# Antenna design

Discussion in 'Wireless & RF Design' started by froten_140, Aug 4, 2008.

1. ### froten_140 Thread Starter Active Member

Jun 4, 2006
30
4
Hello all,

I'm trying to design a radio transmitter at frequency 4Mhz. I have couple of problems but I will post them each in separate thread.

The bigest problem is the Antenna design. At 4Mhz the the half-wavelenght antenna will be 3.75 m, which is not acceptable at all due to space limits, even quarter-wavelenght is not acceptable. The maximum lenght that can be used is 1m which is used with 7.5MHz. I have to use 4Mhz because I have some pre-built circuits to use.

Q1) I need some help on how to design a good ( if possible) antenna to use with 4 Mhz that will provide good transmition.

Q2) I heared about the loop antennas which are tuned using a capacitor, but I don't know what is thier performance for transmition, and how they are fed and what is the thier impedence.

Q3) if I had to switch to 7.5 or 12 Mhz (hupfully not) how can I design and feed the quarter-wavelenght antenna.

Please help me by providing me with practical hints, I have read alot alot of books and web pages but I couldn't get the right ansower I'm looking for.

Thanx in advance for any help.

Apr 5, 2008
15,648
2,347
Hello,

At first you made a calculating mistake.
The wavelenght of 4 MHz is about 300 / 4 = 75 Meters, so a half wavelenght is 37.5 Meters.
A lot of information on designing antennas can be found here.
http://ac6v.com/antprojects.htm
This is a HAM radio page so the antennas over there are made for the HAM radio bands.
You can recalculate them for your needs.

Greetings,
Bertus

3. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
Loop antennas are pretty good, but they are extremely directional. What are you trying to do?

4. ### froten_140 Thread Starter Active Member

Jun 4, 2006
30
4
I'm trying to design a wireless communication system. I don't have any experince with the RF frequencies so using higher frequencies will be a chalenge for me. Using AM band will require a biger antenna.

5. ### froten_140 Thread Starter Active Member

Jun 4, 2006
30
4
If it is possible to use loop antennas for transmitting, what is the impedence of the antenna?

6. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1

You want to stay away from designing a high frequency system because they're hard to understand. But, the common AM band is too low? So, you're arbitrarily choosing 4MHz?

Why not stick to the AM transmission band and use antennas that are commonly available? Or, if the bandwidth is too low, then why not move up to more common FM frequencies?

You didn't mention your modulation method either, which is important for antenna selection.

If you just want simple communications, I would buy a module that will do it. If that is too modular for you and you want to learn some things, then I would move up a bit. Maybe in the lowest ISM band.

Steve

7. ### froten_140 Thread Starter Active Member

Jun 4, 2006
30
4
I'm choosing the frequency arbitrarily, if you have any advises please let me know, and I would like to do it my self to get some experince.

8. ### froten_140 Thread Starter Active Member

Jun 4, 2006
30
4
I'm still reading some pages from the link provided by Pertus, and so far, I think the only choice is the loop antenna. But how to feed it? and what is its imedence? any help.

9. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,694
904
Are you aware that the radiofrequency spectrum is regulated? 4 MHz is right in one of the ham bands. Check Wikipedia for a quick reference. John

10. ### scubasteve_911 Senior Member

Dec 27, 2007
1,202
1
froten,

Why not build a premade FM transmitter? Some members on this site build these and learn a lot from them. Then, you can start making modifications, etc, in order to suit your needs. John has a good point about the spectrum, you need to be careful in what band and power level you transmit it, or you will have the radio police knocking at your door.

Steve

11. ### theamber Active Member

Jun 13, 2008
318
0
Good antenna building is a complex subject more if you want it to radiate to a maximum. You cannot expect us to tell you details how to build an antenna with the information you provided.
What power you want to transmit? Unidirectional or omnidirectional? What whether? What range? What height above ground? What are surroundings?
You will need SWR and field strength meters. And improper antenna can blow your RF output. You may look at antenna turners too. You can be very creative when building an antenna I have seen some very simple designs that did miracles. Check your power limitations in your country for that specific freq. Learn a bit more and come back.

12. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
That is only if you're dealing with high power levels.

There are advantages with going with kits, for one their usually legal. Been said, but you have to be careful lest some federal types with unpleasant motives come a knocking.

The other big advantage of higher frequencies is the antennas get simplier overall. A full dipole for 100Mhz is around 8', give or take, which is a lot easier than trying for a resonant antenna at 4 Mhz. BTW, an antenna at resonance is as if the transmitter was operating at higher power levels, another advantage.