From: moisan@silver.lcs.mit.edu (David Moisan)
Subject: Carpet Loop Antenna
Followup-To: poster
Summary: Indoor loop antenna for small spaces--comes with its own tuner
Keywords: antenna, loop, shortwave, ham, radio
Organization: MIT Laboratory for Computer Science
Date: Wed, 19 Aug 1992 17:17:45 GMT

[Here's something I wrote a while back.  It's never been posted to Usenet to my knowledge.  Comments welcomed---DM]

THE CARPET LOOP II -- A High Performance Indoor Antenna
-------------------------------------------------------
 
Designed by David Moisan, N1KGH
 
Introduction:
-------------

    There are many shortwave listeners who can't, because of location, 
infirmity or a unyielding landlord, put up an outside antenna.  Such 
people are given two choices--random wire or active antenna.  Yet, 
for the serious listener, neither choice is completely adequate.

    Active antennas are expensive, apt to generate as much noise as 
signal, and are prone to overload.  Random wires are cheap (cheapest, 
in fact) and easy to put up, but are unpredictable performers.  Both 
subject the receiver to intermod, spurious signals and other trash. 

    The Carpet Loop II is an ideal step upward for the listener who 
wants something better than a random wire but doesn't want the 
expensive dice roll of an active antenna. 

    The Carpet Loop is made up of two components:  A tuner, and the 
antenna cable itself;  the cable can be either 5-conductor rotator 
cable or 4-conductor flat phone cable, both readily available from 
Radio Shack and elsewhere.  The tuner couples the antenna to the 
radio, forming a (giant!) L-network.  To tune the antenna, you turn a 
switch for best reception. 

    While NO antenna can give a cheap receiver the sensitivity, 
selectivity, or dynamic range it never had, the Carpet Loop will help 
you get the last ounce of performance out of your radio. 

    Two years ago, I was using a random wire.  I had severe problems 
with a local AM station (2 miles away) on 1230 Khz. I was hearing 
intermod from it all over the 9 to 12 Mhz range.                       

    With the Carpet Loop (which was then just the cable), the 
interference was almost completely gone.  Also, the signals I was 
receiving seemed to be just a little bit stronger.  A year later, 
I built the tuner, with much better results.  I'm convinced I have
the best possible antenna for my location. 

   If you're stuck in an apartment, if you have a portable like the 
Sony 2010, the Sangean 803A or the Radio Shack DX400 or 440, if you 
have a tabletop receiver, the Carpet Loop may be for you.  It's 
cheap--around $25 in parts from Radio Shack, *much* less if you shop 
around, and an excellent first project for the technically minded.

 CARPET LOOP II
                                         From Radio
 SCHEMATIC AND PARTS LIST        
                                       Gnd.       Ant.
 S1  1 pole 6 position                  !          !
     rotary switch                      !    D1    !
     (RS# 275-1386)                     !---->I----!
                             +----------*----I<----!
 C1  365 pf variable         !          !    D2    !
     capacitor (see          !          !          !
     text)                   !      o   o          ! #A
 S2  SPST switch             !       \             !   
                            ---   S2   \          ---  C1
 D1, D2  1N914 silicon      |||         o         ---
     switching diodes     (Earth        !          !
     (RS# 276-112)         Gnd.)        !          ! #B
                                        !          !
 J1, J2  6-position terminal            !          !
         strip (RS#274-357)          +G-o <---o----+
                                     !   
 Antenna cable:                      !  o   S1      o 
                                     !  !E          !A
 5-conductor rotator          J1     !  !  o     o  !     J2
 cable (RS#15-1201)                  !  !  !D o  !B !
 (see text)                 1 |o|----*  !  !  !C !  !     
                            2 |o|-------*--!--!--!--!-----|o| 6
 Miscellaneous:             3 |o|----------*--!--!--!-----|o| 7
                            4 |o|-------------*--!--!-----|o| 8
 Aluminum enclosure,        5 |o|----------------*--!-----|o| 9
 spade lugs, connector        |o| (not used)        *-----|o| 10
 and cable to radio                             (not used)|o|
 (see text), knobs                   <- To Antenna ->   

 Note on Schematic symbols:      
                                
   !
  -*-  means:  Intersecting wires are connected
   !

   !
  -!-  means:  Intersecting wires NOT connected
   !

HOW IT WORKS
------------
   
     S1, C1, and the antenna cable connected to J1 and J2 form an L-
network;  when S1 is switched between positions A through F, and C1's 
capacitance is varied, the impedance of the antenna system changes.  
When S1 and C1 are adjusted for best signal, the impedance between 
antenna and receiver is matched.  D1 and D2 provide protection against
static discharges.  The G position of S1 grounds the antenna when not
in use.  S2 disconnects the ground from the antenna, making the antenna
into a random wire.


TUNER CONSTRUCTION 
------------------

PARTS AVAILABILITY:

    With the exception of C1, all parts for the tuner are readily 
available from Radio Shack.  C1, the 365 pf variable capacitor, can be 
gotten out of an old radio.                               

SUBSITUTIONS:

    There are no critical parts in the tuner;  as long as S1 has at 
least six positions, it will do.  D1 and D2 can be any silicon 
diode.  Use any enclosure that's big enough to comfortably 
install components in.
 
    The choice for J3, the jack to the receiver, depends on what 
connector your radio uses for an external antenna.  I used an SO-239 
(RS #278-201);  you could also use a TV antenna terminal strip 
(RS#274-663).

STEP-BY-STEP INSTRUCTIONS:

1)  Mount the components on the enclosure you'll be using--all wiring 
is point to point.  I suggest mounting J1 and J2 on opposite sides, S1 
and C1 on top, and J3 on the other end of the enclosure.

2)  Wire S1 to J1 and J2.  If you use the Radio Shack DP6T rotary 
switch, you'll be using just one of the poles.  The diagram of the 
switch is below:

                                     D  C
  S1  DP6T rotary                 E  o  o  B 
      switch                   G  o        o  A
                           |   o              o   | 
 Viewing from            P |o                    o|        
 bottom of switch          |   o              o   |
                                 o        o     
                                    o  o        
                                         
Wire as follows:

(Note:  If you're using the Radio Shack terminal strip, you will need 
to drill a hole in the cabinet to pass the wires through from inside.  
Use a rubber grommet to keep the wires from fraying)

 S1 Term. --to--> J1 term.     J1 term. --to--> J2 term.
   A                #1            #2              #6
   B                #2            #3              #7
   C                #3            #4              #8
   D                #4            #5              #9
   E                #5


Connect a wire from J2 terminal #10 to the G terminal on S1, and 
this step is done.



3) Install and wire C1.  Connect one terminal of C1 to the P 
terminal on S1.  Connect the other end to J3.  If using the SO239 or 
phono jack, connect to the center conductor.  If using screw 
terminals, connect to terminal #1 on J3.  Skip ahead to step 5.

4) Connect the P terminal of S1 to J3.  If you're using the SO239 or 
phono jack, connect to the center conductor.  If you're using screw 
terminals, connect to terminal #1 on J3.

5) Connect the G terminal on S1 to J3.  Connect it to the ground 
shield if it's an SO239 or phono jack, or to terminal #2 if it's 
screw terminals.

6) Connect D1 and D2 across J3's terminals;  remember that D2 is 
connected opposite of D1.
 
That completes construction of the tuner.

ANTENNA CONSTRUCTION
--------------------

CHOOSING CABLE:


    The kind of cable you use depends on where you're putting it and
and how much you want to pay for the cable.  
                                                                   
    If you plan on running it under carpet, then use the 5-conductor 
rotator cable mentioned in the parts list.  This cable can easily 
withstand being stepped on;  more importantly, there are no exposed 
wires to trip over.  It's also easier to wire than phone cable.

    If you're not running it underfoot, or if you're cheap, you can
use 4-conductor flat phone cable, available nearly everywhere.  It's
a good choice for running around baseboards, around windows or in attics.

    When running the cable around, start at your receiver and go around 
the room--or the house--and back to the radio.  If you're using 
the rotator cable, you can make corners by folding the cable at a 45 
degree angle, like folding paper.

WIRING THE CABLE TO THE TUNER:

    You should have two ends of the cable next to the tuner.  Strip 
the ends and put spade lugs on all the wires.  With the rotator cable, 
mark the *silver* conductor.

    Next, connect the wire to the tuner using the following diagrams:
 
      FOR 5-CONDUCTOR ROTATOR CABLE    FOR 4-CONDUCTOR PHONE CABLE
 
            J1          J2                J1          J2
                              1                             1
         1 2 3 4 5    6 7 8 9 0         1 2 3 4_5   6 7 8 9_0 
         ^ ^ ^ ^ ^    ^ ^ ^ ^ ^         ^ ^ ^ ^     ^ ^ ^ ^ 
 Wire    S C C C C    S C C C C         B Y R G     B Y R G
 colors     
                                     (4&5 jumpered) (9&10 jumpered)

 ((S)ilver, (C)opper, (B)lack, (Y)ellow, (R)ed, (G)reen)
  
GROUNDING:

    Run a wire--preferably a large one--from the ground terminal on 
the tuner (or a mounting screw on the SO239 connector if you're using 
one) to a suitable ground such as a cold water pipe;  I grounded my 
tuner with a short length of RG58 coax connected to a baseboard heater 
via an alligator clip.
 
   Connect the tuner to your receiver;  you are now ready to use it.  


USING THE CARPET LOOP
---------------------

    It's easier to use than to talk about:  Tune your receiver to the 
desired frequency.  Adjust S1 and C1 (or the antenna trimmer on the 
radio) for strongest signal.  For most situations, S2 (Loop/Longwire)
can be left closed in the Loop position;  you may find that setting 
S2 to Longwire may work better for mediumwave listening.
 

CONCLUSION
----------

    The Carpet Loop II is an inexpensive, easily built, high 
performance antenna that can work in almost all apartments.


CONTACTING THE AUTHOR
---------------------

I can be reached at the following addresses:
 
Mail: David Moisan
      86 Essex St. Apt. #204
      Salem, MA 01970-5225
 
Fidonet:  David Moisan, 1:101/165
Internet:  dmoisan@pro-angmar.alfalfa.com
           or moisan@silver.lcs.mit.edu

Your comments and suggestions are welcome.

-- 
| David Moisan, N1KGH    /^\_/^\   dmoisan@pro-angmar.alfalfa.com |
| 86 Essex St. Apt #204 ( o ^ o )  moisan@silver.lcs.mit.edu      |
| Salem. MA 01970-5225     | |                                    |
\-----------------------------------------------------------------/