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).

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 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 ch ange the radiation pattern. The table gives the element length for a constant diameter (20mm) and the element spacing.

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.

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.  

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

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

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. 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.

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

Warning !  The 24 MHz gamma sections have been modified !

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.

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.  

The  18.3m  boom  version  has  the  highest  weight  and wind load, 45,6 kg and  2,32 m² 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.

AO and YO written by K6STI.

Comparison of this design made by W4RNL with EZNEC4 shows very similar results to those obtained with AO. The W4RNL site contains lots of valuable antenna information and really is worthwhile visiting. http://www.cebik.com . My sincere thanks to L.B.Cebik for th verification of this design and the information available on his website.

YAGI DESIGN was written by ON4UN and covers all mechanical issues of antennas. It's a DOS based program, and is extremely easy to use. I wish to thank John for his help as well. Those wishing to obtain this program can always contact John, ON4UN. 

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.

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.