SSB Antenna connection

[Bruce in Alaska]

The best place for grounding is at the ATU. Grounding the TX is not
necessary then. If you only ground the TX, high RF currents will flow on the
outside of the coax from ATU to ground and as a result the coax will radiate
too.

Meindert
PE1GRV

One small caveat that I would make to the above is:

If the RF Ground should prove to be of maginally high in impedance at
the Tx Frequency, then the transmitter, if it is not connected to
that RF Ground by anything but the coax, the transmitter will then be
part of the antenna system, as it will tend to float above the RF Ground
by the impedance at the Antenna Tuners Ground Stud. This can cause the
radio to do very strange things, and on occasion can cause RF Burns
when the operator transmits with the ground connected mic hanger
disc in his palm. Alot more common in, the old days of, fixed tuned
channelized antenna tuners. Autotuners just aren't as efficent at tuning
the antennas as the fixed tuner cannelized tuners.

Bruce in alaska

 

[Jack Painter]

I don't know who taught you your math but 1" cooper tubing doesn't even
come close to the surface area per length of 3" copper foil. It isn't
the DC amps that you need to worry about it is the Impedance of the
connection between the RF Ground System and the Ground stud on the
Antenna Tuner. Lower impedance better Rf Ground. It is the Ground
that makes the Antenna work.

Pi*D is circumference, or Pi2R.

1" pipe exceeds the surface area of 3" strap. As totally impractical as
copper pipe would be as a bonding conductor in almost any application, it
would make an excellent RF ground connection on shore facilities where it
would offer several times the surface area of a solid ground rod half it's
diameter. Copper pipe is commonly filled with conductive salts which leech
through holes in the pipe to maintain high conductivity in ground rod
installations.

Best regards,

Jack Painter
Virginia Beach, Va

 

[Bruce in Alaska]

Much snippage...
.
Is the 1" just a rule of thumb or is there some engineering calculations to
support this distance? I see many sailboats with 1" varnished wooden dowels
or even plastic hair curlers with cable ties used to keep the GTO15 off the
uninsulated backstay wire. I have a feeling wider spacing would be even
better.Comments please.
Doug K7ABX

The 1" is a "Rule Of Dumb", that does allow for some decoupling, but
doesn't really decouple the antenna for the parallel Grounded backstay.
One can calculate the capacative interaction between the two, or one
can just use the "Rule of Dumb" and let the antenna tuner correct for
the extra capacitance. Just understand that most autotuners will not
like the extra capacitance on the output, and this will tend to widen
out the 1/2 wavelength tuning problems that all endfeed autotuners have.

Bruce in alaska

 

[Bruce in Alaska]

Pi*D is circumference, or Pi2R.

1" pipe exceeds the surface area of 3" strap. As totally impractical as
copper pipe would be as a bonding conductor in almost any application, it
would make an excellent RF ground connection on shore facilities where it
would offer several times the surface area of a solid ground rod half it's
diameter. Copper pipe is commonly filled with conductive salts which leech
through holes in the pipe to maintain high conductivity in ground rod
installations.

Best regards,

Jack Painter
Virginia Beach, Va

Jack my boy you need to go back to school and refresh your geometry.

Pi*D is circumfrence This is true. Now what is the outside surface
area for one inch of length of 1" dia copper pipe?
Lets see 3.1416*1"=3.1416 3.1416*1" length = 3.1416 Square Inches.


Now lets look at 3" copper foil.....ok for Inch of length
we have 3.0 inches square inches of surface area on the front
side and 3.0 square inches of surface area on the back side.
That makes a total of 6 Square Inches of surface area per 1"
of length. We will ignore the thickness of the foil, just because
it isn't significant for the terms we are discussion here.

Now which has more surface area per linear inch?

3.1416 Square Inches for the copper tubing........

or

6 square Inches for the 3" foil........

Hmmmmmmm, wonder what could be the answer.........

If you like, you could think of the copper tubing as being flattened
into a bar and measure it again and it still wouldn't approch a 3" foil
for surface area.

You don't get to include the inside surface area of the tubing, because
RF flows on the outside surface only, but even if you did, you would
still come up a bit short on surface area when compared to 3" copper
foil.

We aren't talking about Shoreside systems on this thread as the original
poster specificly asked about shipboard installations. You observations
about copper pipes being used for Grounding Rods in MF/HF systems
instead of solid copper has some merit, but most RF Grounded Antenna
Systems in the Maritime Mobile Radio Service use a 1/4 Lambda Radial
Grounding System designed for the lowest Frequency that the antenna will
be used on, with interspeced radials cut for 1/4 Lambda at the other
major operating bands. These type antennas really fell out of favor
in the 30's and 40's for Non RF Grounded Antennas like the Rhombics
that Pt. Reyes, CA (KMI) used. They had a set of Phased Rhombics that
could be steered to any point from due South to due North, on 4, 6, 8,
12,16, and 22 Mhz. No Grounding required, and a Frontend smoking signal
everywhere in the Pacific.



Bruce in alaska

 

[Jack Painter]

Jack my boy you need to go back to school and refresh your geometry.

Pi*D is circumfrence This is true. Now what is the outside surface
area for one inch of length of 1" dia copper pipe?
Lets see 3.1416*1"=3.1416 3.1416*1" length = 3.1416 Square Inches.


Now lets look at 3" copper foil.....ok for Inch of length
we have 3.0 inches square inches of surface area on the front
side and 3.0 square inches of surface area on the back side.
That makes a total of 6 Square Inches of surface area per 1"
of length. We will ignore the thickness of the foil, just because
it isn't significant for the terms we are discussion here.

Now which has more surface area per linear inch?

3.1416 Square Inches for the copper tubing........

or

6 square Inches for the 3" foil........

Hmmmmmmm, wonder what could be the answer.........

If you like, you could think of the copper tubing as being flattened
into a bar and measure it again and it still wouldn't approch a 3" foil
for surface area.

You don't get to include the inside surface area of the tubing, because
RF flows on the outside surface only, but even if you did, you would
still come up a bit short on surface area when compared to 3" copper
foil.

C'mon ol' salt, you should know the inside of copper pipe is electrically
identical to both sides of copper strap when a bonding connection is made to
either. Skin effect of electrical current is felt equally on both in _that_
condition. Electromagnetic induction on a material from one outside
direction sees skin effect on the outside surface only of a closed
structure, cabinet, pipe, etc. But we are not talking about EMF's. We are
talking about a bonded connection that has RF voltages, and during tuning
applications, RF current applied directly to the conductor. Respectfully, my
geometry and description of the conductor skin-effect condition are both
correct, and 1" pipe exceeds the surface area of 3" strap.

Best regards,

Jack Painter
Virginia Beach, Va

 

[Jack Painter]

made

No it isn't. Consider a massive rod of 1". RF flows at the outside due to
skin effect. No remove the innards of the rod, leaving, say 1/16" of wall.
Why would current suddenly flow at the inner surface? It isn't, for the same
reason it was on the outside when the rod was massive.

Besides, heavy coils in radio stations are all tubes and cooled by running
water through them. Due to the skinn effect, the water is not 'touched' by
the RF.


Yes we are. And EMF is exactly the reason why the electrons start to repell
eachother. And the only place where they are as far apart as possible is on
the outside of the tube.

Meindert, water is not a good conductor, with average tap water having
100,000 ohms resistance across 1 meter of 15mm plastic pipe filled with
water. Even at RF frequencies, where skin effect is most pronounced, a
bonded connection made equally to both inside and outside of a copper pipe
should exhibit skin effect throughout most of the entire cross section of
the copper pipe. This is because the wall thickness of the copper pipe is
not materially different from copper strap.

Example:

For copper tubing used as a inductor in antenna tuners:

coil length
R= ---------------------------------------
conductivity *skindepth*2pi*coil radius

Now, applying voltage to the outer surface only of copper tubing with closed
ends, whether by EMF attachment or bonded connection to the outside only,
would exhibit surface-only skin effect similar to if a faraday cage was
constructed of the same copper strap we are talking about. The outside
surface would carry most current. But if the voltage connection was bonded
to both inside and outside of an opening of the faraday box or the copper
tubing, then current via skin effect would be nearly constant on the inside
and outside surfaces of the box, defeating the faraday effect. The
condition I originally described, that of a bonded connection, applies
voltage equally and carries current equally on the entire skin of the
conductor, inside and out, 360 degrees, as efficiently as a piece of copper
strap of similar cross section.

Best regards,

Jack Painter
Virginia Beach, Va

 

[Bruce in Alaska]

Meindert, water is not a good conductor, with average tap water having
100,000 ohms resistance across 1 meter of 15mm plastic pipe filled with
water. Even at RF frequencies, where skin effect is most pronounced, a
bonded connection made equally to both inside and outside of a copper pipe
should exhibit skin effect throughout most of the entire cross section of
the copper pipe. This is because the wall thickness of the copper pipe is
not materially different from copper strap.

Example:

For copper tubing used as a inductor in antenna tuners:

coil length
R= ---------------------------------------
conductivity *skindepth*2pi*coil radius

Now, applying voltage to the outer surface only of copper tubing with closed
ends, whether by EMF attachment or bonded connection to the outside only,
would exhibit surface-only skin effect similar to if a faraday cage was
constructed of the same copper strap we are talking about. The outside
surface would carry most current. But if the voltage connection was bonded
to both inside and outside of an opening of the faraday box or the copper
tubing, then current via skin effect would be nearly constant on the inside
and outside surfaces of the box, defeating the faraday effect. The
condition I originally described, that of a bonded connection, applies
voltage equally and carries current equally on the entire skin of the
conductor, inside and out, 360 degrees, as efficiently as a piece of copper
strap of similar cross section.

Best regards,

Jack Painter
Virginia Beach, Va

Jeeezzz Louise Jack, where did you learn all this BS that your spreading.

But if the voltage connection was bonded
to both inside and outside of an opening of the faraday box or the copper
tubing, then current via skin effect would be nearly constant on the inside
and outside surfaces of the box, defeating the faraday effect.

Please explain how one "BONDS" a connection to only the inside of a
copper pipe. All of the Physic Professors of the World would really
like to know. Are you saying that if one made a "RF Connection",
to only the inside of a copper tube, that no RF would flow on the
outside of the tube? That is just plain wrong, and a stupid statement
on it's face.

ok, enough of this BS, CFR!!! (Call for Reference) Let's see if old
Jack can actually come up with some documentation that RF flows on the
inside of a connected copper tube or pipe. Lets go for some Peer
Reviewed Documentation here, the straight, No shit, Textbook, kind
of documentation, written by some really Qualified Physics Phd's.

Hmmmm, all the PhdEE's that I asked, just laughed and ask how the
weather and fishing was.........

Bruce in alaska

 

[Jack Painter]

Jeeezzz Louise Jack, where did you learn all this BS that your spreading.


Please explain how one "BONDS" a connection to only the inside of a
copper pipe. All of the Physic Professors of the World would really
like to know. Are you saying that if one made a "RF Connection",
to only the inside of a copper tube, that no RF would flow on the
outside of the tube? That is just plain wrong, and a stupid statement
on it's face.

ok, enough of this BS, CFR!!! (Call for Reference) Let's see if old
Jack can actually come up with some documentation that RF flows on the
inside of a connected copper tube or pipe. Lets go for some Peer
Reviewed Documentation here, the straight, No shit, Textbook, kind
of documentation, written by some really Qualified Physics Phd's.

Hmmmm, all the PhdEE's that I asked, just laughed and ask how the
weather and fishing was.........

Bruce, you're making a totally off the wall argument now, with opposite
assumptions that were never asserted or offered by any of the posters to
this thread. Taking your questions literally as you phrased them would
generate a laugh by all, indeed. If a laugh was your intention, we'll all
have a good one. But I doubt that you are confused about skin effect, or why
a faraday cage works, and specifically what would defeat it's protection
(ie: an opening). So if you seriously think that for instance, a c-clamp
applied across an open end of thin walled copper tubing, contacting the
inner and outer wall in it's grip, would apply voltage differently to the
inside versus the outside of this tubing, then it will be easy to explain
your error in thinking. And since I did not make a joke of your obvious
geometry and math errors in determining the surface area of an object, one
which you continue to be confused about, I would suggest that we either: end
the thread if you do not desire pleasant and professional discussion, or,
omitting the snide comments that do not reflect well on the group or it's
interested participants.

Respectfully,

Jack Painter
Virginia Beach, Va

 

[Bruce in Alaska]

Normally, when tywrapping the feedline to the backstay below
the insulator, the lower part of the backstay is not grounded. Otherwise
small standoff are used.

Doug. k3qt
s/v Callista

I liked the idea, I saw here a while back, of using the new Kevlar based
Backstay material, and not worring about having to ground or not. Seemed
like the logical answer to me. Then just helical wrap the antenna wire
around the Kevlar Backstay and have a really nice "Fully Loaded Antenna
with alot of electrical length........


Bruce in alaska

 

[Gary Schafer]

Bruce, you're making a totally off the wall argument now, with opposite
assumptions that were never asserted or offered by any of the posters to
this thread. Taking your questions literally as you phrased them would
generate a laugh by all, indeed. If a laugh was your intention, we'll all
have a good one. But I doubt that you are confused about skin effect, or why
a faraday cage works, and specifically what would defeat it's protection
(ie: an opening). So if you seriously think that for instance, a c-clamp
applied across an open end of thin walled copper tubing, contacting the
inner and outer wall in it's grip, would apply voltage differently to the
inside versus the outside of this tubing, then it will be easy to explain
your error in thinking. And since I did not make a joke of your obvious
geometry and math errors in determining the surface area of an object, one
which you continue to be confused about, I would suggest that we either: end
the thread if you do not desire pleasant and professional discussion, or,
omitting the snide comments that do not reflect well on the group or it's
interested participants.

Respectfully,

Jack Painter
Virginia Beach, Va

Oh boy! I just got back from vacation and am just now reading this
stuff.

Jack, Bruce and the others are entirely right. I once had a hard time
figuring out why RF would not flow on the inside of a tube too. It
would seem logical that it would do as you say but it doesn't.

Look up "wave guide beyond cutoff". That will answer your question
about why rf dose not flow on the inside of a tube.

It will flow on the inside for only a very short distance from the
opening. Then it gets canceled. This is how many signal generator
attenuater work.
They use a tube of 6 or so inches long with a sliding probe inside fed
from one end. On the other open end is a fixed pickup probe. When the
movable probe is close to the fixed probe on the other end, maximum
signal coupling is obtained. As the other probe is moved away inside
the tube the signal becomes highly attenuated.

It is operating as a wave guide that is much too small for the
frequency involved. If the tube diameter was made large enough to be a
quarter wave length in diameter then the rf would propagate through
it. But that would be in a different mode than the skin effect
conduction being discussed.

By the way did you know that skin effect even comes into play in 60 hz
distribution systems?

Regards
Gary

 

[Gary Schafer]

Steve, you have asked about two distinctly different forms of connection
that require equally different conductors. Additionally, within your
grounding questions there also are two different issues, addressed below:

1. RF feedline from ATU to antenna.

This should be coaxial cable with dialectric and shielding designed for RF.
Never improvise with something such as spark plug wires.

2.(a) Grounding: RF

This does not have to be wide surface area copper, but doing so will not
hurt, and it will allow the combination-use of the RF ground connection to
serve as a lightning protection ground. RF ground does not require a dc-
connection to ground, and is often designed to use capacitive coupling to
ground for sailing vessels and other marine applications where isolation for
galvanic protection is adviseable.

2. (b) Grounding: Lightning protection

Also does not require a dc-connection to ground, but may not use low valued
capacitors such as would be acceptable for RF ground. Lightning protection
DOES require the widest surface area possible, this provides a lower
impedance path to ground. But your radio and auto-tuner and other equipment
are most importantly bonded to each other, and that may be of any standard
braid, #8 wire, etc. Only the single connection of all your bonded equipment
to ship's ground must be of the highest surface area possible. If more than
one connection from bonded equipment to ground must be made, then each of
those connections should be wide surface area conductors.

Hope this helps,

Jack Painter
Virginia Beach, VA

A good lightning ground is also a good RF ground. But a good RF ground
is not always a good lightning ground. (as in the case of elevated
radials)

Most lightning energy is concentrated in the DC to 1 mhz range with
some energy going much higher in frequency. So the ground conductors
and ground system must be treated the same as an RF ground system with
with regard to low impedance leads (large surface area) and, in the
case of lightning, low DC resistance connections. Even a low
resistance connection can develop many thousands of volts across it
with the high current lightning.

An RF ground requires a low impedance conductor as well.

Regards
Gary

 

[Gary Schafer]

Others have covered the GTO-15 question, very well.

There are a number of reasons that copper strap is used for RF Grounding
in the Maritime Radio Installations. One being, that it is desireable
for the RF Ground to have the lowest possible Impedance at the
transmitted frequency.

Two being, that it is desirable that the surface area of the RF Ground
System be as large as practicable, to maximise coupling to the seawater.

Three being, That RF flows on the surface of the conductor, and more
surface area means lower impedance on the Ground.

The antenna wire isn't supposed to couple into the seawater, but into
the ethos, so it should have the least surface area as can practically
handle the RF Current of the transmitter and be tuned to resonance by
the tuner, and as low of resistance as practicable, so that RF Current
can propagate along it's length.

Bruce in alaska Gary S. can chime in anytime on this.....

Hi Bruce,

The diameter of the antenna wire is not too important. Actually the
larger it is the less resistive loss it has and less power will be
wasted in heat. But unless the antenna is significantly shorter than a
quarter wavelength that loss is negligible in the antenna as the
radiation resistance (radiation resistance is where the power goes to
be radiated) is usually much higher than the resistive loss of the
wire.

However in a very short antenna the radiation resistance can be only
an ohm or a few ohms. Then the resistance of the wire would be a
larger percentage and the heat loss would be greater thus warranting a
larger diameter wire.

Otherwise a larger diameter wire has the advantage of greater
bandwidth for given tuner settings. But the difference between #10 and
# 16 would probably not be noticeable.

As you well know, in the case of the ground system as we have said
many times before, it needs to be as short as possible or it becomes
part of the antenna and radiates. "The antenna starts at ground".
Anything above ground is antenna.

Regards
Gary

 

[Gary Schafer]

Chuck, as with Meindert's answer, yes that helps, thank you.

I do end-feed a long wire as I said earlier, but it uses a 4:1 Balun, and
additionally, has one side of that Balun shorted to ground. This is a
noise-limiting design, and while the nice folks at Radio Works (Portsmouth,
Va) maintain that it cannot possibly work this way (their Baluns), the CG
aircraft I worked in Ecuador with it thought otherwise. So does it's
designer, whose name slips my mind at the moment but he was a primary
contributer to "Proceedings", and a Phd in EE with many patented antenna
designs. Anyway, it would be interesting to see some modelling done with
backstay antennas using various feedline approaches. I suspect the
difference varies greatly with wavelength, height above ground (water),
angle, and frequency.

73,
Jack Painter
Virginia Beach, Va

Jack,

Using a balun to feed an end fed wire may help and it may hurt the
situation. It depends on the length of the wire verses frequency.

If the wavelength is greater than a quarter wave length and the
impedance of the wire is high, the balun will transform it down to a
sometimes easier to match impedance. However if you use the antenna on
different bands and you chose a band where the impedance of the
antenna is low, then the 4:1 balun will step the impedance down even
lower than the already low impedance of the antenna. It may well be
that it is too low to match efficiently.

As a general rule that type of balun is not a good idea when using
that type of antenna on multiple bands.

The only good a 1:1 balun would do with that type of antenna would be
to decouple the feed line from the antenna (assuming coax feed line)
and keep the feed line from radiating and or picking up unwanted
signals.

Regards
Gary

 

[Max Lynn]

I am the one who posted that idea. I implemented it and used the setup on a
recent Mexico trip. I am a newbie in this arena, so I can only tell you that
my rig was probably the best in the fleet of several boats. On the subject
of antenna feed wire, I found an old reference on this NG recommending
stripping the braid from coax(RG-8 is what I used) as a substitute for
GTO-15. I was unable to locate a local source for GTO-15, so I went with
the stripped coax. I was unable to do a good job on standoffs for the coax
because of the hydraulics, but it didn't seem to matter a great deal.

I did not think to do a helical wrap of the antenna wire which incidentally
was just standard insulated #16 Ancor about 45' in length. On larger boats,
the antenna wire is buried beneath the UV shield; on mine the wire was taped
to the exterior of the UV shield.

A lot of racing sailboats are switching their rod or wire backstays to
Aramid at this time. The weight savings is dramatic, and the cost is
roughly half of what a backstay with insulators would cost.

 

[Jack Painter]

Oh boy! I just got back from vacation and am just now reading this
stuff.

Jack, Bruce and the others are entirely right. I once had a hard time
figuring out why RF would not flow on the inside of a tube too. It
would seem logical that it would do as you say but it doesn't.

Look up "wave guide beyond cutoff". That will answer your question
about why rf dose not flow on the inside of a tube.

It will flow on the inside for only a very short distance from the
opening. Then it gets canceled. This is how many signal generator
attenuater work.
They use a tube of 6 or so inches long with a sliding probe inside fed
from one end. On the other open end is a fixed pickup probe. When the
movable probe is close to the fixed probe on the other end, maximum
signal coupling is obtained. As the other probe is moved away inside
the tube the signal becomes highly attenuated.

It is operating as a wave guide that is much too small for the
frequency involved. If the tube diameter was made large enough to be a
quarter wave length in diameter then the rf would propagate through
it. But that would be in a different mode than the skin effect
conduction being discussed.

By the way did you know that skin effect even comes into play in 60 hz
distribution systems?

Regards
Gary

Hi Gary, welcome back, and thanks for your replies.

Right principles, wrong application. Trying to apply high power microwave
principles (3-15 gHz) to low power 2-30 mHz) is not the same. Now at 100 mHz
and below, while there would still a small but measurable difference of skin
effect at high transmit power, it ain't much and has nothing to do with low
power 2-30 mHz where a thin walled copper tube has ZERO measurable
difference in skin effect to a copper strap of even slightly smaller gage.
That has been my never paid attention to point all along, that skin effect
involves the entire cross section of thin material, and copper tubing is
more than thin enough to carry current in it's entire (that means from outer
to inner surface) cross section. That's exactly why copper tube is used so
much in AM broadcast components. This is not even related to waveguides
which must by design AVOID all skin effect which causes great resistance and
heating at the current and velocites involved in microwave transmission.

As we eventually got around to research rather than blindly arguing
positions of opinion, then the participants hopefully learned something.
I've learned that applying the math from formulas for skin effect in
conductors of known ohmic value and used with a known frequency can
determine the wall thickness of a conductor which has full cross sectional
current on it. Guess what? The original poster's question about using copper
tubing remains answered. A 1" copper tube has more surface area and carries
just as much low power RF on it's entire cross section as a 1" wide piece of
copper strap that is nearly the same gage.

Best,

Jack Painter
Virginia Beach Va

 

[Gary Schafer]

Hi Gary, welcome back, and thanks for your replies.

Right principles, wrong application. Trying to apply high power microwave
principles (3-15 gHz) to low power 2-30 mHz) is not the same.

Sorry Jack but you are wrong. It has nothing to do with microwave
frequencies. A wave guide beyond cutoff is the mode that the tube is
operating in and it simply tells you that the frequency is too low for
the given size tube to propagate through. The energy inside the tube
gets shorted out. Many 2-30 mhz signal generators use that type
attenuator.

Now at 100 mHz
and below, while there would still a small but measurable difference of skin
effect at high transmit power, it ain't much and has nothing to do with low
power 2-30 mHz where a thin walled copper tube has ZERO measurable
difference in skin effect to a copper strap of even slightly smaller gage.

It has everything to do with it. Skin effect is ever present in all
conductors at ALL frequencies. Note my reference to 60 hz power
transmission where it is also important.

That has been my never paid attention to point all along, that skin effect
involves the entire cross section of thin material, and copper tubing is
more than thin enough to carry current in it's entire (that means from outer
to inner surface) cross section. That's exactly why copper tube is used so
much in AM broadcast components.

That is a contradiction to your point. You say that current flows
entirely through the walls of copper tubing and then say that is why
it is used in AM broadcast components. If that were true then they
would not use copper tubing but instead they would use solid copper
rod for better conduction.

The reason copper tubing is used is that there is no current of any
significance past a certain depth and to use solid rod would be a
waste of copper.

This is not even related to waveguides
which must by design AVOID all skin effect which causes great resistance and
heating at the current and velocites involved in microwave transmission.

Well, microwave transmissions don't travel any faster than HF
transmissions. But you might note that most wave guide inner surfaces
are silver plated to reduce skin losses.

As we eventually got around to research rather than blindly arguing
positions of opinion, then the participants hopefully learned something.
I've learned that applying the math from formulas for skin effect in
conductors of known ohmic value and used with a known frequency can
determine the wall thickness of a conductor which has full cross sectional
current on it. Guess what? The original poster's question about using copper
tubing remains answered. A 1" copper tube has more surface area and carries
just as much low power RF on it's entire cross section as a 1" wide piece of
copper strap that is nearly the same gage.

While skin effect is a gradient and not an absolute barrier, there is
current that flows at all levels in a conductor. Even on the inner
surface of your copper tube. But the amount of current there is so
small that it is immeasurable. It decreases exponentially.

One skin depth is defined as the depth at which the current has
dropped to about .37 times the current at the surface. (If you notice,
this is the same decay rate that a capacitor has when it charges or
discharges.) When you go that same distance (deeper) again the
remaining current will again drop to .37 times the current that it was
at the first skin depth.

So you can see that the current never reaches zero as you go deeper
but it only takes a few skin depths to decrease the current to a very
small value which is insignificant.

..0058" is the skin depth in copper at 200 khz. Skin depth decreases by
10 for each 100 times increase in frequency. So at 20 mhz the skin
depth would decrease by 100 from that. It gets pretty thin!


Skin effect is the reason coax cable works as it does. None of the RF
on the inside of the cable appears on the outside of the cable. Other
than leakage between strands of the shield of the cable. Those wire
strands on coax cable are pretty thin. Much thinner than your copper
pipe. Hard line has no leakage.

Regards
Gary

 

[Gary Schafer]

Welcome home, Gary. Missed your dissertations, really.

Ah, but once again, BZZZT...WRONG.....

I got two real-world examples to show you from my Navy experiences......

1 - Aboard a wooden MSO (minesweeper, ocean), a sailor was nearly burned
alive when he touched a metal handrail just outside the bridge while on
watch! His hands had bad burns, as did his hip, which was touching a pipe
not connected to the handrail.

An electrical inspection found the handrail heavily grounded, per Navy
requirements, to the boat's electrical grounding system, a good
installation with no problems found. However, the burning continued.

I found out about it from the boat's ET gang because I was in MINELANT's
Mine Force Support Group electronics shop, at the time, around 1970? My
EMO asked me my opinion and for me to take a look.

Not far from the handrail was the antenna tuner and 35' whip of the boat's
AN/URC-32 500W HF transmitter. Curious, I got a list of the frequencies
the boat transmitted on the day he got burned. They operated only three
that day, all on RTTY (FSK at full power). I took a tape measure and, as
best we could, measured the length of the ground strap down into the bilge
where it connected to the ship's grounding system. It was around 31 feet,
total length, and didn't really connect to any other points on the way down
into the engine room. One of the frequencies in question was on the 4 Mhz
band just above the 75 meter ham band. At this frequency, the 31' ground
strap was quite close to a 1/4 wavelength resonant ANTENNA with the open
end right under our burned sailor's forearm and coffee cup he threw when
the FSK started.

So, let's test this theory. I took a scope probe with a 6' ground lead on
it and connected the scope between the pipe he was leaning against, itself
some length of antenna at some other frequency, and our burning handrail.
"Key the transmitter.", I called down the passageway. WOW! The trace of
the 4 Mhz RF was TOO BIG TO MEASURE! I could feel the RF in my fingers!

So, moral, this great lightning ground was NOT ANYWHERE NEAR a good RF
ground on 4 Mhz, or any other odd multiple of 1/4 wavelength. It was a
resonant antenna with a handrail capacitor hat waiting to bite someone!

Solution - After months of fighting the electrical engineers at NAVSEA
about SHIPALTs to allow us to install them, we finally won and installed RF
Chokes into all handrail grounds at the handrails themselves to keep them
from becoming antennas resonant at any HF freq the ship used.

2 - Charleston Naval Shipyard, Metrology Laboratory of the Quality
Assurance Office (Code 132.1). I was asked to look at a crazy alarm
problem at the nuclear refueling docks where we pulled out the reactors
from nuclear subs and replaced them with refuelled reactors. (The hulls
are cut open and the core is swapped by a specially-equipped huge crane
that runs on big rail tracks around the docks).

Every time the crane lowered its big hook down into the hull, all the
radiation alarms went crazy, even before the hook got to where it was
supposed to go! Electrical Engineers (not RF engineers by a long shot)
added more and more ground straps between the rails the huge crane sat on
and the hull of the sub to "make sure" we had a "good ground" on
everything. (More grounds were always their answer to everything.)

I made arrangements to get the crane to where it would normally operate,
with the operator at the controls, but with the hook first hanging over the
rail of the crane, then over the hull of the sub in the drydock for
testing. I snatched a portable Tektronix scope from the shop's inventory
that was battery powered so it wouldn't be part of the grounding systems
and met the crane at the appropriate time. I grounded the scope to the
track at a handy pad eye used to hook the sub ground to it and as I
approached this huge steel hook the trace on the scope went off my screen.
My AC voltmeter read over 80VAC between "ground" and that hook. But wait!
What's this MODULATION all about?? I ran back to the shop to retrieve my
portable radio and quickly returned to the test point. Watching the AM
modulation on my scope while tuning around on the AM band, I matched up the
modulation envelope with WNCG AM 910Khz, a 5KW AM radio station some IDIOT
at the FCC allowed them to construct right outside the hospital gate less
than a mile from where I was standing. THE CRANE WAS A GIANT LOOP ANTENNA
and I was standing at the high-impedance FEEDPOINT of that loop!

Identifying the problem was easy. DOING something about the problem was
NOT! NOONE in Rickover's Navy makes any CHANGES to anything without a
fight. This fight I left, gladly, to much higher powers than me, but it
also resulted in a huge strain insulator added to the cable of the crane to
INSULATE the offending signal from the hook lowered into the sub. Wonder
whatever happened to it, now that it's all gone bye-bye....??

Moral....a great lightning or AC line or DC ground is hardly EVER a good RF
ground.....

Ok, as usual, your turn.....
Larry

Hi Larry, Hope you had a good sail.

Well this is an easy one! Your hand rail was not a good lightning
ground even though it may have had a large ground strap connected to
it. The path to ground was too long providing a high impedance.
Same for the crane. Too long a ground lead at the hook.

A good lightning ground has to have a low DC resistance as well as a
low impedance to AC. Remember that lightning has a large AC component
that is very strong. Any impedance in it's path despite how well the
DC ground may be will allow a large voltage to develop on it.

A good antenna ground must also have a low impedance.

Regards
Gary

 

[Gary Schafer]

The diameter of the antenna wire is very important in the antenna's
BANDWIDTH. Go by the CG shore station and look at how WIDE the conical
monopole antenna is:
http://www.tpub.com/content/et/14092/css/14092_35.htm
The whole reason for the wide cone of these broadband HF antennas is to
make it look as if the conductor were several FEET across to the RF from
the feedpoint.

Multiple, parallel conductors are also used to increase antenna wire
apparent diameter in broadband rhombic antennas such as:
http://www.smc-comms.com/rhombic_antenna.htm

To quote the text:
"The simple one wire system has a bandwidth of approximately 2: 1, however
SMC have wide experience in the design of this type of antenna and are
able to offer arrays with 1, 2 or 3 wires per leg to give a bandwidth of up
to 4: 1 and, by careful design, gains of 22 dBi are possible."


Huh?? ANY antenna under 1/4 wavelength long exhibits HIGHER and HIGHER
impedance the SHORTER it gets. The first low impedance of a wire antenna
occurs when its radiator (against a ground, artificial or real) is 1/4
wavelength long. A very short antenna, i.e. a 6' whip on the handrail, has
a very HIGH impedance as frequency decreases on the HF band. That's why we
use an L network to match it to 50 ohms....coil in series, cap to ground to
lower its impedance.


True, that's why we use multiple parallel conductors above.

Actually, in a plastic boat, the radiation from the ground strap is useful
radiation. You've just moved the FEEDPOINT up the radiating element above
the sea. My feedpoint is about 4.8' above ground on Lionheart. It's
signal strength 5, readability 8 in Moscow, Belarus, UAE, Japan, Brazil,
most of Western Europe on 40 meters and 20 meters. Works pretty good!

73, Larry W4CSC

Oh oh, here we go again. :>)

Remember I said that the radiation resistance drops as the antenna
gets shorter. That is the reason the losses go up with a shorter
antenna. Higher current in the antenna and loading coil means more I
squared R loss.

(radiation resistance is equal to the equivalent resistor that would
dissipate the same amount of power that is being radiated) Lower
radiation resistance requires more current for the same amount of
power verses a higher radiation resistance and less current.

The reactance does indeed get higher the shorter the antenna is. With
an antenna shorter than a quarter wave length as you know it looks
like a capacitor. (capacitive reactance) The less capacitance (shorter
antenna) the higher the reactance. The coil in series provides an
equal but opposite inductive reactance to cancel the capacitive
reactance in the antenna.

That leaves only the radiation resistance to feed power to. The coil
AC resistance (not reactance now)is then effectively in series with
the radiation resistance of the antenna. The same current must flow in
both the antenna and coil losses. While the antenna radiates most of
the power it gets, the coil dissipates power in heat equal to the I
squared R loss in the coil.

The capacitor to ground on the other side of the coil and part of the
coil form an L network to match the impedance to the feed line.

Actually we could say that the L network portion really matches the
radiation resistance plus the coil resistance to the feed line.
Because when the coil reactance and antenna reactance are equal we
have resonance and the only component left is purely resistive.

The high reactance in the antenna causes the voltage to go high. But
there is also a phase shift due to the reactance. So the current is
not in phase with the voltage developed across the reactance. That is
why the voltage is high.

Regards
Gary

 

[Bruce in Alaska]

Hi Bruce,

The diameter of the antenna wire is not too important. Actually the
larger it is the less resistive loss it has and less power will be
wasted in heat. But unless the antenna is significantly shorter than a
quarter wavelength that loss is negligible in the antenna as the
radiation resistance (radiation resistance is where the power goes to
be radiated) is usually much higher than the resistive loss of the
wire.

However in a very short antenna the radiation resistance can be only
an ohm or a few ohms. Then the resistance of the wire would be a
larger percentage and the heat loss would be greater thus warranting a
larger diameter wire.

Otherwise a larger diameter wire has the advantage of greater
bandwidth for given tuner settings. But the difference between #10 and
# 16 would probably not be noticeable.

As you well know, in the case of the ground system as we have said
many times before, it needs to be as short as possible or it becomes
part of the antenna and radiates. "The antenna starts at ground".
Anything above ground is antenna.

Regards
Gary

Yep, absolutly right Gary.


Bruce in alaska

 

[engsol]

This topic is interesting. I've seen a lot of opinions expressed,
some pretty startingly. Can you posters to this thread provide
some math and/or references?
Thanks,
Norm B