On 21 feb, 12:54, wrote:
I have been toying with the idea of putting up a 40M vertical antenna.
I have an inverted vee up at 40 feet high but it doesn't seem much
good for hearing European stations. I put up a temporary 40m vertical
today and heard alot more stations than on my inverted vee. I have a
steel shed with a zincalum roof with an area of about 320 square feet.
If I was to mount a 40m quarter wave vertical on that roof area would
I have an adequate ground plane? Or, am I better off ground mounting
it and running out radials? Or for something completely different, how
would I go if I erected an 80m loop at about 30 feet high? I have
tried to reorientate the inverted vee to better capture European
stations but it doesn't seem to help, hence I am looking at other
ideas. Any information gratefully accepted.
Cheers
Max

Hello Max.

You proved yourself that the concept is better then the current
solution. Vertical polarization gives lower elevation angle of the
main lobe. From an antenna standpoint I would place the antenna on
the shed. It raises the feed point, giving lower elevation angle of
maximum radiation.

I suppose the walls are metal also and are electrically connected to
the roof, so you don't get HF inside the shed. The large metal surface
(roof and walls) that serves as counterpoise may be sufficient.
Depending on foundation of the shed and type of soil, some radials may
help you. You can try it yourself. Look to the VSWR or impedance of
the quarter wave as it is now. Add 2 floating radials from opposite
edge of the roof. Vary the length and see what happens with the VSWR
or impedance. When it doesn’t change much, radials will not give a
large improvement.

When the walls of the shed are non-metal, you will create an RF field
between the roof and the soil under it (this will also lead to a
common mode current on the coaxial feed line. In my opinion this is
undesirable (both RF and safety). In that case I would ground the roof
at several places. More thin copper grounding points are better then
some thick grounding points.

Safety is different. When the roof isn't connected to the walls at
sufficient places, I direct hit can be fatal. Though you are pretty
safe in a full metal shed when hit by lighting, when safety grounding
is not OK, you might have a problem in the electrical system. When
your shed lacks its own safety grounding, you will get excessive
lightning current in the mains cable that runs from your home to the
shed.

About radials, buried, laying on the soil or elevated.
Adding radials may reduce ground losses in the vicinity of the
antenna, but will not change elevation of main lobe as this is mostly
determined by soil properties up to many wavelengths from the antenna
itself.

When you make the vertical radiator longer then a quarter wave, it
raises the input impedance and lowers the elevation angle of maximum
radiation (you will like this). You only need a capacitor to tune out
the inductive component. I use this concept for the 40m antenna for
JOTA (16m vertical). The increased input impedance, reduces the
current that must be drawn from the ground system (in your case the
roof with or without radials). You may increase the electrical length
by adding a capacative head.

Some design info from the JOTA logfile. 13m radiator with 8 elevated
quarter wave radials (2m above ground), gives about 60 Ohms in series
with about 200 Ohms inductive part. All over pastoral ground. Last
year we extended the radiator to about 16 m and used a 50 ohms to 200
Ohms transformer and just 3 floating radials. The reason for the many
"abouts" is because of measurements are done with primitive antenna
materials at the grass production ground adjacent to the scouting
camp.

Best regards and good DX.

Wim
PA3DJS
www.tetech.nl
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