Saturday, February 25, 2012

A multiband yagi antenna


A multiband yagi antenna

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, antennagain
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 feedpoint 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.
Part 3 gives a table with the gamma match details
Part 4 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 don’t 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 change the radiation pattern.
The table gives the element length for a constant diameter (20mm) and the element spacing.
Element Length
Description
Position (m)
5.45
Reflector 20
00.00
5.2
Driver 20
02.00
4.9
Director 20
03.60
4.15
Reflector 17 & dir 20
05.25
4.02
Driver 17
06.20
3.8
Reflector 15 & dir 17
07.20
3.395
Driver 15
08.40
3.02
Director 15 & ref 12
09.50
2.91
Driver 12
10.30
2.78
Reflector 10 & dir 12
11.60
2.55
Driver 10
12.45
2.355
Director 10
13.40
2.265
Director 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 22.6 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.
I’m 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.

Element length (m) Description Position (m)

5.45 Reflector 20 0.00
5.2 Driver 20 2.00
4.15 Reflector 17 and director 20 3.05
4.02 Driver 17 4.00
3.8 Reflector 15 and director 17 5.00
3.395 Driver 15 6.20
3.02 Director 15 and reflector 12 7.30
2.91 Driver 12 8.10
2.78 Reflector 10 and director 12 9.40
2.55 Driver 10 10.25
2.355 Director 10 11.20
2.265 Director 10 12.80

Only 20m changes, they other gain figures remain.

Antenna specifications.

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.
  
Element length (m) Description Position (m)

5.45 Reflector 20 0.00
5.2 Driver 20 2.00
4.9 Director 20 3.60
4.15 Reflector 17 and director 20 5.25
4.02 Driver 17 6.40
3.8 Reflector 15 and director 17 7.20
3.395 Driver 15 8.40
3.02 Director 15 and reflector 12 9.50
2.91 Driver 12 10.80
2.68 Reflector 10 and director 12 12.00
2.55 Driver 10 13.014
2.47 Director 10 13.816
2.44 Director 10 15.775
2.31 Director 10 18.25

Antenna specifications.

Frequency Gain (dBi) F/B

14.175 8.3 34
18.118 8.3 21
21.200 8.7 23
24.940 9.6 38
28.350 10.0 29


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 don’t need to be spliced up in this case.

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 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 1-2-3


1700x28x1.5 3000x25x2.5 2750x19x1.5

 
Insert 250x19x1.9

The wind load of this half element is 0.13m². The weight of this half element is 2.6kg
Element sag is 20.5cm. The tip will be adjusted.
Tip lenght ! ( 100mm overlap)
Isolated Non-isolated
Element 1 : 2567mm 2570mm
Element 2 : 2309mm 2312mm
Element 3 : 1998mm 2000mm


Element 4-5-6


1750x25x2.5 **x19x1.5

 


The wind load of this half element is 0.084m². The weight of this half element is 1.85kg
Element sag is 8.4cm. The tip will be adjusted

Total element length !
Isolated Non isolated
Element 4 : 4222mm 4224mm
Element 5 : 4053mm 4056mm
Element 6 : 3852mm 3854mm


Element 7-8-9


1500x25x2.5 ****x19x1.5
 

The wind load of this half element is 0.074m². The weight of this half element is 1.2kg
Element sag is 6cm. The tip will be adjusted

Total element lenght ! ( 100mm overlap)
Element 7 : 3450mm
Element 8 : 3082mm
Element 9 : 2957mm


Element 10-11-12-13

750x25x2.5 **x19x1.5
 


The wind load of this half element is 0.059m². The weight of this half element is 1.1kg
Element sag is 3.5cm. The tip will be adjusted

Total element length  ! ( 100mm overlap)
Element 10 : 2845mm
Element 11 : 2583mm
Element 12 : 2364mm
Element 13 : 2264mm

Wind load and weight of elements.
44.55 kg and a 2.20 m² windload.

If you choose the variant 1 you will have 0.26m² less wind load and will gain about 5.2 kg.
The actual weight of the antenna is function of the choosen boom diameter, the mounting plates and all related hardware.
My antenna uses a 4 inch boom and the weight is around 60 kg.
Part 3.
The Gamma match for the 18.3 m version
A(mm)
B(mm)
C(pF)
D(mm)
D(mm)
14.175
1067
120
180
28
4
18.118
562
120
150
25
4
21.200
735
100
100
25
4
24.930
867
100
100
25
4
28.400
361
100
81*
25
4
(3x27pf parallel)
The Gamma match for the 15 m version
A(mm)
B(mm)
C(pF)
D(mm)
D(mm)
14.175
950
120
180
28
4
18.118
602
120
180
25
4
21.200
631
110
100
25
4
24.930
522
80
270
25
3
28.400
496
100
100
25
4

The Gamma match for the 12.8 m version
A(mm)
B(mm)
C(pF)
D(mm)
D(mm)
14.175
880
120
154
28
4
18.118
598
120
200
25
4
21.200
629
100
111
25
4
24.930
279
100
190
25
3
28.400
497
100
100
25
4

For an output power of 2000 watts one should get capacitors handling 300 volts and 6.3 amps current.

 

The Hairpin match for the 18 m version
 
A(mm)
B(mm)
d(mm)
Shorten driven element
14.175
50
541
4
139mm
18.118
50
393
4
56mm
21.200
50
407
4
88mm
24.940
50
235
4
87mm
28.500
50
317
4
103mm


Part 4.
Is it all worth the trouble ? Looking at the actual cost, YES. The price should be below $800 (US) for the 60ft design.
The design is non-critical can can be easely 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 antennas gives you a 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.
dbi
Type Gain SWR bandedge Description
310-08 7.17 1.31-1.46 3el 10m on 2.3m boom
103BA 7.51 1.53-2.01 3el 10m on 2.3m boom
153BA 7.68 1.45-1.68 3el 15m on 3.5m boom
315-12 7.54 1.49-1.45 3el 15m on 3.6m boom
320-16 7.21 1.27-1.38 3el 20m on 4.7m boom
203BA 7.17 1.22-1.20 3el 20m on 4.8m boom
20-3CD 8.09 2.03-2.90 3el 20m on 6.0m boom
10-4CD 8.58 1.63-1.79 4el 10m on 4.8m boom
412-15 8.40 1.09-1.09 4el 12m on 4.4m boom
415-18 8.24 1.41-1.38 4el 15m on 5.4m boom
417-20 8.52 1.08-1.11 4el 17m on 6.0m boom
204CA 8.25 1.49-1.47 4el 20m on 7.8m boom
420-26 8.60 1.28-1.37 4el 20m on 7.8m boom
20-4CD 8.54 1.78-2.20 4el 20m on 9.6m boom
510-20 9.75 1.49-1.53 5el 10m on 6.0m boom
KLM510 9.21 1.33-1.43 5el 10m on 6.1m boom
105CA 8.38 1.42-1.23 5el 10m on 7.2m boom
155CA 9.70 1.49-1.62 5el 15m on 7.7m boom
205CA 9.23 1.43-1.96 5el 20m on 10.4m boom
KLM520 9.43 1.66-1.25 5el 20m on 12.8m boom

Conclusion.

This design is a valuable alternative for a 4 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 moband yagis.

The real gain, with associated radiation angle is given in next table.

14.150 13.55 dBi @ 12°
18.118 13.64 dBi @ 10°
21.200 13.74 dBi @ 8°
24.940 14.20 dBi @ 7°
28.400 13.77 dBi @ 6°


If you wish more info on this design, or want to share some of your antenna experiences, you can always email me ON4ANT@hotmail.com

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