[HCARC] 10 meter Loop

[Gary and Arlene Johnson]

WOW - that is the $1000 answer for will a 33 foot antenna made as I 
described work - you know will I be able to talk to people???  Secondarily, 
will I be able to use a tuner to get it to work on other than 40 meters, OR 
since the sections are easily replaceable would I be able to add or subtract 
antenna sections to make it work on other meter bands???

I caution the answerer's of questions to remember who they are talking to. 
Bill T used to answer my questions with lots of theory.  Now he has learned 
to answer my questions in high school science terms.  Sometimes - no, most 
times, I am just looking for the answer to WILL IT WORK, and if not, HOW CAN 
I MAKE IT WORK, and IF YES, BUT THIS WILL MAKE IT WORK BETTER terms.  The 
theory behind it is nice, but doesn't add too much to the basic answer to 
the question of WILL IT WORK.  I am struggling with the studying for the 
Extra Exam.  Sometimes the answers given here are harder than the info in 
the Extra Exam materials and that is way over my head.  It's the same reason 
why the erroneously shipped book from ARRL entitled "Experimental Methods of 
RF Design" is going to reside in Dale G's radio library and not mine.  I 
will never and I truely mean NEVER get my knowledge level of RF to the level 
talked about in this book.

I just wanted to know if the vertical antenna I described SHOULD WORK, and 
if not why, and if not then this is what you might choose to do to make it 
work.  And if it will never work I can sell it as scrap at the metal 
recycling place on HWY 27.

Gary J
N5"BAA"




----- Original Message ----- 
From: "Kerry Sandstrom" <kerryk5ks at hughes.net>
To: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>
Cc: <hcarc at mailman.qth.net>
Sent: Monday, August 13, 2012 9:16 AM
Subject: Re: [HCARC] 10 meter Loop


> Actually, Gary, its not exactly either.  The best way to think about it is 
> the antenna is completely enclosed by a sphere of a certain radius.  For a 
> half wave dipole the radius of the sphere is a quarter of a wavelength, 
> for 40 m , thats about 33 feet.  The antennas 'physical aperture' is the 
> cross sectional area of the sphere in the favored direction.  For a 
> dipole, this cross sectional area is the area of a circle of a quarter 
> wavelength radius. The second thing to consider is the 'aperture 
> efficiency' of the antenna. The aperture efficiency is basically an 
> indication of how uniform the the illumination of the aperture is.  For a 
> dipole its pretty bad, the 'illumination' is just a single wire accross 
> the diameter of the aperture. The result is that the 'effective aperture' 
> of a dipole is much less than its physical aperture.  Antennas such as 
> parobolic reflectors illuminated by a horn have effective apertures that 
> are ~70% of their physical aperture. For a Yagi, the effective area is the 
> area of a sphere whose radius is the turning radius of the Yagi.  A yagi, 
> however, doesn't have a very high aperture efficiency.  A stacked array of 
> shorter yagis, does have a higher aperture effiency.  Many years ago large 
> VHF stations usually used 2 or 4 short yagis rather than one long one. 
> Why not now you might ask?  I think it is because it is difficult to have 
> stacked antenna arrays for several bands.  Today's VHF/UHF ops are apt to 
> be on half a dozen bands while years ago most were only on one or two 
> bands.
>
> There are a couple simple formulas that relate the aperture size to the 
> approximate beamwidth and gain.  The beamwidth in radians is approximately 
> the inverse of the diameter of the effective aperture measured in 
> wavelengths.  For a uniformly illuminated aperture, the gain is 
> approximately 4 * Pi * A where A is the area of the aperture in 
> wavelengths. These formulas work best for antennas with uniformly 
> illuminated apertures.
>
> Bottom line:  For a high gain antenna you need the largest aperture you 
> can get and you need to 'illuminate' that aperture as uniformly as 
> possible.
>
> The metal does make a difference.  The metal needs to be a good conductor, 
> at least on the surface.  If the metal has a high resistance, the antenna 
> will have losses do to that resistance.  Because RF doesn't penetrate into 
> a conductor very far, 'skin depth', copperweld wire with a steel center 
> and a copper jacket will work as well as solid copper and be mechanically 
> much stronger.  That is also why silver plating various microwave things 
> works, although the thickness of the plating is very small, it contains 
> essentially all of the RF current.
>
> There are two quantities that are important to losses in antennas.  One is 
> the radiation resistance and the other is loss resistance.  The radiation 
> resistance is the part of the total antenna input impedance that 
> contributes to actual radiation.  The loss resistance is the part of the 
> antenna input impedance that leads to heat.  There is also an 'imaginary' 
> part of the input impedance which leads to the energy stored in the 
> antennas non-radiating near fields.  For an efficient antenna you want the 
> ratio of the radiation resistance to the loss resistance to be as large as 
> possible. The radiation resistance goes as the square of the antennas 
> length while the loss resistance goes as the length directly.  as you can 
> see, for very small antennas, the loss resistance is a larger part of the 
> input impedance and the the antenna generates more heat and less 
> radiation.  Most small antennas use much heavier conductors to try to 
> control the losses.  Thats why you should use as close to a full size 
> antenna as you can.
>
> In order to have an intelligent discussion about loop antennas I need to 
> introduce the concept of duality.  Maxwell's equations have the property 
> of duality.  If you exchange every quantity in Maxwell's equations for its 
> dual quantity, you get the same equations back.  The electric field and 
> the magnetic field are duals of each other.  What this means to us is that 
> a dipole and a loop are duals of each other.  The same equations describe 
> the performance of both antennas.  a dipole is called an electric antenna 
> because the near field is dominated by the electric field components.  A 
> loop is a magnetic field antenna because its near field is dominated by 
> magnetic fields which have the same mathematical description as the 
> elctric fields of a dipole.  Of course, in the far field the electric and 
> magnetic field components are related by a constant, the impedance of free 
> space, so the far field performance of a loop antenna and an equivalent 
> size dipole is identical.  What are the reasons for choosing on type over 
> the other?  In the case of loop antennas, the performance of a small loop 
> antenna is easier to predict and loop antennas are often used for 
> instrumentation where a small calibrated antenna is needed for LF and VLF 
> measurements.  I think dipoles are easier to build mechanically than 
> similar size loops.  Loop antennas are often easier to rotate and work 
> better close to the ground than dipoles.  Some people just like to be 
> different!  As you can see there isn't a real reason to choose one over 
> the other for far field radiation.  The coice is made for other reasons.
>
> I hope you didn't get too confused.  Believe me it all works and the 
> common antennas all are based on these considerations.
>
> Kerry
>
>