Sat nav antenna lead length and losses - Page 1
Forum section: electronics / projects

author: Deathwish
author_rank: Unknown
timestamp: December 10, 2015, 12:11:10 am
content: I am thinking of purchasing a marine sat nav antenna for my GPSDO, I have a choice of 2 cable lengths, 5 or 10 metres. Will I have any signal degradation on the longer length to the shorter one ?.

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2015, 12:26:14 am
content: Yes, but they wouldn't sell it if it didn't work reliably with their own GPSes, which will use an ordinary commercially available GPS chipset, so are likely to have input sensitivity similar to your module.

author: Deathwish
author_rank: Unknown
timestamp: December 10, 2015, 12:29:23 am
content: Thanks Ian. I guess i will throw £12 out there and see what occurs. Its this onehttp://www.ebay.co.uk/itm/391190369647?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

author: Theboel
author_rank: Frequent Contributor
timestamp: December 10, 2015, 12:41:16 am
content: Hi Deathwish,Could You report your experience with this antenna ?the price look attractive just need to ask them if they can post it to Indonesia.

author: Deathwish
author_rank: Unknown
timestamp: December 10, 2015, 12:49:14 am
content: I can yes, but it will be a while, I have a lot of things going on here and I tend to do things slowly.

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2015, 12:54:46 am
content: There's some data here:http://www.rfsupplier.com/active-marinenavigation-antenna-meter-plug-p-2564.htmlWorst case, you'll probably loose an extra 3-4 dB in the extra 5m of cable.Do you have a spectrum analyser that can reach 1.5GHz or can your module report signal levels so you can compare with whatever antennae you are currently using?

author: Deathwish
author_rank: Unknown
timestamp: December 10, 2015, 12:57:24 am
content: No i dont have one of those yet Ian, but it is certainly on my list of must haves. As for the module, no Idea, i havent even switched it on yet , in fact, it hasnt even arrived from China !. Still waiting.

author: cdev
author_rank: Super Contributor
timestamp: December 10, 2015, 01:17:37 am
content: If your antenna has a built in powered LNA, the lead length becomes unimportant. But make sure your GPS provides DC to the antenna terminal (usually 3-5 volts)

author: Deathwish
author_rank: Unknown
timestamp: December 10, 2015, 01:22:03 am
content: This is ithttp://www.ebay.co.uk/itm/121761130751?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

author: ConKbot
author_rank: Super Contributor
timestamp: December 10, 2015, 04:03:15 am
content: Easy answer up front: Since that antenna is active, 5, 10, 30 meters? Go wild, as long as the coax loss isnt greater than 30-35 dB or so.GPS is a tricky bugger when it comes to this kind of thing. Signal/noise ratio or Carrier/noise ratio is way more important than signal level (CNR being the preferred metric for GPS generally). Since the GPS signal is below the thermal noise floor (the decoding provides processing gain to bring it back above the noise) Its all about having a low noise system.The critical thing is having an amplifier either in the antenna (the one you want to use does) or attached to the antenna. Because of the way noise figures work (look up cascade noise calculators if you care to) every bit of loss before the first LNA is unrecoverable, as the thermal noise floor will remain fixed, but your signal slips further below it. After the first LNA, the GPS signal and thermal noise get amplified, the amplifier puts its own noise in, hurting the CNR by whatever the first LNA noise figure is, so you want your first LNA to be a good one. Now your signal and noise is 40dB above the thermal noise floor, so as long as you dont burn that 40dB up in cable loss by time you get to the next amplifier (either inline, or the one in the receiver) the noise of that next amplifier wont add greatly to the total noise. With an active antenna, you'd be pretty hard pressed to tell the difference between a 5 and 10 meter cable as far as signal quality. Atmospheric conditions will have more of an effect than the cable.