----- Original Message -----
Sent: Wednesday, August 10, 2011 4:30
PM
Subject: Re: LF: Re: HB9ASB...
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
Andy
So the FEEDER does form the majority part of this type
of antenna, simply one side of a very poor asymetrical dipole.
It would appear this is not the antenna for a noisy
urban environment
The long element of the dipole is picking up every
possible noise and amplifying it and feeding all this noise plus signal to the
RX, the minute Probe element is insignificant.
This has been my observation all along.
G3KEV
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
