Richard Clark wrote in
:

On Sun, 03 Jan 2010 05:26:32 -0600, Lostgallifreyan
wrote:



Most people I ever came across asserted the importance of a ground
rod local to the antenna to couple with the local water table which is
as close as most ever get to the ocean unless they really like getting
their feet wet while they sit around at home.

In fact, this almost always NEVER happens. Skin effect defines the
layer depth of RF in ground. An 8 foot rod is like a splinter when
you are trying to harpoon a Blue Whale.

Ground rod engineering has been discussed in this forum to great depth
(pun intended, or not). The rods are as well understood as water
witching forks. In the HF region, single or several rods have no
practical RF use whatever. Above HF, absolutely no one gives them any
thought.


True. I'm interested in using a single antenna for all of LW up to around 30
MHz as a simple starting point though, so Dallas Lankford's scheme seems to
fit the bill.

The proximity is as close to the
point where they want to pick up RF as they're going to get, and means
less noise from buildings full of electrical stuff picked up on metal
between antenna and whatever other ground might be provided elsewhere.
This has been the ONE common factor in pretty much everything I've seen
on land-based AM reception. Anything that directly appears to negate
that advice makes it hard to know what to trust, and certainly needs to
be clearly explained.

When you can't do anything else that is effective, a ground rod seems
like more than enough. It is certainly a need for safety's sake,
especially when your vertical could be a lightning magnet. Consider
that same antenna: is it directly GROUNDED? Or is it floating? If
ground is a panacea, I bet most of your advisors immediately isolate
their antenna from it. One has to wonder about faith....


Safety is important, even though lightning strike isn't that big a risk here.
Actually it's risen because nearly all large trees have been removed in the
last two years, and I'd be held reponsible for any damage caused that way.

If you look at that PDF you'll see the 15' whip antenna is directly connected
to ground through 80 turns of wire on a ferrite toroid. I might add a spark
gap in parallel as that wire is not a high current path. So long as it is
much more likely to go to ground rather than along the line in to the house,
I'll have done what I'm supposed to do. The trouble would only exist (other
than unpreventable natural excesses) if it were evident that I had not done
this.

Either design works with equal efficiency. You simply need a coupling
system to the grounded antenna design. One method is using a folded
monopole. Other methods abound (which are often confined to yagi
driven element discussion, but are eminently applicable here).


That PDF shows the coupling in this case. It's similar to other ideas
recommended for similar small SW listening setups. Once I have all I need to
try it I'm just as happy to try experimenting to see what happens as to try
any plan. There are some limits though, the location is too built up to
expect much from anything intended to be directional. I'm just intending to
look around, not looking out there for something specific.

The moment I try to connect to a system that includes a computer,
mixer, multiple supply grounds, as mine does, I'll be using a local
service ground and improving it the same as I would for audio, though
it's currently ok for that, at least. It already uses a star grounding
system where possible, as recommended by audio studio designers and
others. There's actually a supply ground rod outside the front door too,
which presumably helps more than the original wiring 15 years ago which
didn't have that. (But note below, where I mention isolation).

The Star system is great for exactly as you understand and describe
it, but for antenna applications that remote ground could act as a
suicide adapter if it does not have its own path to the service
ground. Yes, this violates the star, but when path lengths include a
lot of resistance and leakage current, voltages can become
considerable when you supply a new avenue through your home. This is
the story of the classic ground loop.


Well, a ground rod isn't going to cost much, and making and breaking
connections to it is one of the easiest and cheapest things I'll be able to
do, so I'll test that empirically when I'm ready. I won't try to predict it
now. Whenever I find some new ground noise problem in anything I do here, I
usually manage to isolate it and solve it acceptably within an hour or less,
so I'll trust my chances. Usually the purpose hasn't been for RF, but quite
often the sources did involve RF too so my instincts might help me more than
my knowledge.

Hence the star network I mentioned, advised for audio setups.. It's kind
of why I wonder about what many suggest, grounding a coax at both ends,
and even in the middle if you want, and certainly to bury it. More
importantly it's why the Dallas Lankford design appeals to me. Isolation
baluns that transfer energy rather than use direct contact coupling look
like a good way to avoid the ground problems while also avoiding local
noise pickup because the twin cable will have good common mode rejection
as it passes into the electrically noisy bulding. (Though I can't help
wondering if Dallas Lankford also tried balanced microphone cable with a
screen grounded at one end, just to see what happened) Such methods have
long been used in audio; is RF below 30 MHz really so different in this
case? So long as that line doesn't have dire resonances of it's own,
isn't attenuation the only big risk? Dallas Lankford certainly thinks it
works after working with it for at least 2 years. He says that if you do
it as described it will be low noise. (As opposed to 'reducing'). I
don't think he's claiming any means of reduction, just saying it's lower
relative to inherently noisier systems, if wired as decribed. Based on
what I know, the claim seems good.

I'm not familiar with Dallas Lankford, but isolation and shielding
techniques are topics I have visited professionally throughout the
years and they are not simple. Without a concommitant discussion of
the noise source, one wrong ground selection can wipe out all pursued
benefits. Let's revisit one of your statements above:
balanced microphone cable with a screen grounded at one end
Which end? Any choice stands an equal chance of being the wrong
choice.


Well, I did think of that. And I didn't state it because I didn't know for
sure. As I imagine that local RF couplings from various digital devices might
place small currents on the local ground, I imagine that grounding a shield
at the remote ground makes sense. Doesn't matter to me though. It's far
easier and faster to experiment than to try to predict because there are only
two ways to try. Dallas Lankford directly states that no shield is even
required, and I doubt he'd have said that if he couldn't demonstrate it, and
as that line is a two-wire loop that has no direct contact with anything, it
should reject any common mode noise that hits it. Even in audio this matters
because the same method is used to reject RF pickup on audio lines. I think
some people persist in baluns instead of op-amp common mode rejection specs
for this reason, despite the chances of modest distortion in audio bands from
the transformers used. Not entirely relevant but it illustrates how people
can find themselves choosing between two less-than-ideal circumstances for
best effect.

I understand that noise context matters for a real attempt to plan for it,
but that's far more difficult that presenting the basic antenna scheme.