is factored in to
the tuning range outcome.)
To use the "standard" 240µH coil value in a loop would cause the tuning
range to be much less with a good portion of the top frequencies not being tuned.
The loop coil is built using somewhere around 160µH
in value, which allows for the top of the band coverage.
But with the smaller maximum to minimum capacitance ratio, a 365pF variable
capacitor and the 160µH coil won't tune to the bottom of the MW band.
There are a couple of ways to tune the whole band. One is to add a fixed capacitor
and a switch which places an additional 200pF or so capacitance in parallel with
the existing variable capacitor. Another fix is to use a higher value variable
capacitor which will restore the higher ratio maximum to minimum capacitance
values.
Knowing this information saves the new builder from an unpleasant surprise
and the necessity for a redesign. Now on to my loop projects …
Loop For My Old Sparton Radio
Spartan Radio & Loop
Here it is friends. I made a loop specifically
for one radio. This my loop #6. I was using my "test" loop (which is shown below)
to see how my Sparton radio would perform. I was impressed and since I use that
my Sparton a lot, it deserved it's own loop.
I decided that this antenna had to be lightened up a little. The two
Garolite® squares had to go. I cut a notch in both pieces of
wood so that they would interlock. The pieces are glued and fastened with a
single flat head wood screw.
The wire is wound with some litz wire. Litz
isn't really necessary but the wire is more flexible than the magnet wire. This
makes a nice looking wiring job when finished. You will need 70 feet of wire for
the tuning coil and about 14 feet for the outer coil. I thread the wire for my
loops outside where there is room to pull the wire. The drilling and other
details are contained in the drawing shown below.
I used litz wire between the loop terminals and the base box. Earlier
I had used 18 gauge PVC covered wire. This was too stiff and the loop would not
stay in the position it was turned to. Just one of those little learning things.
A base box isn't necessary. All the parts could be mounted on the
loop itself, or a small board could hold the loop with a small panel for the
other parts. But I like those basswood boxes that I have.
The Sparton radio is a multi band set and the shortwave bands
require a different antenna. I added a switch and some terminals to accommodate
this need. The radio antenna is switched between the loop coupling coil, or
an external antenna and ground. The other switch is a high low band switch
and adds a 300 pf capacitor across the 365 pf variable capacitor. A larger
value of variable capacitor would tune the whole band in one chunk.
This loop is not real expensive to build, but they require a lot of building
time. Use your imagination and you will be as proud of your loop as I am of mine.
Inside The Tuning Box
Tuning Box Detail
Loop Schematic
Arm Construction Details
R & D Loop
My Research Loop
This is my 4th loop. Sorry for leaving #2 and #3
off the site, but those were so much like my first that I decided I
would wait for something worthwhile to offer. This is worthwhile.
I wanted a loop with lots of bells and whistles to do my radio
experiments. Look at what this loop has to offer.
This loop has two windings. The larger winding (15 turns)
is connected to a variable capacitor for a large tuned circuit. Since
a single 365 pf capacitor will not tune the entire broadcast band,
more capacitance has to be added. In the case of my dad's loop (shown below), he
switched a 300 pf fixed capacitor across the 365 to allow tuning
down to 540 khz. I am using a ganged capacitor and a switch. Included
is a third position of having no capacitor across the coil. I did this
as some of my radios tune the antenna loop by a ganged capacitor inside
the radio.
The other winding (2 turns) is to connect to the antenna
input of the old BC radio. I picked two turns as that emulates the
single turn that my dad had on his larger loop.
This loop has 3 wood pieces. The arms are 12-1/2 inches long,
and the main pole (vertical part) is 30 inches tall. This is a little
larger than my earlier loops so that I would have room for that extra
winding. After the holes are drilled, I stained the oak with Minwax
Red Mahogany stain. After the gloss deft dried, assembled the loop
using two 3x3 inch square pieces of Garolite®. Thin wood, plastic or
anything you have around will work. Before you mount everything,
fit it together and take a look to see if it looks right.
Another change from my first loop is how the ends of
the wires are handled. So that the wire wouldn't work loose, I drilled
an extra hole close to each end hole on my loop. This makes 2 extra
holes I had to drill. What the heck, holes are cheap so I splurged.
The extra holes are about 3/16 inch from the end holes. The wire can
be then looped around a couple of times. This holds the ends secure
and looks neat too.
On the top of the mast pole and each side arm, I drilled
17 holes. I used a 3/32 inch drill but it isn't too important the
size of the hole. Each hole is 3/8 inch from each other and I started
3/8 inch from each end.
Then measuring down from the top of the pole 25 inches,
I drilled a series of 21 holes going back up the pole, 15 holes, one
for each turn, one for the end winding. Then two extra holes close
spaced for ending the tuned winding. . When the wire is
wound on this loop, it will look like a square.
Three more holes are drilled for the untuned winding.
I looped the both wires once through the middle hole as with this
winding, having the ends so close wouldn't matter. I think that
comes out to 21. Lets see.... 15+1+2 +3 = 21. Good thing I stayed
awake in the first grade!
You with me so far? Good. After I wound the wire, I
made a little 4 terminal connection block from another piece of
Garolite®. I also drilled a 1/4 inch hole in the bottom of the
pole and inserted a dowel rod in the hole. This is the weak point
of the loop. If the loop falls over the dowel will break and need
replacing.
Next I built the box that serves as a base for the loop
and holds the capacitor. I used a cut down basswood box, a piece
of 3/4 inch thick oak as a base and some 1/8 inch Garolite® as a
panel. I took a 1x1x3 inch piece of wood and screwed it to the
Garolite®. This is so the 1/4 inch dowel will be stable in the
loop base.
Using that big thick 14 gauge wire, I make the connections
to the capacitor and link switch. In my case, I used a 3 gang capacitor.
That made it easy to use a single link switch to do what I wanted it
to. You could make two spst link switches, one to add a fixed or
second gang and the other to disconnect the capacitor from the
circuit. I built mine the way I wanted, and you can build yours
any way that is handy for you.
So I can use this loop several ways:
Connect a radio to the untuned 2 turn loop and tune the main
coil with the capacitor.
Hook an antenna and ground to the two turns and a diode
and earphone on the tuned side for a cool crystal set. Or if I was
really close to the station, forget the antenna and ground.
Use the untuned winding as a tickler coil in
a regenerative set.
Use only the large loop without the tuning to
connect to a radio. This loop could be used to replace a built-in
loop on a radio.
Connect the coupling loop between a wire antenna
and a crystal set for a wave trap. Make sure the loop is turned
to null the station you want to trap.
Maybe I could use this loop to strain my spaghetti.
Anyway, below are some pictures of how I constructed the loop.
If something is unclear or you have a better idea on something,
please e-mail me. These loops are very cheap to build but take
a long time to make. But they sure look cool sitting on top of
your old radio. Best of luck with yours!
Dave
R & D Tuning Box
Inside The Tuning Box
Inside The Base
Tuning Box Top
Mast Wiring Detail
The Loop Wires
Loop Schematic
Arm Construction Details
