Really Basic Questions

AM Radio discussion. Directional arrays are FUN!
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dummy35
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Really Basic Questions

Post by dummy35 » Wed Sep 14, 2011 6:14 pm

My apologies for the super basic question coming from a less-than-technical person. I have searched and searched and can't find an answer to my question, so I come to you.

True or False: When a tower is broadcasting an AM radio station, the entire tower is acting as an antenna and radiating.

True or False: When a tower that is broadcasting an AM radio station and is outfitted with a folded unipole, the tower is not radiating anything. Only the vertical lines placed parallel to the tower are doing the radiating.

I've probably oversimplified, but basically I just want to know if the tower itself is radiating anything when there's a folded unipole in use.

Thank you to anyone who takes a minute to answer. Much appreciated!

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davedybas
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Re: Really Basic Questions

Post by davedybas » Wed Sep 14, 2011 6:59 pm

The quick answer is: Only the skirt wires do the radiating in a folded unipole. Think of a section of Coax cable with the center conductor grounded and the shield hot with the RF.

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dummy35
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Re: Really Basic Questions

Post by dummy35 » Fri Sep 16, 2011 12:54 pm

Thanks! So, one of the benefits of the folded unipole would be the opportunity to install other equipment on the tower without affecting the AM radio signal (which would be an issue on a more traditional tower)?

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Deep Thought
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Re: Really Basic Questions

Post by Deep Thought » Fri Sep 16, 2011 3:25 pm

Clarifying Dave's answer...the tower is not radiating if the skirt extends the entire length of the tower. If the skirt is bonded to the tower at some lesser height then it is a simple shunt-fed tower and the tower is definitely radiating. Either approach gets rid of the base insulator and simplifies simultaneous use of the tower with other services.

You can mount other antennas on a series-fed tower too. The transmission lines have to be isolated from ground either by using isocouplers, iso-coils (a coil of the antenna's transmission line resonant at the AM station frequency), or 1/4 wavelength stubs where the line is insulated from the tower for the first quarter wave from the ground at the AM frequency.
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boiseengineer(old)

Re: Really Basic Questions

Post by boiseengineer(old) » Mon Feb 27, 2012 11:35 am

BESTA 11/29/2011

Exhibit 34
Description: BASIS FOR REQUESTED STA

THE INSTANT REQUEST FOR EXTENSION OF STA ARISES FROM KGOW HAVING SUFFERED MULTIPLE AND CONTINUED FAILURES OF THE SKIRTED FOLDED UNIPOLE ANTENNA SYSTEM. DUE TO THOSE FAILURES, KGOW IS CONVERTING ITS ANTENNA SYSTEM TO A SERIES FED, BASE-INSULATED ANTENNA SYSTEM INSTEAD OF THE FOLDED UNIPOLE SYSTEM AS CURRENTLY LICENSED.

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Re: Really Basic Questions

Post by NECRAT » Mon Feb 27, 2012 8:08 pm

Here is a tower setup.

Image

AM is on 1340 (FM on 98.1) in New Bedford, MA. As I understand it, the series fed tower is too electrically tall for 1340, and the skirt is used to "shorten" the electrical height of the tower. I could be wrong but I recall that being the reason for this skirt.

(The tower is still series fed with an ATU and the FM and two way tennants have iso-couplers on them.)

(There are also wind speed measuring tools on the tower to have a turbine installed nearby)
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Deep Thought
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Re: Really Basic Questions

Post by Deep Thought » Mon Feb 27, 2012 9:10 pm

That's a pretty standard type of detuning for a too-tall tower. Same theory as detuning from the ground up...it electrically disconnects the part above the bottom of the skirt at the tuned frequency.
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Re: Really Basic Questions

Post by ChuckG » Tue Feb 28, 2012 1:00 am

boiseengineer wrote:BESTA 11/29/2011

Exhibit 34
Description: BASIS FOR REQUESTED STA

THE INSTANT REQUEST FOR EXTENSION OF STA ARISES FROM KGOW HAVING SUFFERED MULTIPLE AND CONTINUED FAILURES OF THE SKIRTED FOLDED UNIPOLE ANTENNA SYSTEM. DUE TO THOSE FAILURES, KGOW IS CONVERTING ITS ANTENNA SYSTEM TO A SERIES FED, BASE-INSULATED ANTENNA SYSTEM INSTEAD OF THE FOLDED UNIPOLE SYSTEM AS CURRENTLY LICENSED.


Finally getting rid of that PITA I see. Good riddance.

OP: I'd much prefer isocouplers to a skirted tower if aux antennas need to be accommodated and your climate includes "winter". Either that or get an old tube xmtr that'll load into a barbed wire fence for an AUX. You'll need it.
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Re: Really Basic Questions

Post by K4WRF » Tue Feb 28, 2012 9:48 pm

In the case of detuning the top of an AM tower like the one shown here in this thread, typically what type of showing does the Commission require to prove that it is effective? Is this considered a sectionalized antenna? WOW! 243.8 degrees! Is that some kind of series fed tower electrical length record?
So many questions...
Many thanks.

w/

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Re: Really Basic Questions

Post by Deep Thought » Tue Feb 28, 2012 11:34 pm

K4WRF wrote:In the case of detuning the top of an AM tower like the one shown here in this thread, typically what type of showing does the Commission require to prove that it is effective? Is this considered a sectionalized antenna? WOW! 243.8 degrees! Is that some kind of series fed tower electrical length record?
So many questions...
Many thanks.

w/
The last one of these I was involved with (sometime in the 1980's...memory's getting kinda foggy) required current distribution measurements on the tower like you'd need for a top loaded tower to prove it is working. We sent a climber up with a loop built on a wood frame with a hook that grabbed the horizontal members, and a detector driving a milliamp meter and had him take relative current readings at each section joint (easier than measuring every 20 feet or so) with about 250 watts into the tower. Worked fine but took forever.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

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