[HCARC] 10 meter Loop

[Kerry Sandstrom]

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