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