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Long antenna cable…how long is too long?
Posted by john-hamilton on June 30, 2020 at 9:41 pmI bought a 120 foot long antenna cable (RG8, the thick kind) in 2016 to put in a conduit. I installed the conduit in 2010 when I built the house, it has a lot of other cables in it, multiple coax for cameras, fiber optic feed, several ethernet cables, etc. All work just fine. When I tried to run the antenna cable through, I realized the conduit was not straight, so I needed another section. I bought another 15 feet (same thickness). Installed it in the conduit to go from my basement to a pedestal. At each end was a short (2 feet) adapter (not thick) to go to the antenna at one end and the receiver at the other end.
It worked fine for 4 years, and just recently stopped working. I checked all of the connections, sprayed some contact cleaner in them, and they worked again. For a day. Tried three different receiver/antenna combos. When I first connected after cleaning the connections, I had 26 satellites. Next time I checked later in the day I had 3 GPS and 3 Glonass. Then 0. I mainly use a Trimble Alloy with a Zephyr 3, but also use R7 with a Zephyr Geodetic 2 and a 5700 with a Zephyr Geodetic.
So, my question is how long is too long? And what would cause it to suddenly stop working? All but 6″ of cable at the antenna end are in a conduit. Do people put amplifiers in long CORS runs? I wonder if a single 150 foot long cable will work, as I know that each time there is a connector inserted it affects the signal strength. But I am puzzled as to why it suddenly stopped working. Part of the conduit is 2″, most is 4″, but I don’t think any animal got in there. And none of the other cables (coax and ethernet) have failed.
shelby-h-griggs-pls replied 3 years, 8 months ago 13 Members · 30 Replies- 30 Replies
About 4 years ago when I had fantasies of setting up an RTK base in the office and a radio comms center to cater for number of large projects that we had on in the area, I looked into this a lot, and bought a lot of cables and connectors, lighting arrestors, … etc. Never installed any of it. Something I couldn’t understand was that connectors are suitable for various “frequencies”, and using the wrong type could be fatal to a particular setup.
http://onlinecablescom.blogspot.com/2011/11/tnc-bnc-n-type-connectors-whats.html
Anyway, as noted in a recent thread, my most recent trouble with air wave communications, involved the amount of data that was being sent/received – in that case it was NMEA data from a depth sounder BT to a DC, with the default setup being about 10 NMEA data items which resulted in a no-go expect that maybe every minute or so one or two depth-data would get through, but cutting the data back to just depth only got it working fine.
If it once worked, then length is not the problem. I once ran some GPS tests with 150 ft of RG8 and saw no problem. That’s why the antenna has a preamplifier in it – to boost the signal making cable loss less important. Conduit size should not be an electrical issue. If you got the cable through undamaged the conduit is fine.
But buried conduits usually fill up with condensed water. If the coax jacket got damaged, then water could get into the shielding and corrode it to lose continuity, causing big problems, but that probably would not vary from day to day. You might test the DC continuity with an ohmmeter and a shorting jumper on the opposite end. Anything more than a very few ohms would mean trouble.
Going from one kind of cable to another can cause issues at these frequencies, but if the right connectors and adapters are used that shouldn’t be serious. All the coax pieces need to have the same rated impedance (not resistance) to avoid standing waves that lose the signal at some frequency bands.
Connections are indeed a common source of problems, but you seemed to have dealt with possible corrosion of the contacts. Is there any possibility of poor assembly/soldering of a connector that now broke loose inside?
.I got an email from a person who operates a lot of CORS. He said antenna cables in conduits are susceptible to damage from water and freezing. He says they prefer to direct bury the cable (a direct bury cable, of course) rather than put it in a conduit. I hate to have to dig up the ground to put a cable in when I already have a conduit, but that sounds like what has to be done.
I have two cat5 cables going over 600 feet to another building in the conduit, and I know for sure at the far end (lower) there is always water in the conduit. Idiots who put in the conduit (electrical contractor) must not have sealed the joints. I periodically pump out the water. I had two cables in there, and I recently added a direct burial cat 6 cable. Of course this is too far for ethernet, which has a 330 foot limitation, so I use an ethernet extender, which is basically an ADSL connection over a single twisted pair. Phone and ethernet extender and several camera feeds, each over a different twisted pair using video baluns, all work fine. It also has an incoming fios fiber optic cable from the road which is probably not susceptible to damage from water/freezing
An electric cable running parallel can also be a cause of disruption due to the antenna signal being very weak at the best of times. My 2 cents. I have used a cable that long in the past with nothing else running parallel (I don’t think network cable even with PoE would cause your issue) and had no issues.
It actually has a 18V dc cable with low current (<1A, to provide power for testing R10’s and R8’s and total station). But I don’t think DC current has any effect. And it has always been there with no problem. But no AC.
But, there is the main power feed coming from a transformer into the house in a separate conduit, not sure how close it is but probably at least a couple of feet away. I hadn’t thought about that.
AC is the one that creates the field and a couple of feet of separation should be fine. As Richard said different connectors are better with different frequencies and even though you have cleaned them it just doesn’t take much. I placed new connectors on my long cable when I did it and then sealed them with dielectric grease
He said it worked once. If it doesn’t work now, something changed and he needs to figure out what.
Was the 120 ft to 15 ft connection among those inspected?
I still recommend the ohmmeter test across the center pin to shield when the opposite end has a short clipped across it. A good 140 ft cable should read less than an ohm (if the meter is even that sensitive). If it is significantly higher, then there is a bad connection or severly corroded braid.
.I always use direct burial cable in buried conduits as an insurance policy. Is you RG8 direct burial and shielded? I know when I set up a CORS for my old firm a couple of decades ago, we had to get a special cable. This stuff was really thick, like around 3/8th, and was what the Trimble engineers recommended back in the day when tech support was calling one of the actual engineers. Maybe your old cable has deteriorated siting in the musty conduit. Bill is correct, do an impedance test on it.
Most physical connection problems I’ve found were at one of the connectors.
I know a guy that always wants to use my phone charger and afterwards the plug in of the cable is nearly worthless because of something he does in his manner of stuffing it in the hole damages the multiple contact points.
Even my trusty HP48GX will hiccup some days because one of the pin connectors is too loose to make a good connection. Some contact cleaner usually clears that problem immediately.
They have these wifi extenders that will fix many cat signal problems.
It does sound like the connectors to me. Re-clean and use dielectric grease would be the first thing I would do. While doing that I would look at the connectors for any evidence of them having tension as I have seen the central pin or the outer connection being broken with tension. I would then run an ohm meter on both wires to see if it just a broken connection, but, keep in mind even a small ohm meter places more signal on a wire than the antenna does. Lastly would be to test each section individually. Antenna’s can be a pain …
Induction….Deduction…..Years ago i used to have access to a IFR COM-120B. With that we could find out exactly where anything and everything possibly wrong with the cable is and was doing..up to about a mile away…Sigh….
That being said, with a random squelching of the signals after deteriorating performance, yes, the connectors are a quick easy test, but just drag a wire direct to the antenna after your test to insure it still works in all other scenarios.
Cable shake tests are difficult at best in a conduit run.
Good luck and let us know what you find.
- Posted by: @norm-larson
even a small ohm meter places more signal on a wire than the antenna does.
While there can be some nonlinear effects with low level signals at contact points, I expect the receiver sends DC to power a preamp in the antenna assembly, and that would break through any millivolt-level barriers just like the ohmmeter.
. John, your assuming its cable and connections for troubleshooting. You might want to rule out the death of the antenna pre-amp by hooking up a short cable that you know works direct to the antenna, and make sure that you haven’t had a failure there. Not real frequent, but have seen it before and went through the pulling hair out trying to find a problem with the cable.
As others have suggested, I assume you have narrowed the problem down to the cable? Cables are notorious for being the root-cause of problems, but not always. But if by moving the receiver/changing antenna, etc. you have narrowed it down to the cable, and now the question is, where in the cable is the problem, there are tools out there to help. Not sure if you know someone who is a ham radio operator, they may have a cable tester. Those work well to isolate the location of the discontinuity, whether the connector, cable, and/or where along the cable.
My 2?, moisture. It??s just a matter of time.
check the impedance with a very good meter.
I have tried three different receivers (Alloy, R7, and 5700) and three different antennas (Zephyr 3, Zephyr Geodetic 2, and Zephyr Geodetic).
Thanks for all of the great suggestions and insights.
Weird coincidence…I have always been interested in radios since I was a kid (my father was a Phd in EE, and I thought I wanted to be an EE as well until I discovered surveying). So just recently I decided I should become licensed and started studying the ARRL Ham Radio License Manual.
be sure to meter the receivers DC output. Trying different antenna is one test, have you tried different receivers? I??ve found (center-to-ground) the DC power failed AFTER trying half a dozen antennae.
And, the measured voltage at the receiver as well as the measured voltage at far end of the cable run.
I think 98% of active antennae require 5 VDC minimum.
Could you not use the existing cable to fish a new one through? Also, I thought since GLONASS was amplitude modulation the length of the cable between antenna and receiver had an impact on inter-channel bias? Maybe Trimble software models that out, though.
I have a pull wire in all of my conduits to make it easy to add new cables. But getting a 135 foot long cable can be expensive, I will buy a new one if I have to but would rather “fix” this one.
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