Jeffrey Angus wrote in news:j60h8s$ao0$1@dont-
email.me:

On 9/28/2011 7:31 PM, Owen Duffy wrote:
The auto tuner can be a path to hiding shortcomings in an antenna
system, in search of the holy grail, low VSWR.

This is a 102' doublet with 40' of open wire balanced feed line.
Except for the input of the tuner looking like 50 ohms to make the
transmitter happy, there is no such thing as "low VSWR" on this
type of antenna.

Well to "make the transmitter happy" is jsut the new language for low
VSWR.

... There
isn't room inside the case for a 4:1 current balun.

But there is probably room for an effective 1:1 Current Balun.


If you disconnect the open wire line from the internal voltage balun
in an MFJ tuner and replace it with an external current balun you will
find that the tuning of the match on the tuner is a lot smoother as
opposed to almost erratic. (But still obtainable.)

Ok, so in your experience, you haven't yet come across a load that could
be matched with the voltage balun, but not with an external current
balun.

Due to the location of my station on the 2nd floor over looking a
covered porch, having a "good RF ground" is not going to happen.
The current balun forces the antenna and feed line into a truly
balanced condition.

Ideal conditions like "forces" and "truly balanced" don't often exist in
the real world. It would be of more interest if you had measured and
reported the differential and common mode current at various
frequencies.

The whole purpose of the tuner is to allow the transmitter to see
a 50 ohm unbalanced load that it was designed for. Accomplishing
that, allows the most power to be transferred to the antenna. The

Yes. It is the meaning of "most" that is relevant. Most doesn't need to
mean 100%, or close to it, you make compromises for frequency agility
and multiband use, but "most" is often unknown. Perhaps some DX QSL
cards can substitute.

102' doublet with open wire line seems to do a remarkable job at
radiating. (And equally well at receiving.)

Yes, they can be a good antenna, but it is not a no-brainer.

For example, a correspondent recently reported problems with just such a
thing on 40m. Turns out his feed line length was such that at 800W into
the feed line, the voltage between the wires was some 4000+V and was
causing flashovers in a 3kW rated ATU. In this case, I recommended that
since he could not lengthen or shorten the feed line enough, that he
shorten the antenna so solve the problem.

He had previously smoked up a CWS Bytemark 5kW rated current balun on
80m with another antenna, caused by unlucky feed line length.


That leads into the question of whether a 4:1 balun is 'better'.

Better than what? A 1:1 balun or none at all,

1:1.

4:1 reduces the voltage impressed on the ATU components, which is good
for high impedance loads, and poor for low impedance loads.

There is no simple thing that always works best on a random set of loads
(which is the case for many multi band antennas).

which seems to be
the selling point of the G5RV antenna and it's clones.

Perhaps to some. There is good argument for use of a balun with a G5RV
and not-balanced transmitter. Varney conceded that in one of his later
articles. Most people who are adament about what Varney did or did not
describe have not read his articles. Antenna manufacturers are not a
good source of factual information.

Owen