Yagi Design Techniques
This section shows the details of an extended boom design. The program Yagi Optimizer [YO] was
used to show the steps required to effect a real-world design. Requested by Charles, W4CO to see what could be attained by lengthening
his existing HG203BA's boom from 16' to 26'.
Stock LJ203BA Simulation Results
The YO chart shows the forward/gain (f/g), front/back ratio (f/b) and impedance (Z).
The data was entered from the assembly manual and YO was set to "W6QHS" taper algorthmn. The specs:
F/G = 7.15 dBi
F/B Ratio = 23dB Max
VSWR @ Res < 1.5:1
Boom = 16' 6"
Boom Dia = 2"
Lng El = 37' 1"
W/Surv = 80mph
Net Wgt = 32 lbs
W/Surf= 5.3 sg ft
W/Load = 138 lbs
As you see from the chart, the mid-band gain shows as 7.14dBi which agrees well w/ the specs. F/B ratio shows as 22.52dB @ 14.180Mhz. Overall, the results show well for YO's simulation capability. The feed impedance shows as 35.1 ohms w/ the imaginary part at -26.9 ohms.
The reason is simple: The dimensions stated are set for use with their Beta Match so when it's included, it should show close to acceptable SWR (1.2:1) or less. This simulation looks ok for a Beta Match.
The driven element must be shortened by apx 0.25" to effect @ 14.180Mhz match (1.01:1) using the supplied "Beta-Rods". The Dimensions are 11" wide by 23.5" long. In the "extended" design below, a more narrow HP spacing will be used b/c experience has shown that matching w/ a wide-spaced rods as supplied is problematic when the Z is lower. See the "Tapered HairPin Matching" Section at the end of this page for more details.
LJ-203BA Extended Yagi - 26' boom
Charles, W4CO, one of the original early experimenters in ESSB, was curious as to what gain could be expected if the boom of his LG203BA was extended from 16' to 26' by using the boom of a HG204BA. So this section shows what can be expected. Note that more gain can be attained at the expense of operatiing BW.
F/G > 8.9 dBi; Z > 19 ohms; F/B Ratio > 16dB; VSWR @ Res < 1.01:1; VSWR @ band edges < 1.4:1
Boom = up to 26; Boom Dia = 2.5"; Lng El = 36'; Wind Surv = 80mph; Net Wgt = 35lbs; Wind Surf= 6sg ft; Wind Load = 140 lbs
YO features a multi-level operating menu where the function in question is highlighted in an highly visible color. The black and white presentation shown here unfortunately. Other key charts can be displayed when called on.
The main operating screen shows the results of parameters manipulated so that you can instantly see the results of a modification of some parameter. This screen shows a quick summary of the yagi's parameters such as f/g, f/b ratio, feed impedance, max gain that this yagi can have are some examples.
This screen also shows the operating BW summarized by three (3) frequencies (low, mid and high) w/ a representation of the E-field pattern at the three frequencies.
At 14.190Mhz, the center frequency, it shows that yagi has 8.9 dBi f/g, 16db f/b and feed impedance of 18.5 ohms. The SWR is 1.01 at the center frequency, rising to less than 1.4:1 at the frequency extremes.
PLOT Screen Chart
Another important output screen is the "PLOT" chart. Pressing the "p" key generates the E-field and H-field plots in 'free-space'. The E-field plot shown here is well 'behaved', ie, the back lobes are small and almost of the same amplitude all away around the yagi's backside.
This chart shows the yagi's gain envelope as if you were looking down on it from the top. The plot is made in 'free-space' w/ the f/g expressed in "dbi" units; gain in dB over an iisotropic source. You have freedom to change the displayed numbers in the "OPTIONS" menu.
The beamwidth of this yagi is 60degs. This is measured from the -3db power points and can be easily read from the chart. Forward gain (f/g) equals 8.9dBi with front/back (f/b) ratio of 16dB at the frequency of 14.190Mhz.
NOTES Screen Chart
YO provides a place in the design file where important data can be recorded. I've traditionally used this space to add the design goals for this yagi along w/ important construction facts such as the construction of the Hairpin Matching section.
In this design, the basic method of increasing the forward gain is done by adding another 4' boom (actually turns out to be only 41" long b/c of the swage) section and replacing the D2 section on the D1 director. This makes the boom 26' long. The Hairpin matching section key dimensions are included so you know where to set the sliding short.
Also the boom to element "bracket" is modeled since it is very large effect b/c each element mounts inside of this bracket. Thus the inductance presented by each element is different than the actual length of the element itself.
ELEMENTS Screen Chart
This screen shows the untapered equivalent dimensions after YO translates the tape schedule into the program. The element spacings are shown as well as the aforementioned equivalent electrical lengths of each element.
With this screen, you can manually move elements or make length adjustments. Any changes made here will change the f/g, f/b, Z as well as the taper schedule. Small changes are ok.
The useable length of the boom length is 26' and the length of the yagi in "lambda", wavelengths. This is 0.38 Lambda antenna.
MATCH Screen Chart
If your starting design using a Hy-Gain product, you normally would use a "hairpin" matching section. Hy-Gain called it a "beta-match" but it's nothing more than a hairpin w/ a different name! I prefer the insulated driven element b/c the final pattern is much cleaner and more symmetrical than a "gamma-match" fed yagi.
The MATCH chart is accessed by pressing "M" on the keyboard. When displayed, the inset chart shows the individual parameters required to ensure low SWR at the yagi's center frequency (14.190Mhz). The driven element is reduced in length to make appear as a capacitive element such that the inductive "beta-rod" can cancel this to produce a 50 ohm feed impedance.
The key parameters of this match are the "length", 13.75" as well as "spacing", 8". The driven element length is manipulated such that the SWR is reduced to unity or close to it (1.01:1) when the proper driven element length is attained simultaneously with the hairpin rod length and spacing. The leads [#10AWG = 0.125" dia] from the balun [Balun Designs] s/b 5" in length as shown and the capacitance loading of the driven element boom-to-element bracket is 50pf shunt capacitance.
Please note that YO's hairpin match calcs assume a 'constant' length of spacing (5.25") from the driven element to the shorting point. So this dimension will always be longer than my linear taper section. It makes the transition from the driven element (11.25") attachment point to the parallel section of HP line (5.25"). The YO hairpin match dimensions are included for completeness.
BRACKET Screen Chart
This screen is accessed by pressing "b" on the keyboard, then "tab" to Hygain clamp This screen lets YO model the HG bracket to ensure that physical dimensions are properly translated into YO's electrical models at the frequency of interest to ensure proper element lengths are attained.
The actual Hygain (MFJ) bracket dimensions are: 7.3125" length and 3.625" width as measured at the center where the elements pass "through". This is where the measurements s/b made.
After entering these dimensions into YO, the equivalent element-to-boom bracket is modeled to produce the dimensions shown in the previous (TAPER) screen for the first section, 2.282" dia + 3.156" length, except for the DE bracket. More in the next section.
TAPER Screen Chart
The dimensions for the TIPS of each element are shown in this chart. Access this screen by pressing "s" on the keyboard. The dimensions are shown for each yagi element but it's the tip section of each that is important. These are lengths that each tip [three (3) for the LJ203BA] should be adjusted. For example, the director's tip length s/b adjusted to 54.875", apx 54-7/8".
The first section, nearest the boom, is a "modeled" parameter of the element-to-boom clamps. The HG antennas use a bracket that's 7.3125" long by 3.625" wide for all elements EXCEPT THE DE. It's thickness is 0.250" for both bracket halves . [The DE bracket is bigger but not modeled!] Inputting this data into YO, gives the "simulated" value of 3.156" wide with 2.282" diameter.
Each element is held by this clamp so that each elements length is reduced by the amount of clamp depth. For example, the director's first element length is 60" but is treated as if it were 61" long. YO recommends that you add 1/2 of the boom dimension to the element length closest to the boom except for the Driven element!. Now that actual amount that is 'shown' is 70.141". This number is entered as the first section's length. You should NOT forget to do this since the element lengths will be incorrect if it's not. Note that all of the other elements are done the same.
An exception is the DE's element/boom bracket. Since it's insulated, there no interaction with the bracket as there is for the other elements. The dimension for this bracket is set to "0.000"! The length of the DE first section is set to the 69.844" in this case. The actual length of this section is 72" but I find that the YO is more accurate if the former dimension is used. For a "hairpin" match, the beta-rod intersects the DE and negates the effect of this bracket. The bracket is essentially removed as far as the electrical calculations are concerned.
YO Text File
For those who have YO, this listing shows the format of a YO file. Copy and paste into a "text-file" editor such as Notepad and save it as "CH203_S1.YAG" file.
Hy-Gain HG203BA - 26'
14.150 14.190 14.250 MHz
3 elements, inches
2.2817 1.2500 0.8750 0.4380
0.0000   3.1562 69.8437 81.0000   60.9589
150.0000   0.0000 69.8440 69.0000   62.2810
312.0000   3.1562 57.8437 69.0000   70.1408
The element-to-boom clamp have been modeled here by
a using the HyGain circular clamp 7.3125" length by
3.625" wide and it passes the element through a 2"
diameter boom. [Dia = 2.282" and 3.156" long]
The object is to see what gain/bw can be had with
the use of a HG204BA boom (26'). The options have
been chosen to give a low SWR over a limited
operating bw of 14.150 to 14.250Mhz.
This design requested by Charles, W4CO, shows that an addition of apx 10' of boom increases the F/G by apx 2dB. The operating bw being restricted, has not been materially effected. The F/B ratio has been reduced by not materially, so it looks like the changes would result in a "large" increase in signal strength. YO has shown the design details that lead you to successfully build the yagi.