In the July 2014 issue of QST, there are plans to build a set of Grid Dip Oscillator or GDO coils that’s titled “Homebrew Dip Coils for MFJ-259/269”. Please read the instructions in that article before starting on this project. I’m not going into details on preparing components, only those components that are different than the article.
I’ve never owned a GDO but I know they can be handy to have while homebrewing. I modified the design from the article because PL-259s are used but my analyzer has a N connector.
I didn’t want to use any adapters on my analyzer to use these coils, so I needed to change the design slightly. Doing some research, I found a version of N connectors for LMR-400 cable. The diameter of LMR-400 cable is very close to a ⅜” wooden dowel. The N connectors are the only thing different from the original article. I chose birch for my dowel as it seemed to be stronger than just pine. Here are all the parts on the bench ready for assembly.
I started out building the high band coil first. The center pin to the N connector needs to be about ½” past the end of the dowel. Cut, scrape, and tin one end of enough wire for this coil, then solder the center pin on it. Put the compression part in upside down inside the body of the N connector so the flat side is facing up. Feed the wire through the center hole of the dowel. Place the dowel center pin down inside the body of the N connector and push it until it stops on the compression washer. After that push on the wire so the center pin is in the correct position. When you remove the dowel, the center pin should be about ½” below the dowel.
Once that’s all set, start making the coil at the top of the dowel. The high-band coil has 5 turns. Once the turns are complete, in order to get the wire under the compression nut, cut a small V into the dowel so the wire can fit. Tap the wire with a hammer if the wire is still sticking up too high to fit inside the compression nut. Extend the remaining wire straight down to the end of the dowel and cut it off at the base of the center pin. Scrape and tin the end, then bend it about 90 degrees then another bend to match the arc of the compression nut.
With the compression washer in the body of the N connector, slide the dowel into the body being careful that the center pin comes out into the correct position. Now tighten the compression nut so it’s snug. Don’t over tighten as you could break the ground wire off, then you'll have to start over.
The low-band coil is assembled in the same fashion, just use a dowel about ½” longer than the high band and a lot more wire. Here’s the business end of the low-band coil:
When assembled, verify their inductance using your antenna analyzer. Values are in the article. To test the coils, wind a coil, and put a cap across the ends of the coil. Using your new GDO coils, measure the resonant frequency by looking for a dip in SWR or resistance. I just wound a coil and grabbed a mica cap out of my junk box. Ended up being resonant for 20m, what luck!
Here are the completed set of coils before covering them with heat shrink and a protective tip:
To give that professional look, use ¾” heatshrink tubing over the coils, then place a furniture tip on the end. You’re new GDO coils will look something like this:
I’d like to thank John Portune, W6NBC for writing this article for QST.
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