OK, I gotta take issue with the part that says,

" A transmission line, even a very good one, generally has a Q of
someplace around 20-75. The definition of Q I am using is reactance
over ESR. Say you need a reactance of 400 ohms to resonate an antenna.
Linear or stub loading would add a series resistance of 5 to 20 ohms as
loss resistance at that point in the system.
"

I know that transmission line Q varies all over the place: it's much
more reasonable to use it in a resonator at high frequencies than low,
and line construction makes a big difference too. To back this up with
numbers, I just ran some calcs (actually put together a little Scilab
program to run them for me) on four different lines: (a) is RG-8/RG-213
type line with solid poly dielectric, (b) is 75 ohm air insulated coax
in an 0.5" ID copper tube, (c) is balanced two-wire line made with
12AWG (~2mm) wire spaced 2" (~5cm) on centers), and (d) is two 0.625"
OD copper tubes spaced 3" on center.

For a 1/8 wave section of line shorted at the far end, the calculated
impedances and Qs a

line a, 10MHz: 0.622+j50, Q=80
line a, 100MHz: 0.197+j50, Q=254
line a, 1000MHz: 0.0622+j50, Q=800

line b, 10MHz: 0.35+j75, Q=215
100MHz: 0.110+j75, Q=679
1GHz: 0.035+j75, Q=2147

line c, 10MHz: 0.972+j469, Q=482
100MHz: 0.307+j469, Q=1526
1GHz * : 0.097+j469, Q=4826

line d, 10MHz: 0.124+j270, Q=2170
100MHz: 0.039+j270, Q=6864
1GHz * : 0.012+j270, Q=21.7k

* -- the open wire lines will likely not work in practice quite this
well at UHF...

With luck, I got all the calcs right; but in any event, I do expect the
Q to go up for a given stub as the sqrt(f), and the Q of open-wire
lines to be considerably higher than that of coax with similar
conductor diameter, just because the impedance is higher.

The Q of an RG-8-type stub at 10MHz isn't wonderful, but at higher
frequencies and with different construction, stubs can work better than
coils. There is a range of frequencies where it can be a matter of
construction preference: the stub may be easier to integrate into a
design, or the coil may be, depending. At high enough frequencies, the
stub often is easier.

Also, I want to point out that in a collinear -- a half-wave dipole,
center fed, in the center, and an additional half-wave element on
either end, coupled through a two-wire-line stub perpendicular to the
antenna performs distinctly better than the same antenna in which the
stubs are replaced by self-resonant coils, or by a coaxial stub which
is made to be collinear with the antenna. That's because the
perpendicular stub interacts with the antenna field to excite the right
mode on the line to get substantial current in the outboard collinear
half-waves. See King for further explanation.

Cheers,
Tom

wrote:
Dave wrote:
How would you guys who are stuck in an endless thread of loading coils like
to take on 'linear loading'?? are the currents the same at each end of the
loading line?? do they cancel completely along the length of the loading
line? does the loading line replace so many degrees of the length of the
elements or cause some kind of delay???


All of the petty arguing and self-promotion aside, linear loading is
just a very poor form of a loading coil. Like any poorly designed
system, the ill effects of design shortfalls can range from very small
to very large.

As a general rule, linear loading reduces efficiency over a lumped coil
of good design. Again the exact amount and the overall effect varies
with where the loading is placed in the antenna, how it is constructed,
and where and how the loading coil compared to it is constructed and
placed.

A transmission line, even a very good one, generally has a Q of
someplace around 20-75. The definition of Q I am using is reactance
over ESR. Say you need a reactance of 400 ohms to resonate an antenna.
Linear or stub loading would add a series resistance of 5 to 20 ohms as
loss resistance at that point in the system.

It would take a very poor coil to have that Q, but it can be done.

Depending on where in the antenna you insert that loss resistance, the
effects can be large or small. Myself, I avoid linear loading. I'm not
a person who likes to gamble.

We have now all seen first hand how a fascination with destroying
others really just destroys the ability to learn anything ourselves
and to help others learn. This loading coil thing has become a mental
illness, like uncontrolled shoplifting. One fellow wrote a nice book on
transmission lines and a long argument about amplifiers and a long
argument about reflected waves on amplifiers did the same thing.

This stuff is more a demonstration of emotional problems or mental
illness than science and education. It's one step below someone going
postal and just shooting everyone else in the world who is responsible
for his failures and unpopularity!

I hope this post gives insight into how arguing or fixations ruin the
educational process, and also sheds light on linear loading. Something
for everyone.

73 Tom