[time-nuts] GPS splitter
kb8tq at n1k.org
Sat Jun 17 11:46:36 EDT 2017
In some cases, you can get away with a 470 ohm resistor on the Tee and leave out the inductor. A lot depends on
the threshold of the detect circuit in the GPSDO. Since the signal is normally well amplified before it ever gets to the
splitter, adding a small amount of loss generally is not a big deal. Depending on this and that, you may see more
loss from the Tee than from the 470 ohm resistor :)
> On Jun 17, 2017, at 10:12 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> At one time I had to design a DC-load since the GPSDO did not experience enough antenna current due to a different antenna being used. So, a BNC-T was quickly converted with a SMD inductor and resistor to add 150 Ohm of more load, and that helped the telecom operator to get their GPS out of "no GPS antenna" warning and actually accept the GPS satellites it was already detecting fine.
> On 06/17/2017 02:40 PM, Bob kb8tq wrote:
>> The DC block requirement depends a lot on the design of the GPSDO’s you
>> are using. With some GPSDO’s a 50 ohm load on the eighth port of a splitter
>> will do a pretty good job of “antenna detect” signaling. In the more general case
>> of “I didn’t design this beast” dc blocks and dc shunts to ground is the best approach.
>> This fairly quickly gets you headed in the direction of the HP / Symmetricom
>>> On Jun 16, 2017, at 11:54 PM, Clay Autery <cautery at montac.com> wrote:
>>> This brings up some interesting questions:
>>> If sharing an active GPS antenna, do you have to DC block all but one
>>> receiver port to prevent multiple receivers trying to supply current to
>>> the antenna?
>>> On say a 26dB antenna (ignoring line loss, power divider insertion loss,
>>> et al), what is the effective gain to each receiver? (Sorry, having a
>>> senior moment)
>>> Should ALL unused ports have 50 ohm +/- 0j terminators on them? I
>>> assume so... Thus, it would be "better" to always use the divider with
>>> the minimum required ports?
>>> I am assuming since this is a receive only situation, it will follow
>>> approximately the same rules of physics that dealing with satellite
>>> antenna installations.
>>> I would LIKE to share one PC-TEL 26dB GPS antenna mounted at the top of
>>> my 38 foot horiz.loop mast right at the shack entrance, using
>>> LMR-400-DB from antenna to Narda 2-way and thence to my current hacked
>>> Nortel GPSDO and my soon to be complete RPi 2/3 w/ Adafruit Ultimate GPS
>>> Hat NTP Server. On that mast, the antenna would have a near 360 degree
>>> view of the sky completely unobstructed. (Eventually, I expect both of
>>> those units to be replaced with commercial units).
>>> I'm assuming that I DC block whichever unit is capable of providing the
>>> LEAST current at 5VDC... I suspect the Nortel unit can supply more
>>> current than the RPi, but that's not a guarantee... And I guess I could
>>> block/turn off DC delivery on BOTH units and add a voltage adjustable,
>>> current limiting DC injection unit into the line.
>>> Clay Autery, KY5G
>>> MONTAC Enterprises
>>> (318) 518-1389
>>> On 6/16/2017 7:26 PM, Tim Lister wrote:
>>>> On Fri, Jun 16, 2017 at 4:40 PM, Gregory Beat <w9gb at icloud.com> wrote:
>>>>> I have reached the point that I need a 4-port splitter for my GPS antenna (outdoor 5 volt). Any recommendations of models (HP/Symmertricom/Microsemi) to acquire OR to avoid??
>>>> As we recommended to me when I asked a similar question, the Narda
>>>> 4372A-4 was a brand I had not heard of before and didn't come up in
>>>> 'gps splitter' searches. I got one on ebay for $24 plus a bit extra
>>>> for DC blocks on the n-1 other ports and it seems to work well and it
>>>> was handy to have an SMA-based solution as most of the gps receivers
>>>> and the antenna pucks seem to use SMA. This meant I only needed 1 N to
>>>> SMA converter cable for an external antenna (which has yet to be
>>>> externalized...). I found it smaller in real life than it looks in a
>>>> lot of the pictures, about the size of a modern smartphone but about
>>>> double or more the thickness (the connectors are on the ends).
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