[MRCA] NVIS on 40M

[ersmar at comcast.net]

Gents:

     Yesterday and today I spent a few hours constructing the inverted-V 
NVIS antenna I had posted about last year.  You know - the one that is based 
on the AS-2259?  I'm getting ready for a Ham radio demo next weekend as part 
of a disaster preparedness public event in a nearby town.  The antenna works 
(I can count QSOs with upstate NY and Ohio this afternoon) but with some 
caveats.

     I used five surplus fiberglass stacking poles that I bought on that www 
auction place last year as the center support.  That brings the top of the 
mast to 44 inches x 5 sections = 220 inches or a little over 18 feet above 
ground.  I thought about going higher, but that would require two levels of 
guying and getting the pole erect is a bit of a gamble, even with the five 
poles and the small amount of wire at the top. Atop this mast I have a 2 
inch PVC pipe cap that I drilled to hold four brass bolts sticking up out of 
the cap, spaced 90 degrees apart around the top.  I jumpered adjacent bolts 
together with lugs and wires and connected a five-foot long piece of RG-8X, 
with PL-259 on the end, to the pairs of lugs, shield to one pair and center 
wire to the other.  This way I can connect two dipoles or V's to opposite 
lugs on the cap and they will be connected properly to the coax.  I also 
drilled a hole and bolted a hook eye in the center of this cap (for possible 
tree suspension of the cap and wires, without the poles.)

     I assembled the poles into one length on the ground and ran a length of
RG-8X through the poles.  I connected this main coax to the coax pigtail
inside the cap with a barrel connector.  At the bottom of the poles I fitted
a PVC closet flange (Oatey Caspers 43507 at Home Despot) and PVC reducing
adapter to serve as a base.  (The closet flange is used as a transition
piece between a new toilet and the waste line underneath it.)  DON'T knock
out the center of the flange that's marked KNOCKOUT.  I learned the hard
way.  Simply drill out a 3/4 hole for the coax and connector to pass
through. Otherwise the flange provides no bottom support for the pole as the
pole passes right through to the ground.

     I started with a seventy-foot long dipole wire for 40M.  I split it in
the center and connected the centers to two opposite bolts on the PVC cap,
using solder lugs on the wire ends and wing nuts on the bolts.  I tied the
dipole ends to lengths of nylon rope (no insulators yet) for guying to 
ground stakes.  I tied
the center of a lenth of dacron cord to the eye on the cap for guying at 
right angles to the wires.

      After I raised the pole and its attached wires and ropes, I tightened 
up all guy lines at the army surplus stakes I had driven into my front yard. 
(Yes, all the neighbors could see me wrestling with this pole!  But they 
should be used to this kind of stuff - they watched me hammer staples for my 
160M inverted L's radials into the grass around my yard a few years back.) 
All this set-up took me about twenty minutes including adjusting the guy 
rope tension.

     I checked SWR - my MFJ259 showed a 1.3:1 minimum at 6.7 MHz or so. 
This, with a slightly (I thought) long dipole.  I'm guessing that the 
antenna's proximity to the ground lowered the resonant frequency even more 
than would be expected from the long wires.  Or the lack of insulators at 
the ends of the wires might cause this lowering of resonant frequency.  This 
will require more investigation.  (I plan to use plastic chain as 
insulators - it rolls up really nicely with the rope and wire.)

     I loaded up the antenna with my Kenwood TS-570's autotuner on 7.250 and 
broke into a QSO with a couple of stations in Ohio.  They copied me above 
the SWBC stations on the air in late afternoon.  I next called a station in 
upstate NY who said I was at par with the SWBC stations.  The antenna took 
full power (appr 85 W as I was operating off a gell cell on the front porch, 
and its terminal voltage was around 12.5 VDC.)

     I'm not sure I have enough room to install a V for 80M or even for 60M. 
I'll have to measure this out a bit more.  If possible, then my plan would 
be to install a combination 80/60M V in the plane that the dacron ropes are 
in now.  I'd split the wires into two segements - 60M with extensions for 
80M - and use a jumper to lengthen the 5.3 MHz wires for operation on 3.9 
MHz.

     As it is, my NVIS antenna occupies a diamond-shaped space on the ground 
measuring 28 feet across between the rope guy anchors and about 56 feet or 
so between the wire's guy anchors.  At this spacing, the wire ends are about 
four feet off the ground (I told you they were low.)  I expect the 80/60 
combo wires will be almost twice that length and even closer to earth.

     My next step (after the public demo next weekend) will be to tweak the 
40M wire lengths for proper resonance at around 7.25 MHz.  I might even try 
setting up two sets of guyed fiberglass poles to test out a dipole 
configuration (wires parallel to the ground) at about 18 feet AGL.  Either 
way I expect that, as long as the wires are close to the ground (NVIS, 
remember?), the wire lengths will have to be shorter than would be 
calculated from traditional formulas.

     I'll keep you guys apprised of progress here.  Hopefully I can bring 
this, and the 80/60M wires, to Gilbert in the summer.

73 de
Gene Smar  AD3F
P.S. Anybody see any photos of the 2005 Meet?