Anyone familiar with antenna theory?

[TecHNooB]

I can't remember a good reason why higher frequencies lead to smaller antennas. I would like a concrete reason linking equations to what's physically happening inside the antenna.TY

 

[guyver01]

I can't remember a good reason why higher frequencies lead to smaller antennas.

really? i thought this was common knowledge.

Due to shorter wavelengths, the use of smaller antennas would be required.

 

[TecHNooB]

really? i thought this was common knowledge.

Due to shorter wavelengths, the use of smaller antennas would be required.

Explain to me exactly what that means physically. I can picture the EM waves and such and I know what you mean by shorter wavelengths. I just can't connect shorter wavelength to smaller antenna.

 

[So]

you need an antenna with a minimum of 1/2 of the wavelength of the signal you're trying to receive to get decent reception...thus, since with higher frequencies, the wavelength is shorter, you can get away with a shorter antenna. IIRC, there is no downside to using an antenna with an integer multiple of the half wavelength, but I never got around to taking the antenna design elective.

 

[So]

Also, wrong forum. This belongs in the compendium

 

[SunSamurai]

I can't remember a good reason why higher frequencies lead to smaller antennas. I would like a concrete reason linking equations to what's physically happening inside the antenna.TY

Is this some sort of metaphor for guys with big cars that peel out of every turn having small dicks?

 

[So]

Is this some sort of metaphor for guys with big cars that peel out of every turn having small dicks?

No, but your post is a metaphor for not paying attention during the emag chapter of physics class.

 

[h8red]

I don't think that there is anything that actually happens inside the antenna, it just collects signal. There is nothing to say that you can't have a larger antenna for shorter wave signals. It is just easier to construct and cheaper to make a smaller antenna than a larger one

 

[shortylickens]

Is this some sort of metaphor for guys with big cars that peel out of every turn having small dicks?

Shens, I have a small dick and I dont drive like an idiot.

 

[PottedMeat]

maybe you should start with the simplest antenna ( to implement and model ) - a dipole. you can probably come up with a general equation showing input received power vs. output power for different lengths at a particular frequency.

 

[Unheard]

I don't think that there is anything that actually happens inside the antenna, it just collects signal. There is nothing to say that you can't have a larger antenna for shorter wave signals. It is just easier to construct and cheaper to make a smaller antenna than a larger one

This is correct. Let us take a look @ TV channel 14. It operates @ 471.250 MHZ. The wavelength for that frequency is roughly 70cm. That is a full wavelength. If you look though, you don't see broadcasters using 70cm antennas. They may have one that is 9' long (280cm) and that would be 4 full wavelengths for the frequency. A longer tuned antenna will radiate better than smaller tuned antenna.

Conversely, my handitalkie for 70cm work (if you haven't figured this out, I'm an amateur radio op) has a 3 1/2" antenna, which is a 1/8th wave antenna for 70cm. It works fine for 5 watts and hitting repeaters on large hills, but it is a less than optimal antenna for our privileges on 70cm (420-450mhz).

As you already know, when you go lower in frequency, the antenna's get bigger. I have a 20 meter dipole in the back yard. 20 meters is the full wavelength for 14mhz transmissions. Knowing this, my dipole also works fine on 40 meters (7mhz) in which it acts as a 1/2 wave, and 10 meters (28mhz) where it is 2 full waves. I can use a tuner to make the antenna match other frequencies, but it is resonant on those 3 bands.

Any more questions let me know.

de KJ4CTS

 

[bobdole369]

you need an antenna with a minimum of 1/2 of the wavelength of the signal you're trying to receive to get decent reception...thus, since with higher frequencies, the wavelength is shorter, you can get away with a shorter antenna. IIRC, there is no downside to using an antenna with an integer multiple of the half wavelength, but I never got around to taking the antenna design elective.

Nah.

a halfwave dipole is simply one antenna design. It's often used as a benchmark of sorts.

Some shorter antennas can work nearly as well as a halfwave dipole, or an end-fed halfwave, but by definition - they will be less efficient. Granted that efficiency might approac 90%.

http://www.qsl.net/va3iul/Antenna/S...ncies/Small_Antennas_for_High_Frequencies.pdf

Give that a read. I don't have time to see if that read solves your original query, but very good info contained within.

 

[Unheard]

maybe you should start with the simplest antenna ( to implement and model ) - a dipole. you can probably come up with a general equation showing input received power vs. output power for different lengths at a particular frequency.

A quick way to know the length of a dipole is to use this equation: 468/frequency.

The number returned is roughly the length of the dipole you will have, divide it by 2 and you get each legs length.