Hello LF,
I'm happy to join the discussion although I'm probably the least
qualified..
The insulation, with a suitable choke, of the antenna from
the feed line is sure a test which can help to determine if in the Mini
Whip of Roelof or any other active antenna the feed line is really a
parte of the antenna.
This was an easy and ultimate test for magic antennas like EH etc.
In this case I don't guess it can make a difference due to low
impedance of the amplifier output which should match the coax and
further the rx input (under such circumstances the feed line should not
radiate or receive but an easy test should give the answer).
An other interesting point, according my experience, is the effect of
"structures" nearby MiniWhip: my MiniWhip is placed about 10m from the
roof (close to a metal mast) and about 1.5 m on one side there is the
feed point of a 160m doublet and a 80m doublet, onthe other side there
is the top hat of the 136/472 TX antenna. The signal level at my rx
(about 50m coax run after the antennas) changes according the MiniWhip
is the only cable connected at the receiver input, or when the MF/LF
antenna is also connected switching from MF to LF on the tx antenna
affects the received signal..
73, Marco IK1HSS
----Messaggio originale----
Da: [email protected]
Data: 5-lug-2013 16.19
A: <[email protected]>
Ogg: Re: LF: VK1OD's analysis of the MiniWhip antenna
Roelof
Without adequate isolation between the feedline and e probe unit
itself, the x meters of vertical
feedline above ground level is part of the antenna.
Jay W1VD WD2XNS WE2XGR/2
----- Original Message -----
From: "Roelof Bakker" <[email protected]>
To: <[email protected]>
Sent: Friday, July 05, 2013 9:09 AM
Subject: Re: LF: VK1OD's analysis of the MiniWhip antenna
Hello all,
Further to this discussion, I have done the ultimate test to
demonstrate that an active whip
antenna with a 1 meter long whip behaves as a capacitance at LF.
For the test I used the ground wave carrier of semi local NDB ONO at
399.5 kHz.
As it is the only station at that frequency the carrier level is
very stable at daytime.
The distance is 59 km, which excludes probably all skywave
propagation.
The antenna was mounted vertical and the carrier of ONO produced a
signal level of -69.1 dBm as
received with a PERSEUS SDR.
Next the antenna was mounted horizontal at the same height as the
bottom of the vertical mount.
This produce a signal level of -72.0 dBm.
I reasoned that the mean height of the 1 meter long antenna when
mounted vertical is 50 cm
higher. So the antenna was then mounted horizontal 50 cm higher from
its previous position. This
produced a signal level of -69.9 dBm, close to the value measured
when mounted vertical.
The main point however, is that a horizontal polarized antenna should
NOT receive a vertical
polarized ground wave signal at (almost) the same strength.
So at LF, there is nothing gained in using a whip instead of a small
piece of copper clad PCB.
Comments are much appreciated!
Best regards,
Roelof Bakker, pa0rdt
