Understanding Log-periodic Dipole Array Antenna theory

[Intell1gence]

Good Day,

After scouring the net for the last couple of days trying to figure out how to improve my 3g signal i came accross a couple of interesting things that i thought i might share here.

To my knowledge the best antenna to use if you want a wide frequency coverage in a rural location without line of sight of the broadcasting tower, is a directional, or more specifically a LPDA (Log Periodic Dipole Array) antenna.

Poynting sells one called the LPDA-92 witch supports frequencies from 698MHz-3000Mhz and has a max gain of 12dBi.

What is the difference between a Yagi and LPDA antenna? - Dr Andre Fourie -Poynting

Dr Andre Fourie explains what the difference between a YAGI and LPDA (Log Periodic Dipole Array) antenna is. Why is an LPDA a better option for 2G, 3G, 4G and LTE?

Now, it seems as though one cannot expect an increase in signal strength by using 2x LPDA's cross polorized (one mounted vertically and the other horisontally, 90deg apart, eg. 2x2 MIMO) in a 3G environment, like one would with LTE to increase your speed.

Ok, so what about increasing the gain by adding a reflector behind/to the LPDA?

In the video above Dr Fourie explains that the LPDA is fed from the tip, a wave is then launched and wherever it finds a suitable length of element it radiates there (from the highest frequency at the tip to the lowest at the back) It scales itself with the frequency.

Wikipedia explains how LPDA's function like this:

...the dipole element closest to resonant at the operating frequency acts as a driven element, with the two adjacent elements on either side as director and reflector to increase the gain, the shorter element in front acting as a director and the longer element behind as a reflector

So the element behind the current driven element acts as its reflector.

What will happen if one adds an even larger reflector behind the last element? Would it then only reflect to the element with the lowest frequency or would it benefit all of the elements?

It seems as though these people think its a good idea to add a grid reflector behind the LPDA to increase the gain:
https://www.signalbooster.com/blogs...bolic-antennas-for-cell-phone-signal-boosters

What would this do exactly?
How does a grid reflector work?
Can it only reflect a certain frequency or can one get a wide band reflector?

I might be a bit presumptuous here, but what would happen if you bolted the LPDA to a Ubiquiti rocket/dstv dish or even a wok?
What would a flat sheet of copper/aluminium/steel do behind the LPDA?

Lots of questions, sorry, but i would really like to understand the mechanics of antennas and hopefully help someone else in the future.

Thank you for your time

 

[ProAsm]

The yagi is pretty obvious in that it is very frequency dependant and does not suit 2G to 4G broadband very well.
However if you only use say, Telkom's 2300 Mhz (like I do) then a yagi is the answer as you can design and tune it for maximum gain on that frequency.
Regarding the LPDA, this is more suited for a broadband requirent like 2G thru 4G, example, 700 Mhz to 2700 Mhz.
One thing you must realize, which the video does not show, is that the gain advertised on any LPDA is the lowest frequency for which it is designed, and the highest frequency will be as good as a piece of wet string.
Adding a large reflector will only effect the lowest frequency, in this case 700 Mhz.
For a suitable LPDA to maintain a good gain say for Telkom's 4G, would be that the lowest frequency is 1800 Mhz and the highest 3000 Mhz.

 

[sajunky]

A conceptual difference between YAGI and LDPA is very small. Both have a series of active, meaning resonant elements that work in total harmony to concentrate the maximum energy on the point that feeds the transmission line. A design is completely different and complicated. For the purpose of this discussion I want to say that increasing number of elements in YAGI leads to increasing gain, while in LDPA to increased bandwith. It is why YAGI type is used in application of relatively narrow bandwith, while a typical LDPA can cover frequencies from 800MHz to 5GHz or more, but signal has more amplitude and phase distortions.

If you know frequency band of the tower, use YAGI for long distance connection. It will give a natural filtration of unwanted radiation and has also a number of parameters which constitute a better signal usability at the decoder.

Ok, so what about increasing the gain by adding a reflector behind/to the LPDA?

Negative, wrong conceptually.

On the other, conceptually different side there is a Dish antenna. A dish is reflecting waves the same way like a mirror is reflecting a visible light, so it is a very simple concept to understand (as opposed to the resonant director/reflector in YAGI/LDPA). A dish is non-resonant, so it can function over a wide range of frequencies. It has two functions: to focus energy on the small receiver in the centre and to assure that all waves reflected from different parts of the dish arrive in the same phase.

If you want to play with dish antenna, put USB stick in the place of LNA/LNB. USB cable should be as short as possible (<1m), or use a self powered USB hub close to the dish. Also search for similar projects like a can antenna or corner antenna.

 

[ProAsm]

Also search for similar projects like a can antenna or corner antenna.

Hell that brings back good memories

 

[ArtyLoop]

When I studied engineering this was my weakest subject... all that math killed me.