[Elecraft] What about Isotron antennas?

[Ron D'Eau Claire]

The isotron will work almost as well as a hunk of wire thrown across the
floor.=20

No one has yet re-written the laws of physics. Until we have
room-temperature superconductors, the efficiency of an antenna will drop
dramatically with its size relative to a wavelength as it becomes really
small. A half wavelength is ideal. One quarter wavelength works as well =
if
there is a good, low loss RF ground available. The wisdom of the =
ancients
says 120 radials 0.2 wavelengths long, arranged in a  circle below a 1/4
wavelength vertical is about as good as it gets. Hams have found that =
one or
two radials will produce fair results, especially if they are elevated.
What's "fair"? We can't be sure, but probably efficiencies of less than =
50%.

Below one-quarter wavelength in overall size, the efficiency drops like =
a
wrench from  the top of a tower. The problem is that the radiation
resistance, that value of "resistance" that represents the power =
radiated
from the antenna for others to hear, drops to very small levels. Low
resistance means relatively high RF currents. Higher RF currents mean =
higher
ohmic losses in the conductors. Very quickly, the best silver or gold
conductor has far more ohmic losses converting RF into heat than can be
radiated. And most of us can't afford to build our antenna out of silver =
or
gold. Copper or aluminum start to look like very dismal conductors. =
Think of
the radiation resistance (the part of the power that is radiated) and =
the
ohmic resistance (the resistance in the conductors that turns RF into =
heat)
as being in series. When the radiation resistance is 50 or 100 or 1000 =
ohms
and the conductor resistance is 5 ohms, very little is burned up as heat =
in
the 5 ohms. But when the antenna is very short and shows a radiation
resistance of, say, 0.2 ohms and the conductor has a resistance of 5 =
ohms,
almost all the RF becomes heat and very little is radiated.=20

There is no antenna that gets around this basic rule. When we make =
antennas
shorter than "full size" (1/4 wave if working against a ground or 1/2 =
wave
long if not) we lose efficiency. All the various designs that are less =
than
full size are compromises trying to get a little better efficiency for a
given size. They use loading schemes to try to make them look =
"electrically"
bigger than they are physically, but those schemes are all lossy. It's =
just
that some are more lossy than others.

The dedicated QRP gang have proven again and again that even a few
milliwatts can span huge distances at times. That's great news for the
manufacturers of tiny antennas who want to suggest that they work like a
much larger antenna. Look at the data and you'll see not a single number
giving you their overall efficiency compared to a full size antenna =
(some
try to say they have efficient loading coils, etc, but don't talk about =
the
antenna system and that's what counts). What they do is print =
testimonials
that so-and-so worked DXCC with their antenna, or some other hopeful
"puffery". It may be true. Put 100 watts in to a tiny antenna and you =
will
probably radiate a few milliwatts. And under the right conditions that =
can
do what the ad says.=20

That takes us back to the basic rule of antennas: put up as much antenna =
as
you can. The old saying that "if you antenna didn't fall down last =
winter it
wasn't big enough" is just a humorous way of putting it. If you are =
going to
put up a horizontally-polarized antenna and want to work DX, get it 1/2 =
wave
up in the sky. If it's down near 1/4 wave up or less, you'll probably do
better with a vertical. That's just because of the way the reflected =
wave
from the earth interacts with the radiation from the antenna. Low =
horizontal
antennas are GREAT for short-haul communications though because most of =
the
signal is concentrated on the first 500 to 1000 miles from your QTH.=20

If an Isotron is all you can put up, save your money and build an =
Elecraft
ATU and use it to load a ten-foot hunk of wire. You'll be ahead overall. =
And
if you can make it a longer piece of wire and put it outside up in the
clear, it'll be far, far better.=20

Ron AC7AC

-----Original Message-----
In my continuing effort to learn about antennas for the K1 and K2... =
what is
your view of Isotron antennas?  Have you had experience with them?  I =
have
yet to buy an antenna (dipole, vertical, Isotron, otherwise) for 40 =
meters.