[HCARC] 10 meter Loop

Sun Aug 12 23:23:46 EDT 2012

Kerry and others as needs be,

Time to be careful with us new guys again!!!

 "Physics says the maximum gain you can get is determined by the physical 
size of the antenna."

I assume by physical size you are talking about length or are we talking 
about diameter of the antenna, or are we talking about a combination of 
both???

What does the metal that the antenna is contructed from have to do with its 
abilities - IF ANY???

Gary J
N5"BAA"

----- Original Message ----- 
From: "Kerry Sandstrom" <kerryk5ks at hughes.net>
To: <hcarc at mailman.qth.net>
Sent: Sunday, August 12, 2012 8:08 PM
Subject: Re: [HCARC] 10 meter Loop

>I agree with Dale.
>
> You have to be very careful when you see gain figures.  Over the 50 years
> that I have been a ham, there have been many many magic antennas. 
> Generally
> they are around for a few years and then they disappear never to be heard 
> of
> again.  Most of my memories are of VHF antennas.  Some of the designs that
> have come and gone are the old Cush Craft colinears, basically 8 half 
> waves
> fed in phase and backed up with reflectors.  The basic design is sound but
> Cush Craft had unique mechanical design in which the ends of adjacent half
> wave elements are embedded in a plastic insulator for there support.  The
> problem is the insulator added  capacitance at a high impedance point and
> they were impossible to match over a reasonable frequency range.  KLM had 
> HF
> and VHF antennas which had 2 or 3 driven elements in parallel.  This was
> supposed to increase the bandwidth.  The idea I first saw published as the
> Swan multi drive beam and was designed for 2 meters.  It worked but not 
> that
> much better than normal yagis and the design hasn't been seen in years.
> Currently there are a rash of dipoles with special properties and 
> verticals
> with unique designs which make them wider band or they don't require a
> ground.  The latest rage seems to be verticals that are 40 feet tall 
> instead
> of 33 feet (1/4 wave for 40 m).  Physics says the maximum gain you can get
> is determined by the physical size of the antenna.  Two well designed
> antennas the same size should perform the same no matter how different 
> they
> look.  Unfortunately the reverse is not true.  While there is a limit to 
> the
> maximum gain, there is no limit to the minimum gain: you can design large
> antennas which just don't work!
>
> Unless you know the operator on the other end, I wouldn't believe any 
> signal
> report I receive.  Too many ops just give you a 599 and not just in
> contests.  Such a report is useless.  The best way to compare antennas is 
> to
> switch back and forth between them and compare the received signal 
> strength,
> but be aware that much of the difference you see may be because the 
> antennas
> are optimized for different elevation angles and the signal is mainly 
> coming
> in on one angle.
>
> Gain is the product of the directivity of an antenna and its  efficiency.
> Its efficiency is a complex concept that includes losses in the antenna 
> and
> the efficiency of the way the antenna aperture is illuminated.  Note that
> yagis and dipoles and vertical are not particularly efficient.  Several
> years ago there was a lot of discussion about "supergain" antennas in the
> professional antenna literature.  What actually was happening is you could
> design antennas which had directivity higher than the conventional rules 
> of
> thumb allowed, however, they were very narrow band, There impedance had
> large reactive components and there losses were high.  The end result was
> you ended up with an antenna which had a narrow beamwidth and hence high
> directivity, but its efficiency was so low that its gain was lower than
> equivalent sized conventional designs.
>
> A dipole has approximately 2 dB gain over a (mythical) isotropic.  Many
> antenna manufacturers like to specify their antenna's gain compared to an
> isotropic antenna.  Sometimes they will use dBi to show its gain in dB
> relative to an isotropic antenna.  Others will use dBd to signify their 
> gain
> is relative to a dipole.  Unfortunately most seem to use just dB and it
> could be either.  For many years QST would not permit the antenna
> manufacturers to include gain figures in their QST ads because the numbers
> were so unreliable.
>
> Dale mentioned that there is antenna modeling software available.  He
> mentioned EZNEC.  I use NEC2 with 4NEC2 as the interface, myself.  It is
> available for free on the internet.  NEC2 was developed over the last ~50
> years by the US government, Primarily the DoD laboratories and the 
> national
> nuclear laboratories.  The current available version runs well on modern
> PC's.  Any model requires a knowledgable user.  Be prepared to spend some
> time learning how to create the input, run the model and interpret the
> output data.  I think the effort is well worth it.
>
> I have been to several antenna ranges.  They have been for UHF and higher.
> Typically they have a transmitter site on a high hill overlooking a valley
> and second site on a high hill on the other side of the valley  for the
> receiver site.  The goal is to elimante as much of the ground reflections 
> as
> possible.  If you get the idea that setting up an antenna range is an
> expensive proposition, you are correct.  It is only worse at HF.  I don't
> believe that any of the amateur antenna manufacturers have an antenna 
> range.
> In years past, a couple of them that also did work for DoD had a range or
> access to one, but those guys are long gone I think.
>
> The local ten meter net actually provides an excellent oppurtunity for us 
> to
> play with antennas.  There are a few participants located at varying
> distances with different terrain between us.  We should be able to make
> reasonable estimates of how we have improved our stations from week to 
> week.
> It should be a lot of fun.
>
> I don't like long e-mails, but unfortunately there is no way to discuss 
> some
> of these topics in one sentence.  Playing with antennas is a lot of fun.
> Most only require some wire and insulators a some rope.  You can learn a 
> lot
> for very little money.  When all is said and done there are really only a
> few different types of antennas.  The ones of most interest to hams is
> dipoles (which includes vertical which are just half a dipole and its
> image), travelling wave antennas (rhombics, vees, Beverage, etc),
> log-periodics, and at the higher frequencies, aperture antenna (parabolic
> reflectors, corner reflectors, etc).
>
> Have fun and see you on ten,
>
> 73,
>
> Kerry
>
>
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