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Thanks to all for your replies. This has been a great learning process.
I will now pass along my experiences, as quite a few of my replies
revealed interest in duplicating this antenna.
The idea for my design came from an article by K5IU and KE7BT in the
Summer 1998 issue of Communications Quarterly. A lot of the theory of
this article is repeated in ON4UN's new book, "Low Band DX-ing", that
just came out.
My tower is 192 feet high, made out of 19 sections of Rohn 25 and a
short mast with a 5 inch corona ball on top. Insulators of my own design
were placed at the 60 foot level. All guy wires are insulated from the
tower and broken up with insulators every 50 feet, except the set at the
60 foot level, which are 154 feet long, which is 120 electrical degrees
on 160 meters. (More on this later)
132.5 feet of Rohn 25 is resonant on 1520 khz as installed (base is 60'
AGL) over East Texas soil. By making the vertical radiator a bit too long
raises the feed point impedance slightly, and allows use of a series
capacitor at the base to resonate it to the desired frequency by tuning
out the inductive reactance of the radiator.
I ended up with a series C of 800 pf to resonate the system on 1830 khz.
The series C is placed between the coax center conductor and the base of the
vertical radiator. The vswr was 1.24:1 at resonance with a measured impedance
at the base of 43 ohms. This is due to the increased length of the vertical
radiator and the 20 degree slope of the radials. My next step is to install
another series capacitor between the coax braid and the three radials and
tune for minimum vswr. The 1.5:1 bandwith of this installation is 145 khz.
Since the radials are floating and elevated, a current balun is necessary
at the feed point. (Ferrite beads over the coax) Also 170 uh RF chokes were
installed from the base to ground and from the radials to ground to keep the
system drained from static build-up. Spark-gap type arrestors are installed
across the three base insulators.
If you utilize radials of 45 or 120 electrical degrees, the current will
be equal on all the radials and will not change as you QSY around the band.
This is not true if one uses quarter wave radials Many have suggested that
three elevated radials are not enough, but there is less than 0.3 db
difference between 3 and 32 elevated radials.
I am concentrating my efforts into a ground screen under the three radials
to eliminate all the ground losses I can.
Again, thanks to all who helped, especially L.B. Cebik, who solved my
problem in one short note!
(((73)))
Phil, K5PC
I hope this is helpful to those of you who have expressed interest in this
project. Experience is a very good teacher!
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