|
Yes, agree with that Rik, but the advantage of the
small antenna is that it can be moved more easily away from local
noise.
Chris, G4AYT.
----- Original Message -----
Sent: Tuesday, January 03, 2012 12:38
PM
Subject: RE: LF: Antennas
Depends on how "gain " is
defined . Larges antennas pick up
more signal , but also more noise . It is the signal to noise
ratio that is important, and this
is no better than with smaller
antennas . With a large antenna
a signal will be S9
and noise at S7 . With a small
antenna
the same signal will be S3 with noise
at S1 . In both cases SNR is the
same .
Like you say with the larger antennas Attenuation is
needed there is so much more gain over the smaller variety.
My antennas both on LF es MF have attenuation
control to reduce the gain, a good position to be in I
suppose.
g3kev
----- Original Message -----
Sent: Tuesday, January 03, 2012 10:48
AM
Subject: RE: LF: Antennas
over the past decade I have tested small loop
antennas , a miniwhip (PA0RDT ) and a
"big" transmitting
antenna for receiving purposes
and found that each of them
has its own
advantages , as well on 137kHz
as on 500kHz .
Loop antenna
- very frequency
selective , can be useful
to attenuate broadcast - 8-shaped pattern can be useful
to null out QRM sources - if large enough you don't need
a pre-amp - you can move the
loop around your property
to find the best (now noise ) location - not
omnidirectional , so you might need
to rotate the loop - single band antenna
-
broadband , can be used from
(V)LF to HF - you can move the
loop around your property
to find the best (now noise ) location - pre-amp (built
in), so you need
to feed it with a DC
voltage. This has
to be done with some care
as it can introduce QRM . - broadband so your RX needs
to be able to handle the all
signals . Can be solved by
a BPF in front of the RX .
"Big" TX antenna (Marconi ): - readily available if you also TX on
137/500kHz - no TX /RX
antenna switching if you also TX on
137/500kHz - no pre-amp needed (in
contradiction , often you will need an
attenuator ). - some frequency
selectivity , but not as good as a
loop - big, often not worth
the effort if you only want
to RX - cannot be moved around
to minimize QRM
If you have a TX antenna and
the local QRM is not too
bad you can use it
as RX
antenna, so no need for an additional RX
antenna. If you use a
loop RX antenna it should be
at sufficient distance from your TX
antenna, otherwise it will pick
up all the QRM from the TX antenna.
I did not notice that
effect with the miniwhip . If you want
to RX only a
loop or miniwhip seems the best
(most economical) option.
I compared the miniwhip
and my TX antenna on many
occasions and could not notice a
significant difference (as RX antenna).
During the winter
2010-2011 Canadian and
US beacons were copied regulary with good
(audible) signals on
500kHz.
Mal, you lost me on this
one . Are you suggesting I (or
we.....here in North
America ) erect large
verticals , inv L systems , Rhombics , and
V beams in order to receive EU LF
signals ?
I thought I
was doing pretty well with my
micro RX antenna!
On Mon , Jan 2, 2012 at 6:28 PM,
mal hamilton <g3kevmal@talktalk.net>
wrote :
Reports from across the pond
and other DX locations
as far as Tenneesee and Kansas using
micro probe antennas
are great for QRSS speeds but not suitable for
audio reception.
Take 160 metres for example where it
is normal
to work world wide on cw but antennas
in use are large verticals, inv
L systems, Rhombics and V
beams, therefore to
have any chance
of receiving signals at audio
level on LF or MF large antennas
of the calibre used on
160 metres are necessary.
As well as TX
at this QTH
I also use large arrays for RX
and often hear signals from
NA that would not be audible with small
loops, micro probes or ferrite sticks
Recently on 500 I was able
to copy a W stn 579 but a
DL stn copied only on
screen, when
I asked what strength
the signal was I got no
reply!!!!!!!!!!!!! using a micro probe
antenna.
If a
proper large antenna system
is not used on LF
es MF then there
is virtually NIL chance
of an audio report from across the
pond for EU stns es vice
versa
Back some years ago I had
audio reports from the Boston area but
the antennas were
proper wire arrays
as used on 160 metres
Small hand
held antennas are fine for High
Power BC strength signals but not for
low power amateur signals to be
heard
No commercial LF/MF station would
even consider an antenna of the micro
variety.
When I was in
the
business some years ago on
LF/MF Rhombics and V-Beams were
the norm
|
