[time-nuts] "Better" gps antennas than a Symmetricom 58532A

lists at lazygranch.com lists at lazygranch.com
Wed Jan 23 10:26:58 EST 2013


For a single frequency use like GPS, the impedance should be close to the target. It is true for scanners and such, 50 ohms is quite nominal. (This notion of DC to daylight and maintaining 50 ohms is fantasy. ) But for a GPS, you know exactly the application. 

The loading could effect the antenna pattern if it were not for the preamp.
-----Original Message-----
From: Jim Lux <jimlux at earthlink.net>
Sender: time-nuts-bounces at febo.com
Date: Wed, 23 Jan 2013 06:07:49 
To: <time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] "Better" gps antennas than a Symmetricom 58532A

On 1/23/13 12:41 AM, Hal Murray wrote:
>
> mspencer12345 at yahoo.ca said:
>> This URL goes into some of the issues involved in using 75 ohm coax in a gps
>> system.  I do acknowledge that several GPS manufacturers have promoted the
>> use of 75 ohm coax so some of the conclusions might be arguable..
>
> If the coax is short, the loss due to mismatch dominates.  If the coax is
> long, the attenuation per X meters will dominate.

And it's not clear that there's actually "loss" due to mismatch.  Most 
antennas/preamps/receivers don't have exactly 50 ohm impedances.  75/50 
is only 1.5:1, and there's an awful lot of antennas and receivers out 
there that only claim 2:1 VSWR.  The usual spec for the antenna is 
1.5:1, so for all you know the Antenna and Receiver is already 75 ohms, 
and RG6 will give you a better match than 50 ohm LMR400.



probably a bigger issue *might* be that reflections will create 
synthetic multipath from the signal reflecting back and forth in the cable.

>
> There should be a crossover length  What is it for 50 ohm RG-58 and 75 ohm
> RG-6?
>
>
> If you are designing your own antenna, you have the option of making one that
> matches 75 ohm rather than 50.
>
>
>
>

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