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Author Topic: Calculating length of ladder line from analyser readings  (Read 2658 times)
G7MRV
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« on: October 11, 2011, 08:50:43 AM »

ok, so the title probably doesnt explain much!

I have a 102ft doublet at average 25ft agl, this is fed mostly by 450 ohm ladder line, which drops vertically for 15ft, then runs horizontally at right angles to the antenna, to a ferrite choke balun, and then the rest of the way to the shack via RG-213. For various reasons, i cannot run the ladder line further, and so am left with rather a long run of coax. The length of ladder line was selected as a compromise where i had the lowest impedence readings on my MFJ-259, across most bands.

I have the opertunity to selectively add short loops of ladder line at the point where it connects to the balun, on the proviso that these are not untidy and dont get in the way (im sure you can guess now who put the restrictions in place Wink )

What i cannot work out is this - how do i calculate the length of a section of 450 ohm ladder line, that will allow me to transform one impedence value to another? For example, say the antenna at that point reads Z=800ohm at 18.135MHz, how do i calculate a length that i can insert to bring that closer to 50 ohm?

The idea is to have a selection of line sections for the various bands that i can insert as required to either provide a good match without the tuner, or to allow at least my MFJ-948 tuner to have an easier time of it! I know i can experiment, checking each section with the analyser in-situ, but im already awash with offcuts of line and dont want to create too many more!


Martin G7MRV
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K1ZJH
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« Reply #1 on: October 11, 2011, 10:04:24 AM »

You can find the velocity factor for open wire line on the web with a Google search, and go from there.
I'd extend that antenna to 55 feet on each leg to improve the 80 meter performance, and use a
length of open wire that is an odd multiple of an 1/8 wavelength at the lowest frequency you intend
to operate on between the feedline and good quality 1:1 current balun.

There was a ham who used knife switches to add or remove short lengths of open line to "tune"
his antenna. I wish I could remember his ham call and webpage. It may have been Cecil, W5DXP?

DX Engineering has an excellent writeup by W8JI on their webpage.

Pete

« Last Edit: October 11, 2011, 10:10:38 AM by K1ZJH » Logged
WB6BYU
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« Reply #2 on: October 11, 2011, 10:11:22 AM »

One way to do it is to experiment with models.  At least that will give you a good idea of
what is going on with the impedances.

You can model the antenna using EZNEC, 4NEC2, or any of a number of other modeling programs.  If you
aren't already familiar with them, you can also use W9CF's online yagi antenna modeling applet with a
single element:  set the element length to 102', use plenty of segments, and get the feedpoint impedance
on each band of interest:

http://fermi.la.asu.edu/w9cf/yagipub/index.html

Then you can enter the feedpoint impedance into VK1OD's transmission line loss calculator and see
what the impedance will be at the balun:

http://www.vk1od.net/calc/tl/tllc.php

This will also let you see the effect of varying the line length and its effect on the SWR on the coax.

You can then take it a step further and feed the calculated impedance at the balun back into the
VK1OD calculator for the 50 ohm portion to see the losses and the actual impedance at the tuner.
From there, W9CF has a handy applet that calculates tuner losses:

http://fermi.la.asu.edu/w9cf/tuner/tuner.html


Using these tools will allow you to see the total losses in the tuner, coax, and open wire line on
each band.  It may be a lot of work to try all the different combinations for each band, but still
faster than cutting lots of short pieces of feedline.
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W5DXP
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« Reply #3 on: October 11, 2011, 10:50:44 AM »

I have a 102ft doublet at average 25ft agl, this is fed mostly by 450 ohm ladder line, ...

Martin, do I have a graph for you. Here's the approximate lengths of 300 ohm twinlead that you need to resonate your 102' doublet on all of the HF bands. Since 450 ohm ladder-line and 300 ohm twinlead have about the same velocity factor, these should be in the ballpark for your required lengths.

http://www.w5dxp.com/G5RVlen.jpg

A lot of information can be had from my 130' notuner dipole.

http://www.w5dxp.com/notuner.htm
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
G7MRV
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Posts: 477


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« Reply #4 on: October 14, 2011, 10:11:09 PM »

Thanks for the replies guys,


Cecil, thanks, yes your system is what i ahd originally intended installing, and now am going back to! You may recall we exchanged quite a few emails about it some time ago.

My eldest son (7) says he will help with it this time, although hes hinted in no subtle way that he expects pocket money for it! Cheesy
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