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Three Band Dipole System




 
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Author Topic: Three Band Dipole System  (Read 11037 times)
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Steve - K4HX
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« on: April 06, 2016, 10:57:19 PM »

There was a nice article in the March 16 issue of QST for a 3-band dipole system, ďA Compact Multiband DipoleĒ, by Zack Lau, W1VT. The system covered 40, 20 and 10 meters by utilizing a specific length of wire for the dipole and a matching section of commercially available ladder line that used #14 wire for the conductors. This line has an impedance of 359 Ohms (although is it often called 450 Ohm ladder line).

I got to thinking that since 10 meters is less and less useful for AM operation with each passing day, and in general, most AM operations seem to take place on 80 and 40 meters, could this design be scaled up to cover 80, 40 and 20 meters. All I did was double all the dimensions from the original article. Iím guessing that this is not optimal, but does produce good and usable numbers.

The doubled dimensions are 96 feet for the dipole (48 feet per leg) and 40 feet for the ladder line matching section. I modeled the antenna with NEC2 at a height of 50 feet over average ground (dielectric constant 13 and conductivity 0.005 mho/m). I used the W9CF transmission line and tuner calculators to run the numbers from the NEC simulations. I assumed another 40 feet of RG-8 coax would be needed in a typical installation to get back to the shack. In most cases, a tuner would be needed on 80 meters. I used a typical C-L-C type with both caps as air variables with a max capacity of 250 pf and the coil rated at 30 uH and a Q of 200 (not a monster tuner) Below are the results.

80 Meters

Frequency 3.705 MHz
Dipole Feedpoint Z: 33.718 - j460.93

Z at the end of 40 feet 359 Ohm line: 16.3 + j30.72
Line Loss: 1.05 dB

Z at the end of 40 feet of RG 8: 49.98 - j78.41
SWR: 4.14
Loss: 0.15 dB

Total Feedline Loss: 1.2

Tuner loss: 0.2 dB

Total system loss: 1.4 dB


Frequency 3.880 MHz
Dipole Feedpoint Z: 37.771 - j392.19

Z at the end of 40 feet 359 Ohm line:  21.92 + j76.7.
Loss: 0.87 dB

Z at the end of 40 feet of RG 8: 9.89 - j34.91
SWR: 7.28
Loss: 0.24 dB

Total  Feedline Loss: 1.11 dB

Tuner loss: 0.7 dB

Total system loss: 1.35 dB



40 Meters

Frequency 7.150 MHz
Dipole Feedpoint Z: 269.17 + j817.75

Z at the end of 40 feet 359 Ohm line: 42.85 - j10.89
Loss: 0.26 dB

Z at the end of 40 feet of RG 8: 51.73 - j13.56
SWR: 1.3
Loss: 0.22 dB

Total Feedline Loss: 0.48 dB

Tuner loss: 0, none needed

Total System Loss: 0.48 dB


Frequency 7.290 MHz
Dipole Feedpoint Z: (297.550 + i 889.340)

Z at the end of 40 feet 359 Ohm line: 41.13 + j13.00   
Loss: 0.29 dB

Z at the end of 40 feet of RG 8: 36.81+ j5.56
SWR: 1.4
Loss: 0.22 dB

Total Loss: 0.51 dB

Tuner loss: 0, none needed

Total System Loss: 0.51 dB



20 Meters

Frequency 14.300 MHz
Dipole Feedpoint Z: (95.708 - i 289.330)

Z at the end of 40 feet 359 Ohm line: 61.84 + j1.63
Loss: 0.37

Z at the end of 40 feet of RG 8: 48.36 + j9.38
SWR: 1.22
Loss: 0.33 dB

Total Feedline Loss: 0.7 dB

Tuner loss: 0, none needed

Total System Loss: 0.7 dB


Yes, a choke balun (1:1) is needed at the junction of the ladder line an coax. So there will be a little more loss there, but if a good one is used the loss will be minimal.

So, you get three bands and no monster tuner needed. For those with limited space, the design is 20-30 feet shorter than a full-sized half wave dipole on 80 meters. Not too shabby. Losses could be reduced further with home-brew open-wire line (I guess 359 Ohm or nearly so could be made) and RG-400 or some coax with less loss than RG8. Anyway, something to think about, if you want three bands with one set of wire and modest tuner requirements.

Please discuss in the Technical section at the link below.

http://amfone.net/Amforum/index.php?topic=41575.msg302570#msg302570
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