[time-nuts] Non-impedance matched antenna cables

Dave Brown tractorb at ihug.co.nz
Fri Jun 13 05:54:30 EDT 2008

----- Original Message ----- 
From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Friday, June 13, 2008 8:37 PM
Subject: Re: [time-nuts] Non-impedance matched antenna cables

> Magnus Danielson wrote:
>> Hi fellow time-nuts,
>> For those of you that isn't aware of it, using 75 Ohm antenna 
>> cables rather
>> than 50 Ohm cables is a bad move, as the antenna cable itself will 
>> create a
>> multi-path system. This article elaborate on reflections i cables:
>> http://tycho.usno.navy.mil/ptti/1998/Vol 30_39.pdf
>> Nothing new, but needs to be said, especially with Thunderbolts 
>> getting more
>> popular, as the manual clearly recommends 75 Ohm cable as I recall 
>> it. Do not
>> listen to that.
>> Cheers,
>> Magnus
> Hej Magnus
> If one uses cheap cable with relatively high attenuation at L1 this 
> may
> not be a problem.
> However the active antenna may need higher gain to compensate.
> The question is just how much one way cable attenuation is required?
> A considerable reduction in this effect can be obtained by making 
> the
> cable length close to an odd multiple
> of one-half the signal wavelength (½λ) in the cable.
> Bruce

Has anyone actually measured the TBolt rx input impedance at L1?
DaveB, NZ

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