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Author Topic: how to cut phasing and delay lines : a couple of questions  (Read 1374 times)
HS0ZFQ
Member

Posts: 99




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« on: December 08, 2012, 07:51:47 PM »

Greetings:
       First of all I would like to give W8JI late and belated thanks for the answers and info he gave me on this topic about a month ago. All that is in a bound book along with an article on Simpfeed.
       Right now I would like to reference an article   N4JTE Antennas Tuesday, July 08, 2008. Beginning on page three of the article he shows how to cut 84 degree and 71 degree lines. Down on line 8 he states " All the 50 ohm coax will be cut to the correct degree length using the MFJ with a Tee connector in parallel with a 50 ohm dummy load " That confuses me because about 9 lines further down he states " So we hook up a 1/4 WL length of coax based on the velocity factor, attach to the MFJ, short out the far end  ... At this point I am wondering what happened to the 50 ohm dummy load?? Can I just hook up the coax, short it, read the freq at low swr, and go on from there without using the 50 ohms in parallel. I cannot get this around my head.The rest I understand perfectly. Any help is really appreciated. I am determined to eventually build two phased verticals for 20, spaced 1/4 WL apart and each vertical being 1/4 WL high, just as W8JI suggested in the beginning.
Yours Truly
Richard
HS0ZFQ
ChiangRai, Thailand
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WB6BYU
Member

Posts: 13019




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« Reply #1 on: December 09, 2012, 09:08:51 AM »

Quote from: HS0ZFQ

...Down on line 8 he states " All the 50 ohm coax will be cut to the correct degree length using the MFJ with a Tee connector in parallel with a 50 ohm dummy load " That confuses me because about 9 lines further down he states " So we hook up a 1/4 WL length of coax based on the velocity factor, attach to the MFJ, short out the far end  ... At this point I am wondering what happened to the 50 ohm dummy load?? Can I just hook up the coax, short it, read the freq at low swr, and go on from there without using the 50 ohms in parallel.



For a quarter wave line you would use the 50 ohm load in parallel with it.  Cutting the
stub for minimum SWR makes it an electrical quarter wavelength.  Without the
50 ohm resistor the SWR would be (theoretically) infinite for any stub length.

For other length stubs you can put them in parallel with the 50 ohm resistor and
trim them for a particular reactance value.

You need the 50 ohm load in parallel to center the measurements close to 50 ohms
where the analyzer is most accurate.
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N4JTE
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Posts: 1152




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« Reply #2 on: December 09, 2012, 01:48:30 PM »

Yeah thats what I meant, hi
Bob
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N4JTE
Member

Posts: 1152




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« Reply #3 on: December 09, 2012, 01:54:10 PM »

Use the following link for cutting phase lines, the article you refer to was in my infancy of phasing, Bud developed this;http://www.va7st.ca/home.html/2009/11/christman-phasing-calculator/
It will help you a lot.
Bob
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HS0ZFQ
Member

Posts: 99




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« Reply #4 on: December 09, 2012, 05:23:11 PM »

 Cheesy   Thank you both N4JTE and WB6BYU for the help in steering me in the right direction. Now all I have to do is find a coax T-Connector in Thailand. I have a Palstar ZM-30 antenna analyzer and have only used it for cutting the length of dipoles in the past, measuring resonant frequencies and SWR. I had never heard of a coax t-connector before. It looks like one end of the T is shorted with a 50 ohm resistor and the other end is for attaching the test coax to so that you can measure, short the end and cut to proper phase angle. Am I on track?
Richard
HS0ZFQ
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WB6BYU
Member

Posts: 13019




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« Reply #5 on: December 09, 2012, 09:18:39 PM »

The T connector has two female fittings on the ends of the top of T and a single
male coax fitting on the bottom of the upright portion.  It simply allows three
pieces of coax to be connected together. 

In this case it allows the stub to be connected in parallel with the dummy
load (which can either be a resistor wired right across the end, or through
a length of coax.)

If you can't find a T connector you can use a 51 or 47 ohm resistor (or two
100 ohm resistors in parallel) and connect them right across the ends of
the coax stub you are measuring.  If you twist the ends of the coax to the
resistor leads you can stick one lead into the center conductor on the
SWR analyzer and the other against the outside, holding them there with
your fingers while you take the measurement.  There are a number of ways
you can make it work without the perfect connectors.
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W6EM
Member

Posts: 710




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« Reply #6 on: December 10, 2012, 05:47:09 PM »

In constructing 1/4 wave shorted stubs, you can use the reciprocity of an open quarter wavelength of line to easily find the correct length, and then apply the short.  An open, 1/4 wave length of transmission line is a dead short, or essentially, R=0 and jX=0 at the design frequency.

If you connect an MFJ 259 and sweep the unshorted length for R=0 and jX=0, you'll find the correct length.  Of course, start with a length a bit longer than you will need, and simply cut off what you need to until you reach the desired frequency.  Then, apply the short and make it what old timers called a transmission line "copper insulator."
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