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I'm trying to mount a ham radio antenna that has a standard(?) 3/8-24 threaded post at the bottom to an existing 5/16" post (built in the past for a different antenna) with 5/16x18 threads.

In other words, I need an adapter that's metal & conductive with a female 5/16-18 cavity/connector/receiver (not sure about nomenclature) at one end, and a female OR male 3/8-24 threaded cavity/connector/receiver or rod at the other (I have a female-female 3/8-24 connector, so the gender of the adapted 3/8-24 end doesn't matter).

The big question: what is this called, and is this something likely to exist as a retail product a local store somewhere in South Florida is likely to sell, so I could get my hands on it quickly & not end up having to spend $5-15 shipping on what's probably a cheap part? If so, what kind of store would I go to? Auto parts? Plumbing supply?

From what I understand, 1/8" FIP/MIP pipe fittings might have 3/8-24 threading, but pipe-threading isn't quite like bolt-threading, because it's designed to seal liquids as well as fasten them together, so screwing an antenna into a "1/8" pipe fitting could/would chew up the threads on the antenna, or fail to secure enough of it to prevent it from tearing out.

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  • Probably want a thread adapter(maybe thread coupling/reducer) from hardware store, maybe auto parts. Would not use pipe fittings(most common), but good hardware store should have or can get what you need. Make sure you explain it is for threaded rod, not pipe, if cannot bring to store.
    – crip659
    Feb 19 at 19:22
  • Is the post structural? If not, just solder some wire between for electrical connectivity.
    – FreeMan
    Feb 19 at 19:38
  • @crip659, define "good hardware store". Around here, there's basically Home Depot and Lowes. Most of the Ace Hardware stores closed a few years ago, and Orchard closed last year. That's my quandry... trying to figure out where ELSE I could go when Home Depot and Lowes ultimately lack it. I mean, South Florida has 6+ million people in an area almost as urban as Brooklyn... somewhere, somehow, there HAS to be a store in the area that sells stuff like this, but I'm trying to figure out which one.
    – Bitbang3r
    Feb 19 at 19:53
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    If any independent hardware stores around still would try them, if they don't/can't get it, bet they know where you can. It is not like the part is that special, just not always in stock. If difficult to get, can get a plain coupling in each size and have local garage weld them together.
    – crip659
    Feb 19 at 20:33
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    Here is the link. mcmaster.com/thread-adapters
    – crip659
    Feb 19 at 21:27
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Some DIY ideas...

1

Get yourself a chunk of mild steel or aluminum, some drill bits, and a couple taps. Build a two-sized coupling nut. You probably have a neighbor who has a tap set. You may want to use jam nuts to lock the rods into the sleeve.

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2

Use a piece of C-channel or box tubing. Drill opposing holes for the two rod sizes and attach them with nuts inside and out.

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3

Drill offset holes in a chunk of steel or aluminum bar stock for each rod size. Nut them each from the top and bottom faces.

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            |=|
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4

Use a 3/4" black iron plumbing nipple and caps. Drill the center of a cap for each of the two rod sizes, nut the rods inside and out, and snugly connect the caps to the nipple.

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  | | _____ | |
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If you search for antique lighting parts supplies, you'll find "hickeys" ready-made that are essentially what @isherwood describes as #1 and #2. The first picture are a couple of hickeys from my box of lighting junk. Neither of these are in the sizes you want but they come in different sizes. There are also reducing bushings such as in the second picture, which I got from https://www.antiquelampsupply.com/ ... poke around there or if you can find a local antique lighting dealer they might have what you need. 3/8 and 5/16 are definitely among the available adapters .. I'm not sure if the thread counts you want will be found in lamp supply but it's worth a look on line.

I should add that you probably should add locknuts for your application especially if the antenna is outside. You won't find actual locknuts in antique lighting supply but you can double up regular nuts on the threaded pipe and/or use thread cement and/or buy locknuts from a regular hardware store.

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  • "Locknut" usually refers to a nylon-insert nut or other type of nut with an integral anti-loosening device. Did you man a jam nut, which would lock the position of the connected pieces?
    – isherwood
    Feb 22 at 19:41
  • I found that "threaded iron hickey" resulted in more hardware and less teenager pastime.
    – isherwood
    Feb 22 at 19:47
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    Come to think of it, an actual lock nut would be difficult to tighten on this arrangement, so whatever I meant, a jam nut is the right approach. Although OP may be able to tighten the antenna mast into the hickey enough that it self-jams against the post and is secure enough, and in that case should avoid reducing bushings.
    – jay613
    Feb 22 at 20:37
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    @isherwood good thing there are no hickeys with set screws to provide additional fodder for your innuendo, as much as it would help OP.
    – jay613
    Feb 22 at 20:40

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