20M Micro-loop Antenna
By Rob L. Dey, KA2BEO 1/9/99
Specifications
SIZE: 2'W x 2'H x 0.5"D approx. (6% the space, with 72% total wire length of a
dipole!) (44% more wire length than a 1/4-wave antenna, in only 12% of the
space required!)
FEEDPOINT IMPEDANCE: Balanced, 50-ohms approx.
CENTER FREQUENCY: 14.100 MHz
SWR: 1.25:1
BANDWIDTH (2:1 SWR): 500 kHz, 14.000-14.350 MHz
POWER RATING: 15 W rms (QRP only), 150 W rms (if 5kV caps are used)
TUNING: None required, pretuned design.
GROUND: None required, balanced design. Also, no balun required for coax feed.
MAINTAINANCE: None required, completely weatherproof.
MOUNTING: Hang vertically in any convenient location, even in the basement.
Feedpoint is at the top.
RADIATION PATTERN: Major lobes in front and back at all elevation angles.
Additional Features of a Small Tuned Loop
* High selectivity increases dynamic range of receiver.
* Sharp nulling can isolate stations apart by 10° to 170° with 60dB
of attenuation or more.
* Approximately 26dB of electrostatic noise reduction quiets man-made,
precipitation, and other noise sources.
* High efficiency performs similar to a full-size dipole.
Parts Required
4 1/2" CPVC 4120 90° Elbow, NIBCO #4707 $0.17
2 1/2" CPVC 4120 Tee, NIBCO #4711 $0.29
1 1/2" CPVC 4120 Slip Cap, NIBCO #4717/GENOVA #50155 $0.25
1 1/2" CPVC 4120 Slip Cap, NIBCO #4717/GENOVA #50155 $0.25,
with 3/8" dia. hole drilled in end
8' 1/2" CPVC 4120 Tubing, GENOVA #500005 $0.24/ft, cut into lengths of:
2 @ 23.250" H, 4 @ 11.000" W, 1 @ 1.000" (for bottom tee), 1 @ 0.750"
(for top tee)
8.5' 18/3 Power Cord/Cable 3 x 18 AWG, cut into lengths of:
2 @ 51" L (to start with)
1 BNC Female Connector, locking solder lug, and nut
2 15pf ±5% 500V Silver-mica Capacitor or equiv.
1 18pf ±5% 500V Silver-mica Capacitor or equiv.
CPVC Primer/Cleaner (Oatey)
CPVC Cement/Adhesive (Oatey)
Solder
Electrical Tape
Assembly Procedure
1. Assemble all CPVC parts without glueing.
2. Insert cable into each vertical side.
3. Strip 2" of jacket off of both cables at top.
4. Cut 1.5" off of 4 of 6 insulated wires, leaving a 2" long WHITE wire on
left side, and a 2" long BLACK wire on right side, for BNC connection.
5. Strip 1/8" of insulation off of all 6 wires at top of assembly.
6. Install BNC connector at top (pointing downward).
7. Solder and tape all 6 top wires as shown in schematic.
8. Entire assembly should be built at this point, except for capacitors at
bottom.
9. Glue all CPVC tubing except for the 6 places at tees.
10. Cut about 1.5" off of each cable at bottom, so that 1/2" is exposed where
tee will be placed.
11. Total length of both cables combined is now 48" long.
12. Strip 1/4" of jacket off of both cables at bottom, so that 1/4" of
jacketed cable remains exposed.
13. Strip 1/8" of insulation off of all 6 wires at bottom of assembly.
14. While installing bottom tee (pointing upwards), solder and tape all 3
capacitors and 6 bottom wires as shown in schematic.
15. Test antenna by transmitting 10-watts (or less) of RF (on 20-meter band)
into antenna, while monitoring SWR.
16. Glue bottom cap to 1" tube.
17. Do not glue the 3 places at bottom tee.
18. Glue top cap (with BNC) to 0.750" tube.
19. Do not glue the 3 places at top tee.
20. Enjoy using your new 20-meter micro-loop for portable QRP operation.
NOTE: Cost should be less than $4.00 using all new CPVC parts, assuming all
other parts (linecord, BNC, and capacitors) can be scrounged.
You can hang your loop with small Bungee cords.
Parameters (theoretical)
A = 1.220m² (4.000ft²)
BW = 58.455kHz
C = 2.4384m (8ft) 0.114-wavelengths
Cd = Cl = 36.58pf
Cf = 48.00pf
Ct = 84.58pf
Cs = 5.294pf (14.448MHz)
h = 1.074m (3.524ft)
Il = 1.335A @ 15W
l = 21.414m @ 14.000MHz
L = 22.92uH
Ll = 0.5118cm (0.2015")
Ls = 60.96cm (0.6096m) 24" (2ft)
n = 47.24% (efficiency)
N = 3-turns
Q = 239.5 @ 14.000MHz
Qc = 300 @ 1.000MHz
Rl = 2.220-ohms (loss R)
Rr = 1.988-ohms (radiation R)
Rw = 0.0925-ohms/ft (ac R of wire)
Vc = 2.691kV @ 15W
Wd = 1.024mm (0.0403") od
Ws = 1.024mm (0.0403")
Wl = 7.317m (24.000ft)
Xl = 2016-ohms
Schematic
