Author Topic: ATSF "Top Hat" Antenna  (Read 295 times)

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C855B

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ATSF "Top Hat" Antenna
« on: December 19, 2022, 10:47:07 PM »
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A minor detail peculiar to Santa Fe locos and cabooses from the late 50's to 1972 is this unique antenna, known as a "top hat". Oddly, it's not because of the shape of the radome, but the guts under it, which are known in the technical radio field as a "top hat"-style antenna.



I rendered it in 3D to detail the handful of 1960's Santa Fe power and cabeese lounging about on my layout. The STL artwork can be downloaded here: http://www.everywherewest.com/Top_Hat_Antenna_N_Scale.stl  The stem below the plate is for drilled-hole mounting; some applications (check proto pictures!) you'll want to nip it off for flush mounting.

These antennas disappeared with the "yellow bonnet" scheme adopted in 1972, replaced as part of the repaint program with 2nd-generation Sinclair "skates". I worked briefly in the Santa Fe radio shop in Los Angeles, so have firsthand familiarity with the design and the unfortunate problem that if the radome was cracked or punctured, the aluminum guts would corrode quickly. I forget who the manufacturer was.

I have no idea if anybody besides Santa Fe used these.

kiwi_al

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Re: ATSF "Top Hat" Antenna
« Reply #1 on: December 20, 2022, 03:35:16 AM »
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I notice the top hat is sitting on a cab, would that be a Topeka cab by any chance?

C855B

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Re: ATSF "Top Hat" Antenna
« Reply #2 on: December 20, 2022, 07:51:26 AM »
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No. That's an Alco S-4.

EDIT: Interestingly, in looking around for CF7s and "Topeka Cab" Geeps that might have had the older antenna, I found this:



It broke my "rule" that yellow bonnet paint meant the newer antenna. All I can surmise is the radio shop in Topeka had a stash of new top-hats sitting around, or the antenna was recovered from the donor Geep. Santa Fe's radio shops at the time were sort of a loosey-goosey operation.
« Last Edit: December 20, 2022, 08:31:52 AM by C855B »

Ed Kapuscinski

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Re: ATSF "Top Hat" Antenna
« Reply #3 on: December 20, 2022, 01:06:04 PM »
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Tangent... but why did they mount them on a plate like that?

It seems like an awful lot of work for something that could've just been bolted to the roof.

samusi01

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Re: ATSF "Top Hat" Antenna
« Reply #4 on: December 20, 2022, 02:25:33 PM »
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Not knowing much about radio, could it be for a ground plane? Would seem redundant in the case of CF7 2068 given the flat roof.

C855B

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Re: ATSF "Top Hat" Antenna
« Reply #5 on: December 20, 2022, 02:36:41 PM »
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I don't know exactly why they didn't mount them on the cab other than the difficulty of somehow making a tight seal between the 7" baseplate and the curved roof. Putting it on a plate behind the cab like that meant that preventing the rain from seeping underneath was a lot less important. IIRC they did something similar with the beacon on the front of the cab, with conduit outside the cab and the wires penetrating the hood.

Not knowing much about radio, could it be for a ground plane? Would seem redundant in the case of CF7 2068 given the flat roof.

Ground plane, yes. With the antenna at the edge of the cab like that it would make for a lopsided radiation pattern without the extra sheet metal. Similar goes for the CF7 - it's on a platform to raise the ground plane above the A/C, which would otherwise mess with the pattern.