I'm surprised so many seem to misunderstand how high
impedance E-field probes work. There is too much hearsay and
false comments being peddled. Stop thinking of the feeder as
'part' of the antenna as if it were all one entity and
consider each stage.... And forget that thing called Ground
completely.
It starts with a high input-impedance amplifier which must
have two connections - ignore any common reference for now, it
just has two input pins. A voltage is imposed across these from
any antenna with two ports, like a short dipole. This input
voltage is buffered, amplified, and sent to the two output pins
from which it travels down a feeder - balanced, coax, or
whatever - and into your receiver. So far we haven't made any
connection between input and output, and they could (and ideally
would) be independent and isolated from eachother
However, they're not. One input connection is usually
common with one output - usually the 0V DC supply pin and the
reference (and please note, I am not referring to this as
ground; ground is taboo, a dirty word, and will not be
mentioned) Which means the other hot-side input pin now has a
voltage imposed on it with respect to the reference. This will
probably come from a probe antenna which is coupling to the
E-field of a radiated signal. Now, bear in mind any antenna
must have two output ports, so where is the other side? As the
reference input pin is connected to the output reference pin,
any other connection to this point will form the other 'half' of
the high impedance short dipole antenna.
Now, the feeder dropping down from the amplifier / probe
assembly (or across, or up and over, or buried in the soil [see,
still didn't say the taboo word] ) is connected to the input
reference pin by virtue of the amplifiers internally linking
them, and must therefore form the other half of the dipole.
This is bad. We have a hi-Z dipole, with one short element in
the air where it should be, and the other element being one
conductor of a probably long length of feeder with its end
connected to we know-not-what.
So what do we now have? A long assymetric dipole plus
amplifier assembly, stretching from some arbitrary shack
connection up into free space. The voltage at the not-middle
of this is the stuff that is amplified and fed to the
receiver. So, as the bulk of the dipole is close to the shack
expect the majority of signal received to be locally generated
noise.
That is the case for a completely unscreened and un-earthed
(still didn't use the word :-) system. Picking up any locally
generated E-fields at high levels onto the dipole.
Now place the receiver and the rest of the local world with
all its noise and QRM generators in a screened room and make the
feed coaxial through a bulkhead connector into teh screened
room and see what happens. All the QRM is contained within the
screened room and cannot pass outside. The antenna is now a
strange sort-of dipole with one end in free space, and the other
connected to a solid mass made from the outside of the screened
room. It can't pick up any QRM from inside the screened room,
and all it will now receive are signals generating an E-field
between the probe and the solid mass. Perfect!
The shack and the rest of the world is not in a screened
room, so there lies the problem. However much you don't want
it to be, with this setup the feeder will always form
the majority of the antenna because there is nothing else
there that can be the other half of the dipole. And
if the bottom end of the dipole intrudes into the rest of the
world, it will pick up stuff it shouldn't
Now, lets control the antenna structure all on its own and
pretend the feeder is not there at all. A true dipole would be
nice although impractical for now, so lets go back to the
classic monopole which is one half of a dipole, and the other
half formed by the reflection of this in a mirror, which is
usually the surface of this planet called Earth. Mount the
amplifier assembly on a conductive pole with the bottom end
firmly connected to the surface of the mirror. Now, we have a
dipole formed of the complete assembly (mast plus probe) with
its reflection. It is not fed in the middle , but
assymetrically near the top where the short probe forms the
other side from teh feed point. The total length of the dipole
is now twice the total height of supporting mast and probe. As
we're using a high impedance input amplifier, if it nad a truely
infinite input Z and zero capacitance, the actual feed point
wouldn't matter, it would always get the same voltage imposed
wherever the non-symmetrical spilt occurred. As Z and C is
finite it does make a difference so longer probes help with
practical amplifiers - but this is digressing.
The antenna so far is ideal, and is no more prone to picking
up local QRM than any other antenna would be. But now we have to
connect this thing called a feeder, which introduces another arm
to the dipole (a tripole now perhaps ) which will ultimately go
close to bad places. If we could isolate the feeder by
inserting a high common mode impedance this would do the job,
but it would have to be a near infinite common-mode impedance,
so is definitely not on. A transformer coupling would help,
but even that has high (relatively speaking) capacitance across
its windings, so will still leak common mode rubbish, gettign
worse at higher frequencies. And there is still the DC power
issue
One solution is to bring the feeder down INSIDE the pole
which must be firmly connected to the reflecting surface of the
mirror, maintain it buried under the mirror's surface for as
long as possible before it sees the Badlands and hope that
burying it will decouple any local QRM. Or make the feeder non
conductive like optical fibre. But there is still the DC power
issue.
Which shows where the problems lie, and where to start
thinking about how to stop them. Separate dedicated mast
firmly connected to the mirror's surface. Feeder fed down
inside it and buried. If coaxial feeder is used - connect the
braid at both ends to the mirror's surface and bring in to
the receiver input port coaxially and screened. DC likewise -
send it up the feeder.
One solution, but its a difficult bit of hardware to build
well, would be a true differential input high impedance
amplifier with a proper equal length short dipole. The
differential input - if ideal - will inherently provide
isolation between input and output ports but it does have to be
a perfect differential input, balanced antenna and so on.
Someone did mention to me once about using two identical active
antennas mounted end to end to form a a dipole, with the outputs
from each combined in a 180 degree hybrid combiner. That could
prove viable, but don't know if he ever tried it.
Phew....!
Andy
2011/8/10 Stefan Schäfer
<[email protected]>
Hi Minto,
Am 10.08.2011 13:10, schrieb Minto Witteveen:
Hi
Stefan, (et al)
Well I beg to differ.. :-)
What I think happens is this: The outside of the coax
picks up electromagnetic radiation like any antenna
(including QRM generated by fluorescent lamps and Alinco
switching power supplies). This signal travels along the
coax to the Miniwhip. (also in the direction of the
receiver but that is not important here as the signal is
on the outside of the coax).
Upon arrival at the miniwhip this signal on the outside of
the coax has nowhere to go but to the _inside_ of the
outer mantle of the coax – it ‘rounds the corner’ at the
end of the coax so to speak.
I think the mechanism is that the unwanted signal on the
screen causes a potential difference between gate and source
of the first (J)FET. So this causes a current flow in the
output stage and so a signal at the RX input.
A common mode choke between RX and the antenna ground should
form a low pass filter for unwanted signals coming from the
shack. Using a common mode choke without a local ground should
have little effect, except the coax is some 100m long (between
choke and probe) ;-)
Ah BTW regarding the discussion "the cable to the E field
probe is the actual antenna": One could just try what happens
if one disconnects the power supply. If the signal is still
present then the cable is the antenna, if the signal is gone:
The probe must be the antenna. Isn't it?! :-)
So
how to avoid the QRM that is picked up by the coax to
‘travel back’ via the inside: for the miniwhip it is
indeed best (as Roelof mentioned) to short these signals
to earth _outside_ the house, preferably as close to the
miniwhip as possible. Grounding there would to the trick,
aided by a (large enough) common mode choke between the
ground point and the house. The QRM that is picked up in
the house would be – after attenuation by the choke -
directed into the ground and not up into the pole and the
miniwhip.
Yes yes, totally agreed.
Whatever
happens in the house would then be largely irrelevant.
Adding a common mode choke close to the rig will do little
extra. (it would only attenuate QRM getting from the
shack’s earth system to the outside of the coax).
It would almost have the same effect (when ignoring the C
between cable and ground along to the choke near the antenna
ground) as placing the choke near the antenna ground, both are
in series and increase the current reducing impedance, yes...
Any
signals picked up by the vertical coax between the
earthing point and the whip will add to the received
signal, but at low frequencies it will not be much.
So far for theory. Now the proof of the pudding: DCF39 is
now > S9+40 dB. My old trusty QRM generator (Alinco
SMPS) generates S9+25 at 135.500. When I switch off the
miniwhip (cut the power) DCF39 drops down to just above
the noise floor. As expected.
Ah yes, that's what i meant above (should have read your mail
completely before answering ;-) ). This is the proof that Mal
cannot be right when saying "the coax is the actual antenna".
But
the Alinco signal only drops down some 15 dB and remains
the only signal that is audible. This is exactly what I
would expect: the QRM travels along the outside of the
coax to the miniwhip, ‘rounds the corner’ and comes back
via the inside of the coax shield. Further proof that it
indeed takes this route: if I disconnect the coax in the
shack the Alinco smps signal disappears also (so it is not
received via any other path).
Hm, i rather expect a galvanic coupling i.e. stray currenty on
the supply cable of the RX. What happens if you run the RX on
batteries? The same dependency?
There could be several reasons apply here...
Last
year I already bought 3 meters of copper pipe to drive
into the ground in the backyard. Bet never got around to
finish the job…
Today it's nice WX here! And in NL?
The main reason the signal strength is much higher with
the elevated miniwhip is (I think) caused by the fact that
I am surrounded by other houses, gardens, trees etc. Not
comparable with an open field…
Yes.
73, Stefan /DK7FC
Regards,
Minto pa3bca
--------------------------------------------------------------------------
Ceterum censeo Carthaginem delendam esse
-----Original Message----- From: Stefan Schäfer
Sent: Wednesday, August 10, 2011 00:03
To: [email protected]
Subject: Re: LF: Re: HB9ASB...
Hi Minto,
Am 09.08.2011 22:48, schrieb Minto Witteveen:
You are right w.r.t. the cable
being (a significant) part of the working of the
Miniwhip antenna. [...]
I don't think so. There should be no difference between
a 5m and 10m
long cable. I think about a capacitive divider. The
probe has about 3
pF, that's one plate of the C. The other one is the
cable and metal
connected. Once if this part of the C has say >10 * 3
pF, the difference
between longer cables become smaller and smaller.
I think it is just the S/N that rises due to lower
becoming noise and
higher signal levels. On a flat field without trees and
houses, you have
excellent reception even with a 2m pole :-)
73, Stefan
