A SMALL AND COMPACT HYBRID ANTENNA SYSTEM FOR 144 MHz

Slobodan Bukvić, YU7XL, http://www.qslnet.de/member/yu7xl/

My recent VHF antenna was a system consisting of sixteen 8-ele yagi antennas, each 2WL long, my own design. Before it, I was using 4x14 ele yagi by YU7EF, each 4.5WL long.

There are many pro et contras for each of these antennas.

The 16-antenna version had higher gain, but higher temperature, too. The main disadvantage is that this configuration requests 5 power dividers (3 at least), which means more energy loss and failure possibilities.

I found the best advantage of 16 small antennas is their behaviour under windy condition. Antennas are self supporting, showing good balance and resistance against wind. Without practical experience I could not believe this, but I was using this antenna system for several years with no wind problems at all.

An American EME ham, getting known that I worked with 16 short antennas, told me: `much aluminum…..`. I disagree, such a system does not contain more aluminum, and is not more expensive at all, than a system of 4 long antennas.

Now I want to describe a realy small antenna system for those who like `do it yourself` building. The antenna is as short as 2785 milimetres, which allows using plastic boom (fishing rod). Such antennas can be mounted in front of supporting frame. Therefore, the antenna can easily be switched between H and V polarisation, keeping good performance in both planes. Finally, fishing rod does not require boom correction, which means good reproducibility. This is also preserved with good broadband antenna characteristics.

ANTENNA QY20806XL4D3

Remarks:

Radiator is made in trapeze (LFA) form, where feedpoints are located on the center of De2 , with 10 mm gap which should be deducted from the complete radiator length, as the following figure shows:

ANTENNA QY20806XL7D3 IN STACK

Every individual antenna has its own stacking properties which cannot be forecasted, neither by someone`s thinking nor by experience. The best way to choose the best stacking distances is to make a stacking table by NEC/EZNEC. The table for QY20806XL7D3 is as follows:

When planning an antenna system you have to look at the table and to choose whether you want to get the best temperature or the maximum gain, or something in between. I suggest choosing the best G/T ratio. Of course, one can be limited with available space in his house yard. If so, it is a good idea to see stacking tables for various antennas and to choose the antenna first. Keep in mind that too small distances can cause SWR changes.

Note, the table becomes invalid if anything in antenna dimensions has changed.

ANTENNAS QY20806XL7D3 IN VERTICAL STACK

As you see in the stacking table, the minimum temperature for this  antenna in vertical stack is around 0.55λ, and this distance is common for most antennas. Such a small distance is not payable because the associated gain is 1.65 dB only over a single antenna. Therefore, we have to choose the distance giving higher gain, sacrificing temperature in a certain extent. Here I choose the distance which gives the best G/T, and it is 240 cm.

Here are the results:

A)    Two antennas verticaly

B)    Four antennas verticaly

Now I took the stacking distance to be 250 cm. Therefore, the total height of the antenna system is 7.5 m.

ANTENNAS QY20806XL7D3 IN H-STACK

Horizontal only stack is not used, even, not recommended, because it requests more space and more work for the same results than with vertical stack. Furthermore, antenna temperature is unacceptable high within small and moderate stacking distances. Instead, there are usualy H-stacks consisted of two vertically and two horizontally stacked antennas. I choose vertical distance of 240 cm and horizontal of 310 cm. This should give the maximum G/T. (Actually, the stacking table says that maximum G/T is at 360 cm, but there is no gain increase above 310 cm).

A)    Four antennas in H-stack, V=240cm, H=310cm

B)    Eight antennas in H-stack, V=240+240+240cm, H=310cm

Note, this system is as small as 7.2x3.1 metres

C)    Sixteen antennas in H-stack, V=250+250+250cm, H=250+250+250cm

In order to take the maximum of short sixteen antennas, vertical and horizontal stacking distances are chosen to be 250 cm. Now you have a system of 7.5 x7.5 meters from boom to boom, giving quite respectable gain.

ANTENNA QY20806XL7D3 FOR X POL

A big advantage of hybrid antennas is their easy adaptation for cross polarisation. Quad/oblong loops are by their nature ready to work in both polarisation planes. You only have to build another set of yagi elements and to put them on the boom in perpendicular positions.

Though not quite necessary, I wanted to optimize vertical elements additionally. As seen from the table 8, vertical elements are placed 20 milimetres shifted in X-axe. This finally have a bit lower gain, compared to horizontal part of the antenna, but better temperature and, even, better G/T. The working bandwidth became bigger, too. Of course, this what is considered horizontal can be turned to vertical and opposite, the choice is yours.

CONCLUSION

This antenna is modelled for different purposes and could be used as a powerful system for  EME, as well as for normal DX-ing or contesting.

Anyway, feeding coaxial cable 50 Ώ should be connected to feedpoints, together with λ/2 balun, 1:4 ratio, because the antenna impedance is 200 Ώ. If the XPOL version used, two coaxial cables are needed, and they must be positioned behind the antenna, out of radiation fields. Also, keep in mind that XPOL elements must cross each other in their centerlines!

I tried also a simulation of the loop elements of this antenna in diamond quad form (see figure 19) , and got a bit better performance. I did not include it, wishing not to make the article more complex and confusing, and it is available by e-mail request from the authoe.

Note: Currently the author is constructing the here described system with 16 antennas and will test it on EME when ready. All data in this artickle are basing upon computer simulations.

16-stack at 260x280: G=24.78 dBi, TA=234.0 K, G/T=+1.09