finding AM ground radials

AM Radio discussion. Directional arrays are FUN!
kcbooboo
Posts: 495
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: finding AM ground radials

Post by kcbooboo » Sat Jun 13, 2015 11:02 am

"...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."

Except these were two IDENTICAL brand new guyed towers, the middle two of a four-tower in-line array. The outer two are self-supporting and they're also around 39 ohms. So while you might say "three out of four ain't bad", it's that 4th tower that we're trying to get back down to 39, which is what all four towers were prior to replacing the middle two.

Bob M.

User avatar
Deep Thought
Posts: 3364
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: finding AM ground radials

Post by Deep Thought » Sat Jun 13, 2015 11:25 am

If the other two are self supporters and have the same Z as the guyed towers, all bets are off. That is almost impossible. The higher base area capacitance is almost certainly going to shift the R and will also affect the open-circuit "detuning" when measuring the others. I'd like to run this just to see what you might expect without the presumption that they should be the same as before. There may have been something going on with the old towers which forced them to be that way. If you don't want to post the details please PM me.

The reason this intrigues me so is that I just set up a close-spaced six tower using measured data to calculate the model (as usual) and the network designs, and it nailed the operating ATU matches within a few percent of 50j0 with calculated network reactance values at four of the six towers (the other two are negative). Here's what the self impedances calculated as (these are all identical 90 degree towers):
557c2083.jpg
557c2083.jpg (29.74 KiB) Viewed 1098 times
I'm not saying you're wrong here because I have seen inline arrays come out close to the same calculated base Zs but I would like to look at this one in particular. Thanks for indulging me.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

kcbooboo
Posts: 495
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: finding AM ground radials

Post by kcbooboo » Sat Jun 13, 2015 2:40 pm

PM sent. We'll continue the base impedance issue off-line.

Thanks.

Bob M.

User avatar
kkiddkkidd
Posts: 690
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: finding AM ground radials

Post by kkiddkkidd » Sat Jun 13, 2015 2:58 pm

I for one would be really interested in seeing the progression of the project... I have worked on a number of MoM projects but it has always been the prep work, sample system replacement, initial measurements, etc.

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

kcbooboo
Posts: 495
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: finding AM ground radials

Post by kcbooboo » Sat Jun 13, 2015 3:43 pm

Without going into details. Mark's analysis showed that the two guyed towers should have the same base impedance, so that gets me back to the original topic. Short of finding a wire and digging it up or just pulling it up out of the ground, I see no other proven way of finding them without getting an expert (and mucho dollarettes) involved.

Bob M.

User avatar
Deep Thought
Posts: 3364
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: finding AM ground radials

Post by Deep Thought » Sat Jun 13, 2015 4:06 pm

I'd give it one more shot measuring directly across the base just to rule out the connection between the ATU and the ground radial bonding ring. You'd have to lose a lot of radials to raise the R that much. About the only other thing I can think of is the guy wires not being properly insulated where they connect to the tower causing a top-loading situation, but that would be clearly visible.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

kcbooboo
Posts: 495
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: finding AM ground radials

Post by kcbooboo » Sat Jun 13, 2015 4:42 pm

I suppose that's possible. Of course a new tower base insulator was installed and there are four copper straps running from it down the sides of the pier. Where they go from there is anyone's guess. New soft copper pipe was run to the tower with lighting wires inside. The antenna current xfmr has the wired pipe running through it. Obviously there has to be a ground at the ATU. There was some damage to the side of the hut between it and the pier, so it could have pierced some wires or even a strap between the two. It would require some digging though. Luckily I can't fit under the hut.

Bob M.

kcbooboo
Posts: 495
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: finding AM ground radials

Post by kcbooboo » Thu Jul 16, 2015 12:40 pm

I borrowed an inexpensive (i.e. hobby-quality) metal detector and tried locating some buried items.

It easily found the buried 4 inch wide copper strap running between two towers.

It totally ignored a handful of copper ground wires sitting above ground.

It totally ignored an aluminum soda can pop-top (the ones that actually pulled out of the can that you had to throw away).

It detected a small piece of scrap copper flashing about 1/2 inch wide by 6 inches long sitting on the ground.

It totally ignored a scrap piece of screen mesh sitting on the ground.

It totally ignored a piece of exposed ground wire but would detect where the wire had been folded back over itself, but only if the sensing coil was resting on the wire.

It almost seems as if the ground wires are aluminum and not copper, and that this unit won't detect aluminum.

I tried various sensitivity settings and calibrated the unit frequently. It had brand new batteries. Didn't help one bit.

I were working around the live (5kw) tower at the time but I don't think that caused the unit to become super insensitive.

Makes me wonder how such a unit would ever find a coin or even a bolt resting on the ground, much less buried under the surface.

Maybe the "professional" models work better. Kevin, please jump on in and tell me what I did wrong (besides not call you).

Bob M.

User avatar
kkiddkkidd
Posts: 690
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: finding AM ground radials

Post by kkiddkkidd » Thu Jul 16, 2015 1:27 pm

You never call, you never write....

I have a fairly high dollar MineLabs Xtera 705 metal detector and it is hit and miss as well. It does find AL (pop tops, cans, etc) and will follow radials but if the TX is on it also finds radials where there aren't any. I use it sometimes if we have cut radials to locate but the TX has to be off for it to give any useful info.

I have found pop tops and .223 rifle casings at 8-10 inches deep but I'm not sure if it would find a radial that deep.

I originally bought one of the $50 RadioShack detectors and as you noticed, it was mute over even exposed wire on a good day with bright sunshine and a head wind.

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

ChuckG
Posts: 971
Joined: Tue Mar 11, 2008 11:12 pm
Location: Moo
Contact:

Re: finding AM ground radials

Post by ChuckG » Thu Jul 16, 2015 4:33 pm

kkiddkkidd wrote: I originally bought one of the $50 RadioShack detectors and as you noticed, it was mute over even exposed wire on a good day with bright sunshine and a head wind.,
Had one. Couldn't locate the steel dumpster I tossed it in.
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

kcbooboo
Posts: 495
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: finding AM ground radials

Post by kcbooboo » Fri Jul 17, 2015 3:15 am

It's possible to switch to another tower, but wouldn't go over well to shut the station off for 15 minutes at a time while I walk around the rough terrain with over a foot of growth. Not possible to do in the dark; too hard to navigate even with a working wire detector.

At least it wasn't the technique. There's got to be another way. I need to give this more thought.

Bob M.

User avatar
kkiddkkidd
Posts: 690
Joined: Mon Dec 03, 2007 11:13 am
Location: Lawrenceburg, TN

Re: finding AM ground radials

Post by kkiddkkidd » Fri Jul 17, 2015 7:13 am

I use a couple different coil/loop configurations w/ diode detector AND 2 different real cable/pipe locators.

What works well at one site may not work at all at the next.

Discussing the metal detector... I think that the problem is that the AM TX totally overwhelms the osc in the detector so it just goes nuts. I tried my Xterria705 on a cold array tower with little success. As Chuck noted, the cheap detector that I tried out didn't work anywhere. It went back to RS the same morning that I picked it up.
--
Kevin C. Kidd CSRE/AMD
WD4RAT
AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
http://www.kkbc.com

kcbooboo
Posts: 495
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: finding AM ground radials

Post by kcbooboo » Fri Jul 17, 2015 12:19 pm

I was thinking of a couple of coils feeding some sort of receiver (possibly an FIM-41) but then taking the signal level and feeding a VCO of some sort to give me a changing audible tone relative to the signal strength. That way I don't have to look at the FIM while watching where I'm scanning.

As mentioned earlier, the problem is rejecting the 5kw signal coming from the tower at the center of the ground system I'm trying to follow. I wonder if I can wire and orient the coils such that I null out the signal from the tower while maximizing the pickup from the radial. I really need an antenna that's sensitive in both vertical and horizontal axes and somehow get rid of the vertical signal and concentrate on just the horizontal one following the ground radials.

By the time I reinvent the wheel, it would be faster to just lay a new ground system for the tower with higher impedance.

Bob M.

User avatar
Deep Thought
Posts: 3364
Joined: Thu Mar 20, 2008 9:23 am
Location: La Grange, IL
Contact:

Re: finding AM ground radials

Post by Deep Thought » Fri Jul 17, 2015 1:32 pm

You're in the near field of multiple towers. Unless the others are floating there is current flow in them which also produces a magnetic field nearby. It'll be nearly impossible to null all of them out.

You can be "missing" more than a quarter of the radials on a particular tower and still not be able to resolve that within measurement error. If the base Z changed the cause is almost certainly within the first 10 feet or so and will involve the measurement point more than the tower itself. Sorry if I missed it, but did you measure the tower directly across the base insulator and compare that with a similar measurement on one of the old towers? You say that there are new straps across the pier but you don't know what (or if) they connect to. This is some fundamental stuff that you need to resolve before you even go looking for ground radials. A single bad connection between the ATU and ground radial terminating ring can cause what you appear to be seeing. A few missing radials will not. You may still be feeding the tower but the RF ground is back through the coax. Seen this more than once and the solution was to fix the ground at the ATU.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

kcbooboo
Posts: 495
Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: finding AM ground radials

Post by kcbooboo » Sat Jul 18, 2015 3:19 am

Yes, the other towers are floating.

When the tower was coming down, it buckled about 1/3rd of the way up. I think this was planned. Unfortunately they hadn't anchored the base and it came off its insulator and hit the ground and skidded until it hit the chain-link fence about 10ft from the pier. This may have torn up some of the ground within that 10ft radius.

I don't know about the ground from the ATU. I suppose one would have to disconnect the feedline and actually measure the DC resistance to see if it IS grounded.

I guess the only way to find out is to dig down to the ground system around the pier and verify it. I'll pass that along. Thanks.

Bob M.

Post Reply