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!
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