Dear Mal,

thank You for gaining interest in active antennas. As several points
have to be answered, I put the answers into the same paragraph where
these points are located:

"mal hamilton" <[email protected]> schrieb:
> Hajo
> Although this type of antenna works I am not sure how because it seems odd
> that the outer braid of the coax has to be connected to Earth at the base of
> the antenna mast, therefore altering the design properties of the active
> antenna. 

As long as an active antenna is used the cable length of which does not
extend to an order of a quarter wave length, which usually is the case
up to medium waves, cable length should have no influence in its
performance. As Roelof is requesting there should be a ground contact to
the cable braid outside the house, and the receiver chassis at the cable
end can also allowed to be grounded for frequencies higher than 50 ...
80 kHz. For lower frequencies an isolation transformer may be required.

Active antennas for HF should have an internal common mode choke (at
least 100 ... 1000uH) at the upper end of the cable and a second short
element connected to the internal RF ground to replace the cable as a
ground. Otherwise, on frequencies close to a cable resonance to ground,
the receive level will have changes of up or down by 20 dB, which can be
demonstrated using a spectrum analyzer! With typical installations
within the HF range the feed cable may develop resonances at a quarter
or even three quarters of lambda. 

I have also been informed that the latest "cry" for strictly commercial
companies to avoid all these RF and cable problems is an active antenna
using two operational amplifiers in push-pull, one for each short dipole
electrode, and their outputs are combined by a transformer which also
cares for the DC feed of the amplifiers over the cable.

> You say no other Earth connection is permitted in the shack, how is
> this possible when the RX in the shack has already an Earth connection
> except an Isolation transformer is always used.
> 
This is a special requirement for the use of active antennas on VLF
(according to my experiments this need will end at about 80 kHz). An RF
current induced in the braid (by either RF signals or unwanted currents
from the environment) will cause a current in the inner conductor too;
the screening effectivity of the cable is getting lost at lower
frequencies (may be not if the braid is replaced by a thick iron tube,
but those kinds of cables are not common). Therefore in order to break
the current in the braid an isolation transformer must be used. This
transformer can be combined with the circuitry feeding DC into the
active antenna; the optimum polarity of this transformer has to be
checked for best noise reduction properties when removing the DC. The RF
choke feeding DC into the cable should have 10 ... 22 mH; and another
less critical RF choke and a protection diode for the intended DC
current direction should also be in line with the DC source.     

> My logic would say that an Active antenna as described should be mounted on
> a pole preferably in a quiet location and the coax connection should go
> directly to the RX in the shack, 

This is ok down to 80 kHz and up to a few MHz depending on heigth and
cable length.

> also height of mast should make no
> difference, but some say the signal is better the higher the active probe,
> why is that the case ?

By doubling the height above ground the received voltage of an active
antenna will rise by 6 dB average. Therefore demands on high linearity
of the amplifier also increase with height! You may find either a 10 kHz
line spectrum (from HF broadcast stations) or a 9 kHz line spectrum
(from LF and MF broadcast stations) even in the VLF range from 9 kHz
upwards in the VLF range, if the active antenna works up to 30 MHz and
its linearity would not be sufficient.  

Just loop antennas for vertical polarization may be built rather close
to the ground (or just as high that people can walk below) without
decreasing receiving efficiency.
> 
> It would be interesting to just have a small vertical antenna mast, with an
> amplifier and compare against a probe type.

My active antenna is just 6 meters above ground, in an rural
environment. Roelof is suggesting 8 meters.
> 
> Perhaps I should build one of these miniwhip antennas and compare it against
> an inv L on 137. I did this with with a small commercial active loop and
> found although it worked it was well below the sensitivity and signal
> capture of my inv L system.

I have also tried the Miniwhip first. But its 50 mA current consumption
may make it difficult to find small RF chokes sufficiently high in
inductance, especially down to 10 kHz. I have changed to operational
amplifiers, such as the OP27, or the LT1252 when to include the HF
range. They operate at lower currents and still have high linearity
because of the direct negative feedback from output to input.
> 
> My 40 metre RX vertical quad loop resonated for 137 is not as good as the
> inv L.  One strange observation is that late afternoon early evening I
> cannot detect DX signals on this loop whereas the inv L system is excellent
> all the time, but late evening and night time the DX signals start to appear
> using the loop, this must have something to do with Angle of arrival of
> signals at night time.

I agree with You that passive wire antennas, even untuned, as long as
they are in a quiet environment, will give higher output for receive
than active antennas. Good selectivity must be provided before the
(average) receiver, of course, to avoid overload problems. I cannot
comment on your differences in the behaviour of inv L versus loops
concerning DX signals, because of lack of experience.

Therefore, after the general die-out of amateur CW operations on 136 kHz
in Europe from 2000 onwards I have connected my LF T-antenna to an 9:1
unun loaded by 68 ohms at the output in parallel to the cable into the
shack, and for the time being I am using this setup as a broadband all
purpose aerial from VLF to 30 MHz. As this T-antenna is not too far from
the house, it is sometimes too noisy for clear reception on lower
frequencies, however.
 
Therefore british NOV stations on 500 kHz (including YOU), even in the
100 mW period, I have received in the winter season in full darkness by
a vertical loop of 17 meters circumference looking north. This broadband
loop also worked fine for SAQ until the neighbour installed its solar
panels and chopper. Because of its 300 Hz line spectrum still
interfering my vertical loops up to 20 kHz the best antenna for SAQ now
is my active antenna.

> 
> 73 de mal/g3kev

Now it is up to You if You really want to try an active antenna. Even if
You should not continue to use it, You will have made the comparison. It
does not need much work to do. 

OK?

73 Ha-Jo, DJ1ZB


> 
> 
> 
> ----- Original Message -----
> From: <[email protected]>
> To: <[email protected]>
> Sent: Wednesday, August 10, 2011 9:58 AM
> Subject: Re: LF: HB9ASB...
> 
> 
> > Dear all, Mal in special,
> >
> > if the cable to the rx would really be the antenna for the active
> > aerial, only local qrm would be received! The screen of the cable has to
> > be grounded bfore entering the house, as Roelof has stated, but avoid
> > any additional grounding within the house.
> >
> > Especially at VLF an 1:1 isolation or separation transformer having an
> > inductivity of 10 mH or higher is needed for active aerials before the
> > receiver to break currents in the screen which the cable has picked up
> > from the environment. And on an ordinary toroid the polarization of the
> > transformer has to be optimized for real noise-free performance.
> >
> > Just by this method I got rid of qrm from a nearby solar panel
> > installation and its 300 Hz spectrum up to 20 kHz which has plagued me
> > since autumn 2010. For my inductive aerials, this interfering field ist
> > still existing. But copy of SAQ last Friday using the active aerial (in
> > my case using an LT1252 as amplifier just needing 10 mA) has been ufb
> > again.
> >
> > 73 Ha-Jo, DJ1ZB
> >
> >
> > "mal hamilton" <[email protected]> schrieb:
> > > Minto
> > > Get rid of fluorescent strip lights from your house, only QRM generators
> for
> > > radio amateurs.
> > > It seems MINIWHIP antennas work better the higher they are installed
> above
> > > ground which makes me think that the cable connecting the probe to the
> RX is
> > > in fact the REAL antenna. !!!!!!!!!!!!!!!
> > > A probe antenna should work equally well at ground level in the clear as
> in
> > > an elevated position.
> > > G3KEV
> > >
> > >
> > > ----- Original Message -----
> > > From: "Minto Witteveen" <[email protected]>
> > > To: <[email protected]>
> > > Sent: Tuesday, August 09, 2011 8:44 PM
> > > Subject: Re: LF: HB9ASB...
> > >
> > >
> > > > .. And he is even visible on my grabber!
> > > > I raised the miniwhip from +4 to +11 meters (large fishing rod in the
> > > > backyard).
> > > > Signal levels increased by 15 dB. Noise level was raised too, but the
> QRM
> > > > from my fluorescent lights in the house no longer show up. so there
> must
> > > be
> > > > some improvement.
> > > > Tomorrow I will again measure de DCF39 to noise ratio.
> > > >
> > > > 73's
> > > > Minto pa3bca
> > > >
> > > >
> > > >
> > >
> > --------------------------------------------------------------------------
> > > > Ceterum censeo Carthaginem delendam esse
> > > > -----Original Message-----
> > > > From: Stefan Schäfer
> > > > Sent: Tuesday, August 09, 2011 21:10
> > > > To: [email protected]
> > > > Subject: LF: HB9ASB...
> > > >
> > > > ...now joins in at 137.695! 25 dB SNR here.
> > > >
> > > > Looking forward to a opening band tonite...
> > > >
> > > > 73, Stefan
> > > >
> > > >
> > >
> > >
> > >
> >
> >
> 
> 
>