Hi Stefan,
Some
comments:
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.
I fail to see how that could
be the main cause… because cutting the power to the miniwhip should then
eliminate the QRM, but it does not! It probably attributes somewhat - only some 10-15 dB, but that leaves
9+20 dB for another
explanation. (i.e. my balanced-unbalanced hypothesis)
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...
But I have to disagree. A
choke (only) at the TX would accomplish nothing (in fact it may even make it
worse). With a choke at the RX end the coax will – acting as an antenna -
still pick up all kinds of noise in the house, and this get transported via
the outside of the coax to the miniwhip. Placing the chocke+ground near the
miniwhip will on the other hand attenuate all the noise that is picked up
along the coax.
An (additional) choke at the RX end _might_ make things
worse at LF because in that case the noise will not be bled to earth there,
with the result that the overall noise voltage on the outside might be still
higher.
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?
Running the 817 on batteries
makes no difference. And galvanic coupling is unlikely because the QRM
completely disappears when I disconnect the coax in the shack…. And the coax
is not connected to anything else up to the miniwhip.
As for the
necessity of a current balun or common mode choke when going form unbalanced
to balanced: picture a classic
dipole fed by coax. (TX) current runs through the center conductor. Kirchofs
law states that the same current must flow in the other direction (on the
inside of the braid). Now at the dipole the current from the center
conductor only has one way to go: into 1 half of the dipole. But the current
on the inside of the braid has two paths: into the second half of the dipole
AND into the outer side of the braid. The actual distribution is determined
by the relative impedances these two other halves of the dipole have at the
specific frequency. Enter the choke, which effectively blocks the path to
the outer side of the braid. The same mechanism is true for
reception.
Today it's nice WX here! And in
NL?
What can I say… at least it
isn’t raining today. But unfortunately I have other duties
(QRL)
Overall an interesting
discussion!! I am curious if there are any other (competing) theories J
Regards,
Minto
pa3bca
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
