Harold:
I missed some of this very interesting discussion.  But I did not see the 
quarter wavelength response.  I am not an antenna expert...I am an antenna 
student    But a very uninformed student.  :-)   Does the quarter wavelength 
at the peak of the cycle (90 degrees) make a difference in the radiation 
pattern and wavelength?  Also, I am aware that some antennas are designed 
for eight wavelength.  Is there a good book or resource available? 
Hopefully for under $100.00.



----- Original Message ----- 
From: "Harold Hallikainen" <harold@hallikainen.org>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Tuesday, February 16, 2010 12:49 AM
Subject: Re: [EE] Antenna efficiency


>
>> A question for the antenna experts.
>>
>> I'd like to know what happens when you go from a half-wavelength to a
>> quarter-wavelength antenna, while trying to maintain the same output 
>> power
>> from the receiver's POV.
>>
>> In other words, let's say that to compensate for shorter antenna length, 
>> I
>> swap out my 1W transmitter with a 2W one. Would that even work? And if it
>> would, what happens to the extra power -- does it get converted to heat
>> inside the antenna, or does poor SWR cause the energy to be lost inside
>> the
>> output stage?
>
> A bit late to the discussion, but I thought I'd make a couple comments.
>
> To me, the geometry of an antenna has two major effects. These are the
> driving point impedance and the directional characteristics. As another
> poster noted, antenna are often rated in dBi or decibels above what an
> isotropic antenna would do. The gain above isotropic is possible by giving
> up some radiation in one direction and getting more radiation in another
> direction. A half wave dipole has gain above an isotropic since it does
> not radiate off the ends. As the length of an antenna varies, its
> radiation pattern varies.
>
> As mentioned above, a half wave dipole does not radiate off the ends. If
> you drive the antenna at its center, it has an impedance of something like
> 70+j0 ohms. If you turn the antenna vertically and still drive it at the
> center, it is now omnidirectional horizontally, but directional vertically
> (no radiation straight up). You can replace the bottom half of the
> vertical dipole with a ground plane. This is the common 1/4 wave ground
> plane antenna that is used for broadcast applications. The driving point
> is about 35+j0 ohms.
>
> As the antenna gets shorter, the vertical radiation pattern changes and
> the real part of the driving point impedance goes down while the reactive
> part goes negative. You can compensate for the negative reactance by
> adding an inductor in series, but the low driving point resistance becomes
> a problem. As the driving point resistance decreases, other loss
> resistance (ground losses, matching network losses) become significant.
> You get more power lost (converted to heat) in these loss resistances.
>
> Harold
> WA6FDN
> Former broadcast engineer
>
>
>
>
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