Paul,
At least in the copy of your message I received, it was a bit hard to
determine where the frequency ended and the SWR began. As I understand,
the SWR was
1.77 MHz 6.21
1.80 MHz 4.31
1.845 MHz 3.37
1.86 MHz 3.77
1.89 MHz 4.88
1.91 MHz 6.08
1.94 MHz 8.19
Since your antenna is tuned for minimum SWR around 1.845 MHz, if you
wanted the lowest SWR at 1.83 (for example), you'd need to increase the
length by the ratio 1.845/1.83 = 1.0082 which is +0.82% or +1.03 feet
for each leg of your 2 x 126 foot dipole.
However, the real issue is why your SWR is so high at resonance. I
modeled a 252 dipole at 30 feet over "Sandy, Dry" soil in EZNEC. You
didn't specify what wire you used for your dipole, so I assumed #14 wire
with 0.05" thick PVC insulation (Er=3.5) and it shows resonance at 1.818
MHz, which is just a little lower than yours. However, the impedance
from my model is 49.5 ohms at resonance, so the match is darn good with
SWR =1.01. It rises to 1.46 at 1.8 MHz and 6.82 at 1.94 MHz, which is
to be expected.
So it's hard to understand why your minimum SWR is so high. You
mentioned a 1:1 balun --- was it mounted directly at the center
feedpoint of the dipole? Is the balun specified for use on 160m? Some
general purpose baluns are only good down to 80m (if that) and may not
function too well on 160m. Is it a "current" a.k.a. "choke" type
balun? So-called "voltage" baluns may not work well if the antenna
itself isn't well balanced (due to some slope of the wire, proximity to
something conductive at one end, etc.). In this case, oddly enough, a
"voltage" balun may be allowing (or forcing) current to flow on the coax
shield, although it would be somewhat unusual for this to cause the
degree of mismatch you measure. Since Murphy never sleeps, are you
absolutely, positively sure it isn't a 4:1 balun? You can check it with
your MFJ-259 with a 50 ohm resistor across the balun terminals in place
of the antenna.
And of course, as you may have read in other Topband threads, MFJ-259's
(and other self contained analyzers) are exquisitely sensitive to RF
pickup, typically from AM broadcast transmitters. They give odd readings
that can vary from day to night, as stations change power and patterns.
I'd suggest using a conventional SWR meter at your transceiver with it
running at least 5-10 watts to rule out interference problems with the
MFJ-259.
Let us know what you find.
73, Terry N6RY
On 2012-11-03 7:01 PM, paul ecker wrote:
Last week I posted:
I put up a 160m dipole but now need to tune it. SWR at low end of band is abt
3 and at top end abt 5. From what I have read this means I need to shorten it.
My question is how much should I take off? A foot off each end?? Any good links
to sites that may explain this in some detail so I can learn?
So in response to a number of questions I got, I remeasured the SWR to find the
minimum SWR
- SWR checked at base of vertical coax run to 1:1 Balun, using a MFJ 259
analyzer:
- Freq- mhzSWRImpedance (X)
- 1.776.2130
- 1.84.3106
- 1.8453.374
- 1.863.773
- 1.894.877
- 1.916.083
- 1.94 8.189
So minimum SWR at
- 1.845 3.374
The dipole antenna length - each leg was cut to 126' and it is 30 ft above
sandy ground
As I may have said previously, I am weak on antenna theory but anxious to
learn. Any advice or direction welcome.
73 Paul kc2nyu
_______________________________________________
Topband reflector - topband@contesting.com
_______________________________________________
Topband reflector - topband@contesting.com
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