Determining Wire Sag

End fed antennas

Since end fed antennas have no concentrated center weight, the antenna sag in feet and % of the span can be read directly from the “0” center weight column on the chart.  All of the sag is produced by the weight of the wire being evenly distributed across the span.  The wire forms a catenary curve between its two fixed supports.

Center fed antennas

 

  • Step 1  For center fed antennas, determine the total weight of the feed line from the shaded area in table 2. Total weight will be the unsupported length in feet multiplied by the weight per foot shown in the table.

  • Step 2  Add the weight of your center insulator and/or balun to the feed line weight, to determine the total weight pulling down on the feed point.

  • Step 3  Go to the wire sag table and select the appropriate antenna span from the left hand column. Wire spans are rounded off to the low band edge and may be slightly longer than your actual antenna.

  • Step 4  Next, go to the “static tension” column and select the tension closest to the tension you intend to use on your antenna, either 25 lbs or 50 lbs. You can interpolate values between 25 and 50 lbs.

  • Step 5  Move across the rows to the two columns that bracket the total weight of your feed line and center balun/insulator. You can interpolate between the weight values shown in the chart or use the highest value.
  • Step 6  Read the sag depth and % of sag directly from the intersection of the “total weight” column and “sag” row.

 

Example:      102’ G5RV antenna, #12 wire

 

  • 33’ 450 ohm matching section feedline           =  33’ x  .026 lbs/ft    =  .858 lbs

  • 20’   50 ohm RG-8X coaxial cable feedline    =  20’ x  .040 lbs/ft     =  .800 lbs

  • center insulator for 450 ohm feedline              =   ---------------------  =  .170 lbs    

  • ladder line to SO239 coax connection             =   ---------------------  =  .170 lbs

                                                                                                                      _______

                                             total center weight  =  2.00 lbs

 

From the 2 lb table column, sag will be:

  • 3.6’ and 3.7% of the span @ 25 lbs tension

  • 2.0’ and 2.0% of the span @ 50 lbs tension

 

                                          Table 2

Wire breaking strength in pounds

Wire wt/ft

 

Feed line wt/ft

AWG

40%Cu clad steel

Hard drawn copper

Soft drawn

copper

Cu Flex Weave

7 strand

clad Cu

 

 

 

 

   RG-6

 

 

 .030

12

917

337

200

170

---

,0198

 

   RG-8    RG-213

.115

14

550

234

125

128

349

.0124

 

RG-8X

.040

16

381

135

70

n/a

---

.0078

 

RG-11

.092

  18

280

85

50

n/a

---

.0049

 

450 ohm

.026

 

RG-58

.029

 

RG-59

.032