On Wed, 09 Jun 2004 00:00:34 -0000, Larry W4CSC
wrote:

Gary Schafer wrote in
:


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.

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."


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.

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.


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.

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

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.


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

In article ,
Gary Schafer wrote:


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
--
add a 2 before @

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

"Gary Schafer" wrote

On Tue, 8 Jun 2004 17:05:53 -0400, "Jack Painter"
wrote:
"Gary Schafer" wrote

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

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.

Hi Gary, the difference that is relevant, I believe, is a waveguide for
microwave broadcast through the inside space of the guide, and there is
minmal current intentionally allowed on the waveguide. As I did explain,
skin effect must be avoided in microwave and it is due to the frequencies,
however it may be exploited in HF conductors which can eliminate wasted
center-core weight and cost. This is because of the drastically different
behavior of microwave from HF. And velocities inside a waveguide are much
faster than HF on a conductor. The attenuator you are describing allows
skin effect (it cannot avoid it either) but the true waveguide avoids it,
with the microwave reflecting off the walls of the guide. Hams can use a
tubing-shield to fox hunt in a building, but it is a stretch of the phrase
to call hiding a hh in the tube a wave guide beyond cutoff.

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.

Sorry Gary, that is not accurate. There is none in DC and very little until
VHF. It has no measureable difference to us for purposes of our discussion
between copper strap and copper tube at HF. Lightning would discover a
different impedance and pick the lower one, whichever that was. You or I or
any of our 150w or 1,000w radio equpment cannot tell the difference. By the
same math, 60hz has no skin effect for home wiring. Long, high power
transmission lines do not enter into a discussion about home wiring, and
neither should mircrowave or skin effect of copper tubing (which there is
none) enter into discussion about an RF ground on a sailboat or other low
power station. It is irrelevant between any copper conductors of similar
surface area and cross section.

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!

Please check your premises. There is no standard depth for any frequency,
rather it varies drastically from one ohmic value of a given material
(conductor) to another. Since we're talking about copper, it's skin depth is
considered fully cross sectional at below 100 megahertz and a thickness of
..0025". At 15mhz on tubing or strap, it is using a full cross section to
carry power, not stray eddy currents. Design of course uses no more than the
proper combination of surface area and cross section to handle the required
frequency and power. Paper thin copper tape has limited usefulness to us,
because it can handle so little current, no matter how great it's surface
area. Copper tape amounts to roughly 1/3 the possible skin depth for copper
at HF, so it is just a cheap and poor alternative for copper strap. Thicker
than that, and we would be wasting center area that would carry little
current. Nobody said coax was the best conductor, it's just the most
economical. ;-)

Cheers,

Jack

"Larry W4CSC" wrote

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!

Larry, we've probably had the details of this antenna system in pieces
across various posts, but would you mind putting in one place here? Sounds
like an intersting and well thought out setup.

Thanks,

Jack

"Jack Painter" wrote in message
news:7exxc.5734$5B2.1970@lakeread04...

Hi Gary, the difference that is relevant, I believe, is a waveguide for
microwave broadcast through the inside space of the guide, and there is
minmal current intentionally allowed on the waveguide.

Wrong Jack. Electromagnetic waves in a waveguide are only possible when
voltages and currents are present. The maximum voltage is between the two
larger sides while currents flow from one side to the other. The entire
field is contained inside the waveguide and therefore the inside surface
must have a low resistance and is silver plated to achieve this. You can
read this in any textbook on microwave transmission.

As I did explain,
skin effect must be avoided in microwave and it is due to the frequencies,
however it may be exploited in HF conductors which can eliminate wasted
center-core weight and cost. This is because of the drastically different
behavior of microwave from HF. And velocities inside a waveguide are much
faster than HF on a conductor. The attenuator you are describing allows
skin effect (it cannot avoid it either) but the true waveguide avoids it,
with the microwave reflecting off the walls of the guide.

Why do you think a microwave reflects on the wall of the waveguide? Because
current flows on the inside wall, which has to have the lowest resistance
possible. It is all skin effect what makes a waveguide tick!

Meindert

"engsol" wrote

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

Norm, because acsii graphics for the formulas you requested do not display
well in newsgroups, here is a collection of the formulas and text from
various websites regarding skin effect:

http://members.cox.net/pc-usa/station/skineffect.htm

Best regards,
Jack

"Meindert Sprang" wrote

"Jack Painter" wrote

Hi Gary, the difference that is relevant, I believe, is a waveguide for
microwave broadcast through the inside space of the guide, and there is
minmal current intentionally allowed on the waveguide.

Wrong Jack. Electromagnetic waves in a waveguide are only possible when
voltages and currents are present. The maximum voltage is between the two
larger sides while currents flow from one side to the other. The entire
field is contained inside the waveguide and therefore the inside surface
must have a low resistance and is silver plated to achieve this. You can
read this in any textbook on microwave transmission.

Hi Meindert, how is that skin effect when, as you said, the currents must
flow from one side to the other? Skin effect would hold currents _on_ the
surface, slow them down, and reduce the reflection that is required for
propagation through the guide.

Why do you think a microwave reflects on the wall of the waveguide?
Because
current flows on the inside wall, which has to have the lowest resistance
possible. It is all skin effect what makes a waveguide tick!

That sounds like a contradiction (current flows from one side to the other,
and current flows on the inside wall, [the latter of which would be skin
effect] ), can you explain please?

Thanks,

Jack

"Jack Painter" wrote in message
news:r_yxc.3506$K45.1736@fed1read02...

Hi Meindert, how is that skin effect when, as you said, the currents must
flow from one side to the other? Skin effect would hold currents _on_ the
surface, slow them down, and reduce the reflection that is required for
propagation through the guide.

With "one side to the other" I meant from the top inside to the bottom
inside, if you lay the waveguide flat on the table. The maximum voltage
inside the waveguide exists between top and bottom of the inside.

Of course, the current never travels from the inside to the outside of the
waveguide.

Below the minimum frequency of a waveguide, no energy can be transported
inside the waveguide and thus no currents will flow at the inside.

Meindert

On Wed, 9 Jun 2004 01:28:26 -0400, "Jack Painter"
wrote:


Hi Gary, the difference that is relevant, I believe, is a waveguide for
microwave broadcast through the inside space of the guide, and there is
minmal current intentionally allowed on the waveguide. As I did explain,
skin effect must be avoided in microwave and it is due to the frequencies,
however it may be exploited in HF conductors which can eliminate wasted
center-core weight and cost. This is because of the drastically different
behavior of microwave from HF. And velocities inside a waveguide are much
faster than HF on a conductor. The attenuator you are describing allows
skin effect (it cannot avoid it either) but the true waveguide avoids it,
with the microwave reflecting off the walls of the guide. Hams can use a
tubing-shield to fox hunt in a building, but it is a stretch of the phrase
to call hiding a hh in the tube a wave guide beyond cutoff.

Please check your premises. There is no standard depth for any frequency,
rather it varies drastically from one ohmic value of a given material
(conductor) to another.


Jack, what velocities are you talking about that are different at
microwaves? The frequency has nothing to do with how fast energy
propagates in a transmission line or anywhere else, regardless of what
you may think you read somewhere.

Electron movement may slow as frequency increases because of the
magnetic forces developed in the conductor but that does not slow the
energy transfer. It only forces the electrons to flow closer to the
surface of the conductor. (skin effect) The electrons deeper in the
conductor are stopped from moving by the counter magnetic fields
developed in the conductor. That is what you are reading about that is
moving slower.

The only reason I even mention wave guides here is that I mentioned
"WAVE GUIDE BEYOND CUTOFF" that is the proper electrical term to
describe why RF does not flow on the inside of a copper tube even if
the end of the tube is open and connected to the outside of the tube.

When the frequency is too low for the diameter of the tube to function
as a wave guide then it is said to be acting as a wave guide that is
beyond the cutoff frequency. Meaning RF will not propagate through it.
And propagation in the wave guide mode is the ONLY way that current
will flow on the inside of a copper tube.

Coax cable must have a center conductor in it in order for current to
flow on the inside of a coax cable. Otherwise it will perform just
like the copper tube.

By the way there are very high currents that flow on the inside walls
of a wave guide. That is why they are usually silver plated inside. It
is a transmission line.

Jack, I don't know what you have been reading in regards to skin
effect but it is very real and present. Any time the frequency is
above DC it is present. In some cases at low frequencies it can be
ignored because it is insignificant but at radio frequencies it does
come into play. And also as I mentioned in power transmission it is a
factor to be considered even though the frequency is only 60 hz. In
home wiring it is not a factor to be concerned with as the conductors
are too small but in large transmission lines it is of concern.

At HF frequencies skin effect is enough that the RF does not penetrate
even the thinnest cable shield of a coax cable. Even typical "hard
line" coax has a thinner shield than typical copper pipe that you are
saying "conducts clear through". Why do you think then that there can
be no RF energy on the outside of a coax cable??

I don't know what you mean "there is no standard depth for any
frequency"? It is well known.

At 60 hz the skin depth is around 1/3 of an inch. Very significant in
a power transmission cable. Or a lightning ground cable..
Look up any large power cable ratings and you will usually find a DC
resistance specified and an AC resistance also specified. The AC
resistance is due to skin effect.

Here are some figures on skin depth for copper: Skin depth (in mils) =
2.602/(sq. root of frequency in Mhz). At 1.8 Mhz it's 1.94 mils or
..00194 inches, just under 2 thousandths. It decreases as the inverse
square root of frequency so at twice the frequency it will be .707
times as deep, and half as deep at 4 times the frequency. At 29.7 Mhz
it's about half a thousandth. At 4 or 5 skin depths any additional
thickness ceases to have additional value.

Now how can you argue with that! :)

Regards
Gary