Need help with the LPDA calculator

[KPMDWhistler]

I am very new to building antennas, and antennas in general. I am also not super knowledgeable with radios, so any help would be greatly appreciated.

So, I want to start planning out my LPDA build, but I'm unable to find any instructions anywhere on how to build one. I have found a website that helps me calculate everything I need to build the antenna. There is a very high likelihood that some of the things that I input wouldn't make much sense, so if you can please give me feedback so I can better understand this tricky subject.

Website: Log‑Periodic Dipole Array Calculator

Input:
Lowest Freq - 100 MHz
Highest Freq - 1000MHz
Diameter of shortest element - 10mm (I wasn't too sure what to put here, I figured 10mm might be a good middle-ground)
Char. input imp. - 50 ohm
Taper - 0.88 (I don't know what this is or how to use it.)
Relative spacing - 0.06 (I don't know what this is or how to use it.)

Output: (Not all outputs included)
Optimal spacing - 0.163 (I assume this is in mm)
Cot of the apex half-angle - 2.000 deg
Number of elements - 21 (Wouldn't it have to be an even number? Or should I just round it to 22?)
Boom Length - 1.383m (54.45")
Length of the dipole element - 1.499m
Distance between element centers - 0.180m

I have not included many things that are probably important, so I would suggest inputting the same variables into the calculator that I provided.

A few more questions:
What does the "Length of the terminating stub []_Zterm = 0.375 m" mean?
What does the "Required characteristics impedance of the feeder connecting the elements Zc_feed = 406.7 ohm" mean?
I assume I don't just straight connect a coax cable to the element loop, what would I use between the element loop and coax cable to the radio?
What would be the recommended element diameter and material?
Would I use some type of aluminum box rod for the boom? If so, what size?

The antenna is not going to be mounted outside, it will be stored inside until I need to use it. When in use it will be vertically mounted to a tripod with a wide stance using hose clamps or a 3/8" fitting.

Any YouTube video links that will help me to better understand what i'm doing would be appreciated.
If my grammar is unreadable, or you need me to clarify anything, let me know.

 

[merlin]

My best suggestion here is find/download the ARRL antenna book. You will need to be good with high school algebra.
The book does cover a section with log periodic antennae.
With search, the USN has some publications regarding LP antennae.
Some youtubes:

Log-Periodic Dipole Array (LPDA)
Wide Band Antennas | Log Periodic Antennas | Antennas Basics and Design
Log Periodic Antenna in Antenna and Wave Propagation by Engineering Funda
I see in your link there are diagrams with the math involved to design an LP antenna. a good reference.

 

[Ubbe]

So, I want to start planning out my LPDA build, but I'm unable to find any instructions anywhere on how to build one.

You can also use this calculator. LPDA - Logarithmic Periodic Dipole Antenna

/Ubbe

 

[prcguy]

You might search for more online calculators and some should have an example antenna to look over as a starting point For things like element spacing ratio (tau) or the other items you are asking about. I like to use 3/4” square aluminum tubing for booms on smaller antennas and when they get longer than 3ft I go to 1” tubing with about 3/4” spacing between booms and that spacing will determine the impedance of the boom feedline which might need to change during testing.

You probably want to use two parallel booms which become the feedline and drill a through hole for each element then a small hole at a right angle in the boom so a screw can go through each element to hold it in. For a single boom you have to insulate each dipole element then make a parallel feedline tat crosses over at each element like on a Create log, and that increases complexity.

I would consider 1/4” dia elements ok for the entire 100-1,000MHz log and if lower frequency go to 3/8” hollow elements below 100MHz. The coax will attach to the booms at the shortest elements and the terminating stub can be a U shaped 3/4” wide length of sheet metal that attaches to the back end of one boom then makes a u-turn and connects to the other boom shorting them out. There is a lot more to know about making a log periodic that is too complicated to share here.

 

[Ubbe]

You can buy a cheap LPDA TV antenna for something like $20 and see how it is constructed and try the calculators on it to see how well it matches. You can probably use the boom and some of the elements as a base and make new holes according to the calculator just to test how well a home made LPDA works using those calculators.

A logperiod are like a self adjusting yagi antenna. From where the coax are connected it is wavelength dependent along the boom where a specific frequency will be picking up the signal, and at that place the elements should match the wavelength. So it's always the highest frequency at the coax point where the wavelength are the shortest.

It's only over a short length of the boom where the frequency will be in resonance so usually 3 elements are within that range so something like a 6dB gain at most. If you want more gain you have to set the elements closer together and adjust some other parameters to make that work but it will then also be much more directional.

It's a balanced antenna type and the coax are unbalanced so usually the coax goes thru the boom that then acts as a sleeve balun.

TV antennas can sometimes be dual band and you can then see clearly where the elements length takes a jump. If you calculate to be using 400-950Mhz range over the whole boom length you then only use the fist 1/3 of the length to make that part a 750-950Mhz range and then calculate for a 400-600MHz range and only use 2/3 of the part of the lowest frequency, to skip 550-700Mhz where you have nothing to monitor. You can probably verify that on a dual band TV antenna that shouldn't have any elements for the 250-470MHz range.

/Ubbe