Antenna design

[froten]

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 half-wavelenght antenna will be 37.5 m, which is not acceptable at all due to space limits, even quarter-wavelenght is not acceptable.

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 antenna which is tuned using a capacitor, but I don't know about its performance for transmitting, and how is it fed and what is the its impedence.

Q3) if I had to switch to a higher frequency (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.

 

[microtexan]

4MHz antenna

What power level are you talking about?

 

[froten]

I'm not sure about the power level, but the communiction distance should be about 100m.

 

[froten]

I'm not sure about the power level, but the communiction distance should be about 100m to 500m

 

[Mikebits]

Sounds like you need a mobile HF type antenna. Check out this site lots of info.
https://www.ac6v.com/antprojects.htm

 

[Boncuk]

How about a selfmade ground plane antenna of lambda/8?

Use coaxial cable shorted to 9.375cm, and make four equal bundles of the same length using the shielding wire. Solder the strands for mechanical stability and and add a drop of hot glue where they terminate.

For a line of sight distance little RF-power is required and the ground plane antenna has omnidirectional radiation characterics.

Boncuk

 

[stevez]

4 mHz is at the edge of the 75/80 meter amateur band here in the US - and probably elswhere in the world. You might consult amateur radio publications or look for websites.

Your desire for a "good" design requires that you define what you mean by "good" as it can mean a bunch of things. You've already indicated that an otherwise good design (halfwave dipole) isn't good. I can guess at what you might say is "good" - small, cheap and efficient. For 100 meters that doesn't seem like much of a challenge but we don't know how much power you have available. The mobile antenna that was suggested might be a good place to start.

Keep in mind that you should check the regulations for your location. Consider getting properly licensed - and talking to local radio operators for advice.

 

[froten]

Thanx stevez for your advise.

The design I need should be small in size (like 1meter in lenght or diameter or a litil bit more) cheap and self constructed, because in my country it is very defecult to get some parts.

To make things simple, My quesion can be stated in the following statment: For transmitting purpose, at bands with wave lenghts from 160m to 30m where outdoor antenna is not possible and the size must be small, what antenna is recomended?

 

[Externet]

You can make a coiled dipole or coiled quarter wave. Wind enameled wire on a pvc pipe of the diameter and lenght that fits your space restriction, will work fine.

Make an appointment with your lawyer in advance in case you are caught interfering with marine band or other radio services.

Miguel

 

[froten]

I will not need a lawer, so don't wory about.
If I make a loop of quarter wave wire, then I guess I will not need a tuning capacitor ? I will need it if I use a shorter wire, correct?

 

[stevez]

Froten - there is quite a bit of knowledge required to design an antenna. There is far too much to tell you here. A small part of the story - any antenna has a feedpoint impedance - a transmiter will only work into a limited range of impedances. Some transmitters will tolerate gross mismatches - others will not. It is common for transmitters to want to see 50 ohms load at the frequency of interest. It is common for antennas of just about any configuration, to present feedpoint impedances quite different than 50 ohms. An antenna system tuner (antenna tuner) is often used to provide a match between the transmitter and antenna system but for gross mismatches more than a tuner is required.

Again, way too much to tell you here in one note.

 

[Externet]

I would not make a loop. I would coil the dipole elements to the size you want, measure radiation strenght and standing waves to trim the lenght/number of turns of the coiled wires, you may end with something like this :

https://members.tripod.com/hanover_technical/coiL_dipoLe_40_003s.jpg

And if the impedance preferred by the transmitting circuit is near the dipole feedpoint, you may get away without the balun:

https://www.nonstopsystems.com/radio/frank_radio_antenna.htm

Miguel

 

[Mikebits]

This sounds like RadioRons area. I am waiting for his input But I do think they sell HF whips as you want. When I was in the NAVY we had these PRC HF radio's that a person wore on their back and they had a short whip.

 

[stevez]

The antenna is one of many parts of the system required to transmit and recieve RF - at 4 mHz. The signal strength required of the receiving system determines the minimum field strength at the receiving antenna (presumes that it's part of the receiving system). So far we don't know what that is. A small antenna and a relatively insensitive receiver along with a potentially noisey environment would require a relatively high signal level.

At the transmitter the radiation pattern of the antenna might reveal how much of the power input at the antenna feedpoint will make it to the receiving system - assuming path losses are determined. If you know what power must be input at the antenna feedpoint you can work your way back to the amount of power you need to generate. The inefficiency of the circuit that couples the transmitter to the antenna, the inefficiency of the transmitter and all other losses do matter.

None of this means you can't just give some things a try - cut and try - so to speak - is the way a lot gets accomplished. Create an antenna that fits your criteria and give it a try. The use of the word "design" implies some planning, calculation, estimation and even guesswork. It is not uncommon to put a stake in the ground - make some assumptions as a place to start. In your case you may choose to start with the antenna - then make the transmitter powerful enough to get the job done. I remind you again of the rules for where you live.

My intuition tells me that a small whip antenna with a crude matching system and 100 milliwatts of power would carry to 100 meters and an modest receiving system at about 4 mHz though it may not overcome extremes in natural or man-made noise/interference. I can tell you with a reasonable degree of certainty that unlicensed operation of this sort is not allowed in the US.

 

[Mikebits]

I can tell you with a reasonable degree of certainty that unlicensed operation of this sort is not allowed in the US.

The 75/80m band has been allocated for Amateur radio use in the US , and the license needed is a minimal form to fill out. No big deal.

 

[Nigel Goodwin]

The 75/80m band has been allocated for Amateur radio use in the US , and the license needed is a minimal form to fill out. No big deal.

Do you not have to take an exam in the USA then?.

 

[Mikebits]

Do you not have to take an exam in the USA then?.

I would be lying if I answered one way or the other. Really not sure about that, but I know a few Ham fellas that mess around with HF, and I once asked about license. They just shrugged and said it was minimal paper work. Perhaps the exam can be done via mail. Really not sure though...

 

[stevez]

I hold an amateur license and while it was not a big deal, a form and the passing of a test was required (actually Morse code was also a requirement back then). I would not consider the test difficult - some people struggle with it - all depends on your background. That only allows operations within the limits of the amateur radio regulations.

 

[Nigel Goodwin]

I hold an amateur license and while it was not a big deal, a form and the passing of a test was required (actually Morse code was also a requirement back then). I would not consider the test difficult - some people struggle with it - all depends on your background. That only allows operations within the limits of the amateur radio regulations.

When I took mine (1970's sometime), which is a class B licence, you had to sit a written exam, two parts, one on electronics, the other on regulations - to pass the electronics exam probably required more knowledge than many of the posters on here

I went to evening classes for the RAE at a local school, and took the exam in the same building, which was then sent away to be marked - I passed, but the friend who went with me failed - he later moved to the USA, Medina near Niagra Falls, don't know where he is now.

To get a class A licence, you needed to take a morse test, and be able to send and receive at a certain minmum speed (I never bothered doing that).

The only difference was the bands you were allowed to use, back then a B licence only allowed 145MHz band and above.

 

[Mikebits]

It seems as though an exam is still required, but no longer is morse code part of it.
http://www.hamradio-online.com/firstlicense.html