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Hello to all,
With the recent questions & talk of 1/4 wave or half slopers 160 meter
I feel I should add some input. If you check the input resistance of a
1/4 wave sloper with a noise bridge or Autek type analyzer on any
resonable height/yagi antenna combination you will find that the reading
is in the range of 10-15 ohms. This is assuming a basic ground radial
system attached to the tower; anything from 10 to 90 random length
radials. Check the input resistance at the antenna feed point not back
in the shack. With an input resistance this low any method of measuring
SWR in the shack with an inline SWR meter will give totally erroneous
readings. SWR bridges don't work at very low resistanaces. Don't worry
about reactances. When you get the radiation resistances up to normal
the reactance will take care of itself.
The answer I came up with is to install a 4:1 W2FMI UNUN transformer
to raise the input to the 40-50 ohm level. My secret (discovered by
total accident) is to install the UNUN 1/4 wave DOWN the coax feedline.
Using RG-213 my UNUNs are installed at about 65' down from the sloper
feed point in the coax feedline. With the UNUN down in the coax line I
have never had a problem reaching a 50 ohm resonance by trimming the 65'
coax 2 or 3 inches at a time to reach the exact resonant point I am
shooting for. The slopers I am using now are the simple Alpha Delta DX-A
but it worked identically with a 125' wire. I mount the sloper at the
highest point on the tower and set its length to the standard length and
don't worry about the angle to the tower (mine on separate towers are 20
and 45 degrees). then I match it by trimming the coax at the UNUN 1/4
wave down the line. My resonant points have never been higher than 1.2:1
(45 ohms) and the bandwidth ranges from 100-140Khz. The sloper now
operates more like a 1/4 wave vertical than the classic sloper. There
does not appear to be anywhere near the directional characteristics of
the 80 meter slopers I use.
My theory of 1/2 slopers is analogous to a variable capacitor. The main
HF yagi on top and the ground below are 2 stators and the sloper itself
is the capacitor rotor. With towers in the 60-80' range this becomes a
very tough situation to try and find a 50 ohm matching point. The coax
cable seems to add extra capacitance that is missing in the sloper
(rotor) section.
I wrote an article on my 160 meter sloper trials and tribulations for
the March/April 1995 issue of DX magazine if anyone wishes to see the
frustrations I went through trying to get 160 meter slopers to work on
60-80' towers.
Best wishes & good DX, Ralph W1ZK
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<HTML>
Hello to all,
<BR>
<BR> With the recent questions & talk of 1/4 wave or half slopers
160 meter I feel I should add some input. If you check the input
resistance
of a 1/4 wave sloper with a noise bridge or Autek type analyzer on any
resonable height/yagi antenna combination you will find that the reading
is in the range of 10-15 ohms. This is assuming a basic ground radial system
attached to the tower; anything from 10 to 90 random length radials. Check
the input resistance at the antenna feed point not back in the shack. With
an input resistance this low <U>any</U> method of measuring SWR in the
shack with an inline SWR meter will give totally erroneous readings. SWR
bridges don't work at very low resistanaces. Don't worry about reactances.
When you get the radiation resistances up to normal the reactance will
take care of itself.
<BR>
<BR> The answer I came up with is to install a 4:1 W2FMI UNUN transformer
to raise the input to the 40-50 ohm level. My secret (discovered by total
accident) is to install the UNUN 1/4 wave DOWN the coax feedline. Using
RG-213 my UNUNs are installed at about 65' down from the sloper feed point
in the coax feedline. With the UNUN down in the coax line I have never
had a problem reaching a 50 ohm resonance by trimming the 65' coax 2 or
3 inches at a time to reach the exact resonant point I am shooting for.
The slopers I am using now are the simple Alpha Delta DX-A but it worked
identically with a 125' wire. I mount the sloper at the highest point on
the tower and set its length to the standard length and don't worry about
the angle to the tower (mine on separate towers are 20 and 45 degrees).
then I match it by trimming the coax at the UNUN 1/4 wave down the line.
My resonant points have never been higher than 1.2:1 (45 ohms) and the
bandwidth ranges from 100-140Khz. The sloper now operates more like a 1/4
wave vertical than the classic sloper. There does not appear to be anywhere
near the directional characteristics of the 80 meter slopers I use.
<BR>
<BR> My <U>theory</U> of 1/2 slopers is analogous to a variable capacitor.
The main HF yagi on top and the ground below are 2 stators and the sloper
itself is the capacitor rotor. With towers in the 60-80' range this becomes
a very tough situation to try and find a 50 ohm matching point. The coax
cable seems to add extra capacitance that is missing in the sloper (rotor)
section.
<P> I wrote an article on my 160 meter sloper trials and
tribulations
for the March/April 1995 issue of DX magazine if anyone wishes to see the
frustrations I went through trying to get 160 meter slopers to work on
60-80' towers.
<P>Best wishes & good DX, Ralph W1ZK
<BR> </HTML>
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