Topband: Tuning 160M Dipole

[Terry Conboy]

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
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