[HCARC] 10 meter Loop

Sun Aug 12 19:29:50 EDT 2012

Gary:

I believe you said you had a copy of the ARRL Antenna Book. There are plenty
of well-tested designs for 10 meter gain antennas you can try out. The
software that comes with the ARRL Antenna Book will let you play with these
designs. One of the fun aspects of ham radio is trying out these different
antenna designs.

I've a few observations concerning the claims of gain for many antenna
designs you find on the internet. 

All antennas operate according to laws of physics. Antenna gain is
essentially a result of directivity, i.e. more power is being concentrated
in one direction than another. The overall power stays the same. The higher
the gain of the antenna in one direction, the lower in another direction.
That's why the Yagi (beam) antenna is so popular at HF and VHF frequencies.
It has good gain in the forward direction and lower gain in the rearward
direction - what is called the front-to-back ratio. 

The actual gain of an antenna is the result of a number of factors: the
efficiency of the antenna (i.e. how much of the input signal is actually
output), the electrical gain of the antenna (discussed above), height of the
antenna above ground (primarily for horizontal antennas), and the type of
ground in the near field (ground conductivity). All of these can be modeled
using relatively inexpensive software, such as EZNEC.

I've found that the exorbitant claims for gain from antenna designs commonly
found on the internet are the product of wishful thinking. At HF it is not
easy to get more than 6-9dB of gain from a reasonably sized antenna compared
to a simple dipole at the same height. High gain Yagis based on National
Bureau of Standards models, top out at about 14.2dB gain compared to a
dipole. However, this requires 15 elements on a boom 4.2 wavelengths long
(see http://tf.nist.gov/general/pdf/451.pdf)! A more reasonably sized
antenna (5 elements on a 0.8 wavelength boom) will give you 9.2dB gain over
a dipole, based on the NBS models.  

Just because Jim Bob puts up a new antenna and starts making lots of
contacts with 20dB over S9 signal reports from other hams, doesn't mean that
his new antenna is really good. A lot will depend on the band and
propagation. It will also depend on the antenna system the other ham is
using. If the other ham has a really good antenna system, of course Jim Bob
is going to sound loud. And without a calibrated receiver, a signal report
is really just a good-faith estimate, not a scientific statement of fact. 

Unless an antenna has actually been tested on an antenna range with
calibrated equipment, a claim for the gain of an antenna is exactly that: a
claim. The only exception are designs that have been computer modeled where
the modeled antenna's characteristics have been validated by range testing.
The National Bureau of Standards has several of these models; a few
commercial manufacturers have also done such validation.

The other factor you need to be concerned about is mechanical design. A
design that is too complicated, difficult to erect, or is fragile in the
wind, is not of much use. Here the KISS principle comes into play. 

Finally, no one design will be optimum for all situations. If you want to
communicate locally and not in a preferred direction, e.g. for a local net,
an omnidirectional antenna, such as a vertical, will be a better choice. If
you want to communicate in a preferred direction a directional antenna, such
as a dipole or simple Yagi, will be a better choice. For close-in
communications you also need to consider the polarization of the antennas of
the other parties and have your antenna be similarly polarized (e.g.
vertically or horizontally).  For DXing you want a low angle of radiation
and probably some gain so either a Yagi type antenna (for 40m/30m and above)
or a vertical or inverted-L (for 160m, 80m, and possibly 40m) will be the
usual choices.

I've built a number of antennas over the years - some of my design and some
by others - and I keep coming back to the basic, tried-and-true, antenna
designs: dipole, Yagi and 1/4 wave vertical/inverted-L. They are simple and
they work.

I apologize for the length of this response to your question. My purpose is
to help you have a better idea of which antenna designs have real merit so
you don't spend your time chasing after designs that won't live up to their
hype.

73,

Dale - K4DG

-----Original Message-----
From: hcarc-bounces at mailman.qth.net [mailto:hcarc-bounces at mailman.qth.net]
On Behalf Of Gary and Arlene Johnson
Sent: Sunday, August 12, 2012 4:27 PM
To: Bob Richie
Cc: hcarc at mailman.qth.net
Subject: Re: [HCARC] 10 meter Loop

This one is sized for 6 meters, but I suspect it could be upsized for 10
meters too.  I thought of making these out of excells 1/2 inch copper water
pipe, or possibly with the soft copper line that they use for refrigerator
water lines or the stuff they use for copper refrigerant water lines.  So
many things to think of - so much frustration not to be able to try them
out.  Someday I will though.

Gary J
N5"BAA"
  ----- Original Message ----- 
  From: Bob Richie 
  To: Gary and Arlene Johnson 
  Sent: Sunday, August 12, 2012 3:15 PM
  Subject: Re: [HCARC] 10 meter Loop

  This antenna has been around for a while.  The problem is finding a place
high enough to hang it around here. Some of the tall trees in the
southeastern pine forests would make this a natural, along with some other
loops.  If you have not been to this site
http://www.hamuniverse.com/antennas.html it has a bunch of antennas and
other interesting articles.

  Bob
  K5YB
  Kerrville, TX 78028

----------------------------------------------------------------------------
--
  From: Gary and Arlene Johnson <qltfnish at omniglobal.net>
  To: hcarc at mailman.qth.net 
  Sent: Sunday, August 12, 2012 1:54 PM
  Subject: [HCARC] 10 meter Loop

  I found the following while looking at antennas on the net.  It's called
the "20db over 9 10 Meter Loop Antenna".  Diagram is on the link.

http://9m2mgl.blogspot.com/2008/10/with-large-number-of-operators-and-wide.h
tml

  "With the large number of operators and wide availability of
inexpensive,singleband
  radios,the 10-m band could well become the hangout for local ragchewers
that it was before the advent of 2-m FM,even at a low point in the solar
cycle.This simple antenna provides gain over a dipole or inverted V.It is a
resonant loop with a particular shape.It provides 2.1 dB gain over a dipole
at low radiation angles when
  mounted well above ground. The antenna is simple to feed-no matching
network is necessary.

  When fed with 50-? coax,the SWR is close to 1:1 at the design
frequency,and is less than 2:1 from 28.0-28.8 MHz for an antenna resonant at
28.4 MHz.The antenna is made from #12 AWG wire (see Fig 22.80) and is fed at
the center of the bottom wire.Coil the coax into a few turns near the
feedpoint to provide a simple balun.A coil diameter of about a foot will
work fine. You can support the antenna on a mast with spreaders made of
bamboo,fiberglass, wood,PVC or other nonconducting material.You can also use
aluminum tubing both for support and conductors,but you may have to readjust
the antenna dimensions for resonance.

  This rectangular loop has two advantages over a resonant square loop.
First,a square loop has just 1.1 dB gain over a dipole.This is a power
increase of only 29%. Second,the input impedance of a square loop is about
125 ?.You must use a matching network to feed a square loop with 50-?
coax.The rectangular loop achieves gain by compressing its radiation pattern
in the elevation plane.The azimuth plane pattern is slightly wider than that
of a dipole (it's about the same as that of an inverted V).A broad pattern
is an advantage for a general-purpose,fixed antenna.Then rectangular loop
provides a bidirectional gain over a broad azimuth region.

  Mount the loop as high as possible. To provide 1.7 dB gain at low angles
over an
  inverted V,the top wire must be at least 30 ft high.The loop will work at
lower
  heights,but its gain advantage disappears.For example, at 20 ft the loop
provides
  the same gain at low angles as an inverted V.

  Sumber daripada ARRL 
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