[Elecraft] dipole antenna efficiency

[N2EY at aol.com]

In a message dated 1/5/08 11:40:39 PM Eastern Standard Time, ron at cobi.biz 
writes:


> We simply do not have conductors that will handle RF with anything like the
> efficiency they will handle DC or low frequency AC. That's because all the
> RF current 'crowds' onto the very surface of a conductor. 
> 
> As we make an antenna physically smaller, the impedance drops. As the
> impedance drops, the RF currents and resistive losses go up. Even silver or
> gold - the best electrical conductors known - are not nearly good enough for
> the sorts of currents we see in small antennas. 
> 
> As an antenna is made shorter the resistive losses far exceed any other
> losses in the system. Of course those resistive losses occur in the matching
> network too. It's not just the antenna itself that converts RF into heat
> better than it makes electromagnetic waves. 

This is so well said that it deserves repeating.

In *theory*, a dipole 1.3 feet long can be made to radiate 80 meter RF almost 
the same as one 130 feet long. But in *practice*, the resistive losses of 
real-world practical antenna systems of those sizes are very different.  

And what we're looking for are practical, real-world antennas that we can put 
up in the limited space we have available.

---

Another factor to remember is that under good conditions we can do pretty 
incredible stuff with very low power.100 watts into an antenna system that has 1% 
efficiency will radiate the same amount of RF as 1 watt into an antenna 
system with 100% efficiency. So even a poor antenna will sometimes let you make a 
QSO.

---

It seems to me that one of the biggest stumbling blocks we amateurs have with 
antennas is seeing and understanding the entire antenna *system* - which 
includes the antenna itself, its surroundings, the feedline, matching networks, 
etc. We have to consider all of them together. And like the links of a chain, 
the weakest one will be the problem. A great antenna in poor surroundings will 
work poorly, etc.

73 de Jim, N2EY


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