Gentlemen and ladies,
I (finally) plan to upgrade my topband transmit antenna from a very poor
(read low) inverted V, to a vertical. Actually, I plan to do this at at
least two of the three stations where I work on antennas. I'd love your
advice and feedback on my plan.
Here is what I know.
1) I will build a 74' vertical out of ~46' of Rohn pushup mast topped
with ~28' of aluminum stinger.
2) Add three our four top hat wires at the 60' level to both guy the
stinger and resonate the antenna.
This should get me an antenna with a feed impedance of around 11 ohms, and
about 90% efficiency, if I read The Book right.
I have one multi-part question - ground mounted radials vs. elevated
radials? My soil, according to the FCC's map, is "excellent", conductivity
of 30 millisiemens/meter.
Ground mounted would likely mean a 15' square mesh of 6" grid galvanized
fence directly under the base of the antenna, with 16 1/7th wavelength
radials (75' or so). From the Antenna Book, this looks like the right
length for that number. It is possible that I might be able to add a few
longer radials, but I wouldn't count on it.
This would have the advantage of using our excellent soil, and of being
fairly "predictable" - that is to say I think I know what performance I
would expect from this configuration.
The major disadvantages seem to be that I have to actually tune the antenna
- that is to say I have to get the length right, which will likely require
raising and lowering it at least once. That is likely to be painful.
One thought I had, stolen from inverted L lore, is to make the antenna a bit
long on purpose, then tune it with a variable capacitor at the base (later
replaced by a door knob once the value is known).
The other option is two elevated radials. I can probably get them up 15',
using the Force 12 gull-wing style (the base of the antenna will be close to
the ground, but insulated from it). My biggest worries here are the
performance (I would be more comfortable with 4, but don't have room for 4
full size radials), and having high voltage on the radial tips near my trees
(the area is quite forested).
What do you guys think I should do? Anyone with real experience with a
small ground radial field compared to 2 raised radials?
Thanks in advance.
***dan, K6IF
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