12 March 2023

Backyard NVIS

Yesterday between yard care sessions (I'm refusing to surrender my Bermuda sod to the weeds), errands and other duties, I found the time to give an initial try to something I've wanted to test for a while now - using a lightweight off-center fed dipole (OCFD) in NVIS mode with either my Icom IC-705.

A few months back I bought a lightweight 10 - 80 meters OCFD from Tim Ortiz, N9SAB (check his eBay store here). This antenna was getting pretty good reviews, and for a time Chameleon was selling them (heck of an endorsement right there). 

Once I got it I began to think, would there be a way to deploy this in my yard, without having to sling lines in trees? Then I spied a small pile of fiberglass electric fence stakes I'd used on another project and thought, "... hmmm, why not give an NVIS seup a try?" So, off to Tractor Supply for more electric fence stakes. In ham radio one can never have too many electric fence stakes. When I laid out the antenna I realized that it was too long to fit in my back yard! I would have to lay about 15' of the antenna over the chain link fence at the back of my property. That wasn't going to work. I ended up dog-legging the last 20 feet or so of the antenna. I figured that would make for an interesting radiation pattern!

Aerial shot of my house showing the dog-leg layout. The blue dot is the feedpoint

Setup was quick and simple - just place the fence stakes at regular intervals and drape the antenna wire over them. The feedpoint was set close enough to my deck that I could reach it with 20' of RG-58.

One end of the OCFD. If you look close you can
see the rest of the stakes stretching out in the distance

The far end of the OCFD, at the end of the 'dog-leg'.
That's my Davis weather station in the background,
which you can monitor on APRS.fi
(just search for W8BYH)

Here's the feedpoint of the OCFD, with a small common mode choke

The feedline is lashed to a deck rail using a large
Nite-Ize gear tie - very handy

In the end I had the antenna set up only 3' off the ground, right up against a stone retaining wall and dog-legged at the far end. Would it tune? Would it get a signal out? Will Ross and Rachel ever get married? (Oops, sorry - the XYL's been binge-watching Friends and that stupid show's just stuck in my brain.)

I hooked up my IC-705 I did some SWR testing without the tuner. I was surprised and delighted to find that this antenna is well below 1.5:1 on 10, 20 & 40 meters, and just a bit less than 3:1 on 80 meters. In fact, on 40 & 20 the SWR was almost dead flat. 

Next it was a trial with Winlink on 40 meters using just the IC-705 factory battery (meaning only 5 watts or less output). The Winlink propagation prediction window said it was a lousy time for 40 meters, at best a 60% chance of hitting any station. But what's life without challenges, eh?

IC-705 running 'barefoot'. I'm guessing my ERP was probably
just 2 - 3 watts

OK, I'll admit it, connecting to a gateway on 40 meters was tough. but I did manage to get to the AJ4GU gateway (about 50 km) and W4MRB (200 km). Because of time constraints I did not get to hook up the tuner and try for an 80 meter gateway. I'll try that another day soon.

What are the take-aways from this? First, that NVIS does work, and works on low power digital modes. The second is that with a handful of fiberglass electric fence stakes I can set up an effective NVIS antenna just about anywhere. And third, this is just a whole lot of fun!

W8BYH out

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