80m/40m Loaded Coil NVIS Antenna

MaverickNH

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There is a 2m Winlink gateway just 5mi away in SoNH that I can regularly hit with portable or base radios but I cannot reliably hit 40m/80m Winlink HF gateways as my main antenna is a 43ft vertical. Even ground wave to HF Winlink stations is very rare, as propagation within a few 100mi is poor on 40m/80m and most 40m/80m antennas are horizontal. I’ve read polarity losses are -20dB or more. That said, polarity randomizes at some point. My vertical just doesn’t send much up at an angle that will reflect back down within a few hundred miles.

So I built a Near Vertical Incident Skywave NVIS antenna with loaded coils to shorten the inverted V dipole legs to fit in my yard. It’s on a 15ft telescopic fiberglass mast. I hammered a galvanized pipe down a few feet with a threaded junction just below ground level, so I can screw a 2ft pipe on to hold the mast up. It’s under a rock in a garden bed, so my wife doesn’t freak out. I just put it up and take it down as needed.

Tuning is rather meticulous as the resonance is pretty steep - about 10kHz per inch of dipole leg. But I got it down to ~1:1 SWR at the Winlink frequencies. The whole rig is vehicle portable - about 6ft long collapsed with dipoles spooled on winders.

Now I’m thinking a used Icom IC-7000 All-Band transceiver to fit in an ammo-can Go-Box with a battery, Win10 tablet, SignaLink and ATU in another small Pelican box.
Why you ask? Why not!

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80m is tough since using wires limits your bandwidth. If you use cage spreaders, this could increase bandwidth.

I whipped this up with the raw dimensions and didn't try to tune at all (Cuz it is fun.)

This is without any loading coil, but it is what to expect at 7.100, surely NVIS. 50 ohm direct feed. Might want to add in a 1:1 balun at the feedpoint.

Good luck! Looks like a real easy portable/deploy-able unit.

uj

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Nice! Thanks for that.

Yep, I have a 1:1 current Balun at the top feedpoint.

Pacific Antenna has a nice little 20/40 kithttps://www.qrpkits.com/dualbanddipole.html I sent for recently. They also plan to release a 40/80 kit I’ll be wanting to try as well. Both rated up to 100W. They should go well atop a SOTAbeams mast for lightweight portability.
 
I built ((an)) antenna ... It’s on a 15ft telescopic fiberglass mast. I hammered a galvanized pipe down a few feet with a threaded junction just below ground level, so I can screw a 2ft pipe on to hold the mast up. It’s under a rock in a garden bed, so my wife doesn’t freak out. I just put it up and take it down as needed.
There's a long history of hiding antennas from neighbors, etc.
 
I was talking to a guy at work who is trying his damndest to get me to get my ass on the air. We started talking about NVIS, which I think is neat as hell. @MaverickNH , did you ever get that kit up and running? Those seem like the ideal level of DIY for me: mostly figured out, but I still have to do some work.
 
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