ANTENNAS FOR 100 POUND DXPEDITIONS 6.4

Force-12 Sigma-5 on 15 meters The match on 15 meters is under 2:1 for the entire band with the dip just before the center of the band. We must remember that these numbers are for the model of the antenna (but the modelâ&#x20AC;&#x2122;s predictions have been verified by direct measurement). -------------- LOADS (RLC Type) -------------No.

Specified Pos. Wire #

Actual Pos.

% From E1

% From E1

Seg

L

C

(ohms)

R

(uH)

(pF)

R Freq (MHz)

Type

1

2

47.00

47.73

11

Open

1.21

Open

21.2

Par

2

2

55.00

56.82

13

Open

1.21

Open

21.2

Par

3

2

50.00

52.27

12

Short

0.52

Short

0

Ser

SWR predicted by model

SWR measured by AntennaSmith

Figure 54 SWR for Force-12 Sigma-5 on 15 meters Though this antenna is relatively short for a 15 meter antenna the resistance presented is still relatively high. Assuming all this resistance is not losses, why would the radiation resistance be so high? Consider the surface area of a 16 AWG wire. The diameter of this wire is about 0.05 inches. The circumference is therefore just under 0.08 inches (0.05/2 * pi). The circumference of the one inch aluminum pipe is 1.57 inches. That is nearly 20 times the surface area of the wire! That is much more metal to radiate.

51

# Antennas for 100 Pound DXpeditions - volume 1

Computer-based antenna modeling and direct experience with lightweight portable antenna systems. Volume 1: Selected high band antennas [20-6...

# Antennas for 100 Pound DXpeditions - volume 1

Computer-based antenna modeling and direct experience with lightweight portable antenna systems. Volume 1: Selected high band antennas [20-6...