can u de-tune a tower near directional array w/o using a skirt?

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Fran3
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can u de-tune a tower near directional array w/o using a skirt?

Post by Fran3 » Wed Mar 02, 2016 8:38 am

I don't consider myself an antenna expert but here is the idea and the question.

The AM station has a 3-tower directional array in line west to east.

There is a utility tower on the property that is about 150 to 175 feet tall. It has a (presently) unused marti antenna on top and a wireless internet dish at about 50 to 75 feet up.

The tower is sitting on an insulator so it is not grounded.

I understand that a skirt is the typical way towers are de-tuned but why wouldn't this work?

Place a parallel L/C circuit between the tower and ground. Tune the circuit to be resonant at the stations frequency.

(In practice you could place an RF Current meter between the tower and the L/C circuit and tune for minimum current... or you could place a scope or RF volt meter across the L/C circuit and tune for max voltage.)

Since a parallel L/C circuit presents maximum impedance at it's resonant frequency... why wouldn't this work?

Thanks for any comments or suggestions.

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kkiddkkidd
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Re: can u de-tune a tower near directional array w/o using a skirt?

Post by kkiddkkidd » Wed Mar 02, 2016 9:39 am

I have worked around a number of situations such as yours where they basically had an ATU between the tower base and ground to detune it. All of these have some semblance of a ground system under the detuned tower.

If you have feed lines or cat5 crossing the base insulator, you will have to deal with that before it will detune. Probably just wrapping a bunch of turns of the cat5 around a form would be good enough. An isocoupler or coil would be required for the feed lines.

I can't speak with 100% certainty to the effectiveness of detuning a base insulated tower but suspect that it is at least as effective as using detuning skirt(s) on a grounded tower.

I'm sure that others here can supply specifics on the network.
--
Kevin C. Kidd CSRE/AMD
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AM Ground Systems Company
http://www.amgroundsystems.com
KK Broadcast Engineering
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Deep Thought
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Re: can u de-tune a tower near directional array w/o using a skirt?

Post by Deep Thought » Wed Mar 02, 2016 9:43 am

It's fairly easy if the tower's guy wires are broken up by insulators; this is how unused towers in a DA are detuned. If not you'll have to get creative.

A skirt is just a resonant L/C circuit with the skirt providing the L. Are you sure the tower doesn't have a connection to the ground radial field? If it does it'll be a lot easier to get a decent detuning null. Since you say it already exists what was done when it was built (or the DA, whichever came first)?
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

Fran3
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Re: can u de-tune a tower near directional array w/o using a skirt?

Post by Fran3 » Wed Mar 02, 2016 10:28 am

Thanks to all for the help.

The 3-tower DA existed first.

Wireless Internet Dish:
This unit has a power over ethernet cable (cat-5?) running from it to inside the building.
- So, are you suggesting that coiling 10 or 20 turns of the ethernet cable around a form at the bottom of the tower?
- Like maybe a 1" or 2" or 3" peice of PVC pipe?

Ground Radial Field:
Not sure if the "utility" tower has ground straps from the other side of the insulator it is sitting on... and not sure if those straps exist that they are connected to the DA ground radial system. We will check that and if best make the electrical connection.
FYI - The utility tower is may be a foot or two inside the DA's ground radial field... or a foot or two outside the DA's ground radial field... either way it is on the extreme edge of the DA's ground radials.

Isocoupler:
If no isocoupler is inline with the Marti antenna we can install one.

So, once we do the above we will just place our L/C circuit between the tower and ground and tune for minimum current... or max voltage... right?

Thanks again for the help !!!

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Deep Thought
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Re: can u de-tune a tower near directional array w/o using a skirt?

Post by Deep Thought » Wed Mar 02, 2016 5:52 pm

Isolating that cat5 is going to be a PITA. You'd be better off stuffing it inside some copper tubing and winding yourself a coil out of it. You can then use that across the base insulator with a capacitor in parallel to try to make the LC circuit at the base. It'll take 30-40 uH to get enough X to work. Commercial units to do this run in the high four figures, which you might want to consider.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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