Trap dipole 80m to 10m

The table above illustrates the trap dipole construction details. The listed variable of "wave", equates to each arm of the antenna design, that is in the overall case, the trap dipole antenna is two wave lengths in width, from the left hand end to the right hand end. The variable "length", equates to the effective length of each arm of the trap dipole antenna.

The table below relates to a 300 ohm feed line, as each section side needs to be 600 ohms so when connected as a dipole, the complete assembly act as a 300 ohm antenna. The 300 ohm to 50 ohm balam thus matches the trapped dipole to a 50 ohm feed line.

The physical width of the trap dipole is 4 metres, each arm of the trap has constructed from two 1 metre wire section, laballed as each part "1st sec" and "2nd sec", with a parallel tuned trap circuit between each 2 metre wire section on each arm of the overall trap dipole. Depending upon the values of "Cshunt" and "Lcoil", component values for the trap circuit design, so the complete antenna behaves differently. With the values shown in the above results list, the max SWR values is around 1·65, equating to 82ohms antenna load for a 50ohm coax line feed after the 300ohm to 50 ohm balam. Increasing "Cshunt" values will reduce the top line SWR values, and stay below SWR = 1·5, but the low end frequency span for the trap is the 80m band, at 3MHz the SWR value of 0·79 equates to just under SWR = 1·3, but at 4MHz equates to a near good perfect match of 50ohms. Each half of the trap dipole relates to a near or more than a full wave, such as on the 10m band, the trap dipole antenna equates to two wavelengths. The 80m band is similar, but the 40m and 20m band equates to a near three wavelengths overall.

The trap dipole antenna is impedance matched by a 1:6 balam requirement, a design circuit found on the web is shown below.

The 1:6 balam can be replaced by a 1:1·5 balam (50Ω unbalanced to a 75Ω balanced), followed by a 1:4 balam increasing the radio set loading match from 75Ω to 300Ω, the 300Ω load impedance equating to the antenna designed load impedance to the radio set.

Text Box

10 Cshunt = 6.8E-12

20 Lcoil = 27E-6

30 segments_A = 1.0

40 segments_B = 1.0

50

60

70 PRINT

80 PRINT TAB(15);"trap dipole to 300 ohm ribbon feed line"

90

100 PRINT

110

120 PRINT

130 PRINT TAB(15);"Loading trap antenna design spec's for each side arm of the dipole"

140 PRINT TAB(15);"1st sec. = ";segments_A;"m "

150 PRINT TAB(15);"Cshunt = ";Cshunt/1E-12;"pF";" Lcoil = ";Lcoil/1E-6;"uH "

160 PRINT TAB(15);"2nd sec. = ";segments_B;"m"

170 PRINT

180 PRINT TAB(15);"performance spec's"

190 PRINT

200 PRINT TAB(15);" freq ";TAB(33)"eqiv. length";TAB(61);" SWR ";TAB(79);"300 ohms ribbon feed";TAB(108);"balam 50 ohms coax feed"

210 PRINT

220

230 FOR f=1 TO 30 STEP 1

240 Xc=1/(2*PI*f*1E6*Cshunt)

250 Xl = (2*PI*f*1E6*Lcoil)

260 seg_A_inductance = segments_A*0.3E-6

270 seg_B_inductance = segments_B*0.3E-6

280

290 Xseg_A = (2*PI*f*1E6*seg_A_inductance)

300 Xseg_B = (2*PI*f*1E6*seg_B_inductance)

310

320 Trap_effect_shunt = ((Xc*Xl)/(Xc+Xl))

330 Trap_effect_inductance = Trap_effect_shunt / ( 2*PI*f)

340 L_effect = Trap_effect_inductance + seg_A_inductance + seg_B_inductance

350

360 resonant_length = L_effect/0.3

370 L_antenna_ohms = 2 * PI * f * L_effect

380 SWR_imped = (L_antenna_ohms)/600

390

400 PRINT TAB(15);" freq = ";f;"MHz";TAB(33)"eqiv. length = ";INT(resonant_length * 100)/100;" m";TAB(61);" SWR = ";INT(SWR_imped*100)/100;TAB(79);"line match = ";INT(SWR_imped * 300 * 10)/10;" ohms";TAB(108);"line match = ";INT(SWR_imped * 50 * 10)/10;" ohms"

410 NEXT

420 REM *CHDIR C:\Users\helen\Pictures\HFantenna

430 REM *SCREENSAVE trap_log_wire_HF_full_wave_160m_20m.bmp 1,1,2000,1100

440