In article .com,
wrote:

It might help the new listener to do some rather simple tests.
Disconnect the antenna from your receiver and tune from the minimum
to maximum freq, noting any birdies or signals you receive.
Many(most?) receivers leak RF from the IF strip that can be received
weakly. Many receivers use 455KHZ so check 455KHZ, and the 2nd
(910KHz), and the 3rd(1365) harmonics. Pay close attention to any MW
or SW stations you recieve. This test is best done with a 50 Ohm
dummy load on the anntena port, but it is still usefull to know what
leaks you have. Many of these birdies will be below the noise floor
when an antenna is connected. But they are usefull to now about
because they could interferr with a very weak signal and can be used
as a quick check if you suspect

your receiver of having a problem.

Now connect your coaxial feedline, but disconnect the antenna. Repeat
the search. I will bet that many will be surpised to find they can
receive several MW outlets and oftenmore then a few SW signals. This
is a weak point for RFI to enter your system.

I found that the braid+foil coax had less ingress. Most braid and
foil coax is 75 ohm, but Belden does offer several 50 ohm versions. I
am using Belden 7807A. But I could detect no difference when using
Belden 1258AM 75 Ohm CATV cable.

John Doty's suggestion of placing the coax feedline underground can
help a lot. (http://www.anarc.org/naswa/badx/antennas/) As can a 1:1
RF transformer before the coax enter the home.
(http://www.members.aol.com/WA1ION/nrants.pdf). John Brynt's work on
feedline ingress is very informative.
(www.dxing.info/equipment/coax_leadin_bryant.pdf) I found that adding
a ferrite torroid with ~20 turns of RG174 at each end helped reduce
the ingress a "lot". To the one MW station I receive with no antenna.

My reasoning is that the best way to improve your RF signal is to
reduce the invading noise as much as posible. Beyond a reasonable
point, adding more wire doesn't help very much and may well hurt. A
longer wire picks up more signal and more noise. By optimising our RF
signal reception and minimising our RF noise pickup can greatly
improve our stions noise floor. And that set the lower limit on what
we can recieve.

"dxAce" mentioned that he has his 9:1 transformer elevated and feed
with his coax. In an RF quite location this is a very good and
workable option. And it worked much better then an unshielded wire.

In "http://members.aol.com/DXerCapeCod/z_transformers.pdf", John
Bryant shows 4 basic ways to use a transformer. I am using a
variation on Circuit A. In between T-storms I think it makes a big
difference.

At the very least, users of the "Doty" system may wish to rethink the
inclusion of an unshielded vertical element. I suspect if more ham
and SWLS knew about Triax much more research would have been made
using this "odd" cable.

I have been given a ~150 spool of used, but good 75 Ohm Triax. When
time permits I plan on doing an feed line using Triax from head to
entry point. I suspect that the most gain will be in the critical
first few feet.

With coax, my standard QRM noise source, a noisy wall wort, would add
noise to the recieved signal when it was brought within a foot or so
of the coax. With Triax I could place the noise source against the
Triax with NO increase in noise. With the antenna disconnected from
the coax no increase in noise was noted. I think the noise is
travelling up the shield and getting into the antenna. Ferrite RFI
split beads helped reduce the ingress on the coax slightly. If one
could get enough ferrite it might choke the noise off completly. I
decided to test something else before scrounging up 50 more beads.

Snip

As I posted in the other thread if you disconnect the antenna from the
coax then you need to put a 50 ohm resistor across the end otherwise the
coax will behave as an antenna. You sure will pick up stations if you
don't terminate it.

With the far end not grounded or terminated from the radio the outer
shield will pick up passing EM wave energy. That energy races up and
down the coax shield where the current induces a complementary voltage
on the center conductor. The result is a differential voltage at the
radio end and yes it works pretty darn good.

When you choke the coax you prevent the RF current on the outer shield.
It should be more effective to just terminate the far end with 50 ohms
than using a number of chokes space along the coax.

--
Telamon
Ventura, California