Running a feedline for a GPS antenna - Page 1
Forum section: electronics / rf-microwave

author: 0culus
author_rank: Super Contributor
timestamp: February 29, 2020, 03:06:25 am
content: Title says it all. I'm looking at installing my GPS antenna that feeds my GPSDO (GPSDO has an internal bias tee that feeds +5Vdc up to the antenna) outside rather than leaning it in the window of my lab.For starters, I'm not too keen on making holes in the wall, and I had seen flat microstripline feedlines that apparently are designed for routing through a window. Are these any good? Probably only need a short section for the window area. The antenna has a female N connector on the bottom.Secondly, lightning protection. I live in an area that has frequent thunderstorms in the summer through autumn. I suppose at a minimum, there ought to be a nice deep ground stake? What else is recommended for GPS antennas?

author: BravoV
author_rank: Super Contributor
timestamp: February 29, 2020, 03:28:01 am
content: For lightning protection, even my antenna (Huber Suhner) already has built in lightning protection, still don't feel safe, added another layer with PolyPhaser DGXZ-06NFNF-A, a RF DC pass surge arrestor, that claims it can withstand multi-strikes, without the need of replacing the GDT as in conventional RF lightning protectors.Here are mine, took shots while ago when freshly acquired.Just fyi, the recommended lightning protection scheme and illustration from HP 58503A/B GPSDO manual.

author: awallin
author_rank: Frequent Contributor
timestamp: February 29, 2020, 08:01:38 am
content: Quote from: BravoV on February 29, 2020, 03:28:01 amWhat is the "7" component?What do you use for grounding the polyphaser, e.g. connecting "4" to "9"? Someone suggested a 20cm wide copper-plate/strip here... not sure how hefty wire this needs to be..

author: BravoV
author_rank: Super Contributor
timestamp: February 29, 2020, 12:32:09 pm
content: Quote from: awallin on February 29, 2020, 08:01:38 amThat is inline LNA with L1 bandpass filter, e.g. 58529A, in my setup, I don't use it.->https://www.microsemi.com/existing-parts/parts/138202The one at the bottom.Quote from: awallin on February 29, 2020, 08:01:38 amI couldn't find a definitive answer for that, but after discussing with local lightning expert that has experienced installing so many cellular BTS towers, I decided to bolt the PolyPhaser at a piece of 20cm x 10cm x 1cm thick copper plate, and then bolted with 3 x 10mm copper wires connected from "10' to "4", and with distance/wire length about 3 meters, while for the main lightning rod's wire, item "2" is using 20 mm wire, and at "4" (red box), the joint was exothermic welded.

author: 0culus
author_rank: Super Contributor
timestamp: March 05, 2020, 12:42:45 am
content: Thanks for the replies about lightning protection. Any opinions on the feedline?https://www.eham.net/reviews/view-product?id=9108This is an example of what I'm talking about. Except I don't want any more UHF connectors, so maybe not that one specifically...

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: March 07, 2020, 01:30:19 am
content: Regarding the diagram above, in my opinion it's a really bad idea to bring your lightning protectioninsidethe building! Keep that outside where it belongs, along with a very low impedance to ground!If your main concern is just where it comes into the building, that flat cable might work OK. I don't know what its loss is -- it would be good to see specs to see if it's worth a hoot at 1.575 GHz. My guess is it doesn't do too well up there.LMR-100A (or cheaper RG-174, or better RG-316) is very thin coax at about 0.110" / 2.8mm diameter and short runs of it are fine even at GPS frequencies. You can transitions from youroutdoorlightning protector to that and bring it into the building, then transition back to something with lower loss, or just go straight to your equipment if it's not too far. Times Microwave has a pretty good onlinecoax loss calculator. If your antenna has an LNA onboard (and you're providing adequate DC bias to power it), you can get away with a pretty good amount of loss in the feedline.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: March 07, 2020, 01:34:07 am
content: Found this about the Comet CTC-50M onDXEngineering:* Frequency Range: DC-1300 MHz* Connectors: SO-239* Impedance: 50 Ohm* Length: 15.75 inches* Max Power: HF 100W PEP, VHF: 60W FM, UHF 40W FM, 900 MHz–1.3 * GHz: 10W FM* SWR: Less than 1.3:1 below 500 MHz, Less than 1.5:1 above 500 MHz* Loss: DC-500 MHz = 0.5 dB, 500-900 MHz = 1.3 dB, 900-1300 MHz = 1.8 dB

author: 0culus
author_rank: Super Contributor
timestamp: March 07, 2020, 05:00:30 am
content: Hmmm, yeah, maybe a run of very thin coax would work better. I'd be curious to get one of those flat ones on the tracking generator though and see how it performs. I also agree about keeping lightning protection outdoors. I will keep looking into mounting options as well as lightning protection.

author: worsthorse
author_rank: Super Contributor
timestamp: March 14, 2020, 03:16:04 am
content: Use a polyphaser for surge suppression (you don't suppress lightning, at best you mitigate the damage caused if there's strike), properly bonded to a ground rod and outside the building. If you are able to use the ground rod at the service panel, that's best. Polyphaser makes a couple of SMA/N and N/N models that pass DC.I tried various incarnations of "flatline around window edges" and never much liked the results, mechanically speaking. You'd probably be better off figuring out how to run RG174, I think.Eventually I had four or five antennas, so I built an grounded entry panel bonded to a grounding rod about four inches away and a cable entry point through a basement window. All of my surge suppressors are directly to the entry panel as is my station ground.

author: El Rubio
author_rank: Regular Contributor
timestamp: March 18, 2020, 05:02:55 am
content: I have worked in wireless communications for 30 years and have worked on GPS timing at wireless sites. At one time, i was responsible for inspecting, approving, and repairing grounding systems at these sites. First, if you install a surge arrestor, any surge arrestor, you must ground it. That ground MUST be bonded to your main entrance ground. If not, you just created a ground loop and another source to ground through your equipment.The method we used for lightning protection started at the tower top. The coax cable shield was grounded to copper bus bar that was electrically bonded to the tower structure via solid tinned #2 wire cadwelded to the tower itself. At the base of the tower, there were one or more cadwelded #2 wires going to a buried ground system. Even the tower ground has to be bonded to the main service entrance ground. Before the coax was routed inside the base station shelter, the shield was again grounded to another copper bus bar bonded to the earth ground system. Inside the building, just below where the cables come in, another copper bus bar this is where the surge arrestors would be mounted or grounded to.This bar is also bonded to the earth ground. It was about 24 inches by 4 inches and was pre drilled with holes for mounting wire terminals. This bar, has a layout. On one side surge Sources were attached ( surge arrestors). The middle of the bar was the Absorber and another solid #2 was cadwelded to the bus bar and routed to earth ground system. The other side of the bar is where equipment grounds were connected. The thought on this is that a surge is taking the most direct route to ground. By putting the absorber between the surge and the equipment, the vast majority of the surge is routed through the short route to the absorber.A surge, such as a lightning strike, is taking every path to ground. Proper grounding gives a low impedance path to route most of that surge safely to ground. Wires, straps, etc should be direct and always routed downward. A surge doesn’t change direction well, so don’t snake the wires up and over things.Back to the GPS in your lab. Chances are, you aren’t going to put your antenna high up, just in a mostly open area. You could probably do fine without the surge arrestor, but if you want the reassurance, make sure it’s grounded properly and no surge arrestor is better than one with its own isolated ground rod.I have a spectrum analyzer that uses a GPS reference for timing. It has about 15 feet of RG174 ( I think, same size) and works fine. You may be able to put a piece of foam in a window and close over it.On my ham station, I used to use a piece of aluminum sheet in a window and put coaxial barrel/bulkhead connectors through it. This way you attach a male connector on each side. It served the same purpose as the outside ground bus bar in my description. It was about 3 inches tall and cut to fit the window opening. The window was just closed over if.