One question about antenna sizes for fm transmitter

[ke5frf]

I think we use different terminology, so perhaps some confusion.

As I was taught, a loading coil is for the purpose of electrically shortening an antenna.
A matching network is for providing a feedpoint impedance match.

A 5/8 2 meter whip with a matching network (coil) will still be approximately 5/8 wavelength long at the element. The matching network is actually part of the feed system, not the antenna. The purpose of the matching network is not to allow for a shorter antenna. An antenna can be matched anywhere in the feed system or at the radio. My radio has an autotuner to serve this function.

Most 5/8 wavelength 2 meter whips are 120-130 cm long.

A base or even center load, designed to allow a shorter antenna, will have irregular radiation patterns with high angle lobes. Much of the power of the system will be wasted as heat. A matching network on an electrically correct element such as a 5/8 vertical on 2 meters will have some loss as heat, but the radiation pattern will not be irregular. The heat loss will be less than a shorter, loaded antenna.

I have no coil on mine. I do realize that some have a small coil for the purpose of matching.

I apologize, I think I assumed we were using the same language.

In my studies, I have come to learn that loading and matching are two different things and important to differentiate.

 

[Nigel Goodwin]

I think we use different terminology, so perhaps some confusion.

As I was taught, a loading coil is for the purpose of electrically shortening an antenna.
A matching network is for providing a feedpoint impedance match.

A 5/8 whip isn't 50 ohms, a 5/8 whip with the loading coil is 50 ohms. So it provides the impedance correction required to match the radio (and coax) - I would consider that providing the matching.

 

[ke5frf]

I'm not trying to argue with you here, because I see now that it is a terminology issue.

Indeed, if a 2 meter antenna that is physically 120 to 130 cm long has a coil on the bottom of it, the purpose of that coil is to provide a 50 ohm match. There are other ways of accomplishing this, but this is a common solution. My antenna doesn't have a coil. It is a simple, 5/8 wavelength whip. My radio automatically corrects for the slight impedance mismatch that it presents.

"Loading Coil" is a slang term, which is overused. I know it is commonly used incorrectly. Properly, a loading coil is for shortening an antenna. A CB, base loaded antenna has a short whip and a coil. It is a modified version of the 102 inch whip, which is a 1/4 wavelength. This would be a loaded antenna. The coil and element are tuned so as to have close to 50 ohm impedance, but that is not why the coil is there. The purpose of the coil is to load the antenna, make it shorter.

Earlier, I used an incorrect term witch I'd like to correct. I said "electrically shorten". I hate when I do that. To electrically shorten would imply that it is no longer a 1/4 wavelength antenna. not true, these base loaded CB antennas are still electrically 1/4 wave, but PHYSICALLY they are much shorter. This is because of the LOADING COIL.

The 5/8 wave 2 meter whip may have a coil, but the radiating element is still about 50 inches (130 cm) long. The antenna is neither physically nor electrically anything but a 5/8 wavelength. The coil is not loading it, it is matching it. thus, it is a matching network.

The coil is not necessary. Coax transformers, baluns, tuners, a variety of techniques could be used to accomplish the same thing. But since it is an easy mobile solution, it is common.

But it still doesn't load the antenna, it matches it. I'm being nitpicky here, because I'm just that way!! I just want to make sure the functional difference is understood by the person doing the asking.

It's kind of like a transistor. In some circuits it can be used as a switch, and in some it can be an amplifier, but we wouldn't call a transistor in a logic circuit an amplifier!

Loading is not the same as matching!

Loaded multiband HF antennas still need an antenna tuner to match them! The coil isn't doing the matching, it is making the antenna short. The tuner does the matching.

A 5/8 whip on 2 meters isn't loaded, it is impedance matched!

LOL. I'm sorry for all the nitpicking, but this is a passion.

 

[ke5frf]

On a loaded antenna, the coil will radiate, with some lobes radiating at high angles.

An impedance matched, physically resonant antenna element will have little energy radiating from any matching components. The matching components simply insure maximum power transfer from the amplifier to the antenna. Nothing more!

 

[Sceadwian]

**broken link removed**

I almost get it =)

 

[ke5frf]

Glad you found some good material to read. I encourage you to read more than just the eHam forum though, there are good articles on eHam with colorful discussions under the comment section. The archive is in the thousands by now. It can be difficult sometimes because nitpicking sessions like this thread sometimes occur ...but over time you get a feel for the ones who know what they are talking about. They're usually the stubborn ones haha.

 

[polishdude20]

ok well i gave up on the transmitter . i made the antenna the right size and then things started going downhill.

the trim cap is so small i need a special screwdriver to turn it but it conducts electricity so when i remove it , it changes. secondly when i start tuning it all it does is tune my radio to different stations with other people on them and never to mine. I can never find a good clean section of the band because the ones that are full are so close to it that when i tune my transmitter it just strengthens the signals of he other stations so they just jump onto my clean one. so basically its crap and if anyone has answers for these problems be my guest and if not i wouldnt care less . thanks for all your help guys but its not worth it anymore

 

[Sceadwian]

polish the simplicity in the circuit you're using is the cause of all it's shortcomings, if you want solid frequency stability and a clean signal let alone stereo you need a much more complicated circuit.

 

[audioguru]

The extremely simple transmitter is crap and your extremely cheap FM radio is also crap.

 

[Warlord_1011]

I have a question

how do you calculate what length the antenna should be?

 

[transistor495]

Its frequency will change if anything moves toward, moves away or looks at the circuit or its antenna ,

What will happen if a pretty girl moving around? Will it change it's frequency?

I think NO

 

[audioguru]

A slender pretty girl won't affect the transmitter much but she will affect and attract me.
A fat ugly girl will probably stop the transmitter and I will run away.

 

[aravindj36]

You crappy guru do you only know how to find the problems in the circuit rather than designing one

 

[aravindj36]

I think you can just google for fm tranmitters and you can find some circuits with acoil in built in the pcb

 

[audioguru]

You crappy guru do you only know how to find the problems in the circuit rather than designing one

What is your problem? Crappy circuits perform poorly.
I designed and built an FM transmitter that has pre-emphasis so it sounds great. It has an RF amplifier so its oscillator is isolated from the antenna.

 

[Hero999]

FM Transmitter