[HCARC] Antennas, Radios and Elmering

[Kerry Sandstrom]

Gary,

There are rules and they indeed are followed.  Its not always obvious what 
rules apply in a given situation.  It takes years to figure out what all the 
rules are.  Actually a college degree does not always help.  College courses 
give you the very basics and teach you how to extend those basics to real 
life situations.  It doesn't matter what your major is/was, its still the 
same.  You have to look at each situation and try to figure out what 
assumptions you can make and therefore which rules apply.  Actually they all 
apply, but in many practical situations they don't matter.  The theory of 
Relativity is an excellent example.  If you're driving your car at 30 MPH, 
relativity still applies, however, the difference in the answer if you 
include relativistic effects and the answer if you ignore them is so small 
that no one includes relativistic effects.  If on the other hand you're a 
subatomic particle going "mach a bunch", near the speed of light, if you 
don't include relativity you will never get close to the right answer.

With antennas, there are two main characteristics you should be interested 
in from an RF viewpoit.  The first is the radiation properties of the 
antenna and the second is the impedance of the antenna where you apply RF to 
it.  These are independent of each other.  You need to be concerned about 
the impedance because you have to get your transmitter to deliver power to 
the antenna.  In EE the maximum power theorem says that in order to deliver 
the maximum power to the load, the impedance of the load (for us the 
antenna) must match the impedance of the source (for us, the transmitter). 
Sinced our transmitters typically, but not always, have an impedance of 50 
Ohm, our load should also have an impedance of 50 Ohm.  At this point we 
have a choice.  We can either use an antenna that has an impedance that 
matches our transmitter and connect the two with a transmission line that 
has the same characteristic impedance (called a matched line) or we can use 
any old transmission line and antenna impedance we want and use a matching 
network (called an antenna tuner) where we want to change from the 50 Ohm 
impedance to whatever impedance the antenna/transmission line has.  We need 
to define 'resonance'.  Resonance is the frequency where the load or circuit 
or whatever has an impedance that is purely resistive.  An impedance in 
general has a resistive part and a reactive part.  The reactive part may 
appear capacitive or inductive.  A wire that is approximately half a 
wavelength long is resonant.  If you break the antenna in two at the middle 
and measure the impedance at that point, you will find it is resistive and 
approximately 75 Ohm.  This is called a half wave dipole and is probably the 
most common ham antenna in the world.  Since it is balanced, the two sides 
are the same, it should be fed with a 75 Ohm balanced line (twin lead or 
coax with a 1:1 balun).  If you don't feed it with a balanced line, ie, you 
use coax cable, you introduce an inbalance in the system.  As a result, the 
feedline , coax cable, will radiate and distort the radiation pattern.  At 
HF close to the ground, you will never be able to measure this.  At UHF you 
will.  Ignore the balance issue at HF.  This is an example of a rule that 
doesn't matter so doesn't need to be followed at HF.

If you take the same dipole and feed it at the end as W$WJ tried to do with 
his DX-35 many years ago, you will find that since the wire is still 
resonant, the impedance at the end is still resistive , but the impedance is 
now ~1000 Ohms.  A 450 or 600 Ohm parallel line is the correct transmission 
line, but what is the DX-35 going to do when it sees ~1000 Ohm load instead 
of the 25 - 200 Ohm load it can handle?  What the DX-35 does is not deliver 
very much power to the load.  You can tell because you can't find a setting 
of the tune and load controls that give the correct readings on the plate 
meter.  If you want to use an end fed wire and an open wire transmission 
line you need something to change the high impedance of the load to a low 
impedance that the DX-35 can use.  Our new rigs, don't have tune and load 
controls, they have a fixed tune output that needs to see a load of 50 Ohm. 
It only makes life harder.

The Windom of any variety, uses a feedpoint some where between the center 
and the end.  Its impedance is somewhere between 75 Ohms and ~1000 Ohms,  if 
its at a frequency where the wire is resonant.  If its not at a resonant 
frquency, its impedance will have both a resistve and a reactive component. 
You have to remove the reactive component and transform the resistive 
component to 50 Ohms.  That is what an antenna tuner does, or in the case of 
the G5RV and Carolina Windoms, what the lengths of transmission line and 
ferrite transformers/baluns do.  Since everything depends on lengths of 
antennas and transmission lines in terms of wavelength, as the frequency 
changes so does the impedance.

While I have talked about a half wave wire, that isn't particularly critical 
to the discussion.  Any hunk of metal will have an RF impedance.  Depending 
on where/how you feed it, that impedance will vary all over the place.  In 
general, you like the hunk of metal to be a half wavelength or more in at 
least one dimension.  At this size it will normally have an impedance that 
is easier to match than extremely small hunks of metal.  The shape doesn't 
really matter.  you would like it has high as possible.  The only way to get 
practical gain out of an HF antenna is to have enough metal in the air to 
cover a volume on the order of a wavelength.  The critical measurement is 
called the antennas effective aperture.  If you know its effective aperture, 
you can estimate its gain and beamwidth from that measurement.

So Gary, to your specific question,  you can put that antenna together 
anyway you want.  You have to figure out a way to match the antenna to the 
radio.  That includes both physically with a coax connecter/cable of some 
sort and electrically with some kind of matching system.  If you put the 
antenna very much higher and try to use it on 20, 15 or 10 meters it will no 
longer be "optimized" for NVIS, but it will still work.  If you want to just 
plug it into a radio without a matching system, i don't it will work very 
well.  If you have an antenna tuner and adjust it properly, it will work as 
well as any othe rantenna of similar size.  The practical stuff is simple - 
just put it up and play with it until it works.  Understanding why and 
understanding the comments others make about an antenna - that's hard.

Kerry