[amsat-bb] Cutting feed lines to 1/2 wavelength

Bob Bruninga bruninga at usna.edu
Sat Apr 11 07:58:02 PDT 2009

> What i want to do is the to have each 
> length adjusted "CUT" for 1/2 wave length 
> at a specific frequency. 

Yes, but remember that the accuracy of the cut for 1/2 wavelenth lets say at 435 MHz probably has to be within say 1/2 inch to not introduce more loss than it would gain in the first place.  But for two 1/2 wavelenghts, the accuracy has to be 1/4 inch.  For 10 feet (about 10 half waves) you have to make the cut within about 0.05 inches.  Make that 50 feet, an dnow your exact length has to be within about 0.01 inches. 

Now lets say that you somehow get extremely lucky and get the exact length just perfect.  That was only at one temperature.  I t would not seem unreasonable to me for a big change in temperature to make more than a 1% chane in velocity factor and then the cable will be out of phase and worse.

Oh, and if you change frequency by even a few Hz, then the length changes again, so... the mutiple-halv-wave feed line only is practical at HF.  Nothing above say 6 meters is even worth considering because of too many varaibleas beyond your control..

SOmething like that.

Bob, Wb4APR

I'm guessing it will not be too 
>hard on HF but could be a bit tricky on 1.2ghz? eg: How far from each cut frequency the cable will retain this 52 ohms matching? 
>I have about more that 500 feets coming from my and neighbourgs back yards when the CATV installers wants to put theses old lines in their 
>scrap container.
>Debate is open!
>P.S. If you see them around your back yards just ask them and offer them to rolled up the line they are just cutting. They will be more 
>than happy to let you take them it is some work they will not have to do. You can have some other extra roll already made by them if you 
>have the right timing as they change them at 10 years intervall or when the technology evolution demands it.
>Luc Leblanc VE2DWE
>Skype VE2DWE
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