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Coax question


The velocity factor is the speed at which an RF signal travels through a material compared to the speed the same signal travels through a vacuum.

The velocity of propagation is inversely proportional to the dielectric constant. Lowering the constant increases the velocity.

Generally, the higher the velocity factor, the lower the loss through a coaxial cable.

Hope that Helps..
 
"...and how does it affect antenna performance?"
For all practical purposes, velocity factor won't affect an antenna's performance at all. The only time it has any importance is when the length of the feed line is used for timing/phasing. Since that's not very common at all, it's not very important at all.
- 'Doc
 
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Phasing large antenna arrays with feedlines is quite common in broadcasting. Velocity factor is also important with quarter wave matching sections used in 4-element vertical dipoles.

http://members.rennlist.org/warren/SplitterCableDesign.pdf


W5LZ said:
"...and how does it affect antenna performance?"
For all practical purposes, velocity factor won't affect an antenna's performance at all. The only time it has any importance is when the length of the feed line is used for timing/phasing. Since that's not very common at all, it's not very important at all.
- 'Doc
Click to expand...
 
VO1KS,
I think I covered that. But since large arrays or stacked arrays aren't too common, it still isn't very important in the given context.
- 'Doc
 
I knew that the velocity factor was the speed that the RF travels through the conductor. I just didn't know what it affected. I noticed one cable was slightly less expensive than the other (both made by the same manufacturer). The only dfference I could see, in performance, was the velocity factor. I wanted to know if it was worth the extra $.15/ft for one over the other. Thanks for the replys and information.

My next question is, has anyone heard of the Davis RF Co.? They are located in Mass. and NH. They sell Belden, Times Microwave, and others. But they also make thier own cable. It isn't cheap, but I am currious about them.
 
roadrage,
With that difference in price I have to think that there's something else that's different besides just velocity factor. The only thing that 'VF' will affect is timing or phasing that the feed line is required to do. And then it determines how you would find the correct length for that timing or phasing. It really has nothing directly to do with how the feed line generally performs.
I know nothing about 'Davis RF Co.' so can't tell you anything about them. I can say that I really doubt if they make their own coax. More like they have their name put on some one else's, which is very common. Ask them who makes their coax, that's not an unreasonable question I'd think.
- 'Doc
 
R.F. Davis has been around for a long, long time, and are the exclusive manufacturer of Bury-Flex cable. Bury-Flex is a direct burial cable the size of RG-8/u, RG-213/U.

Important factors for cables are:
1) Shield density. The higher the % the better the cable. Double shielded, or double shielded with an aluminum foil will provide the best performance at a higher cost.
2) Dielectric. P.E. Foam will provide lower loss. Solid Poly will allow for very tight bends, but give a slightly higher loss. Another good, and rather expensive dielectric is Teflon. (The dielectric material is what affect the Velocity Factor that your asking about.)
3) Jacket material. You will pay more for a non-contaminating jacket, or one that is fire-retardant.
 
Although The velocity factor is the speed at which an RF signal travels through a material compared to the speed the same signal travels through a vacuum.

For most part this is more of an Technical issue rather then a Practical or usable issue
 
Well, I am looking at some RG8 and some RG8U and they are pretty close in electrical spec. The attenuation is very close to each other. One has 97% shielding and the other is dual shielded with 100%. The velocity factor is higher on the RG8 but if that doesn't matter, than they are pretty even in the performance arena. I just found some RG8u for just under $60 for 100' with 2 amphenol 259s vs. $80 for 100' of the RG8 with no connectors. Is there any reason not to buy the RG8U? It seems like a steal. My wife just bought shoes....I have justification.
 
W5LZ said:
Sounds like a deal to me. As for comparing it to buying shoes... go there at your own risk!
- 'Doc
Click to expand...

It does seem logical prid pro quo but they just never seem to think so. She always asks me "How many radios do you need?" or "do you really need more radio stuff?" So I asked her the other day just how many pairs of shoes she has. She just blushed, gigled, and smiled.
 
roadrage said:
Well, I am looking at some RG8 and some RG8U and they are pretty close in electrical spec. The attenuation is very close to each other. One has 97% shielding and the other is dual shielded with 100%. The velocity factor is higher on the RG8 but if that doesn't matter, than they are pretty even in the performance arena. I just found some RG8u for just under $60 for 100' with 2 amphenol 259s vs. $80 for 100' of the RG8 with no connectors. Is there any reason not to buy the RG8U? It seems like a steal. My wife just bought shoes....I have justification.
Click to expand...

rg8 and rg8/u are both ancient no longer used military MIL-C-17 specs that were superceeded by rg213/u about two decades ago, rg213/u itself is now known as M17/74, and rg58 is now M17/28, the better quality dual screened version formerly rg 214/u is now M17/75.

httphttp://www.awcwire.com/ProductTOC.aspx?id=M17/2-RG6-to-M17/77-RG216://

what does all this mean?

well simply put if your offered rg8 or rg8/u nowadays its either inferior most likely chinese made cable passing itself of as mil spec, or just as bad, its cable that's well over 20 years old which means its past its shelf life, look at how much poor quality cable masquerading as military spec there is on sale and you begin to understand why many people have coax problems.

as for amphenol connectors, are you sure they are real amphenol? amphenol is another brand that has been plagued by fakes, amphenol list all their dealers and the stock numbers they hold on their website:

Amphenol RF

an interesting story about amphenol 83-1sp / pl 259 uhf plugs and how fakes appear on e bay:

A story about a fishman, a shark and Amphenol # 83-1SP-1050 connectors


the radio/electronics game is the same as most other businesses, you need to know what your buying to avoid being ripped off, ignorance is one blissful way of definately being ripped off.
 
RG8 is a class of cable that has nothing to do with attenuation, power rating, or material. It refers to the impedance and physical size regardless of what is stamped on the cable. RG8 is 50 Ohm cable that measures .405". The problem is that what materials are used and how it is made changes the electrical specifications of the cable.
Times Microwave makes an RG8 class and RG8X class cable. It is LMR400 and LMR240. Belden also makes RG8 and calls it 9913. RG213 and RG214 are both RG8 class cable as well. So not all RG8 is created equal. Some is much better than others. So it is stamped with a product designation other than its respected class to differentiate between one cable over another for the consumer to choose the correct cable for their particular usage. An example, standard RG8X has attenuation of 2.0 @30Mhz and Times Microwave RG8X (LMR240) is 1.3 @30Mhz.
It makes it difficult for people like me. I can (and have) research and know lots about coaxial cable and still have no idea of who is making it well and who isn't.
Experience trumps book knowledge. I have little experience with coax and only know of a few of the quality companies (Belden, Mil Spec, Times Microwave). Like you said, many lesser known companies sell China made cable that may carry the RG8 designation but be of inferior quality or it could be very old and outdated stock. It is that point alone why I am asking here. People have experience of what companies make quality and what to stay away from. The RG8U I found for $60/100' is made by Advanced Digital which is made in the USA. But I have never heard of them before. I checked them out and the picture of their plant looks less than sophisticated, not that what the plant looks like means much. It may be inferior quality, it may be good. It may just be a lesser known company that cant charge the premium prices. I just don't know. I should stick with a HAM supplier for my cable. That way I know I'm not getting old stock. I guess its worth the extra $20.
 

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