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