[time-nuts] Non-impedance matched antenna cables
jmiles at pop.net
Fri Jun 13 07:00:48 EDT 2008
I don't believe the Thunderbolt is a carrier-phase receiver, is it? If it's not, I'd be surprised if reflections make any difference.
They don't seem to have gone out of their way to match either 50 or 75 ohms (see S11 plot of TAPR unit's antenna jack attached), unless the input Z changes when the unit's powered up.
-- john, KE5FX
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Javier Herrero
> Sent: Friday, June 13, 2008 3:02 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Non-impedance matched antenna cables
> Then, if the input is 50 ohm, at least the reflections in the other
> extreme of the Thunderbolt + cable (i.e., at the antenna side) should
> not be so bad... I'm afraid that since the cable is alredy routed using
> the inside-building tubes (in fact it uses the tubing destinated for the
> TV antenna) is not so easy for me to adjust the cable length :)
> Yes, I was also surprised by the use of F connectors... and that made me
> suspect (with surprise) that the GPS input were 75ohm, but I must
> confess that I did not read more ;).
> The antenna I'm using is a Motorola mainly intended for vehicle
> mounting. Surely not the optimum antenna... but not bad results with the
> Thuerbolt nor the M12's. I've another antenna, but is a cheap one with
> magnetic base, only used for a nixie clock :)
> Björn Gabrielsson escribió:
> > Javier,
> > According to Trimble the Thunderbolt IS 50 ohm at the antenna input.
> > (maybe the sole user of 50ohm F-connectors... ;-) )
> > See page 3-5, Section 3-4 "Antenna Cables" (page 39) in
> > "Note – RG-59 is a 75 ohm coaxial cable. The ThunderBolt and the Bullet
> > antenna are compatible with either 50-ohm or 75-ohm cable. Compared to
> > most 50 ohm cable, 75 ohm cable provides superior transmissibility for
> > the 1.5 GHz GPS signal and a better quality cable for the price.
> > Mismatched impedance is not a problem.
> > Note – The input impedance of the ThunderBolt RF input & its antenna is
> > 50 ohms."
> > --
> > Björn
> > On Fri, 2008-06-13 at 10:47 +0200, Javier Herrero 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
> >> antenna port from 75 to 50 by means of some impedance converter? (of
> >> course, not altering the DC supply to the antenna).
> >> Regards,
> >> Javier, EA1CRB
> >> Magnus Danielson escribió:
> >>> 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
> >>> _______________________________________________
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Javier Herrero EMAIL: jherrero at hvsistemas.com
HV Sistemas S.L. PHONE: +34 949 336 806
Los Charcones, 17A FAX: +34 949 336 792
19170 El Casar - Guadalajara - Spain WEB: http://www.hvsistemas.com
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