Antenna Length, Loading, Inductance, Resonator Related Inquiries

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twinklerip

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For the last couple of days i've been researching about radio and antenna. Now I'd like to make a antenna (with tuner) for different frequencies. There are some confusions that i want to clear up -

1) If I want to use a small antenna for a certain frequency how do I calculate the inductance of the required loading?

2) If I coil up an antenna made of copper wire to save space, will it work for the same frequency as before? (I know it won't because of the inductane, just to make sure)

3) If I use a loading of unknow inductance with an antenna and use a variable capasitor to change the inductance of the loading then won't it work as a series resonator?

4) If I use a resonator then how important is antenna length?

5) How important is antenna lenght for a transmitter?
 

nd5y

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1) Don't know without searching through handbooks

2) No. If you coil up wire it is no longer resonant at the same frequency. In the past I built several rubber ducky and short helical antennas an found that you need somewhere between 1/2 and 3/4 wavelength of wire to get them to act as an electrical 1/4 wavelength.

3) A capacitor can't change inductance. A variable capacitor and fixed inductor will change reactance and resonant frequency.

4 and 5) If you use a loading coil or trap the antenna length is not that important. It will still be resonant but the shorter you make an antenna, the less efficient and more narrowbanded it gets. You can make a short mobile antenna for 2 MHz but it will be nowhere near as good as a ull size dipole or vertical. There should be no difference betweed receiving and transmitting. They both work the same.

Here are some good antenna web sites
http://www.w8ji.com
http://www.k0bg.com
http://www.cebik.com/radio.html
 

twinklerip

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What kind of copper wire can be used to make antennas? Is there any requirement for diameter of the wire?
 

nd5y

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It depends on what kind of antenna you are making. If it will be indoors tacked around a wall or something you can use very thin wire. If it will be a large outdoor HF antenna you have to use wire that won't break win the wind blows it around. If it is something like a VHF or UHF ground plane antenna you need wire that is thick enouhg to support itself.
 

tom_guess

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You ever tried 12 ga or thicker for a wire antenna. I've always just used 16 ga stranded. Primarily for its flexibility.
 

twinklerip

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1) if the shape of a normal bipolar antenna is chnage like the attached pic, how would the shape affect its resonant frequency?
2) does 1/2 wl antennas have to be center fed? what if its fed from one end?
 

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N_Jay

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You have a lot of great questions.
You are going to get a lot of partially correct and partially incorrect answers here.
You will learn more and have much less chance of building misunderstandings by reading a good book an antenna design.
 

Al42

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twinklerip said:
1) if the shape of a normal bipolar antenna is chnage like the attached pic, how would the shape affect its resonant frequency?
Measure the length of the antenna from the feedpoint to the part that's at the top of the picture - that's about the effective length of the antenna - the rest of it is just thickening, which makes it broader banded. Since what you have there is a dipole with the legs parallel, you'll get so much interaction that it's pretty much a dummy load.
2) does 1/2 wl antennas have to be center fed? what if its fed from one end?
An end-fed 1/2 wave antenna has around a 5,000 ohm feedpoint impedance - so you have a VSWR of about 100:1 to 50 ohm cable. Assuming you don't burn the transmitter out as soon as you turn it on, you'll lose aboout 99.999% of the power in the mismatch, and the rest of it in the cable. (Same for receiving - most of the signal in the antenna will never get to the receiver.) If you have an antenna tuner that can match that hign an impedance, you'll have a fairly decent 1/2 wave antenna - no better than a center-fed dipole. (Worse, actually, due to the losses in the tuner, and there will always be some.)

And, as I alluded to above, the diameter of the elements in an antenna determines the bandwidth. Using #36 wire at 1 MHz will probably give you an antenna too narrow to use for voice. If you're making dipoles or long wires, look for copperweld - steel on the inside for strength, copper on the outside (where the RF flows) for low resistance. (Resistance isn't impedance - it burns power that would serve you better if it was radiated.)
 

twinklerip

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One more question -

how efective will a low frequency antenna be if it's used for VHF or UHF purpose
i) by using capacitive loading?
ii) by using proper tuner?
 

kb2vxa

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Hi all,

WOAH HOSSIE!!! Like N_Jay said (more or less) you guys with such questions need to crack the books! Ain't no way Jose you're going to get such an education here, the topic is WAY beyond the scope of discussion ANYWHERE, let alone on a site dedicated primarily to novices and casual hobbyists. Specific questions get specific answers but FIRST you need to understand the answers, you need to lay the foundation before you can build your house and it must have a roof to put an antenna on.

"Come back when you grow up girl, you're still living in a paper doll world." (;->) Now who sung that dumb song, Samsung? I can't get it out of my mind! Thanks guys, I'm gonna KILL someone.
 

gcgrotz

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Both Comet antennas and MFJ have a mobile whip that covers HF bands (depends on which coils you buy) as well as 50, 144 and 432 Mhz ham bands. Cost including mount will probably be around $200-250 depending on which coils you want. This would be a far better solution than trying to build it yourself. Low freq ants get wierd at VHF with lobes going off the ends, high angles, horizontal instead of vertical polorization.

If you do it yourself, you will need test equipment to get satisfactory results that will cost more than a commercial antenna.

Antenna tuners are uncommon at V/UHF because if you have a mismatched feedline, the high losses from a high SWR at those frequencies can be huge.
 

gcgrotz

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Hey Warren!

I think it was Gary Puckett and the Union Gap wasn't it? Oh wait, no it wasn't. Daxx! Now I'll be thinking about it too.

THANKS A LOT!
 

twinklerip

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gcgrotz said:
Antenna tuners are uncommon at V/UHF because if you have a mismatched feedline, the high losses from a high SWR at those frequencies can be huge.
Then how 88 to 108 MHz are tuned in FM radios?
 
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