Reverse engineering an antenna

About 15-20 years ago I bought a Yagi antenna for my cell phone since the carrier was not offering great coverage (it did help). But now I want to use it with my LimeSDR. How do I go about this? It has 4 elements of slightly different lengths that are unevenly spaced. It has a mast mount V. Counting from the V the 2nd element is wired and that element is aluminum on one side and a capped painted (or powder coated) tube on the other end.

I guess 15-20 years would make it a 2G phone. Is that sounding right?

1 Like

Hi! Use this Google search to get an overview of how Yagi’s are designed, read a few of the links to get a good idea.
Then you need to know what frequency you are going to be using, Yagi’s are frequency specific though they can be tuned bit around their resonant centers.
For microwave you might want to thing about log periodic, particularly wide band fractal designs.
Easy to make yourself with a printed circuit board.

I have an antenna and I want to reverse engineer it. i.e. I want to know for what frequencies I can use it for. Maybe you already understood that. I learn as I go and I have looked at Yagi designs but they are in the direction that you answered. i.e. determine what you want to look at then design your Yagi.

This make sense?

Look up NEC2. Maybe NEC3 now.
There are programs where you can enter antenna element dimensions and get Nec2 input files to run through the simulation programs. The names escape me now.
I had designed a lot of antennas this way and it seemed to be very accurate.

Okay, they are up to nec4 now and it costs more than the LimeSDR. But nec2 should be fine.

Seems to be active.

quite easy
longest element is roughly a halfwave
real reverse engineer would be
add a reflection bridge to tx port of lime
do a tx sweep out of that port
with rx port connected to forward and reverse port of the refl. bridge you have an antenna analyzer …
now you can do a swr sweep and see the yagi resonance

homebrew a reference antenna (say a halfwave dipole)

place it away several wavelength distance

now you can calculate the gain of the yagi (and measure directivity, front back ratio etc etc.)

that would be real reverse engineering :slight_smile:

… but i am guessing you just wanna know where you can use that yagi (as an rx antenna) … the rule over the thumb (longest is halfwave) is good enough


Right, okay I should be able to figure that out then.

As a newcomer to the radio world I find the antenna to be fascinating. All the more fascinating as some designs are closely guarded secrets. And the software NEC4 (5??) is not cheap for a hobbiest.

And to take it further if you ask 10 people which antenna you get 10 different answers. That’s interesting because it suggests that there is some knowledge about RF that is still not perfectly understood. Well at least not understood in the sense of conventional scientific “wisdom”.

RF is well understood. If you get what appear to be 10 different answers, it is likely that some at least are telling you the same thing from different angles.

Sweeping the antenna with a reflection bridge and VNA, which the limesdr can do, is the correct approach to determine what the antenna is capable of.

Also, for modelling, take a look at MMANA GAL (weird name i know). It’s easier to use than nec2, and more features than the free EZNEC.

mmana is from makoto mori (first 2 letters of soft)
same wit mmsstv, mmtty …
mmana is superb … easy to use … and brings a few hundred example files that you can then edit to your needs (scale to a different frequency etc.)
if you do not know where to start … drop me a note and i help with the first steps
for structures like a patch antenna i use sonnet (lite) … not so easy but after i found a description how to use it … it is as easy as mmana … grin

for your yagi where you do not have data … hack them in mmana and soft will tell you what the antenna should do … when you then compare it with live results (as noted measure with analyzer) you know how exact (or not) such antenna software is

the neat thing is you can build a dozen antennas on one evening … without grabbing a wire cutter or climbing a tree … grin

1 Like

@gerryk great tip, had not heard of that one.

@dg9bfc may take you up on the offer, I have a tendency to dive into things and make lots of mistakes, learning as I go and once in a while having the lightbulb go on. I prefer this approach because sometimes I stumble into new useful insights/things. If I get stuck I’ll post here so others may benefit.