[time-nuts] Thunderbolt antenna

[Bob Camp]

Hi

How long a run of coax will you have? Do you have a power splitter at the receiver end?

You probably need 10 db of gain between the antenna and the receiver. If the antenna has 25 db of gain, you have 15 db to waste on cable loss and power splitting. With no power splitter, you could run about 200' of RG-6. You likely will spend less on that amount of cable (plus the F connectors) than you will just on the connectors for the Heliax…

As mentioned in an earlier thread, RG-59 may or may not be what the tables say it should be. RG-6 quad shield is made for the same frequency range as GPS. Weather it works broadband or not - who knows. It's pretty likely that it will indeed work at satellite / GPS frequencies.

In a receiving application, impedance isn't a big issue. Since the TBolt is designed for 75 ohms, it's probably happier with 75 ohm cable. The antenna - who knows. Either way 50 or 75 ohms - no big deal. 

Bob

On Mar 10, 2013, at 12:04 PM, Peter Gottlieb <nerd at verizon.net> wrote:

> The seller had a "make offer" so I tried $20 and it was set to auto accept.  I figure for $25 (with shipping) it's worth a shot for a new unit.  The Trimble data sheet says it is good for up to 75 feet of coax, I think I'll end up with about 50 here.  The antenna I have been using is a no-name pole mount unit with 25 feet of attached RG-59 coax that came with the TBolt "kit" I initially got.  I'm not a big fan of RG-59.
> 
> So what coax should I use?  Many people say use good RG-6, although the Dranetz power line units with GPS come with a 100 foot piece of 1/4" Heliax, I have to imagine that would be better.  Here's some on epay: 360492678643, but with wrong connectors.  Expensive, though, here are approx attenuation numbers at the frequency of interest:
> 
> RG-59        10.4 dB/100 ft
> RG-6            8.4 dB/100 ft
> Heliax          7.4 dB/100 ft         FSJ1-50A
> RG-11          5.7 dB/100 ft
> 
> (Yes, I'm aware of the impedance differences)
> 
> It seems, just go with quad shield RG-6 and be done with it.  I even have part of a spool of that laying around.  Maybe more of an issue is how do you properly connect a TNC to that stuff (the antenna has a TNC).
> 
> Peter
> 
> 
> 
> On 3/10/2013 11:01 AM, Volker Esper wrote:
>> 
>> Hi Peter,
>> 
>> Why not. The antenna is optimized for that purpose (receiving GPS L1), omnidirectional and tuned to the GPS frequency, snow skids down, birds can't land on it. As N0UU affirms, there's nothing further sensational inside.
>> 
>> I don't know, how proficient you are with radio frequency stuff, but a gain of 35dB does not guarantee a good reception. You primarily need gain to compensate (cable) losses. The noise figure (NF), for example, can get much more important.
>> 
>> What antenna do you use at the time? If you are using a magnetic car roof antenna a Trimble Bullet surely will be a better choice...
>> 
>> There are lots of GPS antennas on ebay for even less than 30 Dollars. I run four different antennas, which I purchased from ebay and none of them has failed so far.
>> 
>> Volker
>> 
>> 
>> 
>> 
>> 
>> 
>> Am 10.03.2013 06:07, schrieb Peter Gottlieb:
>>> I'd like to get a better antenna for my Thunderbolt.
>>> 
>>> I see Trimble bullet antennas type 57860-00 on ebay for $30 or so, specs look to be 5 volt 35 dB gain.
>>> 
>>> Would something like this be a good choice?
>>> 
>>> Peter
>>> 
>>> 
>>> 
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