Hello Stefan,
Yes, a metal mast will introduce unwanted reception of its own.
I have seen a professional application where a common mode choke was used
over the mast at the bottom to kill unwanted resonances of the mast, which
could be easily coupled to the coax or antenna itself.
Your idea of a optical link is interesting as it would decouple the whip
completely.
In the past I have carried out an experiment with a battery powered SPM-3
and a battery powered whip.
When I raised the whole contraption, the signal level of a local NDB
dropped!
If this also will be the case with a battery powered whip, with the
battery mounted close to the antenna to avoid a stray path to ground via
the power feedline and an optical signal feedline, will to be seen.
Depending on the size of the whip versus the size of the circuit ground it
might still work, albeit in dipole mode.
My standard mini-whip uses leaded parts and the capture area measures 30 x
40 mm. I have also made one with SMD parts, except a RFC and a 2N5109.
This one delivers the same output, whilst the capture area is only 22 x 26
mm.
The smaller SMD parts will introduce less stray capacitance and hence less
signal loss due to a larger C-antenne to C-circuit ratio. So the
capture area could be reduced. Output is ample and I have a passive
four-port power devider in-line with still ample power at the receiver
port.
Regarding field strength, I found that at a field location the antenna
delivered about 10
dB more signal at 400 kHz than received at my city location. However with
the right measures the signal to noise ratio can be (almost) as good at
home as it is in the field. We have bushes and large chestnut trees close
by and it was necassary to raise the antenna up to 4 metre to get
comparable results as in the field location. In the latter 2 metre was
sufficient for a proper signal to noise ratio. The environment adds some
screening, but as long as it does not degrade the signal to noise ratio,
nothing is lost. My antenna is only 6 metre from the house mounted up the
stem of a magnolia tree.
To get rid of the local noise, traveling along the shield of the coax,
grounding the shield at the bottom of the mast has been the most effective
single measure. A second earth stake has been placed at the point where
the cable enters the house. Feeding the antenna from a battery, mounted at
the foot of the mast, will further reduce the local QRM level. However, it
should be stressed that excellent results can be achieved using
traditional feeding methods over a coaxial feedline.
I won't boast, but using this set up, I have received many T/A stations on
137 kHz. So far 102 different non directional beacons from Canada have
been logged as far as British Columbia. This shows that with a bit of
sound engineering decent results can be obtained.
DCF37 is here -57 dBm / S9 + 14 dB.
I hope this helps.
73,
Roelof, pa0rdt
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