Jack Twilley wrote in message ...
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"N2EY" == N2EY writes:

N2EY For all that trouble, it seems to me that a better choice (if
N2EY you want direct coax feed on the non-WARC HF bands) is the
N2EY classic W3DZZ trap dipole. With only two traps and mechanically
N2EY robust construction it is possible to achieve direct coax feed
N2EY and low SWR on 80/40/20/15/10, and the cut-and-try is much
N2EY easier.

Wow. This might work in my limited space, providing that it deals
well with the short height above ground.

How short?

There's nothing magic about a trap dipole of any flavor, be it W3DZZ
or W9INN or whatever. At their very best they're *almost as good* as a
plain vanilla half-wave coax-fed dipole. Their main advantages a

- they can be made mechanically rugged

- they can be directly fed with coax and yield reasonable SWR

- they can be built and adjusted with relatively simple tools and
materials

Now I just have to learn how
to make traps. Any recommended resources for this sort of thing?

W4RNL's site, of course. Some time back I did posts describing
coaxial-cable traps which you can google up.

The main trouble with trap construction is the materials. Ideally we
would use fixed vacuum capacitors and big Miniductor-like coils of
heavy wire or tubing. In practice we often settle for something
less...

1) Conventional LC traps - These are usually made from transmitting
capacitors like the 850 series and Miniductor coils. An alternative to
the Miniductor is to wind heavy wire on a piece of PVC pipe, which can
also serve as an insulator. Main problems with this construction is
getting theparts and weatherproofing.

2) Coaxial-cable traps - These are made from a piece of coax (RG-58 is
probably best) wound on a piece of PVC. Articles by N4UU (QST December
1984, IIRC) and W8NX describe various flavors of coaxial-cable traps,
as do the posts I made here some years back. The main problem with
this method is that the results are highly variable, depending on the
coax used. Depending on the loss of the outer coax jacket, one builder
may have much more loss in a trap than another. Adjusting these traps
isn't easy. Their main advantages are ruggedness and
low cost.

3) Bifilar or "twintraps" - These are similar to coax-cable traps, but
are made by bifilar-winding a pair of wires and connecting the ends
series-aiding. The internal distributed capacitance makes the C and
gthe coil is the L. If wire with low loss insulation is used, these
should be better than coax-cable traps.

Note that not all trap dipoles are "W3DZZ" trap dipoles. That
designation belongs to a specific design, which works as follows:

Each dipole half consists of an outer section (about 22 feet), a
parallel LC trap resonant on 40 meters, and an inner section (about 33
feet) which is also resonant on 40 meters. Direct coax feed or 1:1
balun.

The "trick" is in the LC ratio of the traps.

On 80 meters, the traps are below resonance, and act as loading coils
so that the ~110 foot long antenna is resonant on 80

On 40 meters, the traps resonate and are essentially open circuits.
Thus the middle 66 feet operates as a 40 meter dipole.

On 20 meters, the traps are above resonance and act as capacitors so
that the entire antenna is 3/2 wave resonant.

On 15 meters, the traps are above resonance and act as capacitors so
that the entire antenna is 5/2 wave resonant.

On 10 meters, the traps are above resonance and act as capacitors so
that the entire antenna is 7/2 wave resonant.

The problem is that there are 5 resonance points to hit but only 3
variables to play with (inner section length, outer section length, LC
ratio of trap). So in real life the antenna can only be truly
optimized on 3 bands unless you are very very lucky, or use methods
such as extra "dangle wires" to establish resonance on certain bands,
as is sometimes recommended for getting a 40 meter dipole to work on
15.

If I had my druthers, I'd just use a plain dipole fed with ladder line
(the real stuff, not "Twin Lead with holes" and a true balanced tuner.
But mechanical considerations at my present QTH make coax feed a must,
so I make do with a homebrew trapper.


73 de Jim, N2EY