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
