MY HYBRID LFA YAGI OBLONG ANTENNAS FOR 144 MHZ

            As you  see, I have changed this page again. The main reason for this is that I want to meet  the new standards established by the famous VE7BQH G/T table. Therefore, all antennas are now redesigned and put into the Table easily, for comparison purposes. Also, more details for each particular model are included, specially about stacking and X-pol.

Something on working bandwidth (for SWR 1.5) should be said. It must be as bigger as possible. This way, minor errors in constructive dimensions will not have any important impact on antenna performance. Believe me, all favours which a low bandwidth antenna offers, can easily be lost. Not only by constructive errors, but by surrounding things on the mast, too. Big bandwidth antenna gives much better reproducibility and expected characteristics.

Since very long ago, discussions on yagi versus loop were started, having not been finished yet. Two element quad equals a three element yagi. It was estimated that adding more elements does not show any important difference between them. No, not at all! You can see my pages with quad-oblong antennas, and find out that loop antennas still have better characteristics with bigger number of elements.

Some modellers tried to combine yagi and loop elements, and the quagi and other antenna forms were born. However, the difference between a classic style yagi and them remains not so big.

But, if you take some first elements (reflector, radiator and two to five directors) to be yagi elements, and all other elements to be loops, the difference becomes bigger. This page is devoted to them, to the hybrid yagi-oblong antennas, shortly: the hybrids.

Some can explain the good characteristics of the hybrids in a scientific manner. There is an area of exciting in every antenna, followed by waveguide area, then matching area... Whatever, you can find the hybrid models in the list enclosed down and compare them to some yagi models of the well-known modellers, such as YU7EF and G0KSC, and others.

Some models in the list have LFA radiators, some have classic open or folded dipole. I have chosen their impedance to be 200 Ohms, since this impedance can easily be matched and symmetrized to 50 Ohm coaxial feeding cable. There are some models with 50 Ohms impedance, and this is separately indicated.

Bellow you can find the list of my hybrid models for 144 MHz. Detailed data can be obtained by clicking on the hyperlink of the particular antenna. The models are listed in order of boom length, where you can find the number of all elements, and, the boom length in milimetres, followed by the antenna performances corrected by KF2YN convergence correction. NOTE: these data are given with losses included for aluminum and this explains the difference in EZNEC diagrams. Finnaly, these data are achieved on the central frequency of 144.100 MHz, and you can see the impedance on 144.100 MHz. The antenna bandwidth is indicated by SWR value on 145 MHz.

My marking principles are: the first letters (QY) stand for Quad-Yagi, followed by the number(s) giving the band (2 meters in this case) and the number of elements (two digits). The next two digits stand for the element thickness. The letters XL are a part of my call sign, followed by serial number of the model and, finally, the letter D with a number, showing how many loop elements there are.

My basic modelling principles are:

·        every model should cover the whole 144-146 MHz band with SWR less than 1.5,

·        every model should have vertical side lobe suppression better than 17 dB (as advised by DL6WU, for   good stacking),

·        every model has LFA shaped radiator and 200 Ohm impedance (except separately indicated).

LFA radiator has advantages regarding classic open or folded dipole. It gives much better opportunities for modelling, but also better results because all elements are in line within this configuration. This is particularly good for XPOL.

Not only better characterics are obtained with hybrid antennas. Such antenna can very easily be built for combined horizontal/vertical polarisation, or for circular polarisation. You have just to make another set of vertical yagi elements. Oblong directors by their nature are ready to serve both polarizations. Every antenna model is checked for XPOL in the way that vertical yagi elements are mounted 20 mm back on the boom, behind horizontal yagi elements. Besides, for each particular antenna some more simple corrections are given just to bring SWR to 200 Ohms exactly (usually, on DE2 and position of D1 or D2.

 WHAT IS AGT?

NEC/EZNEC, after having computed characteristics of an antenna, starts checking these data validity. Gain of antenna is now calculated from the formerly computed radiation pattern. If no material loss, these two values for gain must be equal. The equality depends on the segmentation density. Therefore, precision of all calculation directly depends on the proper segmentation for all antenna elements.

The segmentation density is defined as conservative segmentation on 350 MHz, as established by DG7YBN. Since bent elements (LFA radiators and oblongs) make some convergence problems to NEC/EZNEC, KF2YN correction is applied after calculation.

STACKING

            Two antennas stacked should theoretically give 2 times higher gain (3 dB). DL6WU and others found that this is not possible in practice and the maximum obtainable gain is 2.4 to 2.5 dB.

            What is the reason for this? Increasing the distance between two stacked antennas, more energy is concentrated into the main lobe and more gain is produced. In the same time, more energy is going into the side lobes, and this energy is lost for our purposes. The main lobe becomes narrower, but the side lobes become denser and fatter, resulting in higher antenna temperature.

            All antennas designed 30-40 years ago have high antenna temperature. Therefore, stacking such antennas at big distances, at one point give unacceptably high antenna temperature – and this is the reason why the theoretical value of 3 dB cannot be achieved.

            These days modellers pay much more attention to antenna temperature. Such low temperature antennas produce their associated gain which is very close to 3 dB (or 6 dB if four antennas are considered).

             Sometimes, you can see the data showing the gain yield of more than 6 dB! No, this is not error! The total gain is up to 6 dB. The rest is obtained by shifting the complete antenna characteristics down in the frequency due to surrounding impacts, so the area with higher antenna gain comes to the calculated frequency.

I would not recommend taking the stacking distance for maximum gain, but for maximum G/T. This small gain difference between maximum gain and maximum G/T is paid with significantly narrower main lobe!

However, you can see that, within my models, the best G/T ussualy appears together with the maximum gain. Even, especially with very long antennas, the lowest antenna temperature is achieved together with the highest gain!

 OBLONG ELEMENTS

I think the best loop shape is the circle. Modelling with round loops is almost impossible in NEC/EZNEC. The next good shape is the square. I have chosen the rectangle which must be in shape as close as possible to the square.

In the detailed files for a particular antenna, you can see the table with antenna element dimensions. All oblong elements of a particular antenna have the same height, therefore the horizontal side dimension is different and is given in the table, marked DQ1, DQ2.... The complete length of an oblong element is, therefore, 2 x height + 2 x DQ. When building oblong elements, you can bend them with a certain radius, but keeping in mind that bending should not make the complete length extended or shortened. Therefore, an oblong element should be cut in one piece to the complete length, and then bent.

Oblong elements are mounted around the boom, with suitable plastic holders.

 YAGI AND LFA ELEMENTS

Keep in mind that the boom correction is not included. For oblongs this correction is not needed, but yagi elements are very sensitive to correct length. Since there are no absolutely precize boom correction formulas, the best way is to avoid the impact of the boom, by mounting elements on isolators at least 10 mm high over the boom. If you realy must mount them not isolated, apply the SM5BSZ formula for boom correction or find data on YU7EF web site.

Regarding LFA radiator: In the detailed info of an antenna you can see De1 and De2 dimension and the distance between them. You have to calculate the total length of the LFA radiator by yourself:

Total length = De1 + De2 + 2 x V[(De1/2-De2/2)² + d²]

In this length, a feeding points gap is included (usually 10 mm). The calculated total length should be shortened for 10 mm

The feeding point is usually placed on De2.

CROSS POLARIZATION

As mentioned before, just another set of yagi elements have to be placed in the perpendicular plane, 20 mm back on the boom.

There is another approach to avoid impact of the boom and boom correction: just the oblong elements are mounted on the metal boom. Since the length of the boom part with yagi elements is not too big (2 – 3 mtrs), it is possible to use a good fishing rod for this part of the boom – keeping all elements in centerline.

The next problem that must be solved is: both coaxial cables must be lead beyond reflector, not alongside the boom. The cables must not interfere with both horizontal or vertical radiation zone of antenna. (This applies to antenna mast and holders, too, which have to not be of metal, either).

 HYBRID ANTENNAS FOR 144 MHz – YU7XL

Models using 5 mm thick elements

Models using 6 mm thick elements

Models using 8 mm thick elements

Models using 10 mm thick elements