[TowerTalk] CATV / Open wire line convert

[n4kg@juno.com]

Wes,

You neglected to state what SIZE  CATV cable you
were refering to.  I use 3/4 inch cable which has
approximately 0.3 dB loss per 100 ft at 28 MHz.
(<1/3 the loss of RG213)

Yes, you are right about the phase difference between the
CW and SSB portions on 10M.  This is NOT a problem
on 12, 15, 17, or 20 Meters.  The solutions for 10M are
to insert an extra 1/4 WL when switching betweeen CW 
and SSB, use a 'Good Tuner', or just live with the SWR.
(My 3-500 based amps have no problem with a little SWR:-)
Another solution is to match the antennas to 75 ohms :-)

If you want to protect your touchy and expensive 
ceramic tube amp, then you are free to shell out
the Big Bucks for good quality low loss 50 ohm lines.
It's a free country.  (For runs of 200 ft or less, low loss
50 ohm cable is a good simple solution.)

I'm happy with my thousands of feet of FREE  CATV 
runs of 200 to 600 ft :-)

de  Tom  N4KG



On Mon, 18 Sep 2000  n7ws@azstarnet.com writes: 
> 
> With respect to using half-wavelength multiple cable lengths, 
> let me  use the following example to make a point:
> 
> Assumptions.
> 
> 1) 75 ohm CATV coax, Vp = 0.81, 0.82 dB/100' loss @ 100 MHz.
> 
> 2) Zl = 50 +j0 @ 28.5 MHz
> 
> 3) Since this stuff is usually used for longer runs, length trimmed 
> to 223.57 ft.
> 
> With these conditions, the input impedance is 54.35 -j0 for an SWR 
> of 1.09:1.  You expected 1.0:1 but the line attenuation upsets this
> expectation.  The insertion loss for this run is 1.03 dB.
> 
> The phase constant in radians per meter (beta); is determined by:
> 
> beta = (2 * pi * sqrt(epsilon))/lamda,
>  
> where epsilon (1.525) is equal to 1 / Vp ^2.  
> 
> For the example line length, beta * length = 50.27 radians or 2880 
> degrees of total phase shift.  Dividing by 180 degrees 
> (one-half wavelength)  we
> find that the line is 16 half-wavelengths long at 28.5 MHz.
> 
> Now QSY down to 28.055 MHz.  Run the numbers again and 
> you will find  that the line is now 15.75 half-wavelengts long, 
> or an odd 1/4 wavelength  long.
> The new input Z (assuming no change in load Z) is now ~71.1 -j23.5 
> or an SWR of 1.69. 
> 
> Is this meaningful?  I doubt it, but IMHO I believe it is pointless 
> to try and trim the line in the first place.  In the foregoing example,

> the  50 ohm
> input SWR (1.69) is worse at 28.055 MHz than it would be if the load 
> was made to match the 75 ohm cable (1.50). The total insertion loss 
> would be lower as well.
> 
> As the line gets longer or the frequency increases, the effect 
> becomes worse, although the additional line attenuation masks 
> the effect  somewhat.
> 
> There is a case for trimming line lengths and that is where phased 
> arrays are used.  Even there, however, if close phase matching is 
> desired,  it is better to keep the phasing lines short (in terms of 
> wavelength  multiples). 
> 
> As to my take on ladder line, it can be seen at:
> 
>  http://www.azstarnet.com/~n7ws/ladder.htm
> 
> 73,
> 
> Wes Stewart  N7WS

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