I agree with Dave's comments based on measurements and modeling of two
different similarly sized top loaded verticals.
Grant KZ1W
On 11/7/2020 07:49, Dave Cuthbert wrote:
Steve,
*I believe there is a measurement error. *
1. The reactive part of the input impedance should not become more
inductive as the frequency is decreased. 1904kHz --> Z=68.1 + j0.3 and
1844kHz -->48.9 + j21.5.
2. An EZNEC model over 0.005/13 GND shows resonance at 1750kHz and the real
part of the input impedance to be 17 ohms. Real-world will be a bit higher
due to this being NEC-2.
So, the real part of the input impedance is too high and I don't think can
be accounted for by ground loss. Resonance is off by 150kHz.
Questions: How are you measuring the antenna base impedance?
What instrument are you using?
Are there any strong LF/MF broadcast station signals at the
QTH?
Dave KH6AQ
On Sat, Nov 7, 2020 at 5:08 AM Steve HA0DU <ha0du@dx.hu> wrote:
HI All,
Today we finished installing a vertical with a top hat at HG0R, but it
is not perfect (yet).
The vertical part is about 20.5m (67 ft) tall, starts with 10m of 50mm
(2") pipe at the bottom, and ends with 28mm (1.1") OD pipe on top.
There are 8 quarter-wave radials below the antenna at each 45 degs, soil
is wet black earth.
Top hat wires are connected to a small aluminum plate. The four (4) top
hat wires are about 12m (39 ft) long each, and are sloping at an angle
between 60 and 45 degrees.
We measured the antenna, and found the following:
*1904 kHz* (minimum SWR and also ain inflexion point in phase):
*Z=68.1 + j0.3 SWR 1:1.36 Phase 0 deg*
*1844 kHz* (resisitive Z closest to 50 ohms)
*Z=48.9 + j21.5 SWR 1:1.54 Phase 23 degs*
*/One step before, with shorter top hat wires (approx. 10.2m wires) the
SWR was lower at the 0 phase point, but it was at 2005 kHz./*
Please tell me what shall we do:
- tilt the antenna again, add more lengths to the top hat wires, so that
phase 0 point comes down to about 1840 kHz (R will be still higher than
50 ohms), and add more radials, perhaps radiation resiastance comes
closer to 50 ohms?
- add a series capacitor to compensate the +j21.5 at 1844 kHz, and leave
it as is?
Or any aother suggestions?
73/DX
Steve HA0DU
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