Experience w/NRD-535

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Larry Picard

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Feb 12, 1995, 10:02:11 PM2/12/95
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I think everyone is going to have a different opinion as which HF
receiver is their favorite.

Your recent comments on the NRD 535/535D did not mention Sherwood
Engineering's 535 SE which includes their outboard synch. detector and a
speaker to improve the audio.

I would have to say that their is little to choose between the audio
from the earlier 525 and the 535 with or without an external speaker. I
must admit that I have not used the 535 with the Japan Radio sync.
detector (ECSS) unit.

My limited experience with the Drake R8 has confirmed the reservations
of some of the reviewers regarding the ergonomics. I also found the
sync. detector to be a little annoying as it seems to loose lock on deep
fades.

The NRD 525 and NRD 535 both have a true IF notch filter wheras the R8
has a less useful audio notch.

Both the R8 and the 535 come with a built in computer interface. I am
still tweaking my setup but I have found that the value of direct
computer control of the receiver is limited by the RF hash introduced
by the RS 232 interface and cable.

Your approach of using the computer as a memory management tool gets
around this problem and of course frees the computer up for other uses.


While not the equal of the Drake or Japan Radio units, especially for
UTE chasing, the Sony 2010 costs considerably less especially on the
used market.

LP
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| Fidonet : Larry Picard 1:250/930
| Internet: Larry....@odxabbs.tor250.org
| Standard-disclaimer: The views of this user are strictly his own.

Larry Picard

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Feb 13, 1995, 9:05:11 PM2/13/95
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In your recent post you advised that coax should be grounded at two
sites, first at the antenna and then just before entering the house.

Is there an advantage in grounding at more than these sites?

I would also assume that the antenna is grounded when it is connected to
the receiver as the outer braid of the coax is in continuity with the
receiver chassis.

There has been some discussion of grounding problems on this and related
echos. I believe it has been mentioned that electrical codes require
that all grounds be tied together with heavy guage wire.

A little experimentaion with my radio showed that the chassis was
directly connected to the third (grounding) prong of the wall plug. I
am concerned that by connecting my receiver to an outside ground I am
creating a ground loop that involves my house wiring. Can you comment
on this?

This may seem like a trivial point but I recently discovered that the
main ground from the electrical service panel in my house was attatched
to a water pipe which had been painted over. I stripped the paint from
the pipe and re-attached the grounding clamp and I noticed a reduction
in noise from my receiver.

I am also a little confused by what constitues an adequate ground. I
have read that a conducting stake driven into the ground will divert
lightning and provides for electrical safety but that RF grounding
systems have to be a lot more complex with multiple radials with lengths
related to the frequencies of interest. Is this true?

John Doty

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Feb 16, 1995, 11:36:40 AM2/16/95
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In article <825_950...@tor250.org> Larry....@odxabbs.tor250.org (Larry Picard) writes:

> In your recent post you advised that coax should be grounded at two
> sites, first at the antenna and then just before entering the house.
>
> Is there an advantage in grounding at more than these sites?

With grounds the most common experience is "the more the merrier".
As you add more, however, you usually reach a diminishing returns
(no pun intended) situation where there is no *observable* improvement:
that's usually a good place to stop. There are also exceptional
circumstances where grounding increases noise problems, but these,
in my experience, are much rarer than the pundits who preach against
"ground loops" seem to think.

Even a semi-quantitative theoretical treatment of grounding in
oversimplified situations requires heavy math at RF. Experimentation
is thus required even if one has done elaborate calculations. It's
often easier to use the theory as a guide to what to try, and then
experiment.

> I would also assume that the antenna is grounded when it is connected to
> the receiver as the outer braid of the coax is in continuity with the
> receiver chassis.

What's ground? If connect the shield of my coax (which is grounded
outside) to the antenna input of my R8, I hear lots of junk,
indicating that there is an RF voltage difference between the coax
shield and the R8 chassis. Last night this measured about S5.5,
which is about -93 dBm (preamp off, 6KHz bandwidth). That's a lot
of noise: it was 18 dB above my antenna's "noise floor", and 26 dB
above the receiver's noise floor.

This sort of disagreement about ground potential is characteristic
of electrically noisy environments. The receiver will, of course,
respond to any voltage input that differs from its chassis ground.
The antenna, on the other hand, is in a very different environment,
and will have its own idea of what ground potential is. If you want
to avoid noise pickup, you need to deliver a signal, referenced at
the antenna to whatever its ground potential is, in such a way that
when it arrives at the receiver, the reference potential is now
the receiver's chassis potential.

Coaxial cable represents one way to do this. Coax has two key
properties:

1. The voltage between the inner conductor and the shield depends
only on the state of the electromagnetic field within the shield.

2. The shield prevents the external electromagnetic field from
influencing the internal electromagnetic field (but watch out at
the ends of the cable!).

So, it's easy, right? Run coax from the antenna to the receiver.
Ground at the antenna end will be whatever the antenna thinks it
is, while ground at the receiver end will be whatever the receiver
thinks it is. The antenna will produce the appropriate voltage
difference at the input side, and the receiver will see that voltage
difference uncontaminated by external fields, according to the
properties given above.

Unfortunately, it doesn't quite work that way. It's all true as
far as it goes, but it neglects the fact that the coax can also
guide noise from your house to your antenna, where it can couple
back into the cable and into your receiver. To see how this works,
let me first describe how this noise gets around.

The noise I'm talking about here is more properly called "broadband
electromagnetic interference" (EMI). It's made by computers, lamp
dimmers, televisions, motors and other modern gadgets. I have all
these things. In many cases, I can't get them turned off, because
it would provoke intrafamilal rebellion. However, even when I turn
them off, the noise in the house doesn't go down very much, because
my neighbors all have them too. In any case, one of the worst
offenders is my computer, which is such a handy radio companion
I'm not about to turn *it* off.

Some of this noise is radiated, but the more troublesome component
of this is conducted noise that follows utility wires. Any sort of
cable supports a "common mode" of electromagnetic energy transport
in which all of the conductors in the cable are at the some potential,
but that potential differs from the potential of other nearby
conductors ("ground"). The noise sources of concern generate common
mode waves on power, telephone, and CATV cables which then distribute
these waves around your neighborhood. They also generate "differential"
mode waves, but simple filters can block these so they aren't
normally a problem.

So, let's say you have a longwire antenna attached to a coaxial
cable through an MLB ("Magnetic Longwire Balun" [sic]). Suppose
your next door neighbor turns on a dimmer switch. The resulting RF
interference travels out his power lines, in through yours, through
your receiver's power cord to its chassis, and out your coaxial
cable to your MLB. Now on coax, a common mode wave is associated
with a current on the shield only, while the mode we want the signal
to be in, the "differential" mode, has equal but opposite currents
flowing on shield and inner conductor. The MLB works by coupling
energy from a current flowing between the antenna wire and the coax
shield into into the differential mode. But wait a second: the
current from the antenna flows on the coax shield just like the
common mode current does. Does this mean that the antenna mode is
contaminated with the noise from your neighbor's dimmer?

The answer is a resounding (and unpleasant) yes! The way wire
receiving antennas work is by first moving energy from free space
into a common mode moving along the antenna wire, and then picking
some of that off and coupling it into a mode on the feedline. In
this case, the common mode current moving along the antenna wire
flows into the common mode of the coax, and vice versa. The coax
is not just feedline: it's an intimate part of the antenna!
Furthermore, as we've seen, it's connected back through your
electrical wiring to your neighbor's dimmer switch. You have a
circuitous but electrically direct connection to this infernal
noise source. No wonder it's such a nuisance!

The solution is to somehow isolate the antenna from the common mode
currents on the feedline. One common way to do this is with a
balanced "dipole" antenna. Instead of connecting the feedline to
the wire at the end, connect it to the middle. Now the antenna
current can flow from one side of the antenna to the other, without
having to involve the coax shield. Unfortunately, removing the
necessity of having the coax be part of the antenna doesn't
automatically isolate it: a coax-fed dipole is often only slightly
quieter than an end-fed longwire. A "balun", a device which blocks
common mode currents from the feedline, is often employed. This
can improve the situation considerably. Note that this is not the
same device as the miscalled "Magnetic Longwire Balun".

Another way is to ground the coaxial shield, "short circuiting"
the common mode. Antenna currents flow into such a ground freely,
in principle not interacting with noise currents. The best ground
for such a purpose will be a earth ground near the antenna and far
from utility lines.

Still another way is to block common mode waves by burying the
cable. Soil is a very effective absorber of RF energy at close
range.

Unfortunately, none of these methods is generally adequate by itself
in the toughest cases. Baluns are not perfectly effective at blocking
common mode currents. Even the best balun can be partially defeated
if there's any other unsymmetrical coupling between the antenna
and feedline. Such coupling can occur if the feedline doesn't come
away from the antenna at a right angle. Grounds are not perfect
either. Cable burial generally lets some energy leak through. A
combination of methods is usually required, both encouraging the
common mode currents to take harmless paths (grounding) and blocking
them from the harmful paths (baluns and/or burial).

The required isolation to reach the true reception potential of
the site can be large. According to the measurements I quoted above,
for my site the antenna noise floor is 18 dB below the conducted
noise level at 10 MHz. 18 dB of isolation would thus make the levels
equal, but we want to do better than that: we want the pickup of
common mode EMI to be insignificant, at least 5 dB down from the
antenna's floor. In my location the situation gets worse at higher
frequencies as the natural noise level drops and therefore I become
more sensitive: even 30 dB of isolation isn't enough to completely
silence the common mode noise (but 36 dB *is* enough, except at my
computer's CPU clock frequency of 25 MHz).

Getting rid of the conducted noise can make a huge difference in
the number and kinds of stations you can pick up: the 18 dB difference
between the conducted and natural noise levels in the case above
corresponds to the power difference between a 300 kW major world
broadcaster and a modest 5 kW regional station.

The method I use is to ground the cable shield at two ground stakes
and bury the cable in between. The scheme of alternating blocking
methods with grounds will generally be the most effective. The
ground stake near the house provides a place for the common mode
noise current to go, far from the antenna where it cannot couple
significantly. The ground stake at the base of my inverted-L antenna
provides a place for the antenna current to flow, at a true ground
potential relative to the antenna potential. The buried coax between
these two points blocks noise currents.

> There has been some discussion of grounding problems on this and related
> echos. I believe it has been mentioned that electrical codes require
> that all grounds be tied together with heavy guage wire.

I'm no expert on electrical codes, and codes differ in different
countries. However, I believe that any such requirement must refer
only to grounds used for safety in an electric power distribution
system: I do not believe this applies to RF grounds.

Remember that proper grounding practice for electrical wiring has
very little to do with RF grounding. The purpose of an electrical
ground is to be at a safe potential (a few volts) relative to
non-electrical grounded objects like plumbing. At an operating
frequency of 50/60 Hz, it needs to have a low enough impedance (a
fraction of an ohm) that in case of a short circuit a fuse or
breaker will blow immediately.

At RF such low impedances are essentially impossible: even a few
centimeters of thick wire is likely to exhibit an inductive impedance
in the ohm range at 10 MHz (depends sensitively on the locations
and connections of nearby conductors). Actual ground connections
to real soil may exhibit resistive impedances in the tens of ohms.
Despite this, a quiet RF ground needs to be within a fraction of
a microvolt of the potential of the surrounding soil. This is
difficult, and that's why a single ground is often not enough.

> A little experimentation with my radio showed that the chassis was


> directly connected to the third (grounding) prong of the wall plug. I
> am concerned that by connecting my receiver to an outside ground I am
> creating a ground loop that involves my house wiring. Can you comment
> on this?

Yes, you have a "ground loop". It's harmless. In case of a nearby
lightning strike it may actually save your receiver. My R8 isn't
grounded like that, so I had to take steps to prevent the coax
ground potential from getting wildly out of kilter with the line
potential and arcing through the power supply. I'm using a surge
supressor designed to protect video equipment: it has both AC
outlets and feedthroughs with varistor or gas tube clamps to keep
the various relative voltages in check. Of course the best lightning
protection is to disconnect the receiver, but I'm a bit absent
minded so I need a backup.

> This may seem like a trivial point but I recently discovered that the

> main ground from the electrical service panel in my house was attached


> to a water pipe which had been painted over. I stripped the paint from
> the pipe and re-attached the grounding clamp and I noticed a reduction
> in noise from my receiver.

Not trivial. Not only did you improve reception, but your wiring
is safer for having a good ground.

I suspect part of the reason I see so much noise from neighbors'
appliances on my electric lines may be that my house's main ground
wire is quite long. The electrical service comes in at the south
corner of the house (which is where the breaker box is), while the
water (to which the ground wire is clamped) enters at the east
corner. All perfectly up to code and okay at 60 Hz, but lousy at
RF: if it was shorter, presumably more of the noise current would
want to go that way, and stay away from my receiver.

> I am also a little confused by what constitues an adequate ground. I
> have read that a conducting stake driven into the ground will divert
> lightning and provides for electrical safety but that RF grounding
> systems have to be a lot more complex with multiple radials with lengths
> related to the frequencies of interest. Is this true?

Depends on what you're doing. If you're trying to get maximum signal
transfer with a short loaded (resonant) vertical antenna with a
radiation resistance of, say, 10 ohms, 20 ohms of ground resistance
is going to be a big deal. If you're transmitting 50 kW, your ground
resistance had better be *really* tiny or things are going to smoke,
melt or arc.

On the other hand, a ground with a resistance of 20 ohms is going
to be fairly effective at grounding a cable with a common mode
characteristic impedance of a few hundred ohms (the characteristic
impedance printed on the cable is for the differential mode; the
common mode characteristic impedance depends somewhat on the distance
of the cable from other conductors, but is usually in the range of
hundreds of ohms). Of course, if it was lower a single ground might
do the whole job (but watch out for mutual inductance coupling
separate conductors as they approach your single ground).

In addition, a ground with a resistance of 20 ohms is fine for an
unbalanced antenna fed with a high impedance transformer to supress
resonance. Such a nonresonant antenna isn't particularly efficient,
but high efficiency is not required for good reception at HF and
below (not true for VHF and especially microwave frequencies).

Much antenna lore comes from folks with transmitters who, armed
with the "reciprocity" principle, assume that reception is the same
problem. The reciprocity principle says that an antenna's transmission
and reception properties are closely related: it's good physics,
but it ignores the fact that the virtues required of a transmitting
and receiving antenna are somewhat different. Inefficiency in a
transmitting antenna has a direct, proportional effect on the
received signal to noise ratio. On the other hand, moderate
inefficiency in an HF receiving antenna usually has a negligible
effect on the final result. A few picowatts of excess noise on a
transmitting antenna has no effect on its function, but is a big
deal if you're receiving (of course, one might not want to have
transmitter power going out via unintended paths like utility lines:
this is indeed the "reciprocal" of the conducted noise problem,
and has similar solutions).


--
John Doty "You can't confuse me, that's my job."
j...@space.mit.edu

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