Start by disconnecting the feedline. Connect a jumper to the junction
of the vertical wire and the coil. Now use a grid dip meter and tap
down the coil until the antenna is resonant (dip occurs) at the desired
frequency. Now connect the feed line (shield to ground) and tap up the
coil until you find the lowest possible SWR.
If you are not satisfied with the SWR, move the top jumper 1/2 turn one
way or the other and then move the feedline tap for the lowest possible
SWR. If the lowest SWR is higher than it was before moving the jumper,
move the jumper 1 turn the other way and again adjust the feed line tap
point. Keep moving the "tuning" jumper 1/2 turn at a time and adjusting
the feed line tap until you are happy.
Remember, the feedline tap does not 'Tune' the antenna - that is done by
adjusting the total amount of the 'tuning' coil. The feedline tap only
adjusts the impedance (R) seen by the feedline.
The single coil can be viewed as a two coil "L" network. The coil from
the tap point to the vertical wire tunes the antenna to resonance (X=0)
whilst the coil from the tap point to ground (across the feed line) is
responsible for adjusting the impedance (matching to the feed line).
73,
... Joe, W4TV
On 11/1/2017 12:54 PM, MR TREVOR DUNNE wrote:
Hi All
I finally got the vertical up and running, problem I have now is I can't find a
low SWR on the coil, the best I can get is about 2.8:1 no matter where I tap
the coil that's the best SWR, I can move that point up and down the band by
moving the tap but the SWR stays the same,
My current set up is about 55ft vertical I have 2 loading wires in a T shape,
wires are 50ft either side of the vertical and the are pretty level and in line,
I have 38 radials averaging about 45-60ft long, I will add more when time
allows but don't the make the SWR worse ??
With no coil I get resonance at 2.12mhz with R=16 X=2 on my MFJ269,
The coil is 25ft of 5mm copper brake pipe in a 4" diameter, the coil is
attached to base of the vertical and the other end is grounded,
Any pointers on what I can do to get a better match,
Thanks
Trevor
EI2GLB
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