Richard Clark wrote in
:

On Sat, 02 Jan 2010 17:20:24 -0600, Lostgallifreyan
wrote:

I haven't a clue about intermod, yet. One thing at a time.

The term Intermod is probably mis-direction if you research it.
Basically, if an nearby AM/FM/TV transmitter (and nearby can be on the
scale of several miles) happens to excite your antenna; then its
developed voltage will overload the frontend (Intermod follows, but
the products are not what I am emphasizing here). This overload can
be many, many kHz, or MHz from the intended and tuned signal; and yet
this frequency remote signal will develop an AGC that drives down gain
on your intended signal.

This characteristic is VERY common for untuned frontends in modern
receivers. It is not often noted for poor antennas (those whips, when
they are used for SW), but when a real antenna is attached *BINGO*
sensitivity goes down the toilet. By providing a tuned input, the
side-signal that would otherwise silently drive AGC is attenuated, and
AGC is developed only by the in-band signals.


Ok, this is cool, I understand that, and I also see that it doesn't really
concern intermodulation products as the initial problem is a bigger one if it
occurs. Can't help wondering why a receiver doesn't do some tuning before the
AGC for exactly this reason, but never mind...

Right now I see at
least three contradictions (re ground rods, transformers, and feedlines)
with advice from several people, one of which (the guy who wrote the
description of the antenna and balanced line I mentioned) is part of a
group of hams who is turned to for advice by the others. No guarantee of
correctness, perhaps, but if I keep on being told I'm wrong when my
stuff is coming as directly as I can get it from others with experience,
then as far as I'm concerned I'll do what I think best and get out of
the crossfire.

A reasonable posture.

Specifically, many times
I've seen advice that service grounds are not adequate because of common
mode noise and local currents, hence the ground rod you vehemently
negate.

I don't negate its use, I say that it is NOT RF ground. If you tie
this ground rod to the service ground, then that wire will probably
act more in your behalf than either "ground." There is a world of
difference between safety grounds (what those rod-thingies are) and RF
grounds (which often don't go into ground at all).

Ground is a long and rich story that has been celebrated in this group
for years. It deserves respect and attention well beyond these few
words.


True, I don't doubt that for an instant, but it's also a question of what is
practical, and what is recomended by most people I've read words from at
times during the last 30 years or more. While I know that CB'ers would just
stick a magmount on their car's steel rooftop as often as not, and have read
of other schemes that place some small horizontal plate below the antenna,
there's a lot of scope between that and a rod driven into salty ocean
shoreline. 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. 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.

I can
ground to service ground at near end but if the receiver is on
batteries, not connected to anything except a transformer coupling RF
from the antenna, then the ground only needs to be at the antenna end,
according to advice I've seen in several places.

To your specific arrangement - quite true. However, many who have
claimed to have made every precaution then connect their receiver to
an amplifier, computer, what-you-might-call-it and a new path to
ground winds its way through interesting environments that are RF
rich.


I agree. 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).

Even if I do ground to a water pipe or other local ground,
all advice I see until now insists on having a ground rod as close to
the antenna as possible, no matter what else I do, yet now you urge
against this.

I urge against mixing grounds. Such things arrive by the most benign
and seemingly inconsequential actions.


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 will stop asking for advice if all I see is vigorous contradiction
between people who claim knowledge I do not have. Diverting that
disagreement to one with me doesn't alter this, I did not originate the
info behind the choices I am considering. Even if all the various
contributors come here and duke it out between them it appears I'll be
none the wiser.

Attention to one detail at a time helps, but a lot of this arrived
through responding to the query for antenna port Z. Those adjuncts
that massage input/output Z also fold in the discussion of ground.


Agreed. But this is why instead of asking more questions whose answers I am
probably not prepared for, I described the simplest and apparently best
scheme I'd learned of so people see it whole and work from there...

Convention has it that you start a new thread for each side-topic that
drives you into conniptions.

Ah. Well, I thought that's exactly what would annoy people most. If
something directly arises from discussion in a thread, most people tend to
keep it there. I already do start a new one if I'm certain the issue is
different, and if I'm originating it.

Asking about the facts and foibles of
ground would be a good start on a new thread - especially when Art's
wet-dreams descend into discussion of particle duality self
annihilation driving all participation away from antenna design. For
instance "Why are ground rods considered insufficient for RF
application?"

I am content to respond to either discussion.

73's
Richard Clark, KB7QHC


Well, sure, if I am asking a direct technical or practical question. But
while I'm still slightly reeling from what appears to be a dissention with
what otherwise appears to be good advice, I like to keep the discussion in
one place, otherwise confusion reigns and spreads to many threads. Trust me,
that might annoy people. At least in this thread it might be useful to
anyone who has that radio.