Topband: low inv-vee

[HP]

I was going to stay out of this low inv vee antenna - probably should - but here goes. 

The short remark is I believe N6RK comments are true - its a top loaded vertical 
with a little inv vee thrown in . 

The long story is below - read at your own riisk. 

In about 9 years I am at 126 countries cfmed on 160 - have worked virtually everything I can hear 
and of course thats the other shoe - living in a noise pit in Phoenix , AZ. . Highlight was working 
FT5ZM one call on the third morning - I could hear him for about 3 minutes on the greyline . 
Most guys around here would hear him for 30 min or so . Thats the anecdotal BS . 

A few years ago I participated in a RBN test with 3 other guys - one was one element of the N5IA 8 circle 
before the array was up , 126 foot tower with 64 radials , another was 90 foot 25 G top loaded shunt fed 
and 130 radials full length , (that one was spotted in EU during the the test ) ,one was a full size vertical 
above a 40 gfoot high feed point with 32 elevated sloping 132 feet radials - I dont have full details on the 
other but it was a competitive single tower station. We all ran 1 kw input to the system , spaced by 1 khz 
and sent cq for several minutes . The outcome was my antenna was down about 9 dB from a good antenna. 
It was interesting and not totally unexpected however that close in at 100 to 500 miles or so I was down 
far less and quite variable . I was blown away with a VK4 6 dB RBN return last August when I was experimenting . 

N6RK I think has it nailed - at least certainly in my case. My antenna is an 80 meter inv vee with apex at 39 
feet - runs NE/SW from SW corner of the house roof to a shed at NE corner of lot - just makes it . 
Ends are about 9 feet high . It hangs from the side of 40 feet of 25G with ten feet of mast out top with a 
KT34 at 42 feet and a 40M1L rotatable dipole at 50 feet I spaced the feedline to the inv vee out 
about a foot from the tower .. The KT34 and 40M21L feedline and rotor lines are taped to legs inside the 
tower. I have a reasonably fair EZNEC model that duplicates what I measure vs freq with a VNA2180 
very close on 80 and by selecting a loss resistance for the half inch conductior simulating the coax shield 
attached to the NE side of inv vee on 160 and the TL model for 42 feet of RG213 which comes down 
to a remote antenna swutch about 5 feet up the tower . I have pretty good chokes on the rotor / antenna 
swutch and common coax to shack but honestly once the coax is tied to the tower - those chokes make 
very little difference. As Rich says however there is a HUGE difference in what I measure on 160 
if I put a choke with 5 2.4 inch #31 core 6 passes before the inv vee feedline attaches to the switch . 
And RBN testing although hard to be conclusive since it takes too long to make the change looks like the 
antnenna is down about 3 to 4 dB because the curent in the vertical (coax shield) goes way down . 

OK now what does EZNEC say is going on in the wires ? 
At 1500 watts into the system , the current going into the SW side (which is connected to the inner 
condutcor of the feedline is 3.83 amps at -15.5 degrees ) , the current into the SW side connected to the shield is 
1.87 amps at -30.2 deg . The current in the tower is about 360 ma max and actually has a dip down to about 
30 ma near the inv vee feedpoint . The current inwire 4 which represents the shield fo the coax tied to 
the NE side of the inv vee is 6.44 amps at -22.3 degrees and at its base its 5.87 amps at -20.3 degrees 
Ihave yto put 20 ohms loss resistance at thebotton m of that wire to get the measured resistance to agrees withmeasured - 
the reactive part agrees very closely - j 37.23 measured versus - j 36.73 ohms - the 20 ohms loss resistance gets the real 
parts to 5.82 measured vs 5.84 calculated. That does not give me any heartburn as the tower which is the real "ground" 
only has a couple 8 foot ground rods driven in near the base and connected at base. 

Some day I think I will hang a folded "counterpoise" or two at the switch and see if I can tell if I drive down the loss - 
I also have the capability fo attaching "extenders " on each side to get a resonnce at about 1870 - its easier on the tuner 
but not enough different to tell if its better or worse than just hanging 3000 pfd at shack wall onthe 80m inv vee and 
doing a touch up tuning in the tuner Just works out handy 

BTW I have worked 160 WAS in a weekend in contests and just started playing with FT8 seriously and have worked 41 states 
in an about a month - had 3 more from last year for 44 so now as you would expect - need to finish up the 
far NE ME/DE/NH/RI and NE and SD out here . 

73 Hank K7HP