Jim Lux wrote:
At 12:58 PM 11/9/2005, [EMAIL PROTECTED] wrote:
Thoughts on my vertical
2. Got the HF2V vertical in place with 60 radials. Got it "mostly"
tuned
up. Missing: Serious plan for more than 25 KHz of band coverage at a
time, esp. for upcoming CQ WW SSB and also covering both the DX
Window on
SSB plus CW and maybe RTTY.
-- 25 kHz is about right for SWR bandwidth on a single vertical on
75/80, unless there's some huge loss source to kill the Q.
The antenna analyzer seems to agree with you. I _think_ I'm OK. The
ohms part of the reading is about where the manual said it should be.
Given Frank's comment about the gain I should be expecting, everything
is making more sense, in fact.
-- You might seriously consider putting some sort of variable
reactance at the base of the antenna. The radiation efficiency of the
antenna is reasonably constant over the band you're interested in, but
the reactive component varies quite quickly. You really, really don't
want to be tuning out that reactance at the other end of a length of
coax. You might be able to get away with a few switched inductors or
capacitors, and just tune out the last bit (say, to get from 3:1 down
to 1.x:1) at the shack end.
I'd appreciate a link or two or more explanation of this. OK, if doing
it on the shack end is bad, I need something that I can do easily,
quickly, and reliably at the antenna end.
-- The folks at Force 12 have an interesting loading coil for the
middle of their vertical low band dipole that basically stretches and
shrinks a big spacewound coil to tune it. Think in terms of building
yourself a coil out of something like 1/4" copper pipe that's a foot
in diameter and a foot high that can be stretched to 2 feet high. A
motor driven lead screw does the stretching (like in a screwdriver
antenna).
Well, I already have a nice coil on the antenna already that expands and
contracts. The problem is, at least the way it sits, it's kind of hard
to do quickly and easily.
How would I get something like this working by the end of the month and
where would it go?
The capacitance hat probably won't broaden the bandwidth much, but
might raise the radiation efficiency (in that it increases the average
current in the radiator).
That might be worth it all by itself.
This is mostly about the vertical. My main problem of the moment is
better frequency coverage.
My roughed in plan is some combination of the following:
OTOH, the antenna is clearly "out-hearing" the G5RV, by upwards of 40 dB
in at least one case. I am now hearing zone 15 (Croatia), if
faintly, on
SSB and I expect I could probably work a lot of Europe, even as I
have it,
with 100 watts on CW. But, is there something else I have overlooked to
improve the DX radiation angle? I laid out the 60 verticals assuming a
velocity factor of 50 per cent as per ON4UN's book. Did I misread that?
Should it have been longer wire or something?
Wires laying on the ground (or buried in it) aren't really something
you can "tune" for resonance (at least not without knowing a whole lot
more about the EM properties of your soil than you're likely to
know). They basically provide a low impedance return for the "image"
of your vertical, and the length is totally non-critical. (to a very
rough first order, the propagation speed in the buried wire is
1/sqrt(epsilon), but I doubt you know epsilon with an accuracy of
better than 50%, and that would really only apply to a wire that is
immersed many skin depths (in the soil), which yours are probably not)
I get mixed reports on this. ON4UN's book (as I recall) said the
velocity factor was between 40 and 60 per cent "depending on the soil"
and basically said (in practice) no one would know what it was like at
any given QTH. So, that's square with what you say, just about to the word.
It's just I had a few folks claim the velocity factor was more like 100
per cent. But, as you and others are making clear to me, my results are
more like the 50 per cent plus or minus velocity factor is about right.
Or, at least, it is doing me no harm to play it as if it were so.
The basic guideline is that more is better, and that having wires
closer together than 1/100th of a wavelength (or 1/50th, or other
numbers in that vicinity) isn't worth it.
The recommendation of 120 radials 1/2 wavelength long, etc., are
really holdovers from the broadcast industry, as well as measurements
and analysis by George Brown back in the 30s. The goal there is to
produce a particular measured field strength with a certain licensed
radiated power, so as to conform to the terms of your broadcast
license grant.
Jim, W6RMK
ON4UN says much the same. He gives some small difference between 60 and
120 radials (.5 DB or something like that) and in another nearby
section, has an example suggestive that 43 radials is a good number, too.
My reading suggested that "around 60" was about as good as I could
expect. My only question now is whether I need a handful of longer ones.
From the sound of all this expert help I'm getting, that's really not
my problem now. The problem is rigging up something to make frequency
shiting practical.
It is helpful to know it is mostly about reactance, though.
Larry WO0Z
