Multi-Band Fan Dipole HF antenna

The design goal of this project was to build the best possible base station antenna, given the the landscape available: standard 40+ meters lot, with few tall fir trees around. More exactly:

- preferrably omnidirectional coverage

- full legal limit 1.5kWt for possible future rig expansion

- flexible support by trees, with no tower

- reliability, both mechanical and electrical

- single coax obviously

- not relying much on tuner, although tuner is a part of rig

- one man should be able to bring antenna down for maintenance and lift it up

Obvious choice was the multi-band fan dipole. Omnidirectional coverage is impossible with the ideal dipole, but inverted-V design in real environment helps. Off-Center-Fed dipoles were excluded, because on higher bands they have dead zones, which not many hams mention. Originally antenna has been built as the five-band dipole, then two top bands were removed after one year of operation.

Tri-band inverted-V fan dipole: final design, operational now in 2017 (click to enlarge):

Final dimensions of the current tri-band dipole. Risers are those pigtail pieces of wire, connecting the wing with balun (they are included in the total length).

Band

final length of both wings together, in cm

left wing

left riser

right riser

right wing

80 m

3734

1800

67

68

1799

40 m

1958

955

24

24

955

20 m

1023

478.5

33

33

478.5


Dimensions of wire spacers have been calculated in Excel. This is the no-scale view of the right wing of antenna, numbers are drill points (see further), measured from the spacers top:

Spacers have been cut from the plastic pipes, with caps on top (not on the bottom). The first two spacers are stronger than others, because they have to route all wires to the balun. Flex-Weave #12 AWG wire is used for all bands, although for future builds only 80 m wire can be #12 AWG, others can be thinner, at least #14 AWG or less. 80 m wire carries all the load. Especially in my case, when it was no central support initially, so all wires of all bands were hanging on 80 m wire. It was actually U-shaped dipole, "Non-Inverted U".

Thick 5/16" Dacrone rope is used to lift antenna up and hold it year around. For one-man operation, this rope must be thick. It was brought to the tree top with a slingshot.

This spreadsheet shows the drill points, measured from the top of spacers. It was taken as granted from some website, that minimal distance between wires must be at least 5.5" (135 mm), to have not-so-narrow bandwidth. With caps height of 16 mm, excluding the first two of 21 mm, the following table has been created. It matches the no-scale right wing drawing above. The bottom line shows the total length of each spacer - where to cut it off.

With a lot of work for 80-40-20 wires, it was decided to try 10 and 6 meter bands as well, since it was very easy to add those short wires. So with 5 bands, the distance between wires was 5.5". Later, 10 and 6 meters wires have been cut off, so distance between wires now is 11". This can be done right from the beginning.

Spacer number

1

2

3

4

5

6

7

8

9

10

11

12

13

X coordinate, m

0.00

0.80

1.65

2.50

3.30

4.15

5.00

5.83

6.67

7.50

8.34

9.17

10.00

Band

X interval, m

0.800

0.850

0.850

0.800

0.850

0.850

0.834

0.834

0.834

0.834

0.834

0.834

80 m

space from 80m wire, mm

0

0

0

0

0

0

0

0

0

0

0

0

0

to drill, mm:

21

16

16

16

16

16

16

16

16

16

16

16

16

6 m

space from 80m wire, mm

135

118

101

shifted 1/4

to drill, mm:

156

134

117

20 m

space from 80m wire, mm

270

248

225

202

181

158

135

to drill, mm:

291

264

241

218

197

174

151

10 m

space from 80m wire, mm

404

377

349

320

shifted 3/4

to drill, mm:

425

393

365

336

40 m

space from 80m wire, mm

539

507

472

438

406

371

337

303

270

236

202

168

135

to drill, mm:

560

523

488

454

422

387

353

319

286

252

218

184

151

total spacer length

576

539

504

470

438

403

369

335

302

268

234

200

167


Final design works on 3 bands: 80-40-20, as Inverted-V Fan Dipole. Central support rope significantly increated the height of the centrtal point to 18.5 meters (60 feet). Right wing goes even higher, left one lower, to 10 meters fir tree.


Antenna survided all winter storms, not changing SWR much.


Snowfall didn't bring it down.


Antenna is quite visible from 2 blocks away:


This current balun was used. It doesn't carry the mechanical load.

These are SWR charts: overall for three bands, and separate for 80 m band.

SWR for 40 and 20 m bands


Overall, this dipole noise is lower than from vertical HiQ with 102" whip. Also ambient noise was checked by the full power shutdown in the house. This is typical spectrum at my QTH on 80 m. 


40 m band noise at full powerdown:


20 m band noise at full powerdown:

Historical information about the five-band fan dipole:

It was interesting to tune the five-band fan dipole. Top 10 and 6 meters bands were operational on this antenna, as confirmed with local club members. Although during a year, not much activity was happening on those two bands. And also much more efficient directional antennas are available for 10 and 6 meters. So it was decided to cut off those wires, to make the central part lighter, and also lift it much higher with the central rope. U-type of dipole has been changed to Inverted-V Fan Dipole, which was supposed to make better coverage of areas along the dipole axis.

Final dimensions of the former 5-band dipole. Risers are those pigtail pieces of wire, connecting the wing with balun (they are included in the total length).

Band

total length for 5-band antenna, in cm

left wing

left riser

right riser

right wing

80 m

3788

1820

73

77

1818

40 m

1970

960

28

33

949

20 m

1041

473

50

42

476

10 m

526

236

27

36

227

6 m

311

101

54

50

106


Five-band Non-Inverted-U fan dipole: final design, non-operational now (click to enlarge):


Overall view of 5-bands non-inverted U fan dipole:


From another angle:


Right wing (from the back):


Central part:


Both central spacers, balun and coax have been supported by the rope and finally by the 80 m wires. In more details:


Originally, five-bands dipole has been tuned this way, this is SWR chart:

SWR for all bands and separate for 80 m:

SWR for 40 m and 20 m:

SWR for 10m and 6 m:

"Tri-band Inverted-V Fan Dipole" antenna proved to be the reliable most important part of the base station. With 100Wt Yaesu FT-991 and 500Wt Elecraft K3s it brings a lot of fun to work in competitions and everyday DXing and ragchewing.

As a future development, with the deployment of HexBeam for 20 m band and higher, maybe the central 20 m wire will be cut off from the dipole, leaving 80 and 40 meters only.