> Is the single 14AWG too thin causing goofy readings?
Absolutely! Treating the Sky Needle and the #12 wire as a folded
dipole and using 8" for the diameter of the tower, you have a 16:1
step-up with the single #14 wire.
If you made a cage of 4 or 6 wires with a 6 to 8" diameter, back of
the envelope calculations say the impedance with the 67' tap point
would be much closer to 50 Ohms *and* the bandwidth would be much
wider. Even three parallel wires spaced 12" or so would have a much
greater effective diameter and would reduce the step-up you see by
a great deal.
Of course, if all you can do is a single #14 wire, use an Omega
match or an L-network to bring the impedance down to 50 Ohms.
73,
... Joe, W4TV
On 2/14/2014 6:14 PM, Carl Braun wrote:
List
Some of you may have followed my efforts in trying to shunt feed my
90' Tri-Ex Skyneedle with 20 meter yagi at 93'. I'm still unable to
find any sort of resonance point on the tower. To refresh everyone's
memory here are the specifics:
90' Skyneedle that is 12" round at the base and 4" round at the top
13' of mast out the top
5 element Telrex 20M monobander mounted at the 93' level. No other
antennas on the tower
1 ½" copper pipe as a radial ring that surrounds the concrete base
that measures 4' x 8' rectangle. Three 8' ground rods are connected
to the radial ring via 1" copper strap that is .125 thick.
Currently I have 27 14AWG insulated wire radials. Most of the
radials are 20' to 50' long with three at 90 to 120' long and four of
them connected to my 40M vertical array which have 100 count radials
50' to 100' each.
The tower is grounded to each ground rod via 1" copper strap .125"
thick and, as mentioned above, the ground rods are connected to the
radial ring with the same strap with copper clad stainless screws.
When I bolted the gamma arm to the tower at the 90' height I dropped
a single 14AWG wire to the ground where my FLUKE meter read ZERO ohms
between the radial ring and the end of the gamma wire with no
fluctuations so I'm confident that I have good continuity throughout
the tower.
Here are the readings that I saw on the MFJ analyzer with the gamma
arm mounted at the (4) points on the tower that are available...
With the gamma arm mounted at 90' and 36" spacing I saw 425 ohms at
the end of the drop wire on the MFJ
With the gamma arm mounted at 67' and 36" spacing I saw 380 ohms at
the end of the drop wire on the MFJ
With the gamma arm mounted at 46' and 36" spacing I saw 240 ohms at
the end of the drop wire on the MFJ
With the gamma arm mounted at 28' and 36" spacing I saw 120 ohms at
the end of the drop wire on the MFJ
At all of these points I was able to knock down the R with my honkin'
1050pf cap to some resonance sort of resonance at 1.825 MHz but, as
most everyone has indicated, I should be able to find a 50 ohm tap
somewhere on the tower. I can't find it.
When I had the gamma arm mounted at the 90' level. I was able to put
my baby variable 160pf inline to bring the 425 ohm impedance down to
about 60 ohms and the antenna heard very well; especially on the 1700
KHz broadcast band, with a 2.4:1 Vswr. Similar results could be seen
at the other levels too as long as I brought the R down with a
variable cap. Yesterday, with the gamma arm at the 46' level (and
240 ohms on the MFJ) I was able to put the big variable inline to
bring the reading to 24 ohms with a TRUE X=0. With a 22 ohm to 50
ohm UNUN, I saw 1.3:1 Vswr on the output of the UNUN. I worked a W2
in NJ and a W4 in Florida with just the 1000D. BUT...again...I'm
bringing the R down with the capacitor...not finding 50 ohms anywhere
on the tower.
Is my radial field so poor that I'm seeing these goofy readings?
Is the single 14AWG too thin causing goofy readings?
I'm back to scratching my head.
Comments from the list?
Carl AG6X
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