Jim's suggested tuning method works well for me to cover the entire
band. I short/open 3 series capacitors all the same value and can cover
1800 to 1975 < 1.5:1 swr. I use a transmission line transformer before
the caps to match the 25 ohm resonance.
Now for some propagation to EU from suffering sevens land - Redmond,
WA. It's Unobtainium.
Grant KZ1W
On 2/3/2018 20:59 PM, Jim Brown wrote:
I agree with both Grant and Rick, but with this caveat. Current is
greatest near the feedpoint, and an inductor at the feedpoint is right
at the current peak. By contrast, current is least near the end of the
antenna. SO -- this is an important reason why top loading is better
than inductive loading. BTW -- there was an excellent piece of work
published 3-4 years ago studying the relative effectiveness of bottom,
center, and top loading of mobile antennas. The study included
extensive measurements of field strength, which confirmed that top
loading was significantly better, AND that NEC failed to accurately
model inductive loading. As I understand it, NEC models the inductor
is as a lumped component, so fails to accurately take into account
phase shift across it.
Another technique that is often used for matching is to make the wires
much longer than resonant (that is, resonant around the top of the BC
band), so that the feedpoint Z is 50 +jX ohms (that is, inductive) on
160M, and tune the capacitance out by adding a capacitor of equal
value in series. NEC can easily model this.
73, Jim K9YC
On 2/3/2018 7:51 PM, Grant Saviers wrote:
Top loading is a winner to increase antenna efficiency. I agree that
EZNEC is essential since umbrella (slanted) wires have less effect
than a flat top hat and also tend to shield the antenna a bit. The
model needs the segments aligned in each element. Trig or use AutoEZ
to figure that out.
There are a couple of articles worth reading on Rudy Severns web site
antennasbyn6lf.com
This article is for top loaded verticals for the new LF bands but
covers the principles well
http://rudys.typepad.com/files/chapter-3--1.pdf
My T is 10' at the base elevated radials, 85' to the top, and 33' or
so each side, resonates at 1820, 25 ohms.
Grant KZ1W
On 2/3/2018 17:44 PM, Richard (Rick) Karlquist wrote:
You should really model this on EZNEC.
Having done this many times, I can say
that the length of the vertical plus the
length of one of the top wires should be
about 120 feet to hit exact 1830 resonance
without a loading coil. However,
there is absolutely no compelling reason why
this is necessary. A loading/matching coil
is far more convenient if you keep the wire
length to 25 feet. Therefore, it should
never be necessary to put it up and down a
bunch of times to get it "just right". There
is no "just right" with respect to the antenna
proper. Use the taps on the coil to do the trimming
to make the system just right. This has been
discussed many times on this reflector.
Rick N6RK
On 2/3/2018 5:20 PM, Bill via TowerTalk wrote:
Planning on putting up a 160 vertical, but of course a 1/4 wave
vertical is out of the question here in HI since we have a height
limitation.
Here's the question: I'm putting up around 50 feet of 3 inch
aluminum tubing (left over from various antenna takedowns). I
plan on putting up on top of that around another 10 or 15 feet of
smaller diameter stinger for a total height of around 60-65 feet.
I obviously need to make up for the lack of height and I was
planning on making a top hat of four wires coming down from the top
of the antenna at around a 45 degree angle. The problem is how
long to make each of those wires. My guess is around 25 feet based
on my past experience in making Tee-topped 80 meter verticals.
However, it is only a guess and I don't feel like yanking the 3
inch pipe up and down a bunch of times to get it just right.
I know some trimming will be necessary since this is not an exact
science. I just need an educated guess for where to start with the
length. Any ideas?
Bill K4XS/KH7XS
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