AM Mast Height?

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TransmitterMan
Posts: 4
Joined: Mon Jul 28, 2014 12:49 am

AM Mast Height?

Post by TransmitterMan »

A question for the brain trust.

I have been ask to quote for a new 300ft AM mast and ground system for a station in the Pacific operating on 585Khz.

This is somewhat shorter than a quarterwave and I was wondering if there would be much of a decrease in efficiency?

If they agree for an increase in height are there any benefits with regards matching or current distribution in going for a slightly taller than a 1/4WL stick?

Thanks.

David
TPT
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Location: St. Marys, WV

Re: AM Mast Height?

Post by TPT »

From everything I've read a 1/4 wave tower (in this case 420 feet) seems to be a "sweet spot" in terms of matching, efficiency, and cost vs coverage considerations.

There may be other considerations limiting the tower to 300 feet (air traffic, land area, etc.) but the difference between 300 and 420 ft would not be that much in cost, so certainly worth pointing out to the station owner.
R. Fry
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Location: Illinois
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Re: AM Mast Height?

Post by R. Fry »

TransmitterMan wrote: Wed Dec 16, 2020 12:23 am... I have been ask to quote for a new 300ft AM mast and ground system for a station in the Pacific operating on 585Khz. ... I was wondering if there would be much of a decrease in efficiency? ...
Below is an estimate of what might be expected for a small-OD mast and the other conditions/assumptions shown there.

The impedance across the feedpoint terminals with no matching network is about 18 -j258 Ω.

Image
TransmitterMan
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Joined: Mon Jul 28, 2014 12:49 am

Re: AM Mast Height?

Post by TransmitterMan »

Hello Gents,

Thanks for your comments and Richard for the plot. Since posting the client has been asked to include in the quote DRM capabilities so that will probably require a Nautel (10kW) quote as I'm unawares of any other transmitter producer offering this capability.

Richard, I imagine the importance of flat and wide bandwidth antenna for a DRM system would benefit from increase the antenna height. Would you be good enough to use the 420ft figure and draft another plot as I would be interested to compare the two?

Many thanks.

David
R. Fry
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Joined: Mon Jun 23, 2008 7:41 am
Location: Illinois
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Re: AM Mast Height?

Post by R. Fry »

David -

Using a mast height a few percent shorter than 1/4 wavelength can produce a self-resonant radiator that will minimize the I²R loss of the needed matching network to 50 Ω, and flatten the SWR bandwidth. There are many combinations of tower face width and exact height that can produce that result. Other factors are involved in this, as well. Suggest you might work with an active broadcast consulting engineer to generate the details of the antenna system your client is asking about now.

Regards, Rich
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