[time-nuts] Non-impedance matched antenna cables
bruce.griffiths at xtra.co.nz
Fri Jun 13 10:14:49 EDT 2008
Chris Cheney wrote:
>> I've seen that you're right, Thunderbolt is originally supplied with 75ft
>> of 75 ohm RG-59 cable, as indicated in the manual... So I've the inverse
>> problem: the GPS antennas we have in the roof both have 50ohm cable :)
>> So... would be good to change the impedance at the Thunderbolt antenna
>> port from 75 to 50 by means of some impedance converter? (of course, not
>> altering the DC supply to the antenna).
> If a correct match is a must-have, see
> http://www.tuc.nrao.edu/~demerson/twelfth/twelfth.htm on "twelfth-wave
> transformers". Basically, in series:
> The lengths La and Lb are approx one-twelfth wavelength in the cable.
> Continuity for power feed is maintained.
> But, since the L1 free-space wavelength is about 20 cm (cable wavelength
> probably 16-18 cm), one twelfth of a wavelength is rather short - quite
> possibly impractically short as the connectors must be included :-)
Forget the complication of an impedance transformer just trim the cable
to the appropriate length by adding a short section of additional cable
Unless the SWR (or reflection coefficient) of the antenna and the
receiver are measured using an impedance transformer may make matters
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