On Sun, 01 Oct 2006 01:12:02 -0400, jawod wrote:

Owen Duffy wrote:
Some months ago I put some thoughts together to assist our newly
minted 6-hour hams who seem attracted to either short end-fed wires
(although they refer to them as long-wires) or G5RVs.



I ceased efforts when it became apparent that the procedure was beyond
the base competency level for our Foundation Licence,
Comments welcome.

Owen

Hi Owen--great job! But you said "comments welcome", so please don't be offended
by what I have to say.

I have always been curious about all the hype, excitement, and marketing
popularity of the G5RV, So I included a section on this antenna in both editions
of Reflections, ed 1 in 1990, and ed 2 in 2001, in an attempt to educate the
newcomers to its realities. So I invite you to read the pertinent section from
the book below:

The following is a quote from Chapter 20 in "Reflections-Transmission Lines and
Antennas," authored by W2DU.

"Sec 20.2.4 The G5RV Antenna

With this background on random-length dipoles behind us, it seems
appropriate to make a critical examination of a particular 102-foot dipole that
is enjoying a great deal of popularity--Louis Varney's G5RV dipole. In spite of
its popularity, its operation is not well understood among many amateurs, so
I'll shed a little light on the G5RV. First of all, the reason for the 102-foot
length for the G5RV is no secret, but it is not well known. Being unaware of
certain antenna principles, many amateurs have come to believe that there is
some sort of magic in the 102-foot length, and that their all-band success with
this antenna is dependent on this specific length. Nothing could be further from
the truth, because, except for 20 meters (as I'll soon explain), any random
length of at least 3 lambda/2 long at the lowest operating frequency will
perform equally well.
What is the significance of the 102-foot length? Unbeknown to many amateurs
who use it, Varney designed the antenna to be a resonant 3 lambda/2 radiator on
20 meters--that length is 102 feet. He had two specific reasons for selecting 3
lambda/2 on 20--he wanted a four-lobe radiation pattern and a low feed-point
impedance. The 3 lambda/2 was a clever choice, because this length yields a
four-lobe pattern, in addition to a low feed-point impedance that can be matched
to a 50-ohm line with a line transformer without requiring an antenna tuner. As
Varney also intended, this 102-foot length results in a strictly random length
on all bands except 20, so except for the 20-meter considerations I've just
described, there is no magic whatever to this length. The last comment in the
previous paragraph should be taken seriously. It should be noted that the
102-foot length of the G5RV is almost exactly the length I recommended above for
a random-length antenna, 3 lambda/8 at the lowest frequency of operation,
because 100 feet is the length required for 3 lambda/8 at 3.5 MHz.
On 20 meters, the input impedance of the 3 lambda/2 G5RV radiator is low
because the feed point is at the center of the central 1/2 wl portion. Hence,
the impedance (the resonant resistance) is only moderately higher than if the
outer 1/2 wl sections were eliminated, leaving a single 1/2 wl dipole. At the
frequency of mid-band resonance, the free-space feed-point impedance is
approximately 100 + j0 ohms, which reduces to around 90 + j0 ohms at a
convenient height above ground. This results in a mismatch of about 1.8:1
relative to 50 ohms. Varney's choice of the 34-foot line-transformer matching
section, 1/2 wl on 20 meters, was to make a 1:1 impedance- transformer that
repeats the 90 + j0 antenna impedance at its input terminals. Thus, with a
suitable choke balun to make a transition from a balanced to an unbalanced line,
the low 1.8:1 mismatch makes connecting to a 50-ohm line feasible without
requiring an antenna tuner. The SWR on a 1/2 wl matching section of 300-ohm line
is around 3.3:1, while on a 450-ohm line it is about 5:1. Keep in mind that
these considerations apply only to 20-meter operation. On all other bands, the
G5RV antenna terminal impedance is much higher and reactive, resulting in a
higher SWR and making the use of an antenna tuner imperative. Incidentally, the
length of a 3 lambda/2 radiator may be found using the long-wire antenna formula
length in feet = 492(n - 0.05)/f MHz, (Eq 20-3)

where n = the number of half wavelengths in the radiator

It is unfortunate that many amateurs believe that the balun should be omitted.
These people have been misled, because failure to include a balun between the
balanced open wire and the unbalanced coax results in RF radiation in the shack
from current flow on the outer surface of the coax shield.
In addition to the misunderstanding concerning the "magical" 102-foot
length of the G5RV, there are also other areas of confusion focused on this
antenna, some concerning the role of the feed line. There are some who believe
that a particular combination of open-wire and coaxial feed line yields a
perfect 1:1 match on all bands without a tuner. As stated above, this is true
only on 20 meters. Others believe that because the 102-foot dipole length is
shorter than 1/2 wl on 80 meters, a certain length of the feed line is a
folded-up portion of the antenna to make up for the difference in length, and
that the folded-up portion radiates along with the antenna. Still others believe
certain lengths of feed line are to be avoided to prevent "antenna current" from
flowing on the feed line because of line resonance. Patently untrue! I wish I
knew how these myths originate.
My own involvement with the G5RV antenna dates back to the early '70s when
I began lecturing on SWR and reflections on transmission lines. My lectures
promoted the use of antenna tuners with open-wire feed line on random-length
antennas as the best way to achieve all-band operation. I also promoted the
concept that the correct length of feed line is that which is required to reach
from the antenna terminals to the tuner, because, regardless of the length of
the feed line, both the feed line and the antenna are made resonant by the
conjugate matching action of the tuner. Hence, there is no reason to avoid
certain lengths to prevent line resonance, because the tuner makes them resonant
anyway.
I first heard of the G5RV when someone in my audience described his
102-foot antenna with open-wire and coax feed line. He claimed it gave him a 1:1
SWR on all bands without a tuner. I told him he must have a lossy coax to get
1:1, because I knew a 1:1 would be impossible with such an arrangement without
some exceptionally high resistive loss somewhere in the antenna system. After
hearing several more identical claims in later lecture sessions, I analyzed the
antenna on all bands, observing it to be the 3 lambda/2 that it is on 20 meters,
but a random length on all other bands, so I felt confident in rebutting the
ridiculous "1:1 on all bands without a tuner" claims. Incidentally, Varney
published an update of the G5RV in The ARRL Antenna Compendium, Volume 1 (Ref
112), in which he presented the same specifications for the antenna that I
described above, which confirms my earlier observation that his antenna is 3
lambda/2 on 20 meters, and a random length on all other bands.
Let's now examine the other myths and confusion concerning the G5RV that I
mentioned earlier. First, we'll consider the feed-line combination believed to
yield a 1:1 match on all bands. It has been written that the combination of 33
feet of open-wire line, plus 68 feet of 50-ohm coaxial line will yield such a
match. Don't you believe it! A determination of the G5RV antenna-terminal
impedance on all bands shows that there is no length of open-wire line of any
characteristic impedance Zo that will transform the antenna impedance Za to an
impedance that is even close to presenting a match to 50- or 75-ohm coax, except
on 20 meters. However, when fairly long lengths of coax follow a length of open
wire, the high SWR appearing at the junction of the open wire and the coax will
be reduced significantly at the input of the coax because of the attenuation
loss in the coax, especially at the higher frequencies. The longer the coax, the
lower the input SWR, but remember that this method of lowering the SWR is costly
in terms of lost power. Because an antenna tuner is necessary anyway, except on
20 meters, it makes no sense to use any coax at all. Coax performs no useful
function in the feed system, and it consumes power unnecessarily because of the
high SWR. A more sensible method is to run the open-wire line all the way to the
tuner and eliminate the coax entirely.
Second, let's consider the length of feed line believed to be a folded-up
portion of the antenna that radiates. Radiation occurs when the electromagnetic
field developed by current flow on a conductor is not canceled by an opposing
field developed by an equal current flowing in the opposite direction. Hence,
radiation occurs as a result of current flowing on an antenna. However, antenna
current ceases being antenna current at the antenna terminals, because once it
enters the transmission line, the current becomes transmission-line current,
with the current in the two conductors flowing in opposite directions. There is
no radiation from any portion of the line, because the fields developed by the
currents flowing in opposite directions in the two conductors oppose and cancel
each other throughout the entire length of the line. Therefore, no portion of
the feed line becomes part of the antenna."

Owen, I didn't include this to detract from your excellent work--you've done a
great job. But my position is that since an antenna tuner is necessary anyway
for the antenna to be multibanded, why insert any coax at all?

Walt, W2DU