Antenna Length for High Frequency communication

[lasergecko]

It's my understanding that at higher frequencies, smaller antennas are required. (Hence why AM antennas are so large.)

With higher frequencies, such as 2.4 ghz or 5ghz communciations, does a longer antenna length have a harmful impact on performance?

 

[YNos]

the formula is 234/freq for 1/4 wave antenna, and 468/freq for 1/2 wave antenna.
here is the first hit off google.
http://www.scanshack.com/antcal.htm

if you have an antenna that is resonent, you will get better reception, and will have more power actually "make it out" of your antenna. if your antenna is not "tuned" correctly, you will have high SWR's <standing wave ratio's>, causing a percentage of your power to be reflected back into your transmitter.

http://www.intuitor.com/resonance/radioTVres.html
is another link i got when google'ing for antenna resonance.


<edited for spelling>

 

[harrkev]

YNos got it almost right...

As the length of the antenna increases in proportion to the wavelenth, it becomes a less effective radiator. However, at some point it starts to get better, and then worse, and so on. For example, for a simple antenna, you can transmit (using a ground plane) on a 1/4 wave whip. A 5/8 wave whip also work quite nicely, too. So for any given frequency, there can be multiple lengths that can be considered to be "resonant." Some resonances are simple better than others, though.

 

[Machupo]

if you're looking for HF antenna info, check out a document called "ECAC-CR-83-200" (a.k.a. Field Antenna handbook) -- really useful for omnidirectional), dipole), and tons of other antenna designs...

 

[YNos]

but aren't 1/4 and 1/2 wave radiators the only ones that you dont need to have some sort of matching unit on? 5/8 wave you have to have a tapped coil at the feedpoint for your transmitter to se a 50Ohm load .. or some other matching unit to bring it down from ~85-90 Ohms.

 

[tart666]

Originally posted by: lasergecko
It's my understanding that at higher frequencies, smaller antennas are required. (Hence why AM antennas are so large.)

With higher frequencies, such as 2.4 ghz or 5ghz communciations, does a longer antenna length have a harmful impact on performance?

the short answer is "YES". if you go from AM to 5GHz while keeping the antenna size and shape the same, the performance will suffer.

 

[davesaudio]

lets remember radiator length and resonance and impedance are separate properties.
5/8 is a good length we like the lobes or lack thereof
we can also have colinear such- as- a- whole- bunch of half waves stacked but a phasing sections between each, this gives considerable gain over a dipole
then you have your dish stuff- figure out how many wavelengths across a typical dish is , lots

you still need to match impedance , the whole system may be resonant (including the matching components) but the antenna itself may not be....

 

[Smilin]

Originally posted by: lasergecko
It's my understanding that at higher frequencies, smaller antennas are required. (Hence why AM antennas are so large.)

With higher frequencies, such as 2.4 ghz or 5ghz communciations, does a longer antenna length have a harmful impact on performance?

short answer: you can use multiples. As long as the length lines up with the harmonics.

edit: scratch that. I may be thinking of transmitters. It's been too long.