[HCARC] 10 meter Loop

[Bill Tynan]

Gary:

One thing to remember about tuners is that, if they are in the shack, they 
don't do anything to match the feedline to the antenna. All they do is try 
to get a good match for the rig so it can deliver is maximum power to the 
cable and not be damaged. Obvious, the place for a tuner is at the antenna. 
But, of course, that presents other problems.

If there is a high VSWR between the antenna end of the feedline and the 
antenna, less power will be delivered to the antenna and there will be 
increased loss in the feedline.  That's one reason why copper wire open wire 
feedlines were so popular (mostly prior to World War 2). Most hams in those 
days used random lengths of wire for their antennas, or lengths that would 
work on several bands (mostly 80, 40 and 20. There were no 60, 30, 17, 15 
and 12 meter bands in those days and 10 meters was considered the 
stratosphere by most hams, like the microwaves are today. The 10 or 12 gage 
copper wire used for the feeders had less loss than other types of cable 
available then, including the coax we use today.

BTW, if you look at the Website describing the shortened 10 meter dipole 
that  AF5AO referred to, you will see that PY2RML used RG-58 for both the 
antenna and feedline and stated the it had a VSWR of 1.1. That MAY be a 
result of the relatively high losses of RG-58 feedline at 28 MHz. If a 
feedline has high losses, it will make any antenna look good, as far as VSWR 
is concerned. The PY didn't say how long the feedline is. Obviously, the 
longer it is, the higher its losses.

Getting away from VSWR for a moment, there is a very interesting note on a 
balun in the September QST. It's an idea presented by Frank Donavan, W3LPL. 
Frank, BTW, is one of the world's top contesters.

The idea is for a 1/4 - 3/4 balun. A balun is used to feed balanced antennas 
such as dipoles to unbalanced feedline such as coax. BTW, don't be confused 
by the techniques of merely coiling up some of the coax at the feedpoint of 
the antenna. That does NOTHING to balance the amount of RF going to the two 
sides of the antenna. What is does do is act as a choke to help prevent RF 
current from flowing back to the shack on the cable's shield. where it can 
cause problems such as RF feedback.

But back to W3LPL's 1/4 - 3/4 balun which DOES cause equal RF currents to 
flow in the two sides of the antenna. It uses a T connector to split the RF 
into two equal amounts, one half going the 1/4 wavelength length of cable 
and the other half going to the 3/4 wavelength length of cable. Then, 
because of the phase difference between the two lengths of cable, balanced 
currents, 180 degrees out of phase with each other, are fed to each side of 
the antenna. Obviously, this will work only on one band, although it might 
work for a 40 meter antenna on 15 meters.

This is a concept I have never seen before and when I described it on the 
Roadrunners Microwave Group's 2 meter net last evening, several other 
long-time hams said that they had never encountered either.

More fodder for this very interesting discussion. In my opinion, it's a good 
use of the Club's reflector.

73,

Bill, W3XO/5

----- Original Message ----- 
From: "Gary and Arlene Johnson" <qltfnish at omniglobal.net>
To: "Kerry Sandstrom" <kerryk5ks at hughes.net>
Cc: <hcarc at mailman.qth.net>
Sent: Monday, August 13, 2012 10:05 AM
Subject: Re: [HCARC] 10 meter Loop


> 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
>>
>>
>
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