Magnetic Loop‎ > ‎

Assembling and Testing the Prototype Capacitor

Capacitor Assembly (1.5-2 hours)

Required tools: 
  • Fine tooth metal file or rotary file
  • 11/32" small open end wrench
  • 8 mm small open end wrench

Assembly steps:

  1. Deburr raw plates with fine tooth metal file
  2. Base
  3. Screw with nut
  4. Two washers
  5. Plate on each side
  6. Two nuts on each outside rod (thread doesn't catch)
  7. Rotor plate shaft (center) double nut - nuts thread - tighten first, then second nut
  8. Two 8-32 nuts on each of the 4 stator rods (threads do not catch)
  9. Continue steps 5 through 8
  10. When installed the desired number of rotor plates, put nuts (8mm wrench to tighten) the 4 stator rods, then install the plexiglass side piece.
  11. Assure that rotor plates are centered between the stator plates before securing the rotor shaft position with the reduction drive set screws and the nuts on the rear end of the rotor shaft.
  12. Carefully bend the plates as necessary to assure correct position and spacing throughout the shaft rotation.

Assembly of capacitor with 0.1 inch plate spacing using two 8-32 nuts as spacers (Version A)

Capacitor Version A assembly - Click to enlarge

Rotor shaft parts (left to right): (2) 8-32 nuts, rear plexiglass plate (smooth aluminum tube bearing in center hole), (2) #8 flatwashers, 7x[8-32 nut, rotor plate, 8-32 nut], 8-32 nut, 8-32 tapped ¼ inch aluminum tube, front plexiglass plate, reduction drive.

Stator rod parts (left to right): 6-32 nut, rear plexiglass plate, 6-32 nut, 7x[(2) 8-32 nuts, stator plate], #6 flatwasher, 6-32 nut, front plexiglass plate, 6-32 nut.

One 8-32 nut and two #8 washers as a 0.22 in. spacerTwo 8-32 nuts as a 0.24 in. spacerOptionally, you may use a caliper to assure regularity and uniform thickness of the spacers. Given the 0.04" thickness of each plate, two 0.12" 8-32 nuts yield (0.24" - 0.04")/2 or 0.1" rotor to stator plate spacing. One 0.12" 8-32 nut flanked by two 0.05" #8 washers measure 0.22" and yield (0.22" - 0.04")/2 or 0.09" rotor to stator plate spacing. Closer spacing would require custom precision spacers and more critical plate alignment. Using stacked #8 washers as spacers may excessively narrow the air gap between the stator plates and the rotor shaft.

Tuning Enclosure Assembly

Required tools: 
  • Drill, bits and hole enlarger
  • Screwdriver
  • Pliers
  • Diagonal cutter
  • Soldering gun & rosin core solder
1. Here is the prototype Monarch capacitor (with reduction drive attached), built by Chuck W6FT, using 7 sets of 3 inch Monarch plates.  Two #8 nuts are used to space the plates on the 8-32 threaded rod rotor shaft.  The stator plates are assembled on 6-32 threaded rod and also spaced by two #8 nuts.  8-32 tapped ¼" aluminum tubing is threaded onto the end of the rotor shaft.  The measured spacing between rotor and stator plates is 0.1". Prototype Monarch Capacitor
2. The 1.6 L Really Useful Box is drilled.  Two SO-239 connectors are mounted with 6-32 screws and nuts.  Two ½ inch PVC conduit clamps are mounted on the bottom of the enclosure with #10 hardware. Tuning Capacitor Enclosure
3. A large hole is drilled in the opposite end of the enclosure to accommodate the round brass plate of the reduction drive and the ends of the two 6-32 screws are secured to the box with #6 nuts.  (The angle bracket has been removed from capacitor and was not used). Capacitor mounted in box
4. Two 2 inch lengths of 10 AWG stranded wire are tinned and a #6 ring terminal soldered to one end of each. Prepping the leads for the connectors
5. The SO-239 connectors are removed from the enclosure, one side of each shell is prepped and tinned, and the tinned wire soldered to it using a 100 watt soldering gun as shown.  The center pin of each SO-239 is left unconnected. Lead soldered to SO-239 connector
6. One lead with an SO-239 connector is attached to each stator section of the capacitor.  No electrical connection is made to the rotor plates. Leads and connectors are attached to the capacitor
7. The connected parts are mounted inside the enclosure.  Both leads of the NE-2 neon glow lamp are attached together to the same stator section (the high radio frequency voltage present there at resonance is sufficient to make the neon lamp glow with no other connection).
The parts assembled in the enclosure
8. The antenna is tuned with a vinyl Mini Blind Wand that is secured to the reduction drive shaft with a small hose clamp.  Do not touch the bare metal shaft during transmission as any imbalance in the loop may place dangerously high radio frequency voltage on the shaft.
*Adjust the capacitor only with clockwise rotation to avoid loosening the tapped aluminum tube from the threaded shaft.
A Mini Blind Wand is attached to the reduction drive shaft with a small hose clamp for tuning the antenna

Prototype Testing

At 0.1" plate spacing, the capacitance range was measured with an MFJ-259B antenna analyzer to be 16 - 49 pF. This capacitor resonated a 96" long loop of RG-8/U cable from 17.250 MHz to 30.620 MHz. The following impedances were measured at the end of a 36" length of RG-58/U connected to a 22" coupling loop of RG-58/U:

Frequency
MHz		 SWR Resistance
ohms		 Reactance
ohms
18.1 1.1 47 8
21 1.1 55 6
25 1.3 51 17
28 1.3 46 12
30 1.4 55 18

Test transmissions at 100 watts produced no arcing of the capacitor at any frequency.

The measured maximum capacitance of this capacitor with 0.1 inch plate spacing was 49 pF compared to 53.5 pF as expected from the Monarch Capacitor online calculator. The AA5TB loop calculator predicted that this 2.55 foot diameter loop of 0.4 inch diameter conductor should resonate near 15.5 MHz at maximum capacitance, but the measured minimum frequency was 17.25 MHz which is the predicted frequency for a conductor diameter of 1 inch. If with this spacing your antenna does not tune as low in frequency as planned, the antenna frequency may be lowered by increasing the maximum capacitance by spacing the plates closer or adding more plates, or by increasing the length of the loop.

Varying Capacitance and Loop Length

We made the following changes to the capacitor (Version A) described above in order to measure the effect on the the tuning range of the antenna.

  • Version A: the capacitor as originally assembled, 7 complete plate sets with 0.1" plate spacing using two 0.12" 8-32 nuts for spacers.
  • Version B: the capacitor reassembled with 0.09" plate spacing using one 0.12" 8-32 nut flanked by two 0.05" washers for spacers.
  • Version C: Version B with another rotor plate and pair of stator plates added.
  • Version D: Version C with another rotor plate for a total of 9 rotor plates and 8 pairs of stator plates.

The capacitance ranges and the tuning frequency ranges were then measured with each capacitor for loop lengths of 96", 101.5" and 113" and are listed in the table below. You may check with Monarch Capacitors for availability if you wish to purchase additional plates for your capacitor.

 
Loop length in. (diameter ft.)
Frequency Range (MHz)
Capacitor
Version
Rotor
Plates
Stator
Pairs
Plate
Spacing
Capacitance pF
96" (2.55')
101.5" (2.7')
113" (3')
Min
Max
Min
Max
Min
Max
Min
Max
A
7
7
0.1"
16
49
17.25
30.62
16.7
29.45
15.7
27.5
B
7
7
0.09"
16
57
16.5
30.3
15.9
29
14.9
27.1
C
8
8
0.09"
19
68
15.3
29.1
14.8
27.9
13.8
26.1
D
9
8
0.09"
19
72
14.9
28.8
14.4
27.6
13.6
25.8

Typical Standing Wave Ratio Curve at Resonance

Standing Wave Ratio vs. Frequency

Capacitor Version B

Capacitor Version B (7 plate sets and 0.09 inch plate spacing)

Capacitor Version D

Capacitor Version D (8½ plate sets and 0.09 inch plate spacing)
Ċ
Magnetic Loop Antenna Prototype Testing.pdf
(52k)
River City ARCS,
May 2, 2012, 6:46 PM
v.1
Comments