135' OCF dipole - Crazy SWR (Help!)

[Elf36]

So I got home today and went to check 10M, I hit the tuner button on my Yaesu FT-950 and it immediately folded back indicating a very high SWR or internal tuner issue. I factory reset the rig a few times and nothing changes? I got out my NanoVNA and did a sweep over various bands and areas. All showed extremely high SWR reading, except when I did a sweep across a wide range and it has a sweet spot around 88 Mhz? The coax is a newer run of RG213 (50 feet) and no storms or heavy rain in my area lately. The balun/Unun has a small drain hole. I just used it yesterday on 20 & 40M with no issue and have never run more than 100 watts into it?

[ChrisSmolinski]

Something broke     Too many possibilities to speculate.  You need to inspect the entire installation starting at the coax in the shack, out to the antenna itself and the feed point connection.

You can disconnect the coax at the antenna, terminate with 50 ohms, and sweep with the VNA to rule out the coax.

[Elf36]

Thanks for you response. Yes, it is very unusual for sure. I've seen an antenna become un-tuned for various reasons, but never anything like this.

[Elf36]

So far I have only had time to disconnect the coax at the antenna and verify that there in no continuety (Direct short) in the coax. Gonna try to get the antenna down tomorrow. I'm also going to put up an end fed 9:1 with a 29 or 41' piesce of wire and see what happens.

[RobRich]

I suspect you might find one side of the dipole is borked. Either a break in the wire or at the feedpoint.

[Elf36]

Good evening Rob, hopefully, I will find out more tomorrow. It's just unusual for this to happen out of the blue. As stated earlier, the coax appears to not have a short at all. I don't suspect water intrusion, based on the fact that I haven't seen much rain and didn't hear any water sloshing around at all. Hopefully I'll get it figured out by the weekend. I'll definitely have the antenna down by Saturday.

[Elf36]

So I got home today and hooked up an end fed 9:1 balun with 29' of wire. I read this was a small length number that's good for 10-40M. The main sweet spots I found at this length was 10/80 meters. Maybe close on some other bands. At least it's not my radio. I will try and make up another dipole etc over the weekend and see what I come up with. I will also try to open up the guts of the 135' OCF dipole and see what I find out.

[Brian]

Some baluns appear as a short at DC. If yours is of that design, put a meter across the PL259, assuming that's what you use, and see if it's open or short. That will narrow it down.

[Elf36]

I just spoke to the antenna manufacturer and explained the issue and the testing procedures I went through. He was a bit perplexed also, but he offered to send me a brand new balun with pigtails on it, only charging for shipping, which is $8.00 . Cant ask for much more than that. I just cut 2 legs for a temporary 40M dipole on my lunch break. That will at least keep me on 20/40 meters until I can install the new balun.

[RobRich]

About the 9:1 setup, did you deploy a counterpoise? 17' is a popular counterpoise length. Otherwise Kirchhoff's circuit laws say the other part of your antenna is common mode via your feedline. Alternatively, if not wanting to deal with a counterpoise wire, you could try a mix 31 or 43 toroid on the feedline at approximately 17' from the antenna feedpoint.

I suspect you deployed a horizontal end-fed at low to moderate height, correct? If so instead consider making it an vertical, inverted-L, or even a sloper to get yourself some vertical polarization since you already have a decent horizontally-polarized OCFD (when fixed).

Good luck!


Speaking of verticals, I hope to finalize deployment of a second 18' vertical for 11m DX. It will be replacing my 11m rotatable dipole.

My existing 18' vertical is ground mounted with a small radial field. It tends to work great for 11m DX when the band is open, but admittedly, it is also subject to near-field ground clutter due to its low mounting height.

I am planning to deploy the second 18' end-fed atop a ~31.5' mast, though it might end up somewhat lower depending upon the basic guying being used. I doubt the actual takeoff angle will improve much, if at all, but at least it will be above some of the local ground clutter. It also should benefit line-of-sight local comms, assuming such a need occurs.

[Elf36]

I ended up throwing up a quick antenna with a 4:1 balun, 83.2' on one side and 46.8' on the other. It works ok. It's a bit high on 10M though. It will get me through until next week or so. Just worked a California station on 20M PSK31 with it @ 0:00

[RobRich]

Whatever works. In the past I have even dropped a feedline to a magnet mount vertical stuck to vehicle in the driveway for a temporary fix.

The second 18' vertical is initially deployed here, but it likely will be back down for awhile today or tomorrow to tuning. The mast ended up being ~21' due to stability. I am using two 10.5' sections of metal fence top rail with four lightweight guys lines, with the top ~103.5" guy line sections being copper wire radials to hopefully help limit common mode.

[Elf36]

Good deal. I have a perfect fiberglass pole that I want to make a vertical with. I think I'll go with 15M because my tuner will not tune that area due to high swr. I also have a coax run that I'm gonna use for a separate 30M dipole due to the same tuning issue.