Topband: Mixed RF grounds

[Guy Olinger K2AV]

Hi Mike,

For elevated 1/4 wave radials to work well, they need to be carrying the
same RF current away from the common center point. This is to produce near
net zero RF fields at the ground. 1/8 wave elevated radials with an
isolation transformer per k2av.com will do better than the same number and
orientation of elevated 1/4 wave radials. All that the 1/4 wave radials
provide is resonance, but at a cost of increased ground loss vs. 1/8 wave.

The way to model these kinds of things in EZNEC is to do a near field table
100 meters by 100 meters square centered below the vertical wire. Set the
interval to 1 meter for 10,000 readings. Save the table to file and import
it to Excel. In Excel, square the field readings and add them up. Watt loss
in the ground will be proportional to the sum of the field value squares.
On k2av.com click the index button "The FCP Story". This is a PDF of the
original NCJ article on the FCP. At the top of page 22, there is a graphic
showing this arithmetic already done for the sum of field strength squared.
Study that before reading on here.

The additional loss for the 1/4 wave radials is simply the length, **in
feet or meters, not wavelength**. If you had to make your radials out of
nichrome wire, that extra loss would proportional to the length of the
radials in feet/meters, not wavelength. Because of the great length of a
1/4 wavelength on 160, top band is the most sensitive to these issues.

The break even point FCP vs. 1/4 length radials is *four* straight at 90
degree angles without any close local materials to make current unequal in
the radials. **IF** the radials' orientation, shape, length, etc are
"miscellaneous", the FCP will beat it. The literal difference in watts will
depend on the TX power, and particularly on the type and quality of ground
underneath. You can do ridiculously poor top band engineering over certain
highly conductive Midwest USA soils, but here in poor soil NC, or in
urban/suburban extremely poor "soils", that poor engineering will cost you
dearly in ground losses.

To compare these solutions, know what stations on the air in the same metro
area have what for antennas and run what power. In a contest, listen to
them yourself or put them up on an RBN spot analysis to various parts of
the country.

73 and hope to see all of you in the Pre-Stew tomorrow night.
Guy K2AV

On Fri, Oct 19, 2018 at 6:46 PM Mike Waters <mikewate at gmail.com> wrote:

> Iain,
>
> The FCP is a *great* design by Guy, K2AV. It's for Topbanders on a small
> lot, but it sounds like yours is not as small as some. From what I've read
> here, whether an FCP will improve your signal over two λ/4 elevated radials
> at least 10' high is kind of doubtful.
>
> One of my 10' high λ/4 elevated radials was almost straight N, and the
> other one was bent at crazy angles to the S because the neighbor's fence
> was too close.
>
> 73, Mike
> www.w0btu.com
>
> On Fri, Oct 19, 2018 at 5:01 PM <g4sgx at justfans.co.uk> wrote:
>
> > Hi Mike,
> >
> > Yes that seems to agree with most writings.
> >
> > I hope soon to swap the two radials for an FCP to see what all the fuss
> is
> > about. See if its so much better as been told.
> >
> > I fell in love with TB after a recent trip to V31 when I worked some very
> > marginal QSO’s with JA, VK and Russia. Quite a sense of achievement as I
> > watched the greyline sweep across Europe.
> >
> > CW only I’m afraid.
> >
> > I did have an inverted-L over seawater so my TX was good, my Rx antenna
> > wasn’t usable. (Local aircon/floodlights)
> >
> > 73 Iain
> >
> >
> >
> >
> >
> >
> >
> > *From:* Mike Waters <mikewate at gmail.com>
> > *Sent:* 19 October 2018 22:21
> > *To:* g4sgx at justfans.co.uk
> > *Cc:* topband <topband at contesting.com>
> > *Subject:* Re: Topband: Mixed RF grounds
> >
> >
> >
> > Hello Iain,
> >
> >
> >
> > You heard correctly! My inverted-L with 2 elevated radial had no ground
> > rods except on one terminal of a spark-gap lightning arrester.
> Description
> > and photos at
> >
> > www.w0btu.com/160_meters.html (scroll down). You *must* also have an
> > effective common-mode feedline choke near the feedpoint.
> >
> >
> >
> > 73, Mike
> >
> > www.w0btu.com
> >
> >
> >
> > On Fri, Oct 19, 2018, 4:02 PM <g4sgx at justfans.co.uk> wrote:
> >
> > Hi,
> >
> > In my limited garden I can get away with an 17 high inverted-L with 2
> very
> > bent elevated radials and very odd angles. One of them in the farmers
> > field.
> >
> > It has certainly got me out but I have been told if I sick some copper
> rods
> > down at the base and connect to the radials (coax shield and radials)
> this
> > is not going to be beneficial.
> >
> > This seems counter-intuitive.
> >
> > Before I get digging (cos I got to try it anyway and take some analyser
> > readings) can anyone comment on this?
> >
> > Iain G4SGX
> >
> >
> _________________
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