Fixing a rotten AM ground system, quick & cheap..?

[spinjector]

Sorry this is a bit long, but it does get to the point & question at the end.

I have an AM site that's been a dilapidated mess since I started working here. Previous engineers never did much with it and probably made a few things worse. I've fixed a few things, but the power is so low at night, and the ratings so low, I think at this point we're just keeping it on to keep the license.

One of its problems is it's got serious grounding issues at the tower. I never realized how bad until last week. It's a 2-tower site with the AM and an FM/cellular that we don't have anything on. The AM is omni and the FM is detuned with skirts. 8 kilos day & 25 watts at night.

So what happened last week is simply that it rained. The site is normally very swampy, but was dry as a bone for most of the summer, as it hadn't rained much out there. And then it POURED for days after the latest hurricane remnant rolled through, and the swamp came back.

Before: The RF Amp light on the DX-10 was flickering red constantly. There were 9 bad amplifier modules, and last week I finally got the time to repair them all. 7 of them needed new mosfets, and 1 just needed fuses, and the last still has a red light, probably a fried toroid.

Middle: After repairing all the amp modules, the RF Amp light only flickered half of what it was before. I think some of the remaining flicker is due to the remaining bad amp module.

After: I had to go back out there because "someone" forgot to flip that pretty little switch that says "local/remote". This was after it rained for two days straight, and when I walked in the the RF Amp light was almost totally green - even at full power. There was only the barest flicker of red during loud sibilant spikes in program content.

So... It seems the ground system is as rotten as other engineers told me they thought it was.

I have an idea to fix it quick & cheap, but I wanted to bounce it off some of you guys with more RF experience.

What if I were to pound a dozen 8-foot metal rods into the ground around the base of the tower in a big circle, hook them all up with lugs and wire, and then connect it to the ground plane at the base of the tower?

I figure that generic ground rods & lugs from the electrical dept at Home Despot should work kind - the same kind they use for residential & commercial breaker panels.

Am I nuts, or would this might work?

 

[TomT]

We got into a similar problem with an LMA. This site was worse...previous owners had sold off all the land around the tower except for a 25 X 100' parcel the transmitter building and self-supporter stood on.

What we did is build a box around the tower base with railroad ties (you can get them from landscapers). We put down heavy vinyl (again: landscapers) around the base of the tower inside the box. Then we nailed 4" copper strap around the top of the railroad ties and put in several ground rods in the center, and tied them to the outside strap with more strap. Strap into the building and to the ATU.

Since this was an LMA, the next step was done to be cheap (but worked fine). We contacted an electrical wholesaler and bought up a bunch of stub end reels of #12 insulated. There are coal mines around here, they use lots of #12 for the mining machines in various odd colors, leaving the wholesaler with bunch of short length spools that aren't much good to anyone else. We ran the #12 from the strap to the ground rods, until there was a spider web of wire around the tower. We punched holes in the strap, and ran the #12 up and through the hole for a good mechanical attachment, then went back and bonded everything together. Then we filled in the "box" with pea gravel.

Finally, we ran longer lengths around the building & even buried a few lengths in the neighbor's land (which was vacant at the time. However, the "counterpoise" or home made "ground screen" around the tower base proved to be the key to stabilizing this site.

With your site, I would approach it the same way, getting copper around the tower bases first. First build this "box" with the railroad ties and copper strap on top. 8 or even 12' square would work, the ties are about 4' long. This gives you a boundary to anchor the screen inside the box, and, later, longer radials to replace the ones that are now gone. An actual copper ground screen would be ideal around the tower bases, but they are expensive. The "spider web" approach with #10 or even #12 copper --lots of it--is cheaper. Of course, make sure everything is bonded together good, and be sure and tie in the ATU's.

Then next summer see if you can get the funding for new copper radials; again using the copper strap as a tie point.

Once everything is done, you may find your operating parameters on the DA have shifted--would not be surprised if someone in the past has "rocked" things a bit to compensate for changing base current ratios caused by the bad ground system.

One further tip--get some roofing tar and "paint" the copper strap--makes it worthless to the scrap dealers.

 

[ChiefEngineer]

Most of the Am old time systems here which have been successfully repaired were the result of someone telling the ownership how much it would help the signal.

WXLW Indianapolis was built 1959. Moved from 1590. Lyman Ayres of LS Ayres owned the place. 3 towers. Spared no expense. Large copper ground radials.

By 1990 the whole place had deteriorated. Dug carefully to find large gauge copper stilolo there!! A large mesh square was placed around the tower. Bonded to all the radials. Amazing signal improvement.

WERK signed on in 1962. All the towers cut down ( 6 toweres full wave spaced on 90 acres with a creek in the middle of the array). STA began in 1997. Single tower. Unipole. New site. Rotten ground system. Several 15 foot copper ground rods were placed around the base of the tower. It doesn't take care of the radials like it should but it helped.

With the dry conditions we have experienced many transmitter issues with the Gates One. It doesn't like not having a good ground.

Anything you can do to find out if some of the copper is left would help you. Old toy from any ham radio store is an MFJ 259 or 269. Connect the wire to your new found ground radial and find out how long it is and or if it is resonant at your frequency. ($250) Saves a lot of wasted time connecting 10 foot sections to the mesh. You can also use it to find the length of what is left and if you so desire dig it up.

 

[speakerman]

Tom T has a great way of doing it in a cost effective way. The most important part in stabilizing the common point is the high current area close to the tower base. Strap and screen is expensive these days and the spider web design is a great idea. You might be able to find some #12 at a scrape yard. All they want to do is make better than the copper spot price with a decent profit so you can offer 25% over the copper price and usually make a deal with them. They would usually rather get rid of the plastic covered stuff like THHN.

Try to run some full length counterpoise radials in the direction of your primary market if possible also. You may have to sneak some clandestine #12 into shallow ground slits to get it done.

 

[TomT]

Here's one source I found for copper strap:

http://www.gacopper.com/022-CopperStrap.html

$370 for 100' of 4" strap.

 

[Randal Marshall]

Generally, ground rods pounded into the ground do not do much for RF. The other suggestions about improving the ground radials are good. The first 25-30 feet from the tower are the most important for establishing a stable impedance, especially on towers that are greater than 90 degrees. The taller the tower, up to 225 degrees, the more critical the close in ground system becomes. Copper strap can generally be repaired unless it is completely missing. Strap is needed between the tower base, the ATU and the transmitter. Around here, paint and or tar make little difference with the scrap dealers. The best bet is to by a metal stamp and stamp the station call letters in it at various places. If it disappears, alert local police and scrap dealers.

Remember to use silver solder, lead solder will degrade in 2-3 years of weather and you'll be back were you started from.

Also, many tower fields have been allowed to grow in. Try to remove trees and brush out to 1/4 wave length if possible. Again, the area around the base of the tower is most critical, any improvements that can be made there will be worth it.

Good luck, AM work is always a tough sell with the management/ownership.

 

[frankberry]

As Randal said, ground rods will do little to improve your ground. You need the radials.
It's important to remember that the ground system is one-half of the antenna system. It carries the same amount of current as the tower(s).
The station must also meet the terms of the station license. If the station was licensed for operation with 120 equally-spaced ground radials around each tower, that is exactly what the FCC requires. If the original authorization calls for a ground screen under the tower(s), you've got to have the ground screen.
There is no cheap way to replace an aging ground system properly.
The payoff is much improved coverage.

 

[bilco]

Frankberryhas it right. THe ground rods will help a little but they are DC grounds, not RF grounds. There is no such thing as a systemn you describe that meets FCC requirements. The subject of this discussion is part of the answer. I tell my boss "ast, cheap and good, pick two" there is really no good substitute for ground radials. Some of the ideas here are pretty good and most will improve your signal some, but doing it right is the anwer to making that 25 watts effective.

 

[spinjector]

Generally, ground rods pounded into the ground do not do much for RF......The first 25-30 feet from the tower are the most important for establishing a stable impedance, especially on towers that are greater than 90 degrees.

Thanks. That was the major piece of the puzzle that I wasn't sure about. I didn't know if trying to hook into the water table with long ground rods would help, or if it was a surface-area thing. I understand that RF is all interacting waves of energy like water on a pond. I can visualize it all in my head, but I don't have the experience or math to calculate or express it in terms of the equipment involved.

Some of you have spoken about "90 degrees" and such; I'm assuming this is in reference to the phases of a directional pattern. But...this site is an omnidirectional site, so only one "power tower" is involved, unless you count the detuned FM tower 120 feet away.

* Is the length of the radials a function of the wavelength of the signal and/or the height of the tower..?

 

[rfry]

...Thanks. That was the major piece of the puzzle that I wasn't sure about. I didn't know if trying to hook into the water table with long ground rods would help, or if it was a surface-area thing. I understand that RF is all interacting waves of energy like water on a pond. I can visualize it all in my head, but I don't have the experience or math to calculate or express it in terms of the equipment involved.

You might want to read the opening post at http://boards.radio-info.com/smf/index.php?topic=176833.0 .

 

[ChiefEngineer]

When we added deep grounds at a distance from the tower it helped but didn't fix the problem. Thye were in the water table. The only solution is to have access to what's left of the ground system and hopefully add more.

In our case we use an STA because our 6 tower array is gone. We can't operate with 120 ground radials. STA.

There is no magical fix, there are band aids.

 

[TomT]

Two visualizations are helpful:

1. Close in, the tower is like an electron "pump"--you need close in radials/ground screen to gather up electrons for it to "pump out" (I know--terrible physics...)

2. For the antenna to work efficiently, think of those whip CB antennas with the drooping radials...what you had was usually a 1/4 wavelength (i.e., 90 degree) whip, with those drooping radials making up the other half of the antenna.

Now drop the whip antenna on a base insulator on the ground--instead of up on a pole. Those ground radials then would spread out on the ground..right? Scale it up to AM wavelengths and you have a typical 1/4 wavelength vertical antenna (or 90 degrees--360 degrees in one complete wavelength) with 1/4 wavelength ground radials. Except we use more than just 4 because of losses caused by being along the ground.

 

[DanStrassberg]

Some of you have spoken about "90 degrees" and such; I'm assuming this is in reference to the phases of a directional pattern. But...this site is an omnidirectional site, so only one "power tower" is involved, unless you count the detuned FM tower 120 feet away.

You may have figured out the answer since you wrote your post, but although 90 degrees in the context of AM transmitting antennas could--and often does--refer to the phase of the RF signal fed to one of the towers of a directional array relative to the phase of the signal fed to a different tower (usually the reference tower), that is not how the term is used in nearly all of the postings in this thread. 90 degrees, as widely used in this thread, is a measurement of physical distance--1/4 wavelength at the carrier frequency (assuming the velocity of wave propagation is that in free space--299.8*10^6 m/sec). To determine the wavelength in meters, divide 299.8 by the carrier frequency in MHz. For 1 MHz (aka 1000 kHz), the wavelength is 299.8m. 90 degrees = 1/4 wavelength or just a bit less than 75m. To change that into feet, multiply 75m by 3.28 ft/m = 246'. As the frequency decreases, the wavelength increases and vice versa.

 

[Randal Marshall]

Rich has a good point about above ground counterpoises, they do work. I believe Ron Nott of Nott ltd makes kits for these. The only caveat: you will have to file for a new license and do a partial proof, no matter if it is a non directional station or not.

The term 90 electrical degrees refers to 1/4 wave length, which is the typical height for most AM towers and thus most ground radials. Sometimes towers are shorter, sometimes longer, it will be on your license.

If you can, do a 1 KW/1 KM (or 1 mile as the case might be) field strength reading. It should be close to the theoretical and or measured 1 KM or 1 mile value on the license. You can also look up those values on line with the AM search tool if you need to. If the readings are off by more than 10 or 20% then the ground system is suspect. I would also measure the base impedance and make sure that it is substantially as is is licensed. If the base impedance is off, this will throw off your power readings, and it also indicates some issue with the ground system.

As far as fixing the ground system, here are your options:

1. Try to repair what is there, this means digging up and soldering together copper wires. Depending on the age of the wires in the ground and the reactivity of the soil, this may be a loosing proposition. If the soil is acidic, as many are, chances are most of the copper wires and or ground screen are gone. Before you do any of this work, you may want to get the soil acidity tested.
2. Install a new ground system that matches the old one. This is a heavy duty job, especially if there are trees and stumps in the antenna field that need to be cleared away. Copper is hugely expensive right now. The plus side, no modification of the license is required.
3. Do an above ground counterpoise, like Rich suggests. There are many stations around that use these and they appear to work pretty well, check out Nottltd.com. This will require filing for a license modification, and a partial proof. If the ground system is as bad as you say it is, this would be the least expensive way to fix it in a permanent fashion. Still, not cheap though.
4. Do nothing. Eventually the listeners will go away and you can turn off the transmitter.

Good luck.

 

[boiseengineer]

RE: The above ground system.
Wonder how the use of barbed-wire worked out for a NY NY station a few years ago as a reaction to past copper thefts? Never have priced barbed wire so I don't know how it compairs to copper wire.

FCC seems to grant wavers for ground systems when demostrated there's no way to comply. Like when a past owner sells off the land.