Large loop antennas (fwd)

[David V. Rogers]

---------- Forwarded message ----------
Date: Sat, 03 Oct 1998 10:44:01 -0400
From: Dick Rucker <rrucker@clark.net>
To: "Denny Avers, W3DRY" <avers@ibm.net>,
    "Ben Baddley, W4FQT" <BBaddley@aol.com>,
    "Glenn Bilger, W4OCC" <GBilger@juno.com>,
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    Howard Bullock <W4LBM@aol.com>,
    Al Cammarata <w3awu@juno.com>,
    Bob Cannon W4IGO <wigo@aol.com>,
    "Armand Bud Caron, WA3TNQ" <alcaron@erols.com>,
    Wayne Cooper <AG4R@erols.com>,
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    Phil Schroeder <KF4AJZ@juno.com>
Subject: Large loop antennas

Since interest in antennas seems to run high among our members, and Wayne
Cooper AG4R is a builder and user of small loops, the following might catch
your interest:

>Date: 	Sat, 03 Oct 1998 02:46:11 -0700
>Reply-To: utahfolk@konnections.com
>Sender: owner-qrp-l@Lehigh.EDU
>From: utahfolk@konnections.com
>To: "Low Power Amateur Radio Discussion" <qrp-l@Lehigh.EDU>
>Subject: Loops
>
>A lot has been written about loop antennas.  Some of it not so.  
>
>A large loop is one whose circumference is at least one wavelength
>at the frequency of operation.  This discussion relates to large loops.
>
>Reasonably resonant is a term used to mean that a practical network
>can be built to match the antenna feedpoint impedance to a common
>coaxial line.
>
>A half wave dipole at frequency F is resonably resonant at its
>odd harmonics: 3F, 5F, 7F, etc.  The only practical case in the
>HF ham bands is: a 40M (7mhz) dipole works on 15M (21mhz).  Some
>try 30M dipoles on 10M too.  But in general not a lot of multibands
>from a dipole.
>
>Consider now the half wave folded dipole, this is a dipole in which
>the impedance of the feed point goes up by a factor of about 4. The
>antenna consists of two parallel half wave wires spaced quite closely
>and connected at each end.  A feedline connects to the midpoint of
>one of the half wave wires.  The total circumference or perimeter of
>this folded dipole is one wavelength of wire.  The enclosed area is
>equal to one half wave times the distance between the two half wave
>wires.  For our discussion, hardly any area at all ... call it ZIP!
>This folded dipole behaves just like any halfwave dipole when operated
>harmonically ... it is reasonably resonant on its odd harmonics only.
>
>Now take this folded dipole and "open" up the area enclosed by the two
>half wave wires.  The maximum area you can enclose using a fixed length
>of wire is when the geometry is a circle.  If you analyze a circular
>loop you discover it is reasonably resonant on ALL harmonics, you can
>match it quite easily on all the ham bands to coax.  But a circular
>loop takes an infinite number of skyhooks (kinda expensive if perchance
>somewhat impractical) so settle for a square (1/4 wavelength of wire 
>on each side -- at the lowest frequency of operation), takes four
>skyhooks, more likely to be found or accomplished.
>
>The impedance/matching problem has moved back towards that of the
>folded dipole, but a square is still quite large so the characteristics
>are much closer to the perfert loop than the ZIP area folded dipole.
>A square is 80%-ish of the area enclosed by the wire in a circular
>configuration.  Make a loop triangular, rectangular (off square) 
>and you move closer to the folded dipole and it's more restricted
>impedances ... so, what's the rule:  When you put up a loop, 
>enclose the maximum area possible ... use as many corners as you
>can, but don't settle for anything less than a square if you don't
>have to .... stay as far away from the folded dipole ZIP area as
>you practically can.
>
>The plane of a loop is that plane containing the loop wire.
>These loops are thus viewed as two dimensional objects.
>
>A one wavelength loop (this frequency is called the design or
>fundamental frequency) in free space has a dipole pattern normal
>to the plane of the loop ... that is, it radiates max perpendicular
>to the loop plane.  At the second harmonic, the pattern splits into four
>lobes and these lobes move towards the plane of the loop.  
>A similiar effect occurs at each higher harmonic, the number of
>lobes and nulls increases.  The lobes (max values) fold towards
>the plane of the loop.
>
>Result -- here is antenna that operates all bands (tuning wise) and
>whose patterns fold into the plane of the loop as the frequency is
>increased.  What happens if you mount the loop vertically?
>
>It radiates perpendicular to the plane of the loop (thus at a somewhat
>low angle above the earth horizon) on it's fundamental, but the lobes
>fold into the plane of the loop (begin to radiate towards the sky) as
>the frequency is increased.  
>
>What happens if you mount a loop horizontally?  The pattern on the
>fundamental frequency has lots of high angle signal, but not worse
>than a corresponding dipole!, for all practical purposes they are
>the same.  As you incrase the frequency the lobes fold down into
>the plane of the loop (down towards the DX horizon!) and gain 
>increases.  
>
>Conclusion: If you hang a loop vertically you limit it's practical
>use to one band (the fundamental design frequecny band) and most 
>loop users have been taught this use for a loop.  They perform
>super well ... but are single band loops for DX purposes. 
>
>If you hang a loop horizontally, you have dipole illumination of
>the sky but as you move to the harmonics (which are easily matched)
>the lobes move into DX elevations and shazzam you have a multiband
>DX monster!
>
>Thus, if you work one band, hang the loop vertically.  If you intend
>to work many/multi bands, hang the loop horizontally.  The gain is
>actually a bit more in the horizontal mode!  
>
>For years folks have said the horizontal loop is a "cloud warmer" and
>doesn't give much DX results .... they have never USED a horizontal
>loop harmonically!  In fact, until about 1980 or so, most loops users
>were sold the mount them vertical story.  In 1985 a fella wrote an
>article called the LOOP SKYWIRE, a very practical presentation of the
>horizontal loop and an article that has been copied, "borrowed and
>rehashed as original text" by more than one folk out there! First
>published in November 1985 QST and since about 1990 included in the
>ARRL Radio Amateurs Handbook and the ARRL Antenna Handbook.  Hundreds
>and possbily thousands of (horizontal) loop users have discovered 
>this antenna to be the best multiband antenna they've ever used and
>few seldom take it down or remove it!  I know of folks who took their
>beams down before their loops!
>
>The great advantage of the loop is that it can be operated as a grounded
>antenna (feed with coax) and thus does not suffer the major wipeout of
>precip static.  Perhaps the most significant parameter is the loop has
>a considerable improvement in S/N ratio, in that it's response to noise
>is often 3-4 S-units lower than verts and dips.  You hear things you
>haven't heard before.
>
>Feeding the horizontal loop is essentially your choice, the feedpoint
>impedance at the mid of one side of a square loop is a bit higher than
>at a corner, but feed a foot or so off the corner so you don't have to
>combine feedline mechanics with those of corner support mechanics ...
>
>There is a fundamental rule about horizontal loops:  If you can hear
>them
>you can work them!  100 watts will do quite nicely!  Go read the sidebar
>in the original LOOP SKYWIRE article where W8BO comments about his loop!
>5BDXCC!  He uses RG58 coax to feed it!
>
>This writers experience with loops goes back to 1957 and there is no
>antenna
>that beats it in my experience!  I have many first place plaques, certs
>and
>listings in contests, many QRP!  If you are interested in seeing the
>160M
>version, check UPFRONT QST either Feb 96 or 97, the picture of the flat
>loop that took 4th place worldwide, made 50 WAS in 39 hours, and set the
>record for number of ARRL Sections worked (70) in the 1994 ARRL 160M DX
>contest
>.... QRP!  A calibrated 4.8 watts on 160M ... warmin' them clouds!!!! 
>
>The more of you who believe the popular "loops are cloud warmers"
>propagada
>the stronger my signal will remain on the band.  Don't put up a loop! 
>Use
>a dipole, a vertical or a random wire ... loops don't work, they attract
>lightning, bugs and birds.  Neighbors don't like them either, and they
>radiate into your phone lines, cause TVI and BCI .... and one
>case of GDI (Garage Door Interference)  ... I think I heard some Doctor
>say they cause brain twomores and mebbe pizza cravings! So, loops are
>hazardous!
>Don't put up a loop!  Keep your antennas close to the ground.  Load
>your extension ladder .... and getta 8KW furnace AMP !!
>
>.... de Dave NC7W Huntsville, UT