StormgrenNov 9, 2024
Last night, an AA-170 scan of the re-hung doublet dipole is showing some super funky values, including a very flat 10:1 SWR across the top of the band, and dips at 8, 17, and 28 MHz. Given this thing is cut for 80m, something is very wrong. No DC shorts, but Z is 5 ohms or less right now across most of it and that is just downright odd.
Show threadStormgren

Hmmm, so.... I cleaned up the connections at the center of the dipole and added a star washer between the terminals so they won't get too resistive, but DC testing parts of the balun is showing a high-resistance from the outside of the SO-239 (So, the "ground") to the equivalent output terminal. Tearing open the balun, interior resistance was perfect, as was interior connections to the outside screw post. Seems that the bolt connecting the SO-239 to the interior has developed a high resistance fault to the shell of the 239.

That terminal setup (which I'll post pics of in a bit) is...not my finest work... It'll get replaced with a better ring terminal and lots of star washers to keep connectivity going.

Nov 9, 2024 at 8:15pmWeb
Show threadStormgrenNov 10, 2024

Took a crack at figuring out WTF was happening with this balun from yesterday causing me so many problems. Root cause appears to be butyl rubber.

No, seriously.

When I built the balun, I used some butyl rubber tape to seal up the SO-239/box interface. That migrated, due to heat, pressure, or who knows what, down the screw that penetrated through that the ground connection was on, and between the washer and the SO-239 body. This created one heck of a resistive fault at DC, and gods knows what sort of goofy ass impedance/capacitance/what-have-you at AC.

Swapping the screw and washer out for some star washers and a better ring terminal on the coax shield inside the balun box cleaned it right up.

Analyzer screenshot shows the final result in green (it's stretched, so needs some trimming obviously) and red is one of the wonky runs yesterday.

#HamRadio #AmateurRadio #AntennaDIY

Show threadGiovanni - ZL2GXNov 11, 2024

@Stormgren Interesting. The tape getting between the washer is not something I'd expect. I keep thinking I should put a 47K resistor at the antenna end of the feedline for quick continuity tests. This would enable easy short, open circuits as well as connected feedlines checks.

#antennas #amateurradio #hamradio

Show threadStormgrenNov 11, 2024

@gmoretti See my other comment regarding the resistor, but as for the tape, it's tape in form-factor only, really. Butyl rubber tape really is a form of caulking, and as it's really mostly uncured butyl rubber, is subject to flow under heat and pressure, which it was subjected to both hanging on a tree and getting a lot of summer sun on it, so I imagine the balun box got quite warm.

Never thought it'd be quite this flowy, but lesson learned, I think.

Next time, I'll use silicone tape or RTV sealant between the 239 and connector interface, which if that stuff flows, the box probably melted first. 🤣

Show threadJima Nov 12, 2024
@Stormgren I would NOT have seen that one coming. 😳