1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

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Fran3
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1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by Fran3 » Tue Oct 06, 2015 4:46 pm

Checking out underground coax... a few questions...

1 - Is there a relatively inexpensive TDR and/or a Fault Locator to not only measure coax cable length and open/short... but also to find the distance to issues in the cable?

2 - Is the amateur radio insturment MFJ 269 C at http://www.aesham.com/analyzers/swr-meters/mfj-269c/ good enough to determine swr and 50 ohms j 0 for broadcast?

3 - How do you guys splice coax buried in wet soil? Would you put on two make connectors and use a bullet and then heat shrink the whole thing and then coat it in silicone... or what?

The one I'm looking at replacing was done by the guy slipping a 2 or 3 inch section of copper tubing over the shield then soldering the center conductor together, and then clamping the copper tubing to the shield on each section with hose clamps and then covering the entire thing in a black rubber goo... then the guy dug up the dirt under the section and suspended the cable above ground so it wouldn't sit in water or wet dirt !!!

Actually is has been working but I know there must be a better way than this.

Thanks for any comments and suggestions.

Kelly
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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by Kelly » Tue Oct 06, 2015 8:02 pm

Fran3 wrote:Checking out underground coax... a few questions...

1 - Is there a relatively inexpensive TDR and/or a Fault Locator to not only measure coax cable length and open/short... but also to find the distance to issues in the cable?
Depends what you consider inexpensive. There are a wide range of decent TDR's/Megger for anywhere between $300.00-$6,000.00. It depends on how close your accuracy needs to be. I bought a pretty good one from a test equipment place for $700.
Fran3 wrote: 2 - Is the amateur radio insturment MFJ 269 C at http://www.aesham.com/analyzers/swr-meters/mfj-269c/ good enough to determine swr and 50 ohms j 0 for broadcast?
Even in the amateur community, MFJ is commonly known as "Mighty Fine Junk". I certainly wouldn't attempt to rely on readings for an AM or FM broadcast station with an MFJ product. It's fine for amateur radio. Now the devices from Array Solutions? That's pretty good stuff! I wish I could justify buying one of those.
Fran3 wrote:
3 - How do you guys splice coax buried in wet soil? Would you put on two make connectors and use a bullet and then heat shrink the whole thing and then coat it in silicone... or what?
A) Determine what the brand and model of TX line you're trying to splice. B) Purchase the designed splicing kit from said manufacturer. Follow instructions to the letter. C) Walk away with a sense of accomplishment. But, here is the warning part: Some directional AM stations use designed transmission line lengths as part of the antenna phasing system. Adding a splice, adding or removing line may change the antenna system parameters. This includes the individual TX lines from the phasor to the ATU's, or any RF sample lines.
Fran3 wrote: The one I'm looking at replacing was done by the guy slipping a 2 or 3 inch section of copper tubing over the shield then soldering the center conductor together, and then clamping the copper tubing to the shield on each section with hose clamps and then covering the entire thing in a black rubber goo... then the guy dug up the dirt under the section and suspended the cable above ground so it wouldn't sit in water or wet dirt !!!

Actually is has been working but I know there must be a better way than this.
Sounds like a kluge alright!
Skype:kellyalford Twitter: @KellyAlford

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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by ChuckG » Tue Oct 06, 2015 10:13 pm

Last time I needed a TDR I was able to borrow one from the cable company. I forget what brand they had but it got me within a foot of the fault. Might make a few calls before buying.

At AM frequencies perhaps the most common way to determine impedance is with a bridge, either an OIB like the Delta, or a cold bridge like the old GR's. The MFJ is going to be useless if there is any significant RF in the vicinity (or that your antenna picks up).

Agreed with Kelly 100% on using the manufacturer's splicing kit, if available. Problem is for some older lines, they may not be available anymore. In that case I'd think the method you suggest would be fine. This is foam core line in AM service, I assume?
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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by Deep Thought » Tue Oct 06, 2015 10:27 pm

3 - How do you guys splice coax buried in wet soil? Would you put on two make connectors and use a bullet and then heat shrink the whole thing and then coat it in silicone... or what?
I've done exactly that...and then put in PVC pipe filled with copious amounts of RTV-108 at each end.

It's expensive if you don't do this sort of thing on a regular basis but the Array Solutions PowerAIM-120's TDR function works very well. Mine has found breaks and shorts in transmission and sample lines at less than a foot accuracy.

What are you measuring the Z of? Transmission line? Or are you trying to set up a matching network?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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kkiddkkidd
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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by kkiddkkidd » Wed Oct 07, 2015 7:34 am

As far as I know, there are exactly zero coaxial connectors that are rated for below grade use. Even when pressurized. I have checked on various connectors from various manufacturers and they all say the same thing. Connectors are weather proof not water proof.

I have discovered quite a number of buried splices when they failed. Some were buried for years, some for months.

At the minimum, I would use DT's method and make sure that the PVC and coax are clean and dry. There are also splice kits made for buried telecomm cable that I would probably use if I had to do an underground splice. They usually consist of an outer container / mold and containers of 2 part sealer. The mold is placed around the cable to be buried, the sealer mixed and poured into the mold to completely encapsulate the splice. I'm not sure about sizes but I have seen them installed on large telephone cables.

No matter your splice method, Make sure that you accurately locate the exact point of the buried splice. You will probably get to revisit that spot in the future.
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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by Xenirad » Wed Oct 07, 2015 3:54 pm

This is the cheapest one I know of that does all the measurements. I do not own one of these so I cannot attest to its quality. Looks like a decent unit though. http://www.rigexpertusa.com/AA-230_Zoom.html
Fred Francis, Owner Xenirad Broadcast Engineering http://www.xenirad.com

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Dale H. Cook
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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by Dale H. Cook » Wed Oct 07, 2015 4:33 pm

Somehow the "l" got left outside the link above - it should be:

http://www.rigexpertusa.com/AA-230_Zoom.html
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by Xenirad » Fri Oct 09, 2015 9:13 pm

Don't know how that happened but thank you for correcting the link.
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WALKINGMAN
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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by WALKINGMAN » Sat Oct 10, 2015 1:41 am

im using autek analyzer its a cheap gadgets

BigRed
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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by BigRed » Sun Oct 11, 2015 6:09 pm

A couple of thoughts/observations . . .

If you can't find a TDR a cheap and dirty way to do this is if the line is open (either center conductor or shield) and you have a cap checker you can measure the capacitance from both ends. (You can use the same trick with an inductance meter if the line is shorted.) That will give you a ratio that will give you a good idea as to where the break is. You can confirm that with the data for the cable. There should be a capacitance/foot and an inductance/foot spec published for the line whatever the type. They're nominal values but will give you a good idea where to start digging. (And if an AM antenna be very aware of any ground radials; any you cut will have to be repaired.) No matter what instrument you're using though be sure to disconnect the line from the LTU/antenna before taking measurements.

As to splices . . . aside from the fact that they may no longer be available (the last time I tried to find one for CommScope/Andrew 7/8" Heliax line I found that CommScope no longer offered them) they're a real pain to install. Instead, consider using a pair of EIA flanges back-to-back. MUCH easier to install, much easier to "waterproof" and they can be taken apart again for inspection should the need arise. A number of surplus electronics places (The RF Connection in Gaithersburg, MD comes to mind) offer used ones at a good price. You might want to get re-attachment kits (which are basically new rubber glands and "O" rings) which will make the installation good as new. Don't forget to use a "bullet" inner connector and use stainless steel or brass hardware to connect them. If you're using a smaller coax a couple of "N" connectors and a barrel (if both connectors are male) will do the job. Cover them with either "shrink-n-melt" heat shrink or the cold stuff (with the nylon spiral holding the shape that has to be removed) available at most electrical supply houses and home improvement stores. Make sure the stuff you use is rated for direct burial and is waterproof, not just as an insulator.

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Re: 1) Coax TDR/Fault Locator 2)Z meas instrument 3)Coax Repair

Post by Xenirad » Sun Oct 11, 2015 10:24 pm

There are also several places online that have circuits if you want to build your own such as this one. http://www.epanorama.net/circuits/tdr.html
Fred Francis, Owner Xenirad Broadcast Engineering http://www.xenirad.com

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