At 8:34 AM -0800 11/15/02, Jim Apple WB1DOG wrote:
>I'm trying to build a 40/80/160 vertical dipole. Here is the basic idea:
>
>----[80 trap]---[40 trap]--o--[40 trap]---[80 trap]---
> |
> |
> | 35' vertical section
> |
> |
> === 50 ohm coax feed
> |
> |
> |
> |
> |
>----[80 trap]---[40 trap]--o--[40 trap]---[80trap]----
>
> 6' off ground
>
> ---------------- ground ---------------
>
>...is this crazy?
Not at all.
You need a balun at the feedpoint; you should route the coax
horizontally away from the antenna as far as you can; and even with
these two measures you should put a couple of good ferrite
common-mode chokes in the coax at intervals of about 10 meters.
>[How] to estimate the length of the T sections?
Use an antenna modeling program such as EZNEC, not only to determine
the length of the T sections, but also to determine the placements
and the L/C ratios of the traps. All of these parameters interact,
so "cut and try" without the aid of a modeling program could be
difficult and frustrating.
The program will also estimate the feedpoint impedance, which will be
low on 80 m and _very_ low on 160 m. With an estimate of the
feedpoint impedance, you will be able to choose the turns ratio of
the transformer(s) that you'll need at the feedpoint to feed this
puppy efficiently. This transformer may or may not also serve as a
balun. Making a transformer and/or a balun is not difficult or
expensive. Refer to Jerry Sevick's (name?; callsign W2FMI) book
_Transmission_Line_Transformers_, which IIRC is available from the
ARRL.
EZNEC 4 would be best because ordinary EZNEC and all other programs
based on NEC-2 (or on MININEC) are inaccurate when an antenna is so
close to the ground. EZNEC 4, and NEC-4 upon which it is based, are
much better.
73 de Chuck, W1HIS
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