Paul,
I use an Excel spreadsheet with calculations for all the HF bands and the
most common antennas. You will find a copy of my spreadsheet here:
http://www.k8ut.com/tiki-download_file.php?fileId=41
When I am tuning an antenna, I use an MFJ analyzer to determine the best
SWR, and then plug that frequency into the calculation. The spreadsheet then
reveals the difference between the length of my ideal frequency and the
measured frequency. I adjust the antenna by that difference and it usually
gets me real close.
For example: Let's plan to build a 160 meter dipole that is resonant (lowest
SWR) at 1.85 MHz.
Open the spreadsheet and you will see three formulas for 160 meters in rows
37, 38, and 39. The calculated lengths for dipoles on 1.800, 1.850, and
1.900 MHz are in columns O (full length) and P (length per side). Imagine
that you measure the lowest SWR of your new antenna on your MFJ259 analyzer
and find it resonant at 1.765 MHz. Replace the reference 1.900 frequency
entry at cell B39 with your measured resonant frequency of 1.765. Notice
that the calculated length at 1.765 (in cell P39) is 132.58 feet per side.
Since you want your new antenna to be resonant at 1.850 MHz, you can
subtract the calculated length for 1.850 MHz (cell P39 - 126.49 feet) from
your new antenna measured frequency (cell P39 - 132.58 feet). In this case,
you know that your new antenna is 6.09 feet too long, per side. Shorten your
antenna by 12 feet, and measure again.
I hope my explanation and example make sense. 73!
-larry (K8UT)
-----Original Message-----
From: paul ecker
Sent: Saturday, November 03, 2012 10:01 PM
To: topband@contesting.com
Subject: Topband: Tuning 160M Dipole
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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