Re: Wire Antennas

[Rick Hiller]

Dave (and Don),

Not a silly question, Dave, what so ever.

My suggestion was to open it up 90 degrees from the present feed-point, 1/4
way around...this being a voltage maximum (loop) and current minimum (node).


This enables the loop to still act like a loop on 80, if you desire to
maintain it's present performance.  And then on 160 it acts like, as Don
states, "a folded back 1/2 wave dipole on 160 meters" but in this case it
will be off center fed.   Auto 80/160 band switching, once you get the
lengths correct for the two bands, if that is possible in this case, as I do
not know what modes you operate.

If you open it at, as Don suggests, 180 degrees or half way round, the 80
meter loop turns into a bent 80 meter "2 Half-waves in Phase" with a very
high Z feed, if you maintain the feed point as is.  This also shifts the
standing wave current distribution by 90 degrees, which might cause you
problems if your original antenna was set up to favor a particular area of
the world.  Not convinced that is the case though, as it does sound like you
have up 270 feet of wire in a triangular loop, "cause that's where the trees
be".  Wish I had those trees -- hi.

60 feet above ground is low for a horizontal 160 meter sky hook, but you do
what'cha gotta do.

Sounds like a fun modeling and empirical experiment to say the least.

Regards -- Rick -- W5RH

-------------------------------------------------

Dave,

Open it at the point 180 degrees (midway around) from the feedpoint.  If
your existing loop is a full wavelength on 80 meters, opening it will turn
the loop into a folded back 1/2 wave dipole on 160 meters - not as good as
straight out, but it should work since you are feeding the line with a tuner
anyway.
In fact, you may be able to make it automatic by using a trap tuned to 160
meters - that will electrically open the loop on 160 and only add some
inductnce on the other bands.

73,
Don W3FPR

> -----Original Message-----

>
> Not sure where you are suggesting I open it.  I was considering
> disconnecting the feedline on one side where it connected to the balun.
>   Is this what you mean.  My apologies if it is a silly question.
>
> David Wilburn
> [hidden email]
>
>
>
> Rick Hiller wrote:
> > Dave,
> >
> > Another possibility for 160 -- Don't know to what extent/effort
> you want to
> > go to get on 160, but here is an easy way, although, a bit of a
> compromise.
> >
> > Simply open the loop at one of the 80 meter voltage loops (also
> a current
> > node).  The 80 meter standing wave current distribution
> /performance will
> > not change, but it will force the wire to be an off center fed 1/2
> > wavelength for 160.  Although not quite resonant within the 160
> meter band,
> > if you feed it with open wire and run it thru the appropriate matching
> > network, you should be right.

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[Don Wilhelm-3]

Rick,

The only problem with that is the high voltage point on a fullwave loop is
electriclly opposite the feedpoint, not 1/4 wave away from the feedpoint.

It is true that opening it at the 80 meter 1/4 wave point would create an
off-center fed 160 meter dipole that has been bent back on itself, but it
will change the behavior on all other bands too.

I still think the 160 meter trap at the 180 degree point is the best
solution - or use the loop as-is and tie both sides of the feedline together
and operate against ground for 160 meters.  Somewhere on his website, L B
Cebik simulated a 160 meter inverted L antenna with only 4 20 foot radials
and it did not fare too badly - he also simulated with one 20 foot radial
and that did not do badly either, so there is hope even if an extensive
radial system for 160 meters is out of the question in your case.

73,
Don W3FPR

> -----Original Message-----
>
>
> Dave (and Don),
>
> Not a silly question, Dave, what so ever.
>
> My suggestion was to open it up 90 degrees from the present
> feed-point, 1/4
> way around...this being a voltage maximum (loop) and current
> minimum (node).
>
>
> This enables the loop to still act like a loop on 80, if you desire to
> maintain it's present performance.  And then on 160 it acts like, as Don
> states, "a folded back 1/2 wave dipole on 160 meters" but in this case it
> will be off center fed.   Auto 80/160 band switching, once you get the
> lengths correct for the two bands, if that is possible in this
> case, as I do
> not know what modes you operate.
>
> If you open it at, as Don suggests, 180 degrees or half way round, the 80
> meter loop turns into a bent 80 meter "2 Half-waves in Phase" with a very
> high Z feed, if you maintain the feed point as is.  This also shifts the
> standing wave current distribution by 90 degrees, which might cause you
> problems if your original antenna was set up to favor a particular area of
> the world.  Not convinced that is the case though, as it does
> sound like you
> have up 270 feet of wire in a triangular loop, "cause that's
> where the trees
> be".  Wish I had those trees -- hi.
>
> 60 feet above ground is low for a horizontal 160 meter sky hook,
> but you do
> what'cha gotta do.
>
> Sounds like a fun modeling and empirical experiment to say the least.
>
> Regards -- Rick -- W5RH
>
> -------------------------------------------------
>
> Dave,
>
> Open it at the point 180 degrees (midway around) from the feedpoint.  If
> your existing loop is a full wavelength on 80 meters, opening it will turn
> the loop into a folded back 1/2 wave dipole on 160 meters - not as good as
> straight out, but it should work since you are feeding the line
> with a tuner
> anyway.
> In fact, you may be able to make it automatic by using a trap tuned to 160
> meters - that will electrically open the loop on 160 and only add some
> inductnce on the other bands.
>
> 73,
> Don W3FPR
>
> > -----Original Message-----
>
> >
> > Not sure where you are suggesting I open it.  I was considering
> > disconnecting the feedline on one side where it connected to the balun.
> >   Is this what you mean.  My apologies if it is a silly question.
> >
> > David Wilburn
> > [hidden email]
> >
> >
> >
> > Rick Hiller wrote:
> > > Dave,
> > >
> > > Another possibility for 160 -- Don't know to what extent/effort
> > you want to
> > > go to get on 160, but here is an easy way, although, a bit of a
> > compromise.
> > >
> > > Simply open the loop at one of the 80 meter voltage loops (also
> > a current
> > > node).  The 80 meter standing wave current distribution
> > /performance will
> > > not change, but it will force the wire to be an off center fed 1/2
> > > wavelength for 160.  Although not quite resonant within the 160
> > meter band,
> > > if you feed it with open wire and run it thru the appropriate matching
> > > network, you should be right.
>

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[Rick Hiller]

Hello, Don, Dave, et al,

Don said "The only problem with that is the high voltage point on a fullwave
loop is electrically opposite the feedpoint, not 1/4 wave away from the
feedpoint."

Not so. Here's why...

Define a quad, full wave loop, as a square -- geometrically equal lengths on
all sides.  Feed point (zero degree point) is placed on the center of one
side.

On a "closed" full wave loop (360 degrees long) there are always 2 current
maximum points (loops) and 2 current minimum points (nodes).  The same for
the Voltage standing wave -- there are always 2 voltage maximum points
(loops) and 2 voltage minimum points (nodes).  Current and voltage maximas,
on a standing wave antenna, are 90 degrees apart.

On the defined "closed" full wave square loop, the current maxima (loops)
are at the feed point (zero degree point) and 180 degrees around the loop or
half way round the loop on the opposite side.

Then, the voltage maximas are 90 degrees away from the feed point (or 90
degrees back toward the feedpoint from the 180 degree point) -- 1/2 way
between the feed point and the 180 degree point, physically centered on the
adjacent sides to the feedpoint side.

Summary and benefit of this characteristic for a multi-band antenna --
Opening a full wave loop at a voltage maximum (current minimum) does not
change the closed loop standing wave pattern for the fundamental frequency,
but opening it forces a dipole type sinusoid current distribution on the 1/2
sub-harmonic.  Works great for when you are trying to control the current
distribution on both bands for a specific pattern generation.  Hence, my
original suggestion.

TNX and Regards..Rick -- W5RH



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