Topband: relay-switched inverted L for 40/80/160

[Guy Olinger K2AV]

A 160 meter inverted L can be used quite successfully on 80 and 40.  The
reason for its non-popularity on the higher bands is that there is no
handy-dandy premade MFJ box to tune it, and you can't feed it directly with
coax.

On 80 meters it is an end-fed **half-wave** L with the current max up at the
bend and a high voltage feed at the end. It has an omnidirectional
hemispheric pattern good for both local and DX. On 40 meters the antenna
tends to behave like a dipole supported as the horizontal section, with a
very non-intuitive cancellation of the radiation from the vertical section.
For those who have had them all up for comparison, it overall beats inverted
V's, G5RVs, Carolina Windoms, single loops and plain verticals, mostly
because it has no nulls, and apparently because the current max is way up in
the air.

The kicker is that on 160 it is a low Z feed, and on 80 and 40 is a very
high-Z feed which means completely switching matching circuits, and the
matcher must be at the base of the antenna, usually remote. High power
requires high-voltage insulation, vacuum variables, etc.

The performance of a quarter-wave inv L on 160 has been beat to death here.


On 80 a half-wave L is fairly rare because noone wants to do the work for
the hi-Z end match.  It's advantage is that the 160 ground, even a pathetic
one, works very well on the higher bands because even 100 ohms (terrible on
160) ground series R is nothing versus a 2000 ohm feed Z on 80. Ditto on
40.  Using the antenna this way, the length of the wire is deliberately set
to pick the high Z for the ground efficiency on 80 and 40.

The switching relays involved must be able to handle the very high voltage
on 80 and 40, and also the considerable current on 160 for QRO. Do-able, but
there aren't any premade boxes to do it, understandable given the cost of
brand-new vacuum relays.

For the determined ham willing to dig up the components and fabricate his
own device from scratch, the antenna is a killer for a tight situation, that
will do very well 160-30 meters, and with a minor amount of trickery will
tune the entire 3.5-4.0 band. Note that at 1.5 kW levels the voltage level
on 80/40 can easily jump from the center conductor to case in an SO239, so
quite a few mechanical designs suitable for 160 break down under the QRO hi
voltage.

I have had several of these up, and it is by far the best single wire 80
meter antenna I have ever had.  I am dropping my 80m inv L this fall and
replacing it with a new end-fed half-wave L. Going back to the tried and
true.

73, Guy.

On Mon, Sep 28, 2009 at 3:22 PM, Jim Meehan <jmeehan at vpizza.org> wrote:

> I've mulling over a tree-supported wire inverted L for 40/80/160 meters.
> The vertical section would be supported by a high branch on a ~55' Monterey
> pine not too far from my house, and the horizontal section from there to
> another tree.  Lots of radials of various lengths should be no problem.
>
> Originally I had thought to use traps to achieve multi-band operation, but
> then I started wondering about using relays to simply switch out the
> various
> segments.  If I used two-conductor speaker wire or something similar as the
> radiator, I think the same wire could also carry DC to operate the relays.
> I'd use SPDT normally-closed relays.  For operation on 160, no DC is
> applied
> -- both relays are closed, and the entire length of the inverted L is
> active.  For operation on 80, +12V is applied, opening the "far" relay,
> cutting off both conductors of the 160 segment.  For operation on 40, -12V
> is a applied, opening the "near" relay.  This would be for 100 watts only,
> so I don't think I'd need anything too exotic for components.
>
> My hope is that it would have wider bandwidth than traps would allow, and
> be
> easier to construct and tune.
>
> Am I missing anything obvious?  I did a little Googling and didn't find any
> designs like this.
>
> Also wondering about the bandwidth/hassle/cost tradeoff of using copper
> pipe
> or aluminum tubing for the vertical section.  In this case, I'd run a wire
> up the center of the pipe to use as the 2nd conductor for the relay
> switching.  I would put the pipe next to the tree, supporting it with some
> kind of standoffs attached to the trunk of the Monterey pine.  Can I get
> the
> pipe far enough from the tree trunk to minimize interaction, but still
> close
> enough for standoffs to be practical?  Is the bandwidth gain of pipe over
> wire enough to make this worthwhile?
>
> Thanks for your consideration,
>
> Jim Meehan, W6XE
> Oakland, CA
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