Monday, September 26, 2016

NVIS Antenna for State QSO Parties

During my week off of work, I built a NVIS antenna for the IL QSO party and other future QSO parties.  I used the DX Engineering NVIS antenna model for my build but I just couldn't bring myself to purchase all the parts that were overpriced, since I had most of them in my stock.

This antenna is 15' tall, occupies a foot print of 60x60ft in area and is only for the 40m and 80m bands when you spread the dipole in a X configuration.  Not sure where DX Engineering got the 45x45' footprint in their instructions, when each dipole leg is 45'.  This was a rather easy and fast project to compete.

I had some extra RG-213 laying around for the pigtail connector, so I unraveled back the braid and center conductor and soldered two 14 gauge ends with ring terminals to connect to the dipole ends.  The feed line connector end has a soldered PL259 end so you'll have to use a female to female cuppler to attach to your coax.  I used 4 clamp-on ferrites to keep the common mode current off the feed cable, which I clamped just north of the PL259.  I wrapped the RG-213 end where it splits into the dipole ends with waterproof tape to keep water out and also wrapped the small 14awg pigtail ends in electrical tape just to add some strength to it as they are rather weak.

The "T" connector was made out of IKEA cutting boards.  It cuts clean in a band saw, is strong and cheap.  Each board is $1.50, so messing one up isn't going to break the bank.  Drill holes where you have attachment points for your coax pigtails, ring terminals and dipole ends.  The hole the dipoles weave through on each end are smaller 5/8" towards the end and a 3/4" closer to the ring terminal connectors as they provide strain relief for the dipole ends. I used some zip ties just to make sure the dipole ends don't pull out with some tension.

For the mast system, I use a painters pole to elevate the center at 15 foot.  A 1" PVC pipe slips over the top of the painters pole well and I used hose clamps to attach the PVC pipe to the "T" connector.  I'm not going to use guy wires for the painters pole, as the 4 connection points for the dipole ends should provide enough support in the wind.

The directions say each dipole should be a total of 45 foot including guy rope.  Instead of cutting the correct length of guy rope just for this setup, I used the pythagorean theorem to figure out where the stakes should be located on the ground away from the base.  42.4 feet, or about 42 feet 5 inches.  I cut a ring out of the excess cutting board scrap and attached some paracord with a marker with red tape at 42 feet 5 inches.  I can then drive a stake in the ground at the painters pole location and mark all my stakes.  Raise the center mast to 15 feet and then use random length guy ropes to support the dipole ends.

Ring and red maker on cord for 42.5 inches.

You will need to use an antenna tuner with this setup along with at least 75ft of feed line.  To get the ultimate performance, use a band pass filter after your tuner to remove any stray signals.

We'll see how well it does during the next QSO party on the 16th of this month.  Stay tuned.

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