[Elecraft] OT: Vertical doublets (was: Home made Sigma-GT5 & KRC2 or SGC?)

[Guy Olinger, K2AV]

There are two issues here. One is using a vertical doublet instead of
horizontal. The other is tuner losses.

A vertical doublet will instantly drop the sum of your far-field power
by one half or 3 db UNLESS you are over saltwater or a copper plated
meadow.

If the radiation is vertically polarized, for all practical purposes
all radiation below the horizon (e.g., has to bounce) is absorbed by
ordinary ground. This is not true for horizontally polarized
radiation.

Salt water at the "bounce point", or sufficiently dense radials/ground
screen at bounce point overcomes the absorption AT THAT SPOT.

If you can support a forty meter 1/2 wave doublet vertically, the top
is at 65-70 feet. If you hang the doublet horizontally at the same
height, the doublet will be an excellent performer, and will not lose
energy "in the bounce".


Considering tuners, the enemy of tuners is circulating current in the
tuner's components, primarily the inductor(s). What is infrequently
expressed, is that from one angle the best tuners for QRP are the 1.5
kW models. As the matching problems become more unreasonable, the
circulating currents in the inductors go up.

Each configuration of tuner has its Achilles' heel, the matching
situation that causes it to be inefficient, that is have strong enough
circulating currents that the inductor (or other conductor) ohmic
losses become significant.

These losses are in db and equally affect QRP signals as QRO. There
are documented instances of small tuners suffering 3 dB internal
losses and more.  3 db lost out of QRP is 2.5 watts, and out of QRO is
750 watts. The first is barely a night light, the second is a heat gun
on high in an enclosed space. Oddly enough, the answer for either can
be a QRO tuner, which makes it at QRO by using MUCH larger conductors,
reducing ohmic losses to a few tenths of a dB.

The effect on far end for losing 3 dB is a lot worse for the QRP
signal which usually has little margin for readability at the far end.
We just don't hear about the 3 dB losses in a QRP tuner because it
doesn't set the tuner on fire at QRP like it would if you were trying
to run 1500 watts through it.

As to avoiding a tuner's Achilles heel, to avoid the circulating
losses, a knowledgeable ham can always design antenna and feedline to
be a good fit with a given tuner. HOWEVER, using a simple doublet and
balanced feed for multiple bands presents a RANGE of situations, at
least one of which will involve high circulating currents on just
about any tuner.

My 1.5 kW ATR-30 will handle it fine because the inductor is a
monster, and it has very low resistance. Does not ever get even barely
warm at 1.5 kW, even in contest operating. Therefore it will be very
efficient for my K2 QRP operation, where last smidgen of a watt
counts.

73, Guy

----- Original Message ----- 
From: "JAKidz" <jakidz@pullman.com>

>
> W4RNL suggests a 44 foot horizontal doublet fed by ladderline is
> an optimal doublet length for 10-40m. It's tempting to turn this
doublet
> vertical and shorten it with bar hats like the GT-5 if it is
efficient.
> But at least one antenna guru (whose call I can't find at the
moment)
> has stated repeatedly that W4RNL's doublet design is very
inefficient
> because of tuner losses. Who is right? What are the tuner efficiency
> tradeoffs of having the structure resonance low, high or
mid-frequency?
>
> 73,
> John, K7JG