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A multiband yagi antenna

Design en construction ON4ANT-ON4GG  

 

As most of us know, the monoband yagi is by far the best antenna choice. The majority of 

hams have unfortunately no room to put up several towers for all the different monobanders. 

The average ham chooses a trapped multi-band yagi. This antenna type allows him to be 

active on a number of bands,but it has some drawbacks as well ,loss of swr bandwidth,

antenna gain and F/B ratio.

 

Over the past years a number of commercial interlaced designs have been available. These

designs often put 2 bands on a same boom. These interlaced yagis often give a good result 

and can be an excellent replacement for the trapped yagis.

(The W4RNL web site carries an interesting article about these interlaced yagis).

Struggling to get a number of bands with good swr bandwidth and gain on a single boom made 

me decide to develop the antenna described here. The basic principle is to put a number of mono 

band yagis on the same boom, one in front of the other. The first conclusion is that the boom 

length increases rapidly, especially if one wants to cover 20 to 10 meters. The boom length was

limited to 15 meters with an option to shorten the boom to 12.8 meters.This should allow most of 

us to reproduce the design. Those having plenty of room can go for the long design 18.3m(60ft)

boom. The antenna covers the 20 to 10-meter bands. The design has been done with the help of 

AO*, YO*,EZNEC/4*, STRESS*, and YAGI DESIGN*.The electrical design can be found in part 1

It gives full details about element lengths and spacing. The feed-point impedance, free space gain

and swr bandwidths are also given. Two modified designs are described as well. Part 2 gives mechanical 

details, including the tapering detail, wind survival and total wind load. The last part gives you a table

with gain figures from the most common available mono band yagis. These designs have been verified

with the same software as used for designing this antenna. Trapped yagis have not been taken into 

account as these show less gain than their mono band counterparts and usually these trapped yagis

have unrealistic gain figure claims.

Part 1

The basic calculation has been done for an antenna in free-space and all values are in dBi. We donít 

take into account the influence of the earth ground gain, and the reference antenna is an isotropic 

radiator. (0 dBd = 2.15 dBi). If one takes into account the ground gain (as most manufacturers do)

the gain figures will be 4-5 dB higher. However this in influenced by the antenna height. The setup

above real ground will ch ange the radiation pattern. The table gives the element length for a constant 

diameter (20mm) and the element spacing.

Element length (m)

Description

Position (m)

5,45 Reflector 20 0,00
5,20 Driver 20 2,00
4,90 Director 20 3,60
4,15 Reflector 17 & dir 20 5,25
4,02 Driver 17 6,20
3,80 Reflector 15 and dir 17 7,20
3,395 Driver 15 8,40
3,02 Reflector 12 & dir 15 9,50
2,91 Driver 12 10,30
2,78 Reflector 10 & dir 12 11,60
2,55 Driver 10 12,45
2,355 Dir 10 13,40
2,265 Dir 10 15,00

 

What is to expect from this antenna?

Gain is comparable to a 3-4-element monobander, with excellent SWR bandwidth and F/B.

Antenna specifications.

Frequency Gain (dBi) Impedance F/B SWR
14.000 8.1 33.0-j4.1 26.8 1.26
14.175 8.2 30.9+j3.0 29.1 1.00
14.350 8.3 26.0+j12.2 25.9 1.44
18.068 8.1 20.9-j3.6 21.5 1.10
18.118 8.6 22.3-j2.3 22.3 1.00
18.168 8.6 23.5-j1.2 23.2 1.07
21.000 8.4 32.4-j7.8 21.1 1.27
21.200 8.5 34.2+j0.5 21.0 1.00
21.400 8.6 35.7+j8.1 20.9 1.25
24.880 8.5 10.7-j3.6 30.6 1.19
24.940 8.5 10.8-j1.7 30.6 1.00
24.990 8.5 10.8+j0.1 28.0 1.19
28.000 7.9 26.0-j7.2 29.7 1.47
28.350 8.1 26.9+j3.1 25.7 1.00
28.700 8.2 27.6+j13.9 22.3 1.48

 

This design has an almost constant gain over the 5 bands. The swr bandwidth is excellent over the entire

range with exception of 10 meters; here it is limited to 28.8 MHz. Of course this swr is in reference to the 

matching frequency. Iím sure that things still can be improved, but this may have a negative influence on

swr bandwidth and/or F/B. Another disadvantage of getting the last .5 dB out of the design makes it more

critical and less tolerant for small dimension errors (element lengths and spacing). If you really want more 

gain, go for the longer design on the 18m boom.You will get the same bandwidth and F/B (or even better) 

with higher gains.  

Variant 1

A 15-meter boom too big for you? Perhaps this 12.8m antenna is the solution. There will be one element less 

on 20m. The gain will drop to about 7 dBi, which is still good. Only 20m changes, they other gain figures remain.

Antenna specifications

Element length (m)

Description

Position (m)

5,45 Reflector 20 0,00
5,20 Driver 20 2,00
4,15 Reflector 17 & dir 20 3,05
4,02 Driver 17 4,00
3,80 Reflector 15 and dir 17 5,00
3,395 Driver 15 6,20
3,02 Reflector 12 & dir 15 7,30
2,91 Driver 12 8,10
2,78 Reflector 10 & dir 12 9,40
2,55 Driver 10 10,25
2,355 Dir 10 11,20
2,265 Dir 10 12,80

 

Frequency Gain (dBi) Impedance F/B SWR
14.000 7.2 33.5-j11.6 16.0 1.40
14.175 7.1 39.8-j0.9 29.1 1.00
14.350 7.0 45.3+j9.0 14.3 1.30

 

Variant 2

Do you have plenty of room? This 18.3m monster is the solution. It gives you higher gain on the top 3 bands 

with an excellent bandwidth

Antenna specifications

Element length (m)

Description

Position (m)

5,45 Reflector 20 0,00
5,20 Driver 20 2,00
4,80 Director 20 3,60
4,15 Reflector 17 & dir 20 5,25
4,02 Driver 17 6,40
3,80 Reflector 15 and dir 17 7,20
3,395 Driver 15 8,40
3,02 Reflector 12 & dir 15 9,50
2,91 Driver 12 10,80
2,68 Reflector 10 & dir 12 12,00
2,55 Driver 10 13,014
2,47 Dir 10 13,816
2,44 Dir 10 15,775
2,31 Dir 10 18,25

 

Frequency Gain (dBi) Impedance F/B SWR
14.000 8.2 31.9-j0.4 30.0 1.28
14.175 8.2 29.6+j6.9 27.8 1.00
14.350 8.5 25.3+j15.8 23.3 1.43
18.068 8.3 31.5-j5.1 22.4 1.01
18.118 8.3 31.6-j4.9 23.0 1.00
18.168 8.4 31.2-j4.7 23.7 1.01
21.000 8.7 33.5+j1.2 23.8 1.25
21.200 8.9 35.1+j8.6 23.7 1.00
21.400 9.0 36.5+j16 23.5 1.23
24.880 9.8 21.9+10.8 35.2 1.09
24.940 9.8 21.9+j12.7 33.2 1.00
24.990 9.8 21.9+j14.5 21.5 1.09
28.000 9.9 29.6-j9.5 25.7 1.20
28.350 10.0 32.7-j4.6 35.4 1.00
28.700 9.8 24.5-j12.5 35.7 1.49

This design made it at my home QTH. The calculated specification seem to be corresponding really well with 

the on air performance. Initial testing show an advantage as compared to a very large commercial multi band 

yagi.The design is very broadband and allows different kinds of matching.

 

Part 2

Feeding the antenna

The driven elements are all resonated in band. The actual impedance of the antenna is high enough to allow 

different kind of feeding. Personally I use a gamma match; the elements donít need to be spliced up in this case.

Seen the number of questions concerning gamma matches, here are all
calculated gammas:

The capacitor (doorknob type) is mounted in small plastic box under element with connecter on one side 

and gamma rod on second side. The voltage are lower than 500Volts for power output to 2000Watts,

however the current is up to 6.3 amps for this power ! If you can put 2 capacitors in parallel to obtain right value,

do so ! A doorknob capacitor with diameter of 20mm should handle 2 to 3 amps. I have made 2 calculations

for different capacitors, this will give you an idea on where to go is value you find is different from the one calculated.

18.3m boom A (mm) B (mm) C (pF) d (mm)
14.175 120 1128 153 4
14175 120 1067 180 4
18.118 120 596 144 4
18.118 120 562 150 4
21.200 100 672 110 4
21.200 100 562 100 4
24.940 100 702 123 4
24.940 100 867 100 4
24.940 100 911 100 4
28.350 100 462 87 4
28.350 100 361 81 4
28.350 100 387 88 4

For a number of bands several options are possible, I've tried to use standard values wherever

possible !

15.0m boom A (mm) B (mm) C (pF) d (mm)
14.175 120 951 176 4
14175 120 950 180 4
18.118 120 589 200 4
18.118 120 602 180 4
21.200 100 629 111 4
21.200 100 631 100 4
24.940 100 279 190 3
24.940 80 522 270 3
24.940 80 522 330 3
28.350 100 497 100 4

Warning !  The 24 MHz gamma sections have been modified !

12.8m boom A (mm) B (mm) C (pF) d (mm)
14.175 120 905 150 4
14175 120 950 180 4
18.118 120 589 200 4
18.118 120 602 180 4
21.200 100 629 111 4
21.200 100 631 100 4
24.940 100 279 190 3
24.940 80 522 270 3
24.940 80 522 330 3
28.350 100 497 100 4

Element mounting

One can choose isolated or non-isolated element mounting. The boom influence on the element length

is minimal. The use of isolated element can be a disadvantage is you want to use your tower as a toploaded 

vertical on 160m.(* ON4UN). The boom element plate measures 200x100mm. If you wish to mount the 

elements non-isolated you can calculate the influence of the boom on the element lengths with YAGI DESIGN*.

The calculated influence is only a few millimeters for the 20m element. As this design is not critical, one 

can use the isolated element lengths.   

Element tapering

Each element has to be as strong as possible for a minimal windload and weight so we use tapering. Most 

of the available antenna design software programs allow calculating the taper. Only a few allow calculating 

the element strength. Initially I used STRESS*, this software is used by the former Telex/Hygain company.

Afterwards I used a Belgian product, YAGI-DESIGN* by ON4UN. This package can calculate in all circumstances

 the taper of an element that complies with a given wind survival. This for the lowest possible weight and windload.

The element sag is also calculated. The calculated minimal wind survival is 160km/h, (100mph). The antenna is 

mounted on an 80ft tower on top of a 300ft hill here.

Parameters:      EIA-222-C pressure 30lb/sq ft at 86mph.

Shape factor .666

No ice-load

Aluminum 6061-T6  (yield strength 35000)

The table gives us element diameter, wall thickness, length, half element weight and length. The elements will be 

adjusted with the tip end.  Some of these elements are telescopic on the inside. All of the 20m element consist of 

3 diameters.  

Element 4-5-6

 

Element 7-8-9

Element 10-11-12-13-14

The  18.3m  boom  version  has  the  highest  weight  and wind load, 45,6 kg and  2,32 m≤ wind load.

The  actual weight  of the antenna  is function of the choosen boom diameter, the mounting plates and

all related hardware. I use a 4 inch boom ant total weight is about 60 kg.

Part 3

 

Is it all worth the trouble? Looking at the actual cost, YES.

The  price should  be  below  $800  for  the  60ft  design. The design is  non-critical and can be easily

reproduced. The gain is excellent and you will have  a big signal  on  the bands. However  an  antenna

 this size  requires a  strong  tower and  big rotator. If  you  have  the  tower and  rotator  for  it,  its an

 excellent choice.

A comparison  with  some  commercial mono band antenna gives you an idea about the performance

of this antenna. The values indicated are NOT those from the manufacturer, but those calculated with

the  design  software used  for this antenna.  Only  this procedure gives an objective view on the gain,

since all gains were computed in exactly the same way.

 

Conclusion.

This design is a valuable alternative for a 4-5 element monoband yagi, taking into account

the gain and SWR bandwidth. It is obvious that some improvements can be done, depending

on your specific needs. Perhaps you need less bandwidth. I tried to have a broadband yagi

with gain figures close too or better than the common 4 element monoband yagis.

The real gain, with the actual radiation angle is given in next table (antenna at 24m , slooping ground,

and extensive radial system)

References.

What's next?

The design has been reviewed on 2 points.

1. Redesign of the 10m section.

I increased the SWR bandwidth on the 10m section so that it covers 28,000-29,000 with good SWR.

Due to this gain has dropped about 0.4 dB on 10m.

But taking into account the real ground :

 

2. Full redesign on a longer boom.

Yes it still can get bigger.

This time the boom is 24m (80ft). And gain figures are impressive.