The 80 / 40 M fan dipole fed with 300 ohm transmitting line {The Wireman
#562} and then use the balun at the point where the feed line enters the
house would be preferable. That configuration reduces the weight of
the balun on the antenna. From the balun to the station, use a good
quality low loss coax. The shorter length the better.
I bring my balanced line all the way to the operating position where the
balun is located and only about 18" from the tuner. There is no issue
running the insulated balanced 300 ohm line through windows, walls,
ceilings, floors and such. Just maintain about 10" or so from any
metal gutters, metal duct work, AC wiring, phone lines, water lines and
such.
The question often arises about RF in the shack when using balanced
line. If the line is reasonably well balanced the current in each side
of the line is equal but 180° difference in phase. The result is the
field cancels and there is no radiation into the shack. Thus using a
balanced feed system on a center fed antenna is the optimum configuration.
73
Bob, K4TAX
On 12/8/2018 9:03 PM, Robert G Strickland wrote:
Ron...
Would such an antenna cut for 80m, fed with ladder-line, and used on
40m, be a better performer on either band than an 80-40m fan dipole
fed with 72ohm coax? Leaving all other extraneous but influencing
parameters aside. I have the second antenna; the weight of all that
wire and the coax with a ferrite balun results in a significant sag.
I'm wondering if the first antenna, lighter and higher in the air,
would perform better? Thanks.
...robert
On 12/6/2018 14:54, Don Wilhelm wrote:
That statement is true - however ---
The problem is not radiation from the conductor, but how to get the RF
current into the conductor and not in other unintended places.
For those who want to build their own antennas, I suggest starting with
a resonant dipole fed in the center. That provides a good match for 50
ohm coax, and a good current mode choke (balun) at the feedpoint keeps
RF from flowing back onto the outer shield of the coax which can create
a lot of RF in the Shack.
Off Center Fed, and End Fed antennas can be made to work with more care.
The feedpoint impedance does not match coax, and special care must be
used to keep the RF on the radiator rather than coming back into the
shack.
I realize that OCF and End-Fed non-resonant antennas are popular because
they can be used on multiple bands, but there are problems feeding them
while keeping the RF on the radiator and not in unintended places such
as the shack.
For those who want multiband operation, I still suggest a center fed
radiator fed with open wire or ladder-line to a good current mode choke
balun at transition point to coax is a better choice than OCF or end-fed
antennas - keep the coax short and use a tuner that can match the
resulting impedance.
Make the length of the dipole approximately the length of a half-wave at
the lowest operating frequency desired. It will be more "tame" for RF
in the Shack than many OCF or End-fed antennas.
73,
Don W3FPR
On 12/6/2018 8:56 AM, ANDY DURBIN wrote:
"RF current into a conductor will radiate, even if it's at eye level."
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