> Tom w8ji makes some interesting but uninformed comments about the
> Tennadyne feed system. The booms are in fact the feedpoint for the
> antenna and the collins balun belongs at the transition from the
> unbalanced coax line, to the balanced line necessary in any log periodic
> construction. The parallel booms substitute for the balanced line and make
> the antenna much more efficient. The potential of the lower (or upper)
> boom is the same as that of the feed line. The booms are simply aluminum
> feedline, they are not radiators, just balanced line. The idea that the
> balun at the feedpoint forms a dead short is simply untrue when you study
> the construction if that were true every antenna forms a short at some
> point because the shield grounds the antenna. Whether it is a log, a
> vertical, or a yagi.
That is not correct Frank, the statement above illustrates a
common misconception of what a balun does and where it belongs
in the system. Baluns do not always belong at feedpoints, as a
matter of fact there are many cases where an improperly installed
balun can make things WORSE. This is one of those cases.
Since a balun does not always belong at the transition of balanced
(in the case the dual booms) to unbalanced (the coax) where does
it belong?
It belongs at the point where the common mode current would
begin to flow, and where the high choking impedance is required
and effective. In this case, that point is the junction of the lower
boom and the supporting mast.
Let's look at the case of a dual-boom log, with booms (one above
the other) behaving as a balanced feedline. If the coax shield is
directly connected to the lower boom, is at the potential of the
lower boom. Even though that cable is coax if it is routed back
along the lower boom the potential difference between that cable
and the boom is ZERO. There is nothing to introduce common
mode current on the coax (which is the ONLY current than can
make the feedline radiate) as long as it parallels the lower boom
and is away from the side of that boom towards the upper boom.
In other words, if we had an imaginary voltmeter that could
measure voltage referenced to an electrically neutral point in space
around the antenna, the voltage on each boom at any distance
from the feedpoint would be exactly equal and opposite in polarity
at any distance from the feedpoint. With the coaxial cable taped
along the lower boom, the shield of the coax would be well-coupled
to the lower boom and would assume the potential of the boom.
While the coax does have common mode current, it is exactly in-
phase with the current in the lower boom and the shield is for all
practical purposes part of the lower boom! All radiation would be
cancelled by the upper boom, which hopefully carries equal and
opposite currents.
The balance problem, in this case, occurs at the exit point where
the shield leaves the boom. This would be at the point where the
cable moves from the lower boom to the vertical mast or tower
used to support the antenna.
If we measured the voltage with our voltmeter, we would see the
lower boom is not at the same potential as the tower or mast, there
is a voltage difference. (After all, if it was at the same potential
there would be no need to insulate the lower boom from the tower!)
The balun, and it should be a choke-balun, belongs right at the exit
point of the coax from the boom. This is true no matter what other
matching is done at the feedpoint end of the boom.
We can NOT have the cable span the insulator at the boom to
mast point without a suitable common-mode RF choke (balun), no
matter how we might think the balun belongs at the feedpoint,
without unbalancing the antenna and causing common-mode
current to flow on the feedline shield!
Bridging the coax across the insulator from the lower boom to the
tower or mast guarantees the antenna will be unbalanced, and the
lower and upper booms will not have equal currents. It insures the
coaxial feedline has unwanted common-mode current that is not
cancelled.
A choke balun belongs at the exit point of the cable from the lower
boom.
> By the way you do not have to strap the coax to the lower boom if you
> don't want to, it can drape down or use separators if you like, but they
> are unnecessary. It is true that you have to be careful not to have any
> barrel connector touching the boom but a single layer of tape is
> sufficient insulation.
Not true. A single layer of tape will NOT isolate a cable that
parallels another current carrying conductor for several feet! If that
were true, all of our yagis would quit working since they excite
elements many feet away without electrical contact!!!!!
Mutual coupling always works, magnetic and electric fields force
the conductors to the same potential. Tape does not stop the
effect, no more than insulation in a transformer stops coupling.
The feed system, as described, is improperly designed. It may
work in some cases, but it sure gave a friend of mine fits! The
amount of common mode current depends on tower height and
what else is around the antenna. A well-engineered feed system is
not critical for mounting structure height or cable lengths.
Just because you haven't observed problems doesn't mean you
don't have feed system problems, or unbalance in element currents
in the antenna.
73, Tom W8JI
W8JI@contesting.com
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