As the T2FD is growing in popularity,
I hope these issues will be of interest
to more than just a few.


SITUATION

I. I am building a T2FD, for receiving and *maximum* 10W RF out.

II. I have a decent RF and DC ground near the antenna site on top of
the building (huge masses of steel), but not in my shack, which is
some 120 ft. below. This is not unlike having the antenna out on the
lot, and the shack somewhere up in the attic.

III. I want to get rid of any static charges and also to reduce the
risk of EMP travelling down the coax. This too must be a concern for
quite a few people...

IV. Also, I'd like to avoid as much as possible any hums that may
derive from poor double grounding. After all, we mostly do HAVE to
ground stuff inside the shack as well, for electrical safety.


PLAN

1) Make the balun with full DC-insulation between high-Z and low-Z
windings on the toroid-core balun. If everything is nice and
symmetric, only transversal-mode RF will flow.

2) Ground the middle of the high-Z winding to the locally available
ground. NOT ground the low-Z winding - I'll just connect it to the
coax. Route internal wires for maximum insulation. Fill the balun
case with either epoxy or urethane foam. Keep the two leads (coax,
ground) well separated.

3) Insert EMP dischargers right into the balun:
- One across the high-Z winding, for transversal mode.
- One between each high-Z lead and ground, for common mode.
- One across the low-Z winding, a'shunt the coax, for
transversal-mode.
And also one across the top resistor, also for transversal.

4) Put one or more large iron-core chokes on the coax, as to dampen
EMP that could possibly travel on the outside of the coax shield.
(This would also ensure that RF power or noise will not travel on the
outside of the shield, of course)



LIFE IS FULL OF QUESTIONS
Here are just some of mine.

a) For HV DC insulation in the balun, I intend to use as large a
PVC-insulated conductor as I can fit, wind the high-Z winding, wrap it
in insulating tape, and wind the low-Z winding on the top of the
insulating layer.
- PVC is said to be lossy. Is it a dramatic loss or not?
- What tape should I use?
- Is the insulated core of a coax a decent alternative to HV wire?
- What is the implication of making a low-Z winding ATOP the high-Z
one & the insulating layer, instead of interleaving them?

b) For receiving, small 90V neon bulbs should already provide some
protection. But 10W RF on 75 ohm already means some 27+ Vrms, wich
would rise to over 50Vrms across a balanced 300 ohm load - if the
mismatch is modest, that is.
- Are low voltage neons already sufficiently horourable dischargers,
or should I consider some other kind of (comparatively more
honourable) dischargers?

c) For the coax shield choke, I am considering 20 tight turns on a
5in. plastic pipe, the steel core being made with pieces of thin,
insulated rebar - enough to fill the pipe. The core should increase
the inductance considerably. Weight not an issue: the choke will be
hung somewhere safe on the roof, probably 40 ft. away from the balun.
- Any reason to vote for either major party candidate in the coming
election?
- Any reason to use FLAT steel instead of cheap, easy to find rebars?

d) Other than the difference in weight, what are the pros and cons of
potting a balun in liquid-poured epoxy resin vs. filling the balun
case with urethane foam injected from a can?


Any contributed wisdom will be appreciated!

N1JPR/I2

On 12 Aug 2004 10:51:52 -0700, (SpamHog) wrote:

As the T2FD is growing in popularity,

Debatable to say the least....

I hope these issues will be of interest
to more than just a few.


SITUATION

I. I am building a T2FD, for receiving and *maximum* 10W RF out.

II. I have a decent RF and DC ground near the antenna site on top of
the building (huge masses of steel), but not in my shack, which is
some 120 ft. below. This is not unlike having the antenna out on the
lot, and the shack somewhere up in the attic.

Yes, after a fashion. What you mean to say is you are still far from
earth ground, but have plenty of metal.

III. I want to get rid of any static charges and also to reduce the
risk of EMP travelling down the coax. This too must be a concern for
quite a few people...

Certainly are for those who aren't signatories of the Nuclear Test Ban
Treaty.

IV. Also, I'd like to avoid as much as possible any hums that may
derive from poor double grounding. After all, we mostly do HAVE to
ground stuff inside the shack as well, for electrical safety.

Anticipating what follows, perhaps you should simply loop couple all
of that steel at the top of the building. No ground issue there. No
direct connections. No HV problems.


PLAN

1) Make the balun with full DC-insulation between high-Z and low-Z
windings on the toroid-core balun. If everything is nice and
symmetric, only transversal-mode RF will flow.

If you do it right - maybe.

2) Ground the middle of the high-Z winding to the locally available
ground. NOT ground the low-Z winding - I'll just connect it to the
coax.

Here is where the part about doing it right raises its ugly head.
Ground is not simply a matter of being bulk metal. At least not RF
ground, and not always safety ground.

Route internal wires for maximum insulation. Fill the balun
case with either epoxy or urethane foam. Keep the two leads (coax,
ground) well separated.

This is more a matter of cosmetics and personal preference than
engineering consideration. Lead dressing is adequate and mounting
does the rest.

3) Insert EMP dischargers right into the balun:
- One across the high-Z winding, for transversal mode.
- One between each high-Z lead and ground, for common mode.
- One across the low-Z winding, a'shunt the coax, for
transversal-mode.
And also one across the top resistor, also for transversal.

The resistor is already going to snub any "EMP" the rest is window
dressing. It takes quite a few KV to jump an inch.

4) Put one or more large iron-core chokes on the coax, as to dampen
EMP that could possibly travel on the outside of the coax shield.
(This would also ensure that RF power or noise will not travel on the
outside of the shield, of course)

8 turns of 6 inch diameter loops will do just as well, as would a
simple 1:1 Current BalUn. You are too much concerned with Nuclear
Blast. If you suffer EMP, you've got far more to worry about.

LIFE IS FULL OF QUESTIONS
Here are just some of mine.

a) For HV DC insulation in the balun, I intend to use as large a
PVC-insulated conductor as I can fit, wind the high-Z winding, wrap it
in insulating tape, and wind the low-Z winding on the top of the
insulating layer.
- PVC is said to be lossy. Is it a dramatic loss or not?
- What tape should I use?
- Is the insulated core of a coax a decent alternative to HV wire?
- What is the implication of making a low-Z winding ATOP the high-Z
one & the insulating layer, instead of interleaving them?

10W is not going to generate any potentials to offer such prospects of
arc or corona - unless you are constructing a very small loop or
dipole. You have already described you are using a T2FD which
obviates that headache.

b) For receiving, small 90V neon bulbs should already provide some
protection.

And maximum noise generation if they should ever fire. How's that
going to happen? Is this massive metal structure at the top of your
building an AM transmitter tower with a 50KW signal?

Believe it or not, some Neon bulbs will not fire if they are in the
dark (you can certainly force the issue - so I guess EMP will satisfy
that in spades).

But 10W RF on 75 ohm already means some 27+ Vrms, wich
would rise to over 50Vrms across a balanced 300 ohm load - if the
mismatch is modest, that is.
- Are low voltage neons already sufficiently horourable dischargers,
or should I consider some other kind of (comparatively more
honourable) dischargers?

Skip them altogether and get on with radio.

c) For the coax shield choke, I am considering 20 tight turns on a
5in. plastic pipe,

Excessive by twice.

the steel core being made with pieces of thin,
insulated rebar - enough to fill the pipe.

Excessive²

The core should increase
the inductance considerably. Weight not an issue: the choke will be
hung somewhere safe on the roof, probably 40 ft. away from the balun.
- Any reason to vote for either major party candidate in the coming
election?

Vote Status Quo and wait for the night of the Long Knives.

- Any reason to use FLAT steel instead of cheap, easy to find rebars?

Lead may help with the issues of thermonuclear events.

d) Other than the difference in weight, what are the pros and cons of
potting a balun in liquid-poured epoxy resin vs. filling the balun
case with urethane foam injected from a can?


Any contributed wisdom will be appreciated!

N1JPR/I2

Hi OM,

I can only guess this is all about lightning, but you have far more
interesting possibilities with the coax drop to your apartment
brightening up your bedroom. In fact, you would probably get just as
good a signal out if you simply ran 20 feet of coax to a 100 foot wire
going up and tying off (or not) to all this metal.

Basically, there is nothing T2FD about your post whatever. Why don't
you start over with what you are worried about instead. For one,
explain just what EMP means to you.

73's
Richard Clark, KB7QHC

Thank you Richard! I'll try to explain.

II. I have a decent RF and DC ground near the antenna site on top of
the building (huge masses of steel), but not in my shack, which is
some 120 ft. below. This is not unlike having the antenna out on the
lot, and the shack somewhere up in the attic.

Yes, after a fashion. What you mean to say is you are still far from
earth ground, but have plenty of metal.

Yes, it sounds weird, because it is. I have to thank another ham,
I2FZX, for the almost ideal situation.

On the TOP of my building there's a 50 ft steel lattice tower
(HF/VHF/UHF beams) and a tornado-proof 20 ft dish (1/2 kW UHF
moonbounce). They are some 80ft apart, both grounded.

Right now, I still have no real grounding in my flat - I just have a
virtual ground connecting all the conductive material in the shack,
which does help reduce noise and hazards. I intend to at least bring
the wiring up to regulation. That won't help with lightning safety,
though.

I intend to string the T2FD between the fixed part of the tower
(~40ft) and the base of the dish. I could use either for grounding,
but I'd opt for grounding at the dish, which is right atop my place.



EMP
Certainly are for those who aren't signatories of the
Nuclear Test Ban Treaty.

This is a bad neighborhood. Nuke blasts are an almost daily
occurence. As they are not meant for testing purposes, I am afraid the
NTB treaty does not apply. Also, there's a known focus of Al Qaeda
activity just a mile away (go to www.tuttocitta.it and plonk in Milano
/ Viale Jenner 50) but it can't be closed because it would hurt
certain people's sensitivities. I wear lead pants all the time.



IV. Also, I'd like to avoid as much as possible any hums that may
derive from poor double grounding. After all, we mostly do HAVE to
ground stuff inside the shack as well, for electrical safety.

Anticipating what follows, perhaps you should simply loop couple all
of that steel at the top of the building. No ground issue there. No
direct connections. No HV problems.

Please elaborate. Do you mean turning the warship atop my bldg into a
ground reference loop?



2) Ground the middle of the high-Z winding to the locally available
ground. NOT ground the low-Z winding - I'll just connect it to the
coax.

Here is where the part about doing it right raises its ugly head.
Ground is not simply a matter of being bulk metal. At least not RF
ground, and not always safety ground.

Well, not exactly.

A large bulk of metal is usually a pretty good capacitive sink at HF.
In fact, even an insulated structure works decently as a RF ground. I
even built crystal radios in which very few square inches of sheet
metal were enough to provide a cap sink that had a large effect when
worked against a wire antenna.

As for safety ground, NEVER "assume" that a bulk of metal is grounded.
If you did, you could easily be found guilty of manslaughter in court.
In some jurisdictions you have to get an an electrician to sign you
off that, among other things, a piece of eqpt is properly grounded.

In professional radio installations, one sees a "ground reference"
system which is indeed DC-grounded, but for RF purposes, most of the
sink effect comes from strapping together everything in sight,
including cabinets, metal furniture, building structures, drains,
floor support etc etc.



Route internal wires for maximum insulation. Fill the balun
case with either epoxy or urethane foam. Keep the two leads (coax,
ground) well separated.

This is more a matter of cosmetics and personal preference than
engineering consideration.

I would like to maximize the chance that any burst of energy will take
the ground path rather than the coax path. Why should putting, say,
3kV insulation between the antenna and the coax not matter?
Incidentally, you do find kV-range insulation in commercial products
as well.



Lead dressing is adequate and mounting
does the rest.

What do you mean?



3) Insert EMP dischargers right into the balun:
- One across the high-Z winding, for transversal mode.
- One between each high-Z lead and ground, for common mode.
- One across the low-Z winding, a'shunt the coax, for
transversal-mode.
And also one across the top resistor, also for transversal.

The resistor is already going to snub any "EMP" the rest is window
dressing. It takes quite a few KV to jump an inch.

So it snubs any EMP but not an RF signal. What is the spectrum of an
EMP? Is it more like DC or more like all across the RF spectrum?



4) Put one or more large iron-core chokes on the coax, as to dampen
EMP that could possibly travel on the outside of the coax shield.
(This would also ensure that RF power or noise will not travel on the
outside of the shield, of course)

8 turns of 6 inch diameter loops will do just as well, as would a
simple 1:1 Current BalUn. You are too much concerned with Nuclear
Blast. If you suffer EMP, you've got far more to worry about.

Now I understand. FYI, what gets you in a lightning near-hit _is_ an
EMP.

I once had an audible arc in an antenna due to a 1-lightning
thunderstorm. That single hit happened in the midst of a city, and
took out several power stations and countless other equipment. My
antenna was 8 miles away.



a) For HV DC insulation in the balun, I intend to use as large a
PVC-insulated conductor as I can fit, wind the high-Z winding, wrap it
in insulating tape, and wind the low-Z winding on the top of the
insulating layer.
- PVC is said to be lossy. Is it a dramatic loss or not?
- What tape should I use?
- Is the insulated core of a coax a decent alternative to HV wire?
- What is the implication of making a low-Z winding ATOP the high-Z
one & the insulating layer, instead of interleaving them?

10W is not going to generate any potentials to offer such prospects of
arc or corona - unless you are constructing a very small loop or
dipole. You have already described you are using a T2FD which
obviates that headache.

Great! But what about losses?

I NEVER saw baluns with windings atop each other
- they were always directly wound on the core.
And I don't know why.


b) For receiving, small 90V neon bulbs should already provide some
protection.

And maximum noise generation if they should ever fire. How's that
going to happen? Is this massive metal structure at the top of your
building an AM transmitter tower with a 50KW signal?

Nothing but EMP. Lightning arrestors, if not MOV, are mostly gas
discharge devices. All of yesteryear's professional toob radios and
many consumer ones included a tiny neon lamp in their front end.


c) For the coax shield choke, I am considering 20 tight turns on a
5in. plastic pipe,
the steel core being made with pieces of thin,
insulated rebar - enough to fill the pipe.

Excessive²

OK!


- Any reason to vote for either major party candidate in the coming
election?

Vote Status Quo and wait for the night of the Long Knives.

OK! Will do.



- Any reason to use FLAT steel instead of cheap, easy to find rebars?
Lead may help with the issues of thermonuclear events.

Great! But lead will lead to lead poisoning litigation too.

I don't want to litigate a lead-poisoned nuclear-armed jihadi.

Lead poisoning is also rumored to lead people to insanity
- see Goya (the painter, not the beans).

So, I don't want to litigate a lead-poisoned nuclear-armed
totally-out-of-his-mind jihadi either.



In fact, you would probably get just as
good a signal out if you simply ran 20 feet of coax to a 100 foot wire
going up and tying off (or not) to all this metal.

I had thought about that too, esp. for LF! Problem is, between moi
and the warship up on top lie crouching and waiting a few tens of TV
sets and just as many PCs. I already have a wire going up to the 5th
floor (out of 9). Works great, but only after 1 am, and only because
energy-conscious Italians always switch off their gear at night.

73

N1JPR/I2

On 13 Aug 2004 02:16:01 -0700, (SpamHog) wrote:
On the TOP of my building there's a 50 ft steel lattice tower
(HF/VHF/UHF beams) and a tornado-proof 20 ft dish (1/2 kW UHF
moonbounce). They are some 80ft apart, both grounded.

Now how did I suspect that?

Right now, I still have no real grounding in my flat - I just have a
virtual ground connecting all the conductive material in the shack,
which does help reduce noise and hazards. I intend to at least bring
the wiring up to regulation. That won't help with lightning safety,
though.

Well, let's put some closure to this thing called ground. For much of
discussion, ground is actually a large pool of excess charge that is
highly mobile. That is, you can draw on its charge without
significantly polarizing it. When you can't, you get electrical
discharges (ground under a threatening sky becomes polarized and
lightning occurs). In some sense, the bulk of this metal satifies
this criteria, in other senses it does not.

Insofar as RF ground, anything, even earth, which is a signficant
distance (in terms of wavelength) away presents a situation in which
it is isolated and a problematic reference. In this situation, you
can conspire through tuning mechanisms to optimize that path to be a
good conductor (series resonate the lead going there); this works for
RF but for lightning that is another matter (and at different
frequencies simultaneously).

I intend to string the T2FD between the fixed part of the tower
(~40ft) and the base of the dish. I could use either for grounding,
but I'd opt for grounding at the dish, which is right atop my place.

Your need for ground when you are using a dipole is unwaranted in
terms of RF. Where would you connect it anyway? A dipole, by
definition is self complimentary. However, the surrounding metal is
certainly a disturbance to the self. Make sure the ends stand off
from the supports by a good amount - I can only guess, and that would
be 10 feet.



This is a bad neighborhood. Nuke blasts are an almost daily
occurence. As they are not meant for testing purposes, I am afraid the
NTB treaty does not apply. Also, there's a known focus of Al Qaeda
activity just a mile away (go to www.tuttocitta.it and plonk in Milano
/ Viale Jenner 50) but it can't be closed because it would hurt
certain people's sensitivities. I wear lead pants all the time.



IV. Also, I'd like to avoid as much as possible any hums that may
derive from poor double grounding. After all, we mostly do HAVE to
ground stuff inside the shack as well, for electrical safety.

Anticipating what follows, perhaps you should simply loop couple all
of that steel at the top of the building. No ground issue there. No
direct connections. No HV problems.

Please elaborate. Do you mean turning the warship atop my bldg into a
ground reference loop?



2) Ground the middle of the high-Z winding to the locally available
ground. NOT ground the low-Z winding - I'll just connect it to the
coax.

Here is where the part about doing it right raises its ugly head.
Ground is not simply a matter of being bulk metal. At least not RF
ground, and not always safety ground.

Well, not exactly.

A large bulk of metal is usually a pretty good capacitive sink at HF.
In fact, even an insulated structure works decently as a RF ground. I
even built crystal radios in which very few square inches of sheet
metal were enough to provide a cap sink that had a large effect when
worked against a wire antenna.

As for safety ground, NEVER "assume" that a bulk of metal is grounded.
If you did, you could easily be found guilty of manslaughter in court.
In some jurisdictions you have to get an an electrician to sign you
off that, among other things, a piece of eqpt is properly grounded.

In professional radio installations, one sees a "ground reference"
system which is indeed DC-grounded, but for RF purposes, most of the
sink effect comes from strapping together everything in sight,
including cabinets, metal furniture, building structures, drains,
floor support etc etc.



Route internal wires for maximum insulation. Fill the balun
case with either epoxy or urethane foam. Keep the two leads (coax,
ground) well separated.

This is more a matter of cosmetics and personal preference than
engineering consideration.

I would like to maximize the chance that any burst of energy will take
the ground path rather than the coax path. Why should putting, say,
3kV insulation between the antenna and the coax not matter?
Incidentally, you do find kV-range insulation in commercial products
as well.



Lead dressing is adequate and mounting
does the rest.

What do you mean?



3) Insert EMP dischargers right into the balun:
- One across the high-Z winding, for transversal mode.
- One between each high-Z lead and ground, for common mode.
- One across the low-Z winding, a'shunt the coax, for
transversal-mode.
And also one across the top resistor, also for transversal.

The resistor is already going to snub any "EMP" the rest is window
dressing. It takes quite a few KV to jump an inch.

So it snubs any EMP but not an RF signal. What is the spectrum of an
EMP? Is it more like DC or more like all across the RF spectrum?



4) Put one or more large iron-core chokes on the coax, as to dampen
EMP that could possibly travel on the outside of the coax shield.
(This would also ensure that RF power or noise will not travel on the
outside of the shield, of course)

8 turns of 6 inch diameter loops will do just as well, as would a
simple 1:1 Current BalUn. You are too much concerned with Nuclear
Blast. If you suffer EMP, you've got far more to worry about.

Now I understand. FYI, what gets you in a lightning near-hit _is_ an
EMP.

I once had an audible arc in an antenna due to a 1-lightning
thunderstorm. That single hit happened in the midst of a city, and
took out several power stations and countless other equipment. My
antenna was 8 miles away.



a) For HV DC insulation in the balun, I intend to use as large a
PVC-insulated conductor as I can fit, wind the high-Z winding, wrap it
in insulating tape, and wind the low-Z winding on the top of the
insulating layer.
- PVC is said to be lossy. Is it a dramatic loss or not?
- What tape should I use?
- Is the insulated core of a coax a decent alternative to HV wire?
- What is the implication of making a low-Z winding ATOP the high-Z
one & the insulating layer, instead of interleaving them?

10W is not going to generate any potentials to offer such prospects of
arc or corona - unless you are constructing a very small loop or
dipole. You have already described you are using a T2FD which
obviates that headache.

Great! But what about losses?

I NEVER saw baluns with windings atop each other
- they were always directly wound on the core.
And I don't know why.


b) For receiving, small 90V neon bulbs should already provide some
protection.

And maximum noise generation if they should ever fire. How's that
going to happen? Is this massive metal structure at the top of your
building an AM transmitter tower with a 50KW signal?

Nothing but EMP. Lightning arrestors, if not MOV, are mostly gas
discharge devices. All of yesteryear's professional toob radios and
many consumer ones included a tiny neon lamp in their front end.


c) For the coax shield choke, I am considering 20 tight turns on a
5in. plastic pipe,
the steel core being made with pieces of thin,
insulated rebar - enough to fill the pipe.

Excessive²

OK!


- Any reason to vote for either major party candidate in the coming
election?

Vote Status Quo and wait for the night of the Long Knives.

OK! Will do.



- Any reason to use FLAT steel instead of cheap, easy to find rebars?
Lead may help with the issues of thermonuclear events.

Great! But lead will lead to lead poisoning litigation too.

I don't want to litigate a lead-poisoned nuclear-armed jihadi.

Lead poisoning is also rumored to lead people to insanity
- see Goya (the painter, not the beans).

So, I don't want to litigate a lead-poisoned nuclear-armed
totally-out-of-his-mind jihadi either.



In fact, you would probably get just as
good a signal out if you simply ran 20 feet of coax to a 100 foot wire
going up and tying off (or not) to all this metal.

I had thought about that too, esp. for LF! Problem is, between moi
and the warship up on top lie crouching and waiting a few tens of TV
sets and just as many PCs. I already have a wire going up to the 5th
floor (out of 9). Works great, but only after 1 am, and only because
energy-conscious Italians always switch off their gear at night.

73

N1JPR/I2

A rather more complete treatment than the last sent:

On 13 Aug 2004 02:16:01 -0700, (SpamHog) wrote:
On the TOP of my building there's a 50 ft steel lattice tower
(HF/VHF/UHF beams) and a tornado-proof 20 ft dish (1/2 kW UHF
moonbounce). They are some 80ft apart, both grounded.

Now how did I suspect that?

Right now, I still have no real grounding in my flat - I just have a
virtual ground connecting all the conductive material in the shack,
which does help reduce noise and hazards. I intend to at least bring
the wiring up to regulation. That won't help with lightning safety,
though.

Well, let's put some closure to this thing called ground. For much of
discussion, ground is actually a large pool of excess charge that is
highly mobile. That is, you can draw on its charge without
significantly polarizing it. When you can't, you get electrical
discharges (ground under a threatening sky becomes polarized and
lightning occurs). In some sense, the bulk of this metal satisfies
this criteria, in other senses it does not.

Insofar as RF ground, anything, even earth, which is a significant
distance (in terms of wavelength) away presents a situation in which
it is isolated and a problematic reference. In this situation, you
can conspire through tuning mechanisms to optimize that path to be a
good conductor (series resonate the lead going there); this works for
RF but for lightning that is another matter (and at different
frequencies simultaneously).

I intend to string the T2FD between the fixed part of the tower
(~40ft) and the base of the dish. I could use either for grounding,
but I'd opt for grounding at the dish, which is right atop my place.

Your need for ground when you are using a dipole is unwarranted in
terms of RF. Where would you connect it anyway? A dipole, by
definition is self complimentary. However, the surrounding metal is
certainly a disturbance to the self. Make sure the ends stand off
from the supports by a good amount - I can only guess, and that would
be 10 feet.

Now, down in the shack you still have the same problem of ground being
far away in terms of wavelength for RF. However, you are still using
a dipole and the dipole is going to force its own relationships with
the nearby structures. Presumably the T2FD will smooth out those
wrinkles. Insofar as safety ground goes, it is hard to believe that
is not supplied in your apartment - somewhere. Further, with regard
to lightning, you are probably already living inside a faraday shield
(the building's steel, skeletal structure), but make sure you ground
the coax coming into the apartment at the window or where it
penetrates a wall. To answer my earlier question, connect the coax
shield to the building frame before it begins its descent to your
apartment.

Anticipating what follows, perhaps you should simply loop couple all
of that steel at the top of the building. No ground issue there. No
direct connections. No HV problems.

Please elaborate. Do you mean turning the warship atop my bldg into a
ground reference loop?

Think of a Gamma or Delta match. Although these connect to the metal
of the structure they drive, they are inductive loops. You could as
easily build the loop and place it in close proximity to the structure
to accomplish the same thing. This gives you isolation except for the
frequency the loop is tuned to (and would limit you to a narrow band
of operation unless you have some means to remotely tune it).

... for RF purposes, most of the
sink effect comes from strapping together everything in sight,
including cabinets, metal furniture, building structures, drains,
floor support etc etc.

Suffice it to say it is adequate.

I would like to maximize the chance that any burst of energy will take
the ground path rather than the coax path. Why should putting, say,
3kV insulation between the antenna and the coax not matter?

It is based on hope.

Incidentally, you do find kV-range insulation in commercial products
as well.

Much of that is simply opportunistic and chance given the insulation
is suited to other purposes, like holding things together as a
framework made out of plastic.

Lead dressing is adequate and mounting
does the rest.

What do you mean?

Keeping leads apart and away from conductive structures replaces all
the exotic insulation you could imagine. Besides, if we are talking
about your T2FD and coax, I cannot imagine this is a problem in the
first place. You are going to want to drive the coax shield to ground
as soon as possible, not isolate it with insulation. You are going to
want to isolate the T2FD ends, but any induced potentials will have to
reckon with the built in resistor snubbing them. Such potential does
not merit heroic insulation efforts.

So it snubs any EMP but not an RF signal. What is the spectrum of an
EMP? Is it more like DC or more like all across the RF spectrum?

It is mostly Pulsed DC with very little RF over 1MHz. The pulse shape
is characteristically described as several µS rise and about 20 µS
fall. If you consider the strike contains 100,000A and integrate this
pulse over one second, then the duty cycle reduces this to 10 - 100 A.

This is not trivial, but neither is it catastrophic UNLESS that same
current is trying to push its way through Z or R! Remember,
lightning's source is an infinite constant current generator capable
of supporting any potential necessary to maintain that strike level.

Consider this: You have a huge resistor of 1000 Ohms. You are within
reach of one of its leads tied to ground. It is supporting 100,000A
through it - hence there is 100 Million volts across it. You reach
out and grab that lead going to ground. Are you going to be hit with
that same gazillion volts? The ground lead presents all of 0.001 Ohm
and with that same current, for 10's of µS you feel the tingle of 100V
- Maybe. Your body resistance is easily 1-10 KOhms and the current of
that 100V is enough to kill you if sustained (10's of µS hardly
qualifies) but probably a burning pin-prick sensation. If we convert
this burn into power, it renders you sinking (or simmering) 100
mW-seconds. In equivalent heat experience, this is like grabbing a
Christmas tree bulb (the big ones) for a hundredth of a second.

Now I understand. FYI, what gets you in a lightning near-hit _is_ an
EMP.

I once had an audible arc in an antenna due to a 1-lightning
thunderstorm. That single hit happened in the midst of a city, and
took out several power stations and countless other equipment. My
antenna was 8 miles away.

You heard the arc. That is an expression of power. This power
conversion is easily measured in the microwatts. It may sound loud,
but hearing is deceptive because it is subjective.

Still and all, microwatts for the front end of a receiver can be
overpowering. You can rest assured that this power was generated due
to the current encountering either Z or R such that it was absorbed by
the heat conversion.

Great! But what about losses?

I NEVER saw baluns with windings atop each other
- they were always directly wound on the core.
And I don't know why.

To maintain a wide bandwidth and to preserve balance.

All of yesteryear's professional toob radios and
many consumer ones included a tiny neon lamp in their front end.

I have repaired many such sets, professional and consumer alike. Yes,
some included the NE-2. That practice was discarded just as many
years ago too. There are probably a billion TVs out there, and not
one of them with this kind of protection.

Returning to the issue of lightning, I would suggest you mine the
archives of rec.radio.amateur.antenna for Richard Harrison, KB5WZI
for thousands of commercial and amateur tower installations over a
career spanning 50 years. He also discusses feedlines and lightning.

73's
Richard Clark, KB7QHC

What part of "electromagnetic pulse" refers to nuclear blast?

"Richard Clark" wrote in message
...
.. You are too much concerned with Nuclear
Blast. If you suffer EMP, you've got far more to worry about.

"CW" no adddress@spam free.com wrote in message
...
What part of "electromagnetic pulse" refers to nuclear blast?


For many that is what is thought of. Just as when someone mentions dropping
THE bomb. It is thought of as nuclear.
An air burst is suspose to cause the EMP that will fry most electronics.

On Sat, 14 Aug 2004 18:29:56 GMT, "Ralph Mowery"
wrote:


"CW" no adddress@spam free.com wrote in message
...
What part of "electromagnetic pulse" refers to nuclear blast?


For many that is what is thought of. Just as when someone mentions dropping
THE bomb. It is thought of as nuclear.
An air burst is suspose to cause the EMP that will fry most electronics.


For All,

reference "The Electromagnetic Bomb (E-Bomb)":
http://www.airpower.maxwell.af.mil/a...pp/apjemp.html

"The ElectroMagnetic Pulse (EMP) effect [1] was first observed
during the early testing of high altitude airburst nuclear weapons
[GLASSTONE64]. The effect is characterised by the production of a
very short (hundreds of nanoseconds) but intense electromagnetic
pulse, which propagates away from its source with ever diminishing
intensity, governed by the theory of electromagnetism.
...
"Even if the pulse is not powerful enough to produce thermal
damage, the power supply in the equipment will readily supply
enough energy to complete the destructive process. Wounded devices
may still function, but their reliability will be seriously
impaired. Shielding electronics by equipment chassis provides only
limited protection, as any cables running in and out of the
equipment will behave very much like antennae, in effect guiding
the high voltage transients into the equipment."

Note, the EMP of the Flux Compression Generator, an example of a
tactical E-Bomb, is VASTLY richer in current unlike a lightning
strike:
"The FCG is a device capable of producing electrical energies of
tens of MegaJoules in tens to hundreds of microseconds of time, in
a relatively compact package. With peak power levels of the order
of TeraWatts to tens of TeraWatts, FCGs may be used directly, or
as one shot pulse power supplies for microwave tubes. To place
this in perspective, the current produced by a large FCG is
between ten to a thousand times greater than that produced by a
typical lightning stroke [WHITE78]."

However, these intense currents are not widely suitable for mission
purposes for the same reasons of the Lightning's current pulse:
"Whilst FCGs are potent technology base for the generation of
large electrical power pulses, the output of the FCG is by its
basic physics constrained to the frequency band below 1 MHz. Many
target sets will be difficult to attack even with very high power
levels at such frequencies..."

Details abound in this paper, which I will leave to the student to
absorb. Of particular note are the statements about coupling
efficiency (unpredictable) and range (hundreds of meters).

73's
Richard Clark, KB7QHC

Richard,

Thank you for the exhaustive reply!

[...] this works for RF but for lightning that
is another matter (and at different frequencies simultaneously).

LOL!

Your need for ground when you are using a dipole is unwarranted in
terms of RF.

Yes and no. A "stand alone" antenna such as a dipole is not worked
against a RF ground. (BTW, I remember seeing Hertz's original dipole
in a museum in Wuerzburg... & I swear it wasn't grounded). But a
ground is a good pulse sink if it is very permeable across the
spectrum. In that sense, a pulse sink should be a decent RF ground
too.

Make sure the ends stand off
from the supports by a good amount - I can only guess, and that would
be 10 feet.

Absolutely!


Insofar as safety ground goes, it is hard to believe that
is not supplied in your apartment - somewhere. Further, with regard
to lightning, you are probably already living inside a faraday shield
(the building's steel, skeletal structure), but make sure you ground
the coax coming into the apartment at the window or where it
penetrates a wall. To answer my earlier question, connect the coax
shield to the building frame before it begins its descent to your
apartment.

I wish...

In this part of the world steel structures are unheard of 'cept in
industrial buildings. The tallest building in the country is only 30
floors, and its structure is reinforced concrete. Also, walls are
always some form of brick & mortar ot at least gypsum. This explain
why fire safety & property insurance are almost an afterthought here.
We do have disasters, but very few in post 1930's high rises - mostly
it's cheap built 19th century buildings collapsing after a gas
explosion. Otherwise, one flat may burn out but others won't be
damaged.

Also, a power grounding is very effective for low frequency events. If
you get zapped by a pulse, all bets are off if your ground is high
impedance - and an end-fed thin ground wire is. In my case, the
shortest route to the building ground connection on my floor is almost
100 ft with lots of zigzagging.

Think of a Gamma or Delta match. Although these connect to the metal
of the structure they drive, they are inductive loops. You could as
easily build the loop and place it in close proximity to the structure
to accomplish the same thing. This gives you isolation except for the
frequency the loop is tuned to (and would limit you to a narrow band
of operation unless you have some means to remotely tune it).

I had never thought of that. So a broadband / omni antenna puts a bit
more punch into the coax than a narrowband / mispointed beam. But I'd
still rather put up a simple, forgiving, uncritical broadband, even if
known inferior.


Keeping leads apart and away from conductive structures replaces all
the exotic insulation you could imagine. Besides, if we are talking
about your T2FD and coax, I cannot imagine this is a problem in the
first place. You are going to want to drive the coax shield to ground
as soon as possible, not isolate it with insulation. You are going to
want to isolate the T2FD ends, but any induced potentials will have to
reckon with the built in resistor snubbing them. Such potential does
not merit heroic insulation efforts.

OK, I'm abandoning the heroic insulation efforts. I'll ground the
coax up on top, and introduce an insulating transformer to decouple
grounds only if double grounding introduces noise - that can be done
later at the shack end of the coax.


What is the spectrum of an EMP?
It is mostly Pulsed DC with very little RF over 1MHz. The pulse shape
is characteristically described as several µS rise and about 20 µS
fall. If you consider the strike contains 100,000A and integrate this
pulse over one second, then the duty cycle reduces this to 10 - 100 A.

Now I know what to expect when I wile away a thunderstorm sucking an
antenna plug.



I NEVER saw baluns with windings atop each other
To maintain a wide bandwidth and to preserve balance.

Got it!


I have repaired many such sets, professional and consumer alike. Yes,
some included the NE-2. That practice was discarded just as many
years ago too. There are probably a billion TVs out there, and not
one of them with this kind of protection.

Things change. Just as many more people now enjoy the other side, most
TVs get their storm thrills through the power socket nowadays -
especially if served by cable :0. Open a PSU, and you'll likely find
MOV's. Not good for RF work because they tend to have too much
capacitance....

And TV antennas tent to be 1) tiny 2) narrow beam, 3) tuned to UHF. No
protection againt a direct hit, but non that sensitive to near hits.

The market for gas discharge devices for many kinds of RF applications
is still quite lively though, and it's not for protection against
nearby transmitters.


mine the archives of rec.radio.amateur.antenna
for Richard Harrison, KB5WZI

I'm grabbing my steel hat as I type!

73, thanks again.

Filippo N1JPR/I2

On 14 Aug 2004 15:18:31 -0700, (SpamHog) wrote:

TVs get their storm thrills through the power socket nowadays -
especially if served by cable :0

I once fixed a TV that suffered a direct hit. The tuner in the corner
of the cabinet was enveloped in a charred, molten mass of plastic but
still worked fine.

This was back in the tube days however, and they were far more robust
against EMP.

73's
Richard Clark, KB7QHC