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Topic: How to build an suitable RF antenna for Receiver/Transmitter? (Read 3 times) previous topic - next topic

JohannesTN

First of all I'll have to say that I don't know much about RF other than the stuff I Googled so bear with me...

I Brought this of Ebay:
http://www.ebay.com/itm/251174142265?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

It works very well, but I would like to extend the range of these two units (by placing the receivers antenna on the opposite side of a brick wall). I have already tried to extend the Remote control antenna but without any good results, therefore my goal now is to extend the build in antenna of the receiver unit. The frequency is 315MHz and I calculated that my antenna should have a length of 23,81cm for everything to be right.

This is the receiver unit, I've marked the antennas, the white coil-like antenna is approx 23cm when streched which is right according to my calculated antenna length.


I've already red this well documented PDF about RF Antennas:
http://www.hoperf.com/upload/rf/antennas_module.pdf

But still I can't figure out which antenna would be the best fit for me and my application?
How do I know which antenna would be the best fit, I don't really know the difference of which can and which can't be used between the ones described in the PDF.

I already tried building a simple antenna with just a piece of wire (23cm) but with my little knowledge I don't know if I've done it correctly and the best way. I soldered the piece of insulated wire to a pretty heavy coaxial cable and soldered the coax screen to the Receiver units ground. I all seemed to work, and it extended the distance by 30m.

-According to the PDF there is something called a "Groundplane" what exactly is that, and where/how should I use it?
-does it matter how thick the coaxial cable is?
-does it matter how thick the antenna wire (23cm) is? what is the best

I think that must be more than enough questions for you to answer first, for me to supply you with more information. I really hope that some of you RF guys would take a little time trying to help me out, I would appreciate that. Thank you.

regards
Johannes

AndyCC

#1
Aug 10, 2013, 01:31 am Last Edit: Aug 10, 2013, 01:38 am by AndyCC Reason: 1
The antenna you want depends on your application. To start with, there are two classes of antenna: omnidirectional (where the signal radiates out from the antenna with equal strength regardless of the angle you are from the antenna feed point) and directional (where the signal is stronger in one direction than in other directions). If you have a fixed transmitter and a fixed receiver, then you'd want directional antennas pointed at each other. If you have a moving receiver or transmitter, then omnidirectional antennas will be easier.

After that, antennas have orientations: they can be horizontally polarized, vertically polarized, or circularly polarized. In general if one antenna is horizontally polarized and the other is vertically polarized, you'll get signal loss.

A half-wave dipole is a simple antenna. It consists of two equal lengths of wire, each 1/4 wavelength long (approx; start a little longer because wire is not a perfect conductor). One wire is connected to a coax centre conductor and the other connected to the braid. Here's a picture:


If you mount it vertically, it will be omnidirectional. If you mount it horizontally, then it will be directional broadside to the wire (it won't see a signal that's at the front or back of the wire).

Here's a picture of the radiation pattern of a horizontal dipole. It will see (and transmit to) everything in the two circles:


A 1/4 wave vertical is half a dipole, mounted vertically. The other "half" is a surface that reflects radio waves in the same way that a mirror reflects light waves. Because it's tricky to make a vertical 1/2 wave dipole (with a feed point halfway up the antenna), people often make a 1/4 wave vertical with the feed point at the base, and the coax braid conducted to conductors running flat along the ground to act as the reflector. The reflector is the ground plane. Often, rather than have a perfectly flat conductive surface, people use a number of wires radiating out from the centre.

Here's a picture of a vertical:


To start with I'd avoid adding coils and capacitance to your antenna. Also, be aware that metal objects near the antenna will change its radiation pattern. Get them as clear as you can (typically 1 wavelength). If you can't do that - not a big deal, you just might have "dead zones" (nulls) that aren't in a typical dipole or vertical.

Thicker antenna wire won't do much for you on a fixed frequency application. (It could give you a better bandwidth if you have multiple receive frequencies.) The coax won't make too much difference at the power levels you're using as long as you keep it short.

Robin2

I read somewhere that you can have very significant power losses in the cable between the transmitter and the antenna which can negate all the benefit of the antenna. If so it may be necessary to have the transmitter and antenna very close together (i.e. both on the same side of the wall).

...R

JohannesTN

To AndyCC

I think the omnidirectional antenna would be best for me as the receiver i stationary but the transmitter isn't. I'll try to build the half-wave dipole antenna with these instructions http://nathan.chantrell.net/old-stuff/radio/radio-scanning/how-to-make-a-simple-dipole-antenna/ does it seem alright? I've calculated each of the "antennas" to be 22,6cm with this http://www.radiobrandy.com/dipole1.html then I just have to connect the one end of the coaxial cable to the antenna (on the receiver) and the braid connected to the receivers ground?

When I Google "315mhz antenna" I see a lot of fabric made whips, since they are fabric made they must be all right and good to use, but as far as I can see, non of them have a groundplane but just the whip, what is the reason for this? shouldn't they have a groundplane to work right?

thank you

regards

JohannesTN

Just to post some pictures of the antenna I tried to build quickly.
I measured and cut two solid copper wires at approx. 22,6cm


This antenna works great and extends the range with approx 60 meters even when I use 10 meters of coaxial cable to connect it to the receiver.

Here is how I connected it


I still doesn't know if the ground of the antenna is supposed to be connected to the receivers ground as i did here, but it works. Is it okay to have two antennas connected at one time? as you can see I have my DIY antenna connected and the little white "coil"-antenna connected both at the same time

thank you

C2

You are likely loosing half your power in the coax cable. You can look up cable loss charts for your particular coax.

Make the leads shorter at the PCB and disconnect the other antenna.

For a more omni directional antenna and better 50-ohm match, bend the antenna elements at the center to make a 60 degree up to a right angle, known as inverted V antenna.

JohannesTN

I'll try to cut of some of the length of the coax cable when I make the final antenna. Is it necessary to disconnect the white coil-antenna? can't i just leave it there to have some signal on the other side of the brick wall?

thank you

C2

Antennas work best when they are tuned for resonance. The dipole is much superior to the coil antenna, which will detune your dipole if left in the circuit. Leaving them both connected ruins the performance of them both. There are ways to phase multiple antennas, but that gets considerably more complex.

AndyCC

#8
Aug 12, 2013, 07:50 am Last Edit: Aug 12, 2013, 07:54 am by AndyCC Reason: 1
Most of the 315 MHz antennas I can find are either "rubber ducky" whip antennas or antennas with a magnetic base. In either case, there's a ground plane (typically an automobile roof underneath the magnetic base, or a handheld radio and the user's hand underneath the rubber ducky - not the best of ground planes, but it's a ground plane  :) )

Coax loss can indeed be dramatic, especially with certain types of coax. It's hard to tell what you're using, but if it's RG-58 coax, then it will lose somewhere between 7.3 dB and 11.2 dB every 30 m or so. That equates to a loss of about 58% of the signal in the worst case. Some types of coax are better than others at high frequencies. Here are some useful charts if you're trying to decide what coax to use:

http://www.w4rp.com/ref/coax.html

It should be fine to connect the ground of the antenna to the circuit ground.

mauried

Its not essential to have a ground plane if you have the right kind of antenna.
A end fed 1/2 wave antenna doesnt need a ground plane .
These types of antennas are used on some cars as on glass type antennas where one end of the antenna
is simply stuck on the glass, and a small coupler is stuck on the other side.
The tricky bit is making the coupling circuit to drive the antenna from one end , as a end fed 1/2 wave
presents a high impedance feed.
Some of the rubber ducky types can be like this with the coupling circuit on the base and a loading coil
in the antenna proper so that its overall length is reduced to a manageable length.

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