finding AM ground radials

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Radio Ranger
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finding AM ground radials

Post by Radio Ranger » Mon Jun 08, 2015 3:52 pm

I've been tasked with locating ground radials on a late 1940's era non directional stick, probably sometime later this summer. I'm not optimistic that I'll I'll find any radials there, but...

To prepare for that task, today I took an FIM 41 to another site with known good radials (new, professionally installed ground system in 2009) attached the field meter to a cut off shovel handle to make it operate upside down, and walked 'across' the radials in the tower field to see if the meter would deflect upwards as I crossed a radial. I was about 100 feet away from the tower, but didn't see anything like what I expected with the loop oriented at either 90 degrees from or inline with the tower base. Maybe part of the problem is that this is a two tower DA with the towers spaced about 90 degrees apart, so that when I oriented the field meter to null one tower I got a peak on the other?
This was complicated by the 18" high grass that covers the tower site, that makes it hard to read the meter when it's upside down and close to the ground.

The next thing I tried was mounting an old broadcast receiver ferrite rod that is wound with maybe 100 turns of 28 ga wire on the end of a wooden broomstick and connecting the loop wire to the external input BNC on the FIM-41 via a 6 foot piece of coax. Again, I couldn't find any place with any orientation of the loop where I could tell I was passing over a radial.

What am I missing?

Years ago I rented a cable locator from a company called Rycom (?) that used a VLF frequency generator connected to the radials of an AM (nonD) and the detector on that worked great----cross the radial with a wand and see the peak. Maybe it's time to find something like that now for better results?


Steve Brown
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kkiddkkidd
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Re: finding AM ground radials

Post by kkiddkkidd » Mon Jun 08, 2015 4:10 pm

Did you short the FIM loop shield?

Did you contact your chiropractor for any appointment?

Or you could just call us down here in hot humid TN to locate and mark them for you...

You should be able to see the radials with an FIM (with the shield shorted) if you walk around the ground field. At 100ft, the radials will only be about 5ft apart so move out as far as you can. Shorting the FIM shield is essential to seeing anything close in to the tower.

Later,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

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Re: finding AM ground radials

Post by kkiddkkidd » Mon Jun 08, 2015 4:26 pm

Also... As I have found over the years, full 1/4wave radials are easily locatable even if they aren't connected to the tower base grounding. You have to electrically and visually verify that they are really firmly connected.

Regards,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

Radio Ranger
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Re: finding AM ground radials

Post by Radio Ranger » Mon Jun 08, 2015 8:14 pm

Thanks for the replies, Kevin. I'm not sure how to short the shield on the FIM---what's the process?

Have you tried the method I mentioned with the loopstick? It sure is easier than carrying the FIM upside down.

Steve Brown

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Re: finding AM ground radials

Post by ChuckG » Mon Jun 08, 2015 10:54 pm

kkiddkkidd wrote:Also... As I have found over the years, full 1/4wave radials are easily locatable even if they aren't connected to the tower base grounding. You have to electrically and visually verify that they are really firmly connected.

Regards,
I can vouch for that statement, having spent weeks locating radials on a 1940's three-tower DA only to find most of them had snapped not far from the tower base ring.
I didn't use a FIM, swinging a 13 lb Nems Clarke inches from the ground wasn't my idea of practical. Instead I made a simple homemade loop antenna at the end of a stick with a tuning cap and a diode driving a small meter. Much like a metal detector, only you don't find anything fun. :lol:
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Re: finding AM ground radials

Post by kcbooboo » Tue Jun 09, 2015 6:48 am

ChuckG said: "I made a simple homemade loop antenna at the end of a stick with a tuning cap and a diode driving a small meter. Much like a metal detector..."

I need to do the same thing on a 5kw station where most of the assumed breaks are probably within 50 ft of the day tower.

When using such a tool, will the meter reading increase or decrease when you get over a ground radial?

Does the loop antenna have to be oriented a certain way to minimize pickup from the 5kw 240ft stick behind me? In other words, should the loop be flat to the ground or vertical? And if vertical, should the loop line up with the tower or be at right angles to it, to minimize direct pickup of the radiated signal?

It's just not practical to turn the station off during the day, and it's too hazardous to be walking around the rough terrain at night.

Bob M.

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Re: finding AM ground radials

Post by ChuckG » Tue Jun 09, 2015 3:32 pm

kcbooboo wrote:When using such a tool, will the meter reading increase or decrease when you get over a ground radial?

Does the loop antenna have to be oriented a certain way to minimize pickup from the 5kw 240ft stick behind me? In other words, should the loop be flat to the ground or vertical? And if vertical, should the loop line up with the tower or be at right angles to it, to minimize direct pickup of the radiated signal?

It's just not practical to turn the station off during the day, and it's too hazardous to be walking around the rough terrain at night.

Bob M.
I did it with the loop vertical, about 2" off the ground and in line with the tower. When it crosses a radial there's a slight bump in the signal strength. Problem is there is still a bump when the radial is broken or disconnected, so the end result is you don't know a lot more than you did when you started, other than which muscle relaxers work best on back issues. :?
If I had to do it again I think I'd locate ONE radial, do the geometry and measure to locate the others, then dig up each end and check continuity. At leas that way I'd know if the radial was still continuous.
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Re: finding AM ground radials

Post by kcbooboo » Wed Jun 10, 2015 4:44 am

"If I had to do it again I think I'd locate ONE radial, do the geometry and measure to locate the others, then dig up each end and check continuity."

That's a lot of work when there could be 30-40 radials with issues. We think the problem is between the two towers, so any wire could be connected to ground either back at the ring around the base of the tower, or at the strip between the two towers, where intersecting wires would cross. There's probably going to be continuity to each end, unless a wire is broken in two spots.

If the station was off the air because of this, there'd be money available to hire Kevin to find and maybe fix the ground system, but since it's running OK (good enough to meet the competition), there's no money to "make it right" because that doesn't affect the air product. Yes, the poor ground raises the base impedance several ohms, meaning it's less efficient, and the signal isn't quite as strong in that direction, but there IS a signal, so that makes it good enough.

Bob M.

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Re: finding AM ground radials

Post by ChuckG » Wed Jun 10, 2015 9:07 pm

kcbooboo wrote: We think the problem is between the two towers, so any wire could be connected to ground either back at the ring around the base of the tower, or at the strip between the two towers, where intersecting wires would cross. There's probably going to be continuity to each end, unless a wire is broken in two spots.
Ouch. You probably aren't going to find that sort of problem by checking continuity or with a FIM/ tuned loop. The only way I know is to dig it up and inspect. Maybe Kevin has some better suggestions?

Are the DA parameters unstable with changing weather? That's what prompted me to grab the shovel, the DA would go out of whack whenever the ground conditions changed from wet to dry. Had to repair probably 2/3 of the connections to the tower base rings and replace the strap from the towers back to the phasor.
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Re: finding AM ground radials

Post by kcbooboo » Thu Jun 11, 2015 6:17 am

I don't think there's any instability. All of this happened when two (guyed) of four towers were taken down and replaced. One tower just has a higher base impedance (44 ohms) than the others (39 ohms). We suspect that parts of the tower pierced the ground system when it came down.

If we have to dig up about 1/4 of one tower's ground system, it would almost be easier to just pull new wires in that quadrant, but I'm not the one who has to spend the money, and since the station is operational (although not optimal) mgmt won't spend any money to "make it right" because no listener will ever notice it.

Bob M.

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Re: finding AM ground radials

Post by kkiddkkidd » Thu Jun 11, 2015 2:18 pm

ChuckG wrote:
Ouch. You probably aren't going to find that sort of problem by checking continuity or with a FIM/ tuned loop. The only way I know is to dig it up and inspect. Maybe Kevin has some better suggestions?

Are the DA parameters unstable with changing weather? That's what prompted me to grab the shovel, the DA would go out of whack whenever the ground conditions changed from wet to dry. Had to repair probably 2/3 of the connections to the tower base rings and replace the strap from the towers back to the phasor.
Severe DA instability wet to dry is the #1 sign of a bad ground system. Poor efficiency is #2.

I use 3 different methods to locate radials. An RF pickup coil will tell you something about the health of a ground system but definitely not everything. You can find radials that are totally disconnected with an RF pickup (especially if they are near 1/4wave). I can often tell the difference between connected radials and disconnected radials simply from identifying a few good radials and a few bad to compare. Then again, sometimes there is no discernible difference even knowing which is which.

I also use a modified cable locator to put a signal on individual radials and trace them to their extent. A broken radial at any point will become pretty obvious.

And most importantly... The old Mark 1 eyeball. In almost 20 years of doing ground system work, I haven't found anything more reliable than the eye peering at a bunch of wires (hopefully) bonded to a strap.

Most of the ground system problems that I find are what I call "manufacturing defects" unless there is obvious physical damage. Many, many old ground systems were built using low (no?) silver content solder, poor heat or poor bonding techniques. The wire and other bonding materials may be in good condition but not connected to each other in any way.

And then of course you have someone on a trencher taking the short cut across the AM ground system for a phone or power line that cuts dozens of radials. IF they are repaired at all, often, they are improperly repaired and fail shortly. This is one of the most time consuming and expensive repairs to implement due to the scattered nature of the damage.

I have seen severe damage to ground systems "allowed" by really good engineers that weren't fully aware of what was in the ground around their sites.

And yes, ground systems wear out. It fully depends on the soil makeup and original materials used but there is a point where the little white flags start to wave. I typically use 30 years as a rule of thumb but have found well built, 15 year old ground systems that were not repairable AND have found 50+ year old ground systems that needed nothing more than a few minor repairs to be good for many more years.

Later,
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

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Re: finding AM ground radials

Post by Deep Thought » Thu Jun 11, 2015 4:40 pm

Keep in mind that the four towers in a DA like this will generally NOT read the same base Z even if they are otherwise identical and are floating (<90 degrees tall) or detuned unless the spacing is very large. Depending on where it is in relation to the others it could read lower or higher, and things like differences in static drain chokes and connecting tubing can kick it a couple ohms either way if you are measuring it at the ATU output jack. Before I got too far down into this rabbit hole I would measure the towers directly across the base and also run a MoM model to see what you should expect. If you need the model done just let me know (with the tower geometry) and I'll send you the results.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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Re: finding AM ground radials

Post by kcbooboo » Fri Jun 12, 2015 4:42 am

And therein lies the problem. It's a Catch-22. The two new towers are basically identical and the whole idea was to be able to go with MoM, but with one tower's base Z so different, that makes it a bit difficult. Other work was done on the existing towers to make them better (as far as MoM calculations go) and they are doing fine. We can't file for MoM with one tower so far off, and it wasn't this way with the old towers.

I'm not the CE; I just do contract work for him and thought I'd run this past the experts.

I did find one patent on a 150-200 kHz ground radial locator system that used a pair of coils (not all of the details were provided, of course) and a sensor fed a signal to an audio oscillator and amplifier, so you could hear a tone change as you passed the coils over the radials. This is a lot easier to use than trying to view the meter on a FIM while swinging the coils over the ground back and forth to locate the wire. Now, one could use an inexpensive voltage-to-frequency IC to generate a tone based on the DC output from the FIM, then use a speaker or headphones to scan for the radials. You could almost search for them with your eyes closed, although they're better suited to watching where you're walking.

Bob M.

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Re: finding AM ground radials

Post by kkiddkkidd » Sat Jun 13, 2015 8:32 am

Don't over complicate the locator... An AM loop antenna, a diode detector and a set of hi-z headphones is all you need. I have mine setup to use a VOM if needed, but I use the headphones almost 100% of the time. Depending on where along the radial you are working, you may hear a peak OR a null as you cross the radial. I have never been able to explain this phenomenon but don't be surprised if you hear it.

I use a Tenna tunable AM loop (not ferrite loop) but it is very similar to others sold at Radio Shack, etc. But remember, you can find radials all day and still not know if they are really working.

On the subject of the Base Z's, I may be wrong but I thought that you adjusted the model to bring it to measured values. Whatever they are. I have never heard that there was a range that the Z's have to fall within.

Regards,
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Kevin C. Kidd CSRE/AMD
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http://www.amgroundsystems.com
KK Broadcast Engineering
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Re: finding AM ground radials

Post by Deep Thought » Sat Jun 13, 2015 10:23 am

kkiddkkidd wrote:I have never been able to explain this phenomenon but don't be surprised if you hear it.
It's mostly a function of the relative phase of the signal coming from the ground radial vs. the field from the tower. There is a time displacement and even disconnected from its brethren the radial has current flowing in it which the loop will detect. Which makes me wonder if there is some mathematical relationship you could use to help determine whether the radial was indeed connected at the base ring by observing whether it is a peak or null at a certain distance from the tower. Hmmm...
kkiddkkidd wrote:On the subject of the Base Z's, I may be wrong but I thought that you adjusted the model to bring it to measured values. Whatever they are. I have never heard that there was a range that the Z's have to fall within.
You do adjust the model, and in kcbooboo's example the difference is within what I would consider normal variation especially if the towers are not identical. 5 ohms out of 39 doesn't seem to indicate a major issue to me. Conversely, I've measured shorter towers which should have been around 12-13 ohms that came in at ~25, which had no connection to their respective ground system at the ATU in spite of the (very respected) tower contractor swearing up and down that they had connected everything properly. We added a 4" strap from the tower pier ground strap directly to the ATU strap and everything went back to normal. This is why I also suggested measuring directly across the base insulator. It's also easy to take a heavy cable with clamps to connect between the two while watching for changes in base current or tower Z at the ATU.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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