[Elecraft] Long wire balun question

[Ron D'Eau Claire]

Ya' make my drool, Jim, thinking that I'd have a yard nearly big enough to
stick up a 60 or 120 foot full-size vertical plus a bunch of 60 to 120 foot
radials around the base for 80 or 160 meters. I even have to dream of having
enough room to do that on 40 at some future date.

If an end fed wire is 1/2 wavelength long, the ground is unimportant for
efficient operation. The impedance at the feed end will  be in the thousands
of ohms. Unfortunately, the whole rig is likely to be at the same voltage as
the end of any dipole and that's not so good unless you relish the feel of
RF burns and a tendency for logic circuits to go "freaky" with stray RF <G>.


I look at an antenna system as three resistors in series - which it is once
the reactance is tuned out. One resistor is the radiation resistance of the
antenna. All the power consumed in this resistor becomes electromagnetic
waves - the good stuff! 

The other two resistors are the ground resistance and the resistance in the
conductors of the antenna, including the conductors in the antenna tuning
network, any loading coils, feedlines, etc. 

In most cases, the conductor resistance is small - a total of a couple of
ohms. Still that can be significant if hundreds of amps of RF are flowing
through the wire, such as can happen on a feedline at very high SWR's.
That's basically why open wire feedline is so much better than coaxial
feedlines. It has a higher impedance than coax, so the currents flowing at
the current loops is proportionately less at the current loops and so the
resistance losses are less. 

The ground resistance is a different matter, and where most of the losses in
an antenna working "against ground" come in. Of course, the easiest way to
eliminate the ground resistance is to use a balanced antenna that does not
require an RF ground, but sometimes that isn't practical. 

If a ground connection needed, how good it must be depends a great deal on
the radiation resistance of the antenna. 

For example, if you drive a stake in the ground, or throw a hunk of wire on
the ground, you can expect to see a ground resistance of at least a couple
of hundred ohms. Say 300 ohms for a working number. If the antenna is 1/2
wavelength long, it might show a radiation resistance of say 3000 or 4000
ohms. Remember, those resistances are in series. So, for a given RF current,
about 10% of the power is dissipated in the ground and 90% of the power is
dissipated in the radiation resistance and becomes the electromagnetic wave
headed off to that DX we're trying to raise. 

90% is an very efficient antenna! 

On the other hand, if the antenna is only 1/8 wave long, or less, the
radiation resistance might be as low as 2 or 3 ohms. Now, with a 300 ohm
ground, about 99% of the RF power is dissipated in the ground connection and
only 1% or 2% becomes the electromagnetic wave "tickling the ionosphere".
"Tickling" is the right word unless you're running a huge amount of power. 

Of course, most antennas are somewhere in between, but all end-fed antennas
live by the same rules. Divide the ground resistance into the radiation
resistance, and the bigger the number the more efficient the antenna.

Unfortunately for most of us, when we use an end-fed wire we're restricted
to a length of 1/4 or less, so the ground resistance is crucial for decent
efficiency. Most of us have a hard time measuring ground resistance, but one
way to estimate it is just like Jim did. He knows that a 1/4 wave radiator
should show a radiation resistance of about 35 ohms. When he sees a total
resistance of about that using an antenna analyzer at the feed point, he is
sure the ground resistance is a fairly low value. 

One might ask, why not use a half wave end fed wire on 20 meters and above
if it's so efficient? That's a wonderful idea if your shack is up on the
second floor or higher and the wire is out in the clear. Remember, the
maximum radiation is taking place at the current loop on the antenna, and at
20 meters that's only 16 feet from the voltage loop. It doesn't make much
sense to have an antenna that's 99% efficient but so close to the house,
fences, trees and the ground that 95% of what it radiates is absorbed before
it can get over the back yard fence! When an end fed wire like that is used
on the higher bands, it's common to work out a way to put it up in the clear
and feed it with a transmission line. One technique that used to appear in
all the handbooks that hasn't been seen a lot lately is to put the half-wave
antenna up in the clear, then couple one end to a parallel-connected
capacitor and inductor resonant at the operating frequency. A low-impedance
feedline is link-coupled to this "tank" circuit. It works FB, even though
nothing is connected to the "ground" end of the tank circuit. 

Another very famous variation is to connect a 1/4 wave length of open-wire
feedline to the end of the 1/2 wave long radiator. One side of the feedline
connects to the end of the antenna wire and the other side of the open wire
line connects to nothing at all. It is insulated. The 1/4 wavelength of
feeder converts the very high impedance at the end of the antenna to a low
impedance at the rig end, where it can be fed by any low impedance balanced
source. This configuration is famous as the "Zepp" (short for Zeppelin)
antenna used on the big airships of the 1930's where the wire trailed behind
the dirigible and was fed at the end. In theory, if the antenna is exactly
1/2 wave long, the impedance at the end approaches infinity so no current is
drawn; It's a strictly "voltage fed" wire. If there was no current, then
there'd be no current flowing in either side of the open wire line at that
feed point, so the feeders would be balanced and would not radiate. However,
a real antenna doesn't approach infinite impedance and does draw some
current. But neither are insulators perfect, so there's some leakage from
the unconnected side. In practice the balance is good, provided the antenna
is exactly 1/2 wavelength long. 

The problem with both of these configurations is that the antenna length or
matching network up at the end needs to be adjusted when changing
frequencies! That's not a popular idea today. 

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jim Wiley

...For field day, we often use vertical antennas for 80 and 40 meters.  
These are full size verticals, made from aluminum tower sections plus 
tubing "stingers" to get the right length.  There is no reason that 
these same ideas cannot be used for the higher bands as well. 


Each vertical antenna is surrounded by a batch of 16 to 24 radials...We then
put an 
antenna coupler box right at the base of the antenna, and use the MFJ 
antenna analyzer to adjust the system for resonance at the desired 
operating frequency.  BTW - if the MFJ box is connected directly to the 
antenna, it usually shows about 35-40 Ohms impedance at resonance - 
exactly what a 1/4 wave vertical should read!  ...

If you are using an end fed wire (frequently mis-named a long wire), the 
principles are exactly the same.  Any end fed antenna, if operated 
against earth ground, needs a fairly good ground system to be 
effective.  Antennas with one radial (or two, or three or four) may tune 
up easily and show good SWR, but then so does a dummy load.  An 
interesting aside - to be a true "long wire", an antenna must be at 
least one wavelength long, preferably several wavelengths.