[CQ-Contest] Station Re-Build - Coax

Fri Mar 15 13:22:19 EDT 2019

Hi Scott, 

In most single op stations d ouble shielded coax is an unnecessary 
expense if common sense RFI avoidance practices are followed, 
such as: 

- not bundling coaxial cables with other cables, and 

- not routing coax carrying microvolt signals in close proximity 
to strong RFI generators such as computers, computer monitors, 
lighting fixtures, switching power supplies and heating and air 
conditioning equipment. Its best to use double shielded or solid 
shielded coax if you must route coax cables in close proximity to 
RFI generating electrical equipment. 

If you for some reason you must bundle coax cables, always avoid 
bundling coax cables carrying microvolt signals (cables from receiving 
antennas or antennas used for both transmitting and receiving) with 
- coax cables used for transmitting. 
- video monitor cables 
- computer network cables 
- AC power cables 
- or any other cables potentially carrying high level RF signals. 

Double-shielded coaxial cables such as RG-223, RG-400 and 
RG-214 carrying microvolt signals can usually be bundled with 
with double shielded cables carrying high level signals but great 
care must be exercised when installing connectors on all of the 
bundled cables to achieve the greatest possible shield integrity 
between the coax cable shields and the connectors. I ts never good 
practice to bundle coax carrying microvolt signals with unshielded 
AC power cables. 

73 
Frank 
W3LPL 

----- Original Message -----

From: "K9MA" <k9ma at sdellington.us> 
To: "Alan Higbie" <alan.higbie at gmail.com> 
Cc: cq-contest at contesting.com, donovanf at starpower.net 
Sent: Friday, March 15, 2019 2:36:34 AM 
Subject: Re: [CQ-Contest] Station Re-Build - Coax 

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 at 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 at 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 at gmail.com> 
>> *To: *cq-contest at 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 at gmail.com 
>> _______________________________________________ 
>> CQ-Contest mailing list 
>> CQ-Contest at contesting.com 
>> http://lists.contesting.com/mailman/listinfo/cq-contest 
>> 
>> 
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