headers
Steve Flood | 8 Jul 2012 16:54

Topband: Loss comparison between two antennas

I have a 73.5-ft. wire vertical over a decent field of radials (about 60 or
so, 50 to 120 ft. long).

I have configured this vertical two ways over the past several years and am
wondering if they are equal enough in radiated power that I should just use
the simpler of the two.

The configurations differ only in the topload "T", and the feedpoint
matching.

Configuration 1:   70-ft. flat horizontal topload wire with vertical
attached at center.  Feedpoint is 25+j0.  Matching is through a 2:1 UNUN
wound as per Sevick on a FT-150K.

Configuration 2:  186-ft. flat horizontal topload wire folded back on itself
to fit in a 60 x 6-ft space.  (From the center vertical attachment  point,
each topload leg goes out 30 feet, turns 90 degrees for 3 feet, then turns
back on itself another 90 degrees and goes 60 feet.  Looks like a
squared-off "S".)  This resonates the antenna below 1.8Mhz with a feedpoint
Z of 37 +j245.  Matching is through an L-network.  My intention was to
increase radiation resistance and efficiency and move the current maxima
away from the feedpoint and to the center of the vertical for lower takeoff
angle.

My question is how much am I losing in each system.  Is the UNUN
more/less/same loss as the L-network?

I run 1200 watts and the L-net components are sized adequately.   A 1-minute
keydown produces no arcing, and the coil and cap do not get hot.
(Continue reading)

Permalink | Reply |
headers
Jim Brown | 8 Jul 2012 17:06

Re: Topband: Loss comparison between two antennas

On 7/8/2012 7:54 AM, Steve Flood wrote:
> This resonates the antenna below 1.8Mhz with a feedpoint
> Z of 37 +j245.  Matching is through an L-network.  My intention was to
> increase radiation resistance and efficiency and move the current maxima
> away from the feedpoint and to the center of the vertical for lower takeoff
> angle.

About five years ago, I increased the top-loading on my 86 ft Tee 
vertical so that the feedpoint impedance was 50 + J xxx, and tuned it 
with series capacitance. It works "well," but there's no good way to 
compare it to anything, other than, perhaps, a model.  Like you, I have 
a lot of radials on the ground.  A couple of years ago, I added a second 
vertical wire, spaced from the first by 6-8 inches, wired in parallel at 
both ends. The second conductor increases the SWR bandwidth nicely.

73, Jim K9YC
Permalink | Reply |
headers
Guy Olinger K2AV | 8 Jul 2012 17:07

Re: Topband: Loss comparison between two antennas

The losses in a 160 meter vertical antenna system are almost
completely controlled by what you do for the antenna's counterpole
(radials or counterpoise) and the electrical characteristics of the
dirt under the antenna.  So the most important thing, the overwhelming
thing, you have not described at all.  What are you connecting the
coax shield to?  Describe the counterpoles for your two antennas in
detail.

73, Guy.

On Sun, Jul 8, 2012 at 10:54 AM, Steve Flood <kk7uv <at> bresnan.net> wrote:
> I have a 73.5-ft. wire vertical over a decent field of radials (about 60 or
> so, 50 to 120 ft. long).
>
> I have configured this vertical two ways over the past several years and am
> wondering if they are equal enough in radiated power that I should just use
> the simpler of the two.
>
> The configurations differ only in the topload "T", and the feedpoint
> matching.
>
>
>
> Configuration 1:   70-ft. flat horizontal topload wire with vertical
> attached at center.  Feedpoint is 25+j0.  Matching is through a 2:1 UNUN
> wound as per Sevick on a FT-150K.
>
>
>
> Configuration 2:  186-ft. flat horizontal topload wire folded back on itself
(Continue reading)

Permalink | Reply |
headers
Steve Flood | 9 Jul 2012 03:59

Re: Topband: Loss comparison between two antennas

>>>What are you connecting the coax shield to?  Describe the counterpoles
for your two antennas in detail.

Both antennas were built over the same radial system, just at different
times.  (see my original message).

I have 60 + radials of 14awg aluminum high-tensile electric fence wire.
They vary from 50 to 120 feet long and are buried an inch or two in acidic
pine forest mineral soil.  Ground quality/conductivity is probably quite
poor since it is very rocky.

The inner ends of the radials connect to a 10 x 10 inch, 0.1" thick
aluminum plate.  Each radial wire is wrapped twice around a sheet metal
screw and then the screw is tighten into the plate.

The L-network is enclosed in a large plastic box 6 inches above the radial
plate.   The coax connection is a standard panel-mount SO-239 with a wide
copper strap six inches long connecting the SO-239 'shield' to the aluminum
plate with a 1/4" st,.steel bolt and 1 inch fender washers - all dissimilar
metals treated with Penetrox.

Steve, KK7UV
Permalink | Reply |
headers
Guy Olinger K2AV | 10 Jul 2012 18:15

Re: Topband: Loss comparison between two antennas

Hi, Steve.  It has taken some time to get to this and do it carefully.

And sorry, some of the needed info WAS right there in your first post.
 Some days I just read right over tops of things and can't see them to
save my life.  But the specifics you returned in your second post are
significant beyond your first post.  I still would have asked you for
additional information.  You didn't say whether you were supporting
this with trees or a tower, significant because of issues with tower
supported 160 antennas. But I have mostly left that alone.

Short answer: If your 60 some-odd radials were uniform length and
spacing all around, your pushing up the current max to the top of the
vertical probably costs you.  Since your 60 odd radials are
miscellaneous lengths, the ground field cancellation under the radials
is missing and it's a different ball game.  See long answer.

The term counterpole is used as a general term for "radials buried or
elevated, or counterpoise."

Long answer:

The resistance of the ground medium you describe will probably vary a
LOT with moisture content, the decaying material, if wet, will be
pretty conductive, and if dry pretty lossy.  As to the varying lengths
of your radials, the current in the radials is an
apportion-to-the-individual-radial exercise based on the impedance of
each radial one at a time versus the others, a parallel Z calculation
problem.  This means generally long radials with their lower Z get the
lion's share of current and little ones with the higher Z are starved,
especially so in dry weather.  During drier weather, the longer
(Continue reading)

Permalink | Reply |

Gmane