Al Baker wrote:
> Please tell me more about your fine antenna.
The "capacity umbrella" was formed from 6 radial wires, each 30ft long
coming away from the top of the mast at 45 degrees to the vertical.
From the 30ft point onwards they became insulated guy wires. Six
perimeter wires joined up the ends of the 30ft radial wires, and if my
maths is correct they were each about 21ft long. I believe I was using
#16 wire. The mast was made out of odd lengths of telescoping aluminium,
starting at 2" diameter at the bottom and thinning to 1.5" at the top.
Looking back at my references, I believe it was a 1982 Ham Radio article
by Jack Belrose, not an IERE paper, which showed that extending the
umbrella more than 43% of the way down the mast began to cause Rrad to
rise again because of current cancellation. Although not as effective as
a horizontal hat would have been, the attraction of the sloping wires
was the ease of mechanical implementation.
If I recall correctly the hat brought the whole thing to resonance at
about 2.2MHz. I used a small tapped series L, switched with relays, at
the bottom to trim the resonance into band.
I calculated Rrad to be about 5 Ohms. First resistance measurement with
just a 4ft earth stake was 33 Ohms - efficiency 15%. I began adding
radials approx 50ft long buried in the lawn, measuring the resonant
feedpoint resistance each time. After putting in 16 radials I was
getting tired, the resistance was down to 7 Ohms - efficiency 71% - so
I stopped :) I reckoned I was in the area of "diminishing returns".
I matched the 7 Ohms with a 9:1 transformer - trifilar wound on a
ferrite core. The 2:1 VSWR bandwidth was only 90KHz which is why I used
the switched series L to move the resonant frequency around the band. I
used 3 inductor tap positions which gave me1.83MHz, 1.93MHz and 1.99MHz
which suited my 160m activity at the time.
First contact on the vertical was a CW QSO "across the pond" which I'd
not managed before.
Hope that answers the questions.
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