Use of stranded wire versus solid wire for RF purposes

Thread Starter

drc_567

Joined Dec 29, 2008
1,156
When working with higher frequencies, such as RF, it has been suggested to utilize stranded wire when larger diameters are required. The reasoning behind this is that the RF current tends to flow on the external surface of the conductor, whereas direct current propagates through the entire cross section of the wire. Typically, this might be of consequence when sizing a ground wire for some type of apparatus. As an example, consider a 1 ft length of #12 wire:
for solid wire, the peripheral surface area for 1 foot of wire length is :
3.046 square inches
for #12 stranded wire with 7 strands, the relevant surface area is:
8.05 square inches
for #12 stranded wire with 19 strands, the area in question is:
13.25 square inches

work sheet attached ...please excuse the handwriting
 

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nsaspook

Joined Aug 27, 2009
13,086
For short distances it's unlikely to make much difference. The propagation on a ordinary stranded wire is usually slower than solid at RF and the loss is higher. For a RF grounding point you want the lowest possible inductance. That's usually a smooth flat surface.
 
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ian field

Joined Oct 27, 2012
6,536
For short distances it's unlikely to make much difference. The propagation on a ordinary stranded wire is usually slower than solid at RF and the loss is higher. For a RF grounding point you want the lowest possible inductance. That's usually a smooth flat surface.
At high RF; the wire in an inductor is only a short length - its common practice to use silver plated wire for minimum resistivity at the surface where the skin effect is.
 

Hypatia's Protege

Joined Mar 1, 2015
3,228
When working with higher frequencies, such as RF, it has been suggested to utilize stranded wire when larger diameters are required. The reasoning behind this is that the RF current tends to flow on the external surface of the conductor, whereas direct current propagates through the entire cross section of the wire. Typically, this might be of consequence when sizing a ground wire for some type of apparatus. As an example, consider a 1 ft length of #12 wire:
You will realize very little if any advantage where the 'strands' are not electrically insulated from each other --- On the other hand, 'cables' formed of individually insulated strands so arranged as to mitigate (magnetic) proximity effects are highly effective to a few megahertz... Construction info and theory may be found under 'Litz wire' --

Note: I do not recommend reference the Wikipedia article on this subject (for it's over-qualitative, generally 'dumbed down' treatment of same)!:rolleyes:

Best regards
HP:)
 

Thread Starter

drc_567

Joined Dec 29, 2008
1,156
So, which material would be the better choice to connect a 10 ft. ground wire from a transceiver to an earth ground, operating at about 14 MHz?
1. A length of #12 gauge solid wire
2. a length of 1/2" wide copper braid
 
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nsaspook

Joined Aug 27, 2009
13,086
So, which material would be the better choice to connect a 10 ft. ground wire from a transceiver to an earth ground, operating at about 14 MHz?
1. A length of #12 gauge solid wire
2. a length of 1/2" wide copper braid
What is the reason for the ground on the transceiver? If the connection to the antenna is a matched shielded coax/balanced feeder and the transceiver is shielded then the absolute RF properties of the ground connection at the transceiver IRT communications might not be very important.
 
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Thread Starter

drc_567

Joined Dec 29, 2008
1,156
In my limited experience, there is not ever a perfect match. The goal is usually to minimize the SWR. It would be interesting to see what happens with an ungrounded chassis though.
 

nsaspook

Joined Aug 27, 2009
13,086
In my limited experience, there is not ever a perfect match. The goal is usually to minimize the SWR. It would be interesting to see what happens with an ungrounded chassis though.
Generally if you have appreciable RF energy moving from the radio case to ground instead of the transmission line feeder then your match is not even close or something is completely out of whack.

http://www.w8ji.com/rfi_rf_grounding.htm
RF in the Ham Shack
It is commonly assumed "RF in the shack", interference to consumer devices like telephones or stereos, or even RF feedback or RF burns result from poor operating position equipment grounding. There is also a belief that a good equipment RF ground improves our transmitting signal strength, and the lack of an RF ground causes poor reception. Another popular belief is each piece of desktop equipment needs to have separate leads to a grounding point, so RF doesn't move from the chassis of one piece of gear to the chassis of another through daisy-chained cabinet grounds. Sometimes we read that beads or isolators are recommended on coaxial cables running between pieces of equipment. Like most of us do at one time or another, I strongly believed in RF grounds on my desk.

The beliefs above are actually not true except in very specific (and uncommon) installations! Eventually I learned RF grounds, very often, didn't do anything but mask more significant system problems.
 
You will realize very little if any advantage where the 'strands' are not electrically insulated from each other
This is an assertion often found in articles on Litz wire. However, recent research (http://thayer.dartmouth.edu/inductor/papers/stranded.pdf) has shown that using stranded wire with uninsulated strands can give a substantial performance advantage over solid wire in the hundreds of kilohertz frequency range, such as commonly used in switching power supplies. The advantage is not as great as with true Litz, but it is had at much lower cost than Litz.
 

SLK001

Joined Nov 29, 2011
1,549
So, which material would be the better choice to connect a 10 ft. ground wire from a transceiver to an earth ground, operating at about 14 MHz?
1. A length of #12 gauge solid wire
2. a length of 1/2" wide copper braid
At 14MHz, either would suffice, but I would lean toward the solid copper wire (the lower the gauge, the better).
 
At 14MHz, either would suffice, but I would lean toward the solid copper wire (the lower the gauge, the better).
FWIW -- Geometries that maximize surface area (e.g. 'straps' or 'flat' braids) are most suitable -- Note that while 'poly filament' arrangements increase surface area, they offer little in the way of mitigation of proximity effects at frequencies much above 5 MHz and, hence, are hardly worth the 'complexities'/expense of fabrication...

Best regards
HP
 
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