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Re: [RFI] Daisy-chain grounding [was DigiKeyer II]

To: rfi@contesting.com
Subject: Re: [RFI] Daisy-chain grounding [was DigiKeyer II]
From: donovanf@starpower.net
Date: Fri, 20 Dec 2013 15:42:34 -0500 (EST)
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
Note that the Motorola Standards and Guidelines for Communications Sites (page 
8-3) states that: 

"Braided wire shall not be used..." 


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

From: "EDDIE J EDWARDS" <eedwards@oppd.com> 
To: "jim@audiosystemsgroup.com" <jim@audiosystemsgroup.com>, rfi@contesting.com 
Sent: Friday, December 20, 2013 6:06:27 PM 
Subject: Re: [RFI] Daisy-chain grounding [was DigiKeyer II] 


Actually, you are correct in the sense that the R-56 grounding guidelines are 
not based on circuit board designs, but they are based on real world problems 
and experience on lightning protection at commercial transmitter comm sites so 
a different set of "circuit fundamentals" are applied for a different signal 
source. As they say, one size doesn't always fit all for differing electrical 
circuits. AC power circuits are based on different fundamentals from RF 
circuits or from audio circuits depending on the signal used. And RF ground is 
different from AC safety ground, etc... 

Motorola expanded their original R56 guidelines from around 50 pages to 518 
pages in 2000 in response to repeated equipment failures at radio sites related 
to poor installation practices when following their old R56 booklet. Lightning 
is not equivalent to a DC circuit or even a more complex signal like an audio 
or data circuit. Lightning protection, as I understand it, is based on creating 
a single current path with the lowest possible "impedance" for a lightning 
strike covering from DC to RF energy. 

The theory is "daisy-chaining" or series connections that create multiple 
connections on each device can create impedance "bumps" increasing overall 
impedance all along the chain. Even if only two wires are connected to the 
exact same point on a device, it is considered a "multiple-connection", i.e. 
more than one conductor/wire making contact. Lightning will find the path of 
least impedance (not least resistance) so these little impedance bumps might 
cause the lightning path to go somewhere unexpected like through some portion 
of the devices' circuit boards instead of the desired path. This is especially 
true if the multiple ground wires are incorrectly connected to multiple 
locations on the same device leaving a new possible path through the device's 
circuits instead of around it. 

Another problem with daisy-chaining in commercial set-ups is what happens when 
a technician temporarily removes one of the devices in the middle of the 
daisy-chain for repairs. The result is most of the other devices are left 
unprotected until the missing device is returned to service. You just cannot 
predict what service techs will do on a given day, but especially on a Friday! 

When Single-Point grounding is done correctly, it will greatly reduce (but not 
eliminate) the chance of lightning damage to equipment that operates 24-7. 
Motorola expanded use of single point grounding requirements with Motorola's 
R-56 in 2000 (updated in 2005) which can be found on-line at this link: (see 
Chapter 5 section 5.4.1 page 5-33,34 for specific daisy-chain info): 

M/A Com (back in 2001) also had or has a shorter 56 page version (ironically 
numbered), but also requires single-point grounding that meets the National 
Electric Code (NEC). (See 
http://www.repeater-builder.com/ge/lbi-library/t4618r3a.pdf see section 1.5.1) 
(Note: The M/A Com guide still refers to use of an internal comm room "Halo" 
which has been abandoned by most vendors today.) I believe NEC also forbids 
multiple wire connections (similar to daisy-chaining) for safety reasons in 
electrical circuits. 

All that being said, I'll repeat what I said originally: The typical or simple 
ham radio station can probably get away with some very short-runs of 
daisy-chaining (especially on computers) since most hams have human-operated 
layers of lightning protection such as disconnecting all antennas and grounding 
all the cables at the entry port during possible thunderstorm periods. If a ham 
does all that, he reduces his risk of the daisy-chaining having any impact. 

On the other hand, a multi-multi contest station should probably employ the 
same single-point grounding standards that the commercial guys do whenever 
possible. Also, I would not daisy-chain computers and RF devices together, but 
keep them separate if at all possible. Physical equipment layout to separate RF 
devices from computers would make this possible. I keep my computers on station 
left and RF on station right or middle with cable entry from the right. 

QST has had some good primer articles on lightning protection and grounding for 
ham shacks: a search for lightning in ARRL's Periodicals Archive found an 
article in 2002, June & July. I know there are others more recent. 

73, de ed -K0iL 

-----Original Message----- 
From: RFI [mailto:rfi-bounces@contesting.com] On Behalf Of Jim Brown 
Sent: Thursday, December 19, 2013 4:20 PM 
To: rfi@contesting.com 
Subject: Re: [RFI] DigiKeyer II 

On 12/19/2013 1:39 PM, EDWARDS, EDDIE J wrote: 
> But for anything connected to RF equipment, daisy chaining violates the 
> "guidelines" and specs for single-point grounding systems. This may be 
> acceptable as long as the overall shack has proper lightning protection in 
> all other cases. 

That is NOT correct. These so-called guidelines are based on a 
misunderstanding of circuit fundamentals. 

73, Jim K9YC 
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