[amsat-bb] Re: Polarity questions

[Joe]

Howdy all,

I Have a question,,

Will the below set up  work with 300 ohm TV type of twin lead?

Joe WB9SBD

i8cvs wrote:

>Hi Gus
>
>To know the velocity factor of a coax (known or un-known ) with good
>accuracy I make the measurement at about 30 MHz because the velocity
>factor do not change with frequency and a low frequency measurement
>make the error the smallest as possible.
>
>Get a N coax T  and connect the male port of it to the vertical Y channel of
>an oscilloscope.
>
>A spectrum analyser is not necessary because any oscilloscope works at
>30 MHz
>
>Connect one female port of the T connector to a RF generator and set it
>to 30 MHz
>
>If you don't have a RF signal generator use a low power 30 MHz TX at
>1 or 2 watt output power or less using few suitable 50 ohm attenuators.
>
>Connect the other female port of the T connector to an estimated 1/4
>electrical wave long of coax cable under test and live open the other
>end of it.
>
>   ______________    _______________
>  |                              |     |         30 MHz         |
>  |    Oscilloscope    |     |  signal generator    |
>  |                              |     |  or low power TX |
>  |___________Y_ |    |_______________|
>1/4 Lambda       /\|                                \/
>open o------------|------------------------|
> end            T connector
>
>
>If you suspect that the velocity factor is 0.66 like for solid polyethylene
>(PE) make the 1/4 electrical wave at 30 MHz 1.65 meters  i.e. 65" long
>
>The measurement plane of the coax cable under test is the middle of
>the T connector just between both female ports where the generator
>and the coax under test get in contact each other so that the real lenght
>of 1/4 electrical lenght must be taken from the above measurement
>plane including the male N connector up to the open end of it.
>
>Because of the theory of transmission line and following the Smith
>Chart we see that a 1/4 electrical wave long transmission line becames
>a short circuit at his resonating frequency when the other end of it is
>open.
>
>While looking at the CRT of the oscilloscope adjust the frequency
>of the signal generator until the amplitude of the signal on the CRT
>falls abruptly to zero.
>
>The frequency shown by the signal generator is the resonant frequency
>of the 1/4 electrical wave.
>
>Knowing the frequency and the physical lenght of 1/4 wave it is
>easy to calculate the real velocity factor of the coax cable under test.
>
>No special instruments are necessary because about everyone own a
>signal generator for 30 MHz and a CRT oscilloscope.
>
>Using a GDO or an antenna analyser the measurement can be wrong
>because it is difficult to know if the above instruments made for
>amateur use are well calibrated or not.
>
>73" de
>
>i8CVS Domenico
>
>----- Original Message -----
>From: "Angus" <angus at young5769.freeserve.co.uk>
>To: <AMSAT-BB at amsat.org>
>Sent: Sunday, September 21, 2008 7:57 AM
>Subject: [amsat-bb] Re: Polarity questions
>
>
>  
>
>>Hmmmmm might get shot down in flames for this but.
>>
>>If you want to know the velocity factor of a coax (known or un-known) try
>>getting two eaqual lengths of coax which are physically 1/4wavelength long
>>i.e. 50cms for 2mtrs.
>>Short out ONE end (join the inner and outers together with a short a link
>>    
>>
>as
>  
>
>>possible) and do this for  both pieces of coax.
>>We are going to make a coaxial dipole so these two shortened ends go on to
>>    
>>
>a
>  
>
>>piece of feeder (try and keep this feeder as short as possible also but
>>    
>>
>the
>  
>
>>dipole needs to be somewhere clear but does not to be 100 feet up either).
>>Yes you will need to support the dipole elements as they will just hang
>>    
>>
>down
>  
>
>>so perhaps a piece of garden cane or some other RF invisible material.
>>Trim back both OPEN ends of the dipole until you get the lowest possible
>>VSWR (on the centre frequency you want to use) and you should have very
>>close to an electrically 1/2wave dipole for the coax you used (each side
>>    
>>
>of
>  
>
>>the dipole will be a 1/4 wave long). You can use these figures for making
>>    
>>
>a
>  
>
>>1/4wave delay line or if your making a polarity control box you will need
>>    
>>
>to
>  
>
>>know the lengths of both 1/2 and 1/4 of the coax you are going to use, the
>>dipole will be considerably shorter than a normal wire dipole for the same
>>frequency because we have used the velocity factor to help reduce its
>>length.
>>
>>If you think that the feeder you have used is radiating either do perhaps
>>    
>>
>6
>  
>
>>turns around a small 1" tube to make a balun at the feedpoint or alter the
>>length of the feeder slightly, if theVSWR alters when you change the
>>    
>>
>length
>  
>
>>of the feeder then the feeder is radiating.
>>
>>My two pennys worth, should help you get very close to the figures you
>>    
>>
>need
>  
>
>>without the use of a GDO or an antenna analyser.
>>
>>regards
>>Gus
>>
>>
>>--
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>>
>>
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>>    
>>
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