Topband: Still in search of resonance

[Joe Subich, W4TV]

 > 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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