Highland Ham wrote:
wrote:
On Sep 3, 3:56 am, Andiroo wrote:
All

Can anyone point me to some simple designs for a home made 17 metre
antenna. I am looking for some simple half wave dipole type designs.
468/18.1mhz=25.85 total feet for a half wave dipole. Divide by two
for
each leg.
Feed with 50 ohm coax. Can't get much simpler than that. Can't get
much more
efficient either unless maybe you have to run 500 ft of coax. .
MK
===================================
A current balun near dipole in the shape of ferrite cylinders or ,when
using RG58 type of coax, 7 - 9 windings of the coax on a larger toroid ,
will ensure no RF interference in the shack /house (eg no RF current
flowing along coax braid)
It is even better to use a balanced feeder instead of coax.
You then either need a matching unit for a balance feeder or use a balun
as described above near the transmitter ,connecting coax to feeder
through a dual plastic connecting block .

Personally I would always use a balanced feeder for any wire antenna.

That reel of cable marked "balanced feeder" is a myth!

Twin feeder is not self-balancing. It never will be "balanced" unless
your installation has MADE it balanced.

As it comes off the reel, twin feeder is capable of carrying unwanted
common-mode currents just as easily as the intended differential mode.
All practical antenna installations are unsymmetrical (either the
antenna, its environment or both) so there will always be some unwanted
common-mode current. Common-mode current will cause the feedline to
radiate and will conduct unwanted RF back into the shack, where it often
flows through into your house wiring... all of which is regarded as a
Bad Thing.

Twin feeder will only become more "balanced" if you - the user - have
taken some positive action to suppress the common-mode current. When you
install a typical link-coupled ATU designed for "balanced" feedline, the
low capacitance across the link coupling creates a high impedance to
block the entry of common-mode current into the shack. A suitable balun
would also create a high impedance to block the common-mode current, so
in this case you would need a choke balun (aka feedline choke or line
isolator).

When you insert this high impedance in the path of the common-mode
current, the current distribution over the whole antenna-feedline system
readjusts itself to take account of this new factor. The new current
distribution will be forced towards better symmetry on the antenna
itself, with a smaller common-mode current on the feedline... so the
feedline has now become more "balanced".

What should take the credit for this good result? It was the ATU or
balun that you installed - so all the credit goes to you. The so-called
"balanced" feeder deserves no credit at all, for it did nothing more
than react to the change that you made.

Many people confuse the cause and effect here. If they believe that the
feedline is in some sense "ready balanced", they will install a suitable
ATU or balun "because that's what balanced feedline needs". Very often,
it all works out OK because they have done exactly the right things,
even though they misunderstand the reasons. But if it doesn't quite
work, and something more is needed to suppress the common-mode currents,
this wrong thinking comes completely unstuck.

Coaxial feedline has the major advantage that you can insert a choke
balun right at the feedpoint, which substantially prevents common-mode
current from being launched onto the outer surface of the cable[ 1]. The
entire current distribution will then change, to take account of the
fact that you have forced a minimum in the common-mode current at the
feedpoint. Because the feedline is in the near field of the antenna,
radiative coupling may cause common-mode current to reappear on the
surface of the coax, starting with a new maximum a quarter-wavelength
down the line from the feedpoint. But the size of this maximum is
generally much smaller, and so too will be the residual current arriving
at the shack - where, if needed, you can insert a second feedline choke.
In really desperate situations, you can have as many common-mode chokes
along the coax as you like.

With twin feeder, you don't have those options. Your only practical
options are at the ATU, or where the twin feeder makes a transition into
coax. That restriction makes it even less likely that twin feeder will
ever be truly "balanced".

Bottom line: twin feeder has many advantages, especially for multiband
operation... but automatic balance certainly isn't one of them.




[1] In coaxial cable, the inside and outside of the shield behave as
two independent conductors for RF current, giving a total of three
current pathways. The centre conductor and the inside of the shield have
very tight coupling between their electric and magnetic fields, which
ensures that their RF currents are exactly equal and opposite (the
differential mode) and their net field on those two conductors remains
completely inside the cable. The third conductor is the outside of the
shield, and behaves a a single fat wire carrying an RF current that is
totally independent of what's happening inside. Although strictly
speaking this is not a common-mode current, that's what it is generally
called by analogy with twin feeder.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek