On Sat, 03 Jan 2004 22:11:39 GMT, "Vaughn Combs"
wrote:
Hi Richard,
If this helps both my XYL and I will be in your debt ;-)
It comes with the usual expectation of your first born....
So I should solder a shielded copper wire to the near end (aka the feed
point but on the other side of the insulator --- near the house), let it run
down the side of the house (about 22 to 25 feet down) and then to a ground
rod? Then connect both the wire from the point where the new ground wire
attaches and the wire (formerly the feedline) that connects to the "long"
wire to the balun. Then its on into the house and the tuner via RG-8.
Correct?
Yup. That is what I was describing, but it could work dangling (with
a sealed, insulated end that is mechanically tied down; however, this
is not anymore efficient as the floating wires along your rafters,
merely different).
View with fixed font:
BalUn longwire
rig --- tuner ant ---OOOOO-----------------------------
rig --- tuner gnd ---OOOOO-+
| |
| | drop
| |
----- -----
--- ---
- -
Ground loop issues arise due to there being two paths with a
significant distance (in wavelength) between them (from the +
down, through ground, and back up to the rig) and an impedance between
(the BalUn which serves this purpose by design). This usually comes
about when the ham shack is elevated, and still has some form of
ground return that is an appreciable distance (in wavelength,
basically more than 0.1 wavelength) from true ground.
The rig ground may be deliberate, or it may be passive through metal
connections and continuity with the power supply which in turn
inherits it from the wall socket. Even if there is no safety ground
from the wall socket, there is still a capacitive coupling in the
wiring to fulfill the mission (and 40/80M probably plays hell with the
VCR).
What do I connect to the ground lugs on the tuner and rig? The inefficient
RF ground wire that I have now? That wire currently goes to a ground strip
that has 1/4 wave length radials that were cut for each band running through
the attic and an additional wire that is attached to a pipe that I found in
the attic that is a plumbing drain pipe vent.
This may still be a viable option. Don't discard anything, simply
isolate one or all and experiment (it's the name of the game).
BTW, would there be any advantage to using other than 1:1 for the current
balun? Any recommendations for the balun? I have seen some that can be made
simply from coax.
Simple coax chokes work just as effectively. Half a dozen to eight
turns of six to eight inch turns will be adequate for 40-15M, you may
need to experiment with a few more turns outside of this. Current
BalUns are simply Ferrite donuts put over a 1 foot length of RG-52
coax (one or a couple dozen donuts). The same thing can be achieved
with a large torus with four or five turns of coax through the center
(if its big enough). This is square law time, so four turns equals 16
donuts. Stacking two large toroids with 3 turns each would substitute
(or maybe three with three turns).
Using ferrite makes for a smaller device that works over a larger span
of frequencies, but winding air wounds can be tested immediately at no
more cost than the cable already handy.
I hope that I am not being a pain but this RFI prob is a bugger and it would
be REALLY nice to squash this one. It is bugging the XYL quite a bit and it
limits my operating time ;-)
Also, the ground loop that you are referring to would be current in the
shield (or in my case the outer skin of the copper wire ;-) coming through
the feed line and into the equipment. From there it is possibly getting into
electrical ground through the power supply?
Again, Many thanks for your help and 73,
Vaughn, N2BHA
Hi Vaughn,
I've described the ground loop above. Isolating it means "busting"
ground somewhere. I use a battery. Voila! Opening any path does
just as well, but you have a dangling radiator somewhere (like the
drop). This may help, or it may change nothing. RFI problems are
unique to every situation and having your shack up high is the major
contributor, but to keep peace in the family try to limit RF exposure
to the end of the pipe (the purpose of the choke).
The choke inhibits RF from traveling back (conduction) into your shack
(but your rafter mounted "radials" contribute RF exposure through
radiation). You might try wrapping your power supply AC leads through
a ferrite (or use a split core). This is choking your rig from going
into the House AC. If you do this, take care to observe ALL
connections to the rig. Do you have a packet connection to the rig
from a computer? This is another path to ground, Choke it!!! Do you
have a modem on the computer? Ditto. Computer Display? Ditto. A
powered microphone? Ditto. The shack is a hydra of wires all heading
to ground eventually and all it takes is one - and sometimes it melts.
There are three forms of power transfer: Conduction, Radiation, and
Convection. Clearly convection is out as we are not heating soup.
Conduction comes through wire, and radiation through the air. Your
problems (as for most) usually comes by virtue of shared paths in the
AC distribution and this is a conduction issue. If you move your
rig's power supply to another wall socket ON A DIFFERENT BREAKER, you
might solve one problem (and discover a new one). Use an extension
cord (choke it) to experiment with this possibility. Having the
antenna feed point close to the house brings about the possibility of
radiation coupling. Move it (the choke point) away from the house.
Remember this is square law time too and with each doubling, you
quarter your exposure.
73's
Richard Clark, KB7QHC
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