Many years ago, I designed a pulse amplifier for one of the Hubble Space
Telescope instruments. It had a lot of gain and a 100 MHz bandwidth, so I did
the cable shielding calculations, concluding that it needed a double shielded
cable from the detector to the preamp. It was a pain to get the double shields
into the connectors, so the technician used a single shielded cable, assuming I
was just an overly anal engineer. The output was terribly noisy, until we
replaced that cable with the double shielded one. Sometimes it really does make
a difference.
73,
Scott K9MA
----------
Scott Ellington
--- via iPad
> On Mar 14, 2019, at 6:03 PM, Alan Higbie <alan.higbie@gmail.com> wrote:
>
> Frank ~
>
> Agreed. Bundling cables (especially single-shielded coax) is a terrible
> practice.
>
> The original question in this thread was whether to use single-shielded or
> double-shielded coax.
>
> My set-up is for SO2R - - and I have recently realized that my RADIO 1 and
> RADIO 2 output cables run "bundled" for about 25 feet. A terrible practice
> (I admit).
> So obviously I have an opportunity to mitigate leakage between these
> cables.
> And, upon reinstall I will also separate the cables from each other (and
> from a parallel AC line!!).
>
> Additional context from the US Navy study:
>
> "7. MITIGATION TECHNIQUES
> . . .
>
> 7.5 Cable Leakage
>
> Examples of signal and noise leakage into coaxial cables are provided in
> Section 5.3, and Figure 11 in that section provides measured values of
> cable-to-cable isolation for typical flexible coaxial cables. Note that a
> 0-dBm signal in a single-shielded cable with only 80 dB of isolation to
> another similar nearby cable will result in a -80 dBm signal in the second
> cable. If a receiver connected to the second cable has a noise floor of
> -130 dBm for a 3-kHz bandwidth, the leakage signal will be 50-dB above the
> noise floor of the receiver.
>
> The following procedures will eliminate emission leakage problems in RF
> paths.
>
> · For long coaxial cable runs, use low-loss and solid-shielded coaxial
> cable such as Times LMR series or Andrew Corporation Heliax cable.
> Carefully check the total attenuation for the length needed from the
> manufacturer‚s literature, and use an appropriate size cable.
>
> ***· Never use single-shielded coaxial cable for any application in a
> receiving site, even for very short coaxial cables.***
>
> ***· Always use double-shielded coaxial cable. ***
> Where flexible cable is needed for short 50-Ohm runs, use a cable such as
> MI7/84-RG-223/U or one with equivalent shielding."
>
> 73, Alan K0AV
>
>
>
>
>> On Thu, Mar 14, 2019 at 4:53 PM <donovanf@starpower.net> wrote:
>>
>> Hi Alan,
>>
>> Your quotation from the Navy study omitted the important details
>> described in the next few paragraphs.
>>
>> The context was multiple single shielded coaxial cables bundled
>> with cables radiating interfering signals into the victim cables.
>>
>> Double shielding greatly reduces this problem but its also very
>> effective if coaxial cables feeding microvolt signals to receivers are
>> not bundled with cables carrying high level signals, a terrible
>> practice.
>>
>> 73
>> Frank
>> W3LPL
>>
>> ------------------------------
>> *From: *"Alan Higbie" <alan.higbie@gmail.com>
>> *To: *cq-contest@contesting.com
>> *Sent: *Thursday, March 14, 2019 2:59:07 PM
>> *Subject: *[CQ-Contest] Station Re-Build - Coax
>>
>> Relevant to this discussion is an extensive study conducted by US Navy:
>>
>> THE MITIGATION OF RADIO NOISE AND INTERFERENCE FROM ON-SITE SOURCES
>> at RADIO RECEIVING SITES (November 2009) by Wilbur R. Vincent, George F.
>> Munsch, Richard W. Adler, Andrew A. Parker. (By the way, each of the
>> study's authors is a ham.)
>>
>> The authors surveyed noise floor problems at 40 Navy receiving sites around
>> the world. They drew on data from thee Navy's
>> Signal-to-Noise-Enhancement-Project.
>>
>> They had many recommendations and conclusions. But the most relevant to
>> the original question of this thread is this one:
>>
>> *"5.3 Cable Leakage*
>> *Leakage of noise and other spectral components into RF cables running from
>> antennas to receivers has been noted at all receiving sites that use
>> single-shielded coaxial cables. Receiving sites using high-quality
>> double-shielded coaxial cable and properly-assembled coaxial connectors
>> seldom encounter cable-leakage problems." *
>>
>> I am currently in the midst of replacing my "single-shielded coax" with
>> "high-quality double-shielded" RG400.
>>
>> I recently purchased 200 feet of Harbour Industries RG400 from Electro
>> Enterprises, Inc. for $1.43 per foot. It came with certification as
>> factory new MFG certificate.
>>
>> A link to the US Navy study can be found at:
>> http://www.arrl.org/radio-frequency-interference-rfi
>> (scroll down to bottom of page under heading Naval Postgraduate School RFI
>> Handbooks)
>>
>> 73, Alan K0AV
>> alan.higbie@gmail.com
>> _______________________________________________
>> CQ-Contest mailing list
>> CQ-Contest@contesting.com
>> http://lists.contesting.com/mailman/listinfo/cq-contest
>>
>>
> _______________________________________________
> CQ-Contest mailing list
> CQ-Contest@contesting.com
> http://lists.contesting.com/mailman/listinfo/cq-contest
_______________________________________________
CQ-Contest mailing list
CQ-Contest@contesting.com
http://lists.contesting.com/mailman/listinfo/cq-contest
|