eeee hmmm, guys dont kill me now but to be in phase, and I
guess that´s what you want, the feedlines has to be EXACTLY
the same lenght.
They "don´t need to" be mults of electrical wl, they can be
whatever lenght, however it´s easier to control your impedances
if they are mults of electrical half waves since the cable wount
transform the impedance then.
73 Jim SM2EKM
PS Be careful so you don´t radiate all RF straight up.
Bill Tippett wrote:
>>The question however, is this. Must the two pieces of Heliax be exactly equal
>in length? Or, can they a multiple of a wavelength longer as measured by my
>MFJ 259? If so, do they have to be a full wavelength longer or can it be
>just a multiple of a half wavelength? My gut feeling tells me that
>everything would be fine if added an extra full electrical wavelength. I
>think just using multiples half an electrical wavelength would put them into
>out of phase.
> Your gut feel is correct. A feedline 360 electrical degrees
>more than the others will work OK. 180 electrical degrees (0.5 wl)
>will put the antennas out-of-phase which is NOT what you want.
> I very precisely matched the feedlines on my 3-stack using the
>MFJ-259. Later, I once was considering replacing my top KLM-610
>monobander with a KT-34XA, so I modeled everything to try to understand
>what the effect would be due to the booms not being the same length and
>the feedpoint not being physically in the same place. I discovered
>phase match is NOT very critical as long as it is within about 10 degrees
>(I think the number may be closer to 30 but I didn't keep my data).
> If I were you, I would first carefully measure the velocity
>factor of your hardline (each one separately if they are from different
>material). Then simply calculate the additional length of feedline
>for 360 electrical degrees using [(983.56/f) * Vf] where f = frequency
>in MHz and Vf = velocity factor. For example, assuming 28.4 MHz and
>Vf of .87, you would need to make the longer feedline 30.13' longer
>than the short feedline. Assuming you measure correctly, you could
>have an error of +/- 30.13/36 or ~10" before you approached a 10 degree
>error. If you are a purist, you can use your MFJ to match exactly,
>but it is not nearly as critical as I once believed.
> 73, Bill W4ZV
>P.S. For example, assuming phase shifts of 0, +10, and +20 degrees
>for the 3 antennas going up my tower, gain only changes by a negligible
>-0.08 dB and vertical beamwidth decresases by a negligible 0.1 degrees.
>Self Supporting Towers, Wireless Weather Stations, see web site:
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