# RDF and Dipole antenna help

Discussion in 'The Projects Forum' started by GrassCube, Apr 1, 2015.

1. ### GrassCube Thread Starter New Member

Apr 1, 2015
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0
Hi,
im a bachelor's mechanical engineering student, and im currently working on a project that involves a robot finding, locking on, tracking and steering the robot to the target.
Basically i figured out the 2 methods i would like to use for the tracking, but i need help with the transmission and receiving the signal, as i have zero knowledge in this field .

This is what i want to achieve:
-Setting up a transmitter, with a continues transmission (with on/off switch).
-Building a receiver using an Arduino board so that i can track the direction of the signal.
-The transmitter will be set at a max distance of about 20-25 meters.
-The robot should get up to 20 cm from the target (so it should be accurate)

The methods:
I found 2 methods that use radio waves (instead of GPS) for tracking a target, i will begin with the one i think will work the best.

1)Method 1: RDF - Radio Direction Finder

This is what im trying to use and modify for my needs, link.

-Can i build a transmitter with the same parameters as the walkie-talkie by myself, and how hard would it be?
i need it to just transmit a signal.
-The robot must fit in a 30x30x30 cm cube, so the length of the antenna of 11.5 inches is exceeding the parameters. In the instructions it says that the antenna should be 1/2 a wavelength long or less, what happens if i shorten the antenna, and what is the minimal length i can use the retain the accuracy and range?
-Dose the dipole antenna has to be straight or can it be a bit curved backwards (in case i wont be able to shorten it and will have to place it inside the robot casing)?
-I know that if i get higher Mhz i can shorten the dipole antenna, how can i achieve that?

I understand this method a bit less.

-On what kind of signal this method works?
-Can i build a transmitter?
-Can i make the helical antenna shorter or replace it with a flat one like in this video

General questions:
-Which method is more accurate, at long and short range?
-Which method will work best for me, depending on the requirements?

I like the first method better because it has a detailed plan of how to build it, and it looks much more responsive then the RSSI, but i might be wrong. But what worries me, is that the antenna is to long and i prefer not to buy the walkie-talkie but to build something by myself, as i have a lot more components that i need for the project and i want to minimize the overall cost.

Hope i gave you guys all the info you need, so you could help me. If i missed something i will gladly provide the necessary info.

Tnx,
Simon

2. ### Papabravo Expert

Feb 24, 2006
11,067
2,152
Building a transmitter:
There are a couple of issues here and the first is that it is illegal to spew RF energy according to your whim. There are specific allocations for unlicensed operation that have power and other limitations. Your best bet is to find a usable module that you can adapt to your purpose.
The second is that it is extremely unlikely that you can design and build a functioning transmitter from scratch. Most electronic assembly is done by highly specialized machines to achieve uniform quality in the finished product. You cannot duplicate this level of quality using manual assembly techniques without a great deal of training and practice.
Third is that it takes multiple transmitters to use direction finding to accurately locate a receiver, unless there is some method of determining rage for a given azimuth.

I'll take a break now and research RSSI since I've not heard of that before.

3. ### GrassCube Thread Starter New Member

Apr 1, 2015
4
0
I didn't get the last part, i want to locate the transmitter not the other way round, like the guy did in his video (with the robot and walkie-talkie).

4. ### Papabravo Expert

Feb 24, 2006
11,067
2,152
You concerns about the antenna are valid. Normally you can go up in frequency to allow the use of shorter smaller antennas. It might be helpful to let you in on a simple calculation:
The product of frequency and wavelength is a constant. That constant is the speed of light and is approximately 3E8 meters per second. An FRS walkie-talkie operates at a frequency of 462 MHz. What is the wavelength?
3E8 = 462E6 * W ⇒ W = 3E8 / 462E6 = .649 Meters ≈ 25.5"
A typical antenna would be either a half wavelength or a quarter wavelength meaning 12.75" or 6.4"
Problem is that I'm not sure a continuous carrier is legal in the FRS band.

5. ### Papabravo Expert

Feb 24, 2006
11,067
2,152
I suppose there are circumstances where you can do that by looking at changes in signal strength as you change the orientation. That makes the two methods almost the same in my mind. How is it that they are different?

6. ### BR-549 Distinguished Member

Sep 22, 2013
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724
Maybe 20-25 meters is too far, but have you considered sonic transducers?

7. ### alfacliff Well-Known Member

Dec 13, 2013
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the time delay on recieve would fit your aplication better, it uses two vertical antennas spaced about a 1/2 wave apart, switched at a known rate. the incoming signal is fm modulated gy the switched antennas, and gives a left/right signal. look up "fox hunting" we hams have been doing it for years, looking for hidden transmiters and developing lots of ways to do it. the advantage of the TDR is that it works all the way to the transmitter, signal strength does not affect it.

8. ### GrassCube Thread Starter New Member

Apr 1, 2015
4
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Well now i know that i dont have to use the full required length of 1/2 wavelength but can do just fine with 1/4, this solves me the size issue.

About the legality of the of the continuous transmitting, i think that if i use the commercially sold walkie-talkie and occupy a band by chating with someone or transmit a continuous blank signal it'll be the same .

Well my knowledge in electromagnetic waves and transmission aims to zero, i just thought that if the types of antenna are different the transmitting/receiving method must be different. This was my deduction, maybe stupid but i tried .

Im not realy sure how they work, i'll check it out but im guessing that i need a clear line of sight (without any obstructions) between the transmitter and the receiver for them to work.
And i dont think that the distance in an issue because the walkie-talkie can transmit up to 16 miles away and the RSSI method is used by RC plane pilots for FPV (first person view), to receive a video feed from the RC plane. I am worried about the accuracy at close range.

Well i liked this RDF method a bit more than the RSSI (not even sure why) before i knew what would work best for me. So i'll experiment with that, and for the wort case scenario i have the RSSI to try and make it work.

Thank you all for your help!

I'll probably will have more question but until now i have enough to work with...

9. ### Papabravo Expert

Feb 24, 2006
11,067
2,152
And you might be wrong. You have to read the specific rules that apply to the band you intend to use. I think, but I'm not sure, that FRS regulations specifically forbid the use of a continuous carrier without modulation. That is not the same thing as continuously talking. In any case you should consult Title 47 CFR Part 95 Subpart B for relevant regulation on FRS. I think the reason for this regulation is to proscribe simple jamming devices.

https://www.law.cornell.edu/cfr/text/47/part-95/subpart-B

10. ### alfacliff Well-Known Member

Dec 13, 2013
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the tdt method uses the phase of the detected signal (phase modulated by the two antennas at their switching rate,) you dont have to have a continous carrier, it gets its phase info every time theres a transmisssion. the rssi method has a few problems, reflections cause peaks and nuls in the signal strength. even standing waves do too. the phase of two square waves is very easy to detect.

11. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,070
And you would be wrong. The 1/4λ monopole is an efficient antenna only if it is mounted at right angles to a metallic plane which about 1/2λ in diameter. Look up "Ground Plane antenna". Otherwise, an antenna needs to be overall 1/2λ in length.

Think of a VHF antenna such as is mounted on a metallic car body. It is possible that your robot is big enough to act as a ground plane under your transmit antenna. The receiving array still needs to be 1/2λ dipole(s) or a Yagi antenna, where the elements are ~1/2λ long.

I am with PapaB. I take a dim view of unlicensed/illegal operation of transmitters on frequencies that require a license, or have restrictions on allowed use.

I am still finding lost high-altitude balloon payloads for ANSR using radio direction finding techniques, both on the ground, and from my airplane. Nowadays I use one of these as the DF receiver. I use one of these as a DF antenna (on the ground). The transmitter is a ~10mW low-power beacon that operates near 146MHz. Useful DF range is up to several miles on the ground (depending on terrain) and up to 50mi from the air.

Here is some stuff I did a long time ago...

Last edited: Apr 2, 2015
12. ### GrassCube Thread Starter New Member

Apr 1, 2015
4
0
Tnx! i'll check it out.

Now im confused, what is the difference in 1/2 dipole and 1/4 dipole antenna, what will i loose in terms of accuracy, errors...? i hope i asked the right question.
Even the guy whit the RDF robot wrote in the instructions for the antenna that it has to be 1/2 wavelength or shorter.

This will be a one time experiment, just to show proof of concept, and it'll be at a remote area. But i'll check the legality of the subject and find out what are my options.

13. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
429
if you use a loaded antenna, it can be shorter. not as efficient, but at a short range sufficient. hw about using some radio control equipment for testing, most of that is unliscensed.