Trimble Thunderbolt GPSDO question

[SpencerTC]

Hello all!

I have been trying to expand my home lab over the past year. Thus far I don't have a dedicated clock source [10 MHz] for my lab, and I have decided on using the proven Trimble Thunderbolt GPSDO as my source.

I mentioned this to a friend of mine and he gave me a Thunderbolt module that he had laying around and that wasn't being used. The one I was given has an F connector for the antenna port. The "bullet" antenna that Trimble sells comes in 2 versions, one with an F connector and one with a TNC connector and although it isn't mentioned in their datasheet, I would expect [based on the connector choices] the antenna(s) to be matched for 75Ω. The Thunderbolt module itself has components that [according to the internet] are expecting a 50Ω impedance [from the antenna], and Trimble recommends their bullet antenna, which (as I mentioned) I reasoned was probably 75Ω.

My questions are this; Does Trimble indeed recommend a 75Ω antenna for use with their Thunderbolt module's? For what it's being used for, I've heard that it [matching] didn't matter and to just use whatever antenna I wanted on the Thunderbolt....have any of you guys heard similar advice, if so, what are your thoughts on ignoring the impedance of the antenna (and antenna feed line)? What impedance do some of you more experienced guys think the Thunderbolt hardware is expecting? What impedance{mis-match} would be acceptable without damage and/or noticeable loss of performance?

Thanks for your time and consideration!

[texaspyro]

Just use 75 ohm RG-6 coax.  It has less loss than 50 ohm stuff and this is a good thing for GPS usage.  The impedance mismatch is not a problem.

[tkamiya]

Impedance mismatch does NOT cause any damage in this type of use.  It matters if you are operating a HUGE transmitter, though.  50 ohm/75 ohm mismatch is not a big deal, especially if your run is not long.  It does increase loss but it's not significant.  Active nature of GPS antennas more than covers it.

For receiving applications, 75 ohm has less loss than 50 ohm type, everything else being equal.  Plus, it's a lot cheaper and available.  I  use a very good 50 ohm type called Andrews Heliax, only because I had it.  I wouldn't hesitate to use RG6.

Since it isn't possible to attach F type connector to a 50 ohm coax, that is not a choice.  I also read Trimble once recommended 75 ohm.  I don't know what their current recommendation is.  It really doesn't matter.  Don't overthink it.  Most receivers are not well matched anyway.

[SpencerTC]

Thanks for the input!

The GPS antenna I already have (Motorola GCNTM20A3A) is 50Ω with an N connector, so I think I'm going to swap the connector on the board for something else (possibly BNC/TNC) that fits the same footprint and just use some of the 50Ω feedline I already have laying around in combination with it. Thank you for alleviating my concerns.

Best regards,
Trent Spencer - KM4YTU

[Xmaxyama]

It's very good with what you are doing right now.

[tkamiya]

I do not like F connectors.  Too many rotations before it fully seats and center "pin" is prone to damage. 

I much prefer BNC and N.  Besides, it's more of lab standard, so cuts down on number of adapters and cables.  I think you'll be happier with either one of those.  Just be careful that both of those connectors have 50ohm and 75ohm variety.  BNC will mate fine but N will not.  Telling them apart is not always easy. 

[tsman]

BNC will mate fine but N will not.

I've always been told that the 50 Ohm pin is slightly thicker and can cause damage to a 75 Ohm female connector.


edit: Nope. I'm wrong. 50 and 75 Ohm BNCs are mechanically compatible.

[texaspyro]

I do not like F connectors.  Too many rotations before it fully seats and center "pin" is prone to damage. 

All my GPS antenna connections and wiring use "push on" F connectors.  No rotating involved and swapping a connection takes less than a second.

[SpencerTC]

BNC will mate fine but N will not.

I've always been told that the 50 Ohm pin is slightly thicker and can cause damage to a 75 Ohm female connector.

50Ω N(m) has a much thicker pin than 75Ω N(m) and installing a 50Ω N(m) onto a 75Ω N(f) will cause permanent damage, the other way around isn't damaging, however nether combination will make a good connection.

50Ω and 75Ω BNC and TNC however have the same size pin and do not cause damage to each other, the only way I have been told to tell the difference between the two is to look at the PTFE around the little fingers that make up the shields primary contact....I can never remember which one has the teflon and which doesn't, if I am ever in doubt, I just put a termination on it and either sweep it on VNA or put a fast pulse through it on a scope.

Thanks everyone for your input and advice!

[tsman]

50Ω and 75Ω BNC and TNC however have the same size pin and do not cause damage to each other

Thanks. It looks like there is some confusion about this. Finding several sites that have the same mistaken idea as me that 50 and 75 Ohm BNCs have a different size pin. Looking at a datasheet for a 75 Ohm BNC from a connector manufacturer however does clearly state "Mechanically mates with common 50 Ohm BNC".

[tkamiya]

I do not like F connectors.  Too many rotations before it fully seats and center "pin" is prone to damage. 

All my GPS antenna connections and wiring use "push on" F connectors.  No rotating involved and swapping a connection takes less than a second.

That, of course, is a choice.  Personally, I want positive lock of some kind.  Personally, I wouldn't want to use push on with no locking at all. 

[tkamiya]

With CCTV and security camera industry mucking things up, BNC situation is a mess. 

When I was shopping for short harnesses, I looked into inexpensive solutions.  50 ohm and 75 ohm cables used with 50 ohm and 75 ohm connectors in whatever combination were wide spread.  If I were to trust pictures, RG58 was used with obvious 75 ohm connectors.  I didn't think it was even physically possible....  I have no idea what's going on anymore.  I know they are NOT SUPPOSED TO cause any damage when mixed but RG58 is not supposed to fit connectors made for RG6, RG59, etc either. 

I just use adapters and convert oddball ones to standard kind and go from there. 

Sounds like you are just starting to build up your home lab.  For me, standardizing connectors became a big issue.  Better to think this through now than 6 months later. 

[texaspyro]

Personally, I want positive lock of some kind.  Personally, I wouldn't want to use push on with no locking at all.

I've been using them for over 20 years without any issues.   They make swapping things around very easy... and I do that a LOT.  If you have a situation where the outdoor connection is under a lot of stress they might not be the best choice, but for indoor use they work great.   I hate screwing down F connectors.

[tkamiya]

I bet you do switch connections often for testing LH.  BNC is great for that and that's my standard for this kind of application.  Then N.  UHF/SO239/PL259 is banned from my lab.  F is on "restricted use only" status.  That's my own standard anyway.  I like SMA but it lacks robustness.  Also, too many "absolute crap" stuff floating around, I had few situations caused measurable problems. 

Anyway, thanks for your hard work on LH.  I am surely benefiting from all of that.  (as it is probably obvious from my questions)  On your spare time, can you make it touch screen capable, please?  (kidding!)

[texaspyro]

On your spare time, can you make it touch screen capable, please?  (kidding!)

It already is.   Click or touch on most things on the screen and something happens (usually zooms to a full screen view).  Click on the upper left corner and you get a touch keyboard.

[tkamiya]

What I would like to get actually are statistical information on locking.

For the run, how long it has been since the lock was last obtained, and how long it has been running total. 

Like
Running: xxxx hours
Last Lck:  yyyy hours
lck lost:  zzz times

They aren't already here, are they?  I have a reciever that loses lock.  I would like to know that even if it is not obvious from graph.

[texaspyro]

I would like to know that even if it is not obvious from graph.

Your best bet is to look at the red holdover status line at the top of the plot area.  Also, holdover events are noted as comments in the log files.