Topband: Effect of current max not at base of vertical.

[Guy Olinger K2AV]

Using an accumulation of remote beacon network measurements is probably the
most reliable, and the only sky-wave measurement available to us for real,
unbiased measurements for ham testing of this sort.

The thimble analysis of George's setup, is that the missing half of the
radial field will cause double the current on the west side and obviously no
current on the east.  The double current on the west will not cancel the
ground induction field from the vertical resonator which has it's field
uniformly distributed around the compass.  So induced current-in-the-dirt
losses in immediate vicinity below the radiators will be significantly
without radial field cancellation benefit.  The 40 radials will most likely
be a satisfactory low R current sink, without significant effective series
resistance losses, for the feed coax shield.

The greatest potential induction loss is always in the immediate vicinity of
the vertical radiator, due to the inverse cube aspect of magnetic field
strength vs. distance.

It's a guess on my part, but from Google maps satellite view, since the
seaward edge of the property (nice digs!) appears to be a deck and not
antenna supporting palms, this puts the antennas' feed point back onto the
property a bit, implying loss-able material in the 20-30 feet of potential
maximum losses around the base before seaward below-the-horizon radiation
encounters friendly sea water. So the usual ground induction loss potential
is in play, depending on where the verticals actually are on the property,
possibly nearly as much as if the sea were not there.  And the "soil"
characteristics won't be midwest rich pastoral soil for the field, but
buildings, concrete, etc.

This sets up the site as a ground induction test bed.  No series R
counterpoise issues, but has exposure to ground induction losses from the
vertical radiator due to non-uniform distribution of counterpoise current
around the compass.

The longer vertical, with it's current center probably up at the coil, will
have a lot less ground induction than the coil-less wire, which will have
its max at the ground. We have George's results, which follow on results of
better than expected results with 80 meter end-fed halfwave "L"s with their
current max up at the bend.

73, Guy.

On Thu, Sep 22, 2011 at 9:08 PM, GeorgeWallner <aa7jv at atlanticbb.net> wrote:

> On Tue, 20 Sep 2011 17:17:58 -0400
>   Guy Olinger K2AV <olinger at bellsouth.net> wrote:
> > I share the frustration over the very minimal amount of
> >data out there.
> >
> > However...
> > Erection of a 260 foot vertical in a testing
> > environment...
>
> G'Day Topbanders,
>
> I am not sure how general a conclusions could be drawn
> from my experience, but I have a set up that is somewhat
> relevant to this thread, and have done some on the air
> testing with it.
>
> I have two verticals, about 2 meters apart. One is 21
> meters tall and the other one is 28 meters tall with a
> high Q center loading inductor to make it resonate at 1900
> kHz (this is my 160 m antenna). This antenna is fed via a
> low loss antenna coupler. The two antennas share a common
> ground system, which is salt-water to the east and a
> buried field of 40 radials of varying length between 30
> and 120 feet long to the west. On 80 meters the shorter
> antenna is a 1/4 wave vertical, while the longer one could
> be considered to be a half-wave vertical.
>
> I have done extensive tests on 80 meters, comparing the
> two antennas towards the east. I have used the reverse
> beacon network, and a couple of friends' SDR-s in Europe
> for these comparisons.  In tests from my Florida QTH,
> towards the east (towards Europe) and the side where the
> salt water is, the taller antenna has almost always been
> better by 2 to 3 dB. Towards the west (and the land side)
> I have not done enough testing to draw conclusive results,
> but I feel that the 1/2 wave vertical is better in that
> direction too.
>
> I understand the 80 meters is not 160 meters, but...
>
> I would be happy to set up a test sched with anyone to my
> west or north-west, who is interested in carrying these
> studies further.
>
> 73,
>
> George, AA7JV
>
> PS: BTW, I almost always use the 1/4 vertical on 80
> meters, even towards the east, as going through the
> coupler is a PITA (as Guy has pointed out).
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