[R-390] SWL antenna

[Dan Merz]

Tim,  the use of twinax interests me.  Can two lines of coax be used and be
equivalent to twinax ?    It might be easier for me to find coax than
shielded twinax at reasonable cost  It would seem the usual problem with
coax is that the sheath brings the noise in from whatever noise source it
goes by,  because the sheath is involved in getting the signal from the
antenna as part of the ground return for the antenna.  It would seem the
users of coax argue that putting the ground return out of the local house
noise field at the antenna helps eliminate this problem,  with a transformer
at that location and no direct connection to the coax sheath at that point.
Shielded twinax puts both legs inside a shield and to  the extent that the
shielding works seems a more direct approach rather than relying on a
dirt/soil ground connection for the antenna circuit with the receiver.  I'm
thinking all this has been beaten to death before but I still need some
education and appreciate the responses.  I have one 20 ft length of the
shielded twinax that I obtained to get the two connectors to mate to my 390
and 390a receivers.  It's about .330 in. diam. and I use it with a jumper to
connect to my twin lead coming from the antenna I now have.      Dan.  

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Tim Shoppa
Sent: Wednesday, November 08, 2006 5:55 AM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] SWL antenna

John Kolb wrote:
>At 04:37 PM 11/6/2006, Dan Merz wrote:

>>  But I've been
>>wondering about the transmission line across and inside the attic
house
>>about 40 feet across and down about 16 feet into the basement.  My
first
>>idea was to use one of the better .240 diam coax types either 50 or
75 ohms
>>but then I starting reading about losses when mismatch exists and
thought
>>maybe RG8 might be worthwhile.  Or will I be just as well off using
the
>>cheap 300 ohm foam twin lead that I now use.

>The RG-8 won't work any better than the smaller coax and will be a lot 
>stiffer and harder to handle

>>  I don't expect much noise from
>>things in the house but do know the current washer is a problem when
it's on
>>- probably has more electronic controls than the older one that I had
for
>>years with mechanical switching.  My current system picks that up
because
>>the lead-in wire drapes over toward the laundry room.  But I can hear
it on
>>a portable am radio operating on its internal antenna as well.  I
usually
>>lose interest in listening when the washing machine is running.

>Coax instead of twin lead should help with pickup from local QRM such
as
>washing machines.  Noise of your current washer probably comes from the 
>motor, not the controls.

To build on top of what John and Roy have said:

Even better than coax is twinax or whatever they call shielded balanced line
today. I got a bunch and it works wonders on MW and LW along with a
magnetically shielded loop antenna.
Loops have a very real advantage in that they have a deep null that is
useful for eliminating a single point source of interference. For the higher
SW bands I have a dipole in the air connected via the twinax. In all cases
the reduction in local RFI is profound compared to coax (worst) or coax and
a balun (slightly better than just coax but still not wonderful).

And the twinax is wonderful for use with the 390A, with it's balanced
antenna jack. You're using it exactly as it was designed. Remember that the
390A was often used with noisy mobile generators and in environments with
lots of noise.

Twinax is often found surplus (meaning you should dumpster-dive!) from IBM
mainframe deinstallations, or you can buy it in quantity and new on E-bay or
other places.

Tim.
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