[Elecraft] EH antenna??

[Ron D'Eau Claire]

Sorry. I didn't mean to cause a dust-up about antennas on the Elecraft
reflector. I simply tried to answer a question about something I have
looked into and found more questions than answers myself, Hi!

I CAN answer your question about the bandwidth, Trev. Again, it's
straightforward antenna theory. All you need for adequate bandwidth are
adequate losses.  Now, in a truly big antenna, such as really long "long
wire" or a loop many wavelengths around, most of the r-f gets radiated,
so there is very little to be "reflected" back to the source. Such an
antenna has the best kind of losses: radiation "losses". The signal all
got launched! Unfortunately, that takes antennas many, many wavelengths
long, so most of us live with our doublets and loops and other
intermediate sized antennas and, by being careful, we get very good
efficiencies out of them. Most of the r-f is consumed in the "radiation
resistance" -- that fictional resistance that represents the r-f that
got radiated.

But, as an antenna gets smaller relative to the wavelength of the r-f
signal, the reactance changes more quickly with frequency. Hence, the
"bandwidth" over which radiation occurs efficiently will be narrow.
Looking at it another way, the "bandwidth" over which the r-f voltage
and current are in phase is very narrow, and unless they are in phase
the antenna cannot take the r-f power. So "happiness" in a small antenna
is a small bandwidth. It's a good sign that the antenna is working
efficiently.  

Even so, it's easy to make a small antenna show a wide bandwidth. All
you have to do is to increase the resistance. For example, if you put a
3 d-b attenuator in between the transmitter and any antenna (or even NO
antenna), it will only absorb 1/2 of the transmitter's power and the SWR
will never exceed 2:1. But that doesn't mean a thing in terms of
radiation efficiency.

B&W has for years sold a "wide band" doublet that works just that way
for Ham and marine use. You give up half the transmitter power, but you
don't have to worry about matching networks, ATU's etc. The B&W has the
resistive element out in the center of a "folded dipole" configuration,
so r-f flows through the antenna wires and much of it is radiated. It's
a workable "compromise" antenna where bandwidth is more important than
efficiency. 

Small antennas with big "loading" (or as the EH calls it "phase shift")
networks frequently show very wide bandwidths when the network is lossy.
And as the antenna gets smaller the matching network can be carrying
hundreds of amperes of r-f, even at QRP power levels.  Again, such an
antenna may work halfway around the world on a milliwatt under the right
conditions. The point is that you might be putting 5 watts into and
getting microwatts out, and sometimes microwatts will travel a LONG
distance. That's why I find that "on-the-air" tests that are not
conducted under carefully controlled conditions with the proper
instrumentation are virtually useless. 

Evaluating an antenna objectively requires setting up a situation where
the performance can be carefully measured.  I don't own the acres of
land or the test equipment for an "antenna range", so I'm out of luck on
that score.

I am NOT suggesting that people who hook up a new antenna and work
fantastic DX with it are lying, but I am saying that the "real world" is
too variable to collect valid data about one antenna being better
without carefully-controlled conditions where antennas can be compared. 

I'm as interested in "why" something works as I am in operating it, so
that's a question I often ask. When I built my first crystal set many
moons ago, the first question I had was "How does this thing work?". I
haven't changed. 

Ron AC7AC
K2 # 1289

-----Original Message-----

   ...The most significant thing about this antenna, and the factor that

makes it so obviously different from an ATU loaded chunk of metal, is 
its bandwidth.  Once tuned (I use an MFJ Analyser) its bandwidth on 40m 
between the 2:1 SWR points is in excess of 100 kHz.  Try and achieve 
that with a commercial mobile whip, never mind less than 2 feet of metal

tubing.  If anyone can explain it within conventional antenna theory 
then I would be pleased to hear it.... I see many postings about 
this antenna from people who have read the alleged theory and come to 
their own conclusions but have never tried it for themselves..
Trev G3ZYY