There seems to be some confusion, Carl!
First of all, since the gamma match (regardless of the diameter of the gamma
"rod") is a shorted transmission line, less than 1/4 wavelength it WILL have
series inductive reactance that you will need to tune out with a series
variable capacitance.
Second, I don't understand the "R" readings that you are reporting at
various tap points with the MFJ, that come down when you apply the series C.
That suggests to me that what you are reporting as "R" is R+jX or [R+jX],
and it's coming down as you tune out the series reactance jX. If you find a
tap point that results in 50 ohms real when you use the series-C to make
X=0, that IS the 50 ohm tap point that you are looking for. You will NOT
find a point that gives you 50 ohms real without the series C to tune out
the inductive reactance,
Other opinions notwithstanding, you CAN do the gamma match with 14 ga. wire,
The only effect of using a thin gamma "rod" or gamma "wire" is to increase
the losses a bit in the gamma, and to increase the inductance per unit
length of the gamma.
Finally, take care. That you don't have enough broadcast signal on the Sky
needle to screw up the MFJ readings.
ON additional note: In some cases guys use a shorter gamma that resultsn in
a resistive real part LESS than 50 ohms. In this case only a portion of the
series inductance is cancelled with the series C and theremainin inductive
reactance is used in conjunction with a shunt variable C to form an
L-network to match the real part UP to 50 ohms!
Sounds like you are hitting all around it, Carl. Just remember that what you
are searching for is R=50, and X=0, or R+jX = 50 + j0.
GL!
73,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Carl
Braun
Sent: Friday, February 14, 2014 6:15 PM
To: '160'
Subject: Topband: Still in search of resonance
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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