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
