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Cable Lengths vs. signal

When wiring from a transceiver thru filters, amps, switches, couplings to the antenna

  • The lengths of cable should be kept as short as possible.

    Votes: 20 100.0%
  • Thelength of cables and run needs to be (some) fraction of the wave length.

    Votes: 1 5.0%

  • Total voters
    20
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jassing

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I have tried to do research (on the web) with regards to a question who's answer seems obvious to me; but I have been known to be wrong. Yesterday, at a gas station; got to talking to a guy with a ham rig in his car -- and we got around to discussing cb's.

It seems I can go to one site and read one way of thinking; and read another site and get a different way of thinking -- both ways are mutually exclusive. Even from vendor websites, it seems to vary.

I remember a psych class lesson that said as long as "hysteria" doesn't enter into it; collectives usually are right (collective as in a group of people will usually come up with the right answer to a given problem).

So -- Which is right? If you have math/physics/electrical laws to back it up; please explain.

I think it's pretty obvious; but maybe the guy with a "technicians license" does know something I don't. When it got down to "doing the math" he suddenly had to leave.
 
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j0nnyb1aze

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I think I missed what the actual question was.

From the topic: Cable length will have an effect on signal strength. The longer the cable the more it will attenuate(weaken) your signal due to the resistance of the cable. Coax is gennerally rated in dB of loss per 100ft.
 

mpddigital

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CB like most RF transmission antenna coax should be 50 Ohms. The cable run should be as short as possible. Transmission loss is determined by the type, construction and quality of the cable used. Longer runs need better (larger) coax for less loss as a general rule. CB is low frequency and has less propensity for loss than higher frequencies.

Hope this helps
 

jassing

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I think I missed what the actual question was.

From the topic: Cable length will have an effect on signal strength. The longer the cable the more it will attenuate(weaken) your signal due to the resistance of the cable. Coax is gennerally rated in dB of loss per 100ft.
So you're vote is for "shorter =better" not the "needs to be 18' or fractions there-of (like 3') " to be a partial wavelength. (that's what I think) but this guy said "you need to use 18' from the antenna, and anytime you insert something , it needs to be a fraction (3', 6' or 9')"
 

jassing

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The story about using a fraction of a wavelength of cable is an old wives tale perpetuated by CB shops.
Exposing the 18' CB Coax Myth
And apparently some "ham technicians" as well... I was honestly shocked when I went to some websites of manufactors of antennas and the like, and they pushed this as well.

I'm glad I'm not alone... And it just proves, "a technicians license" doesn't mean they ARE technicians.
 

W6KRU

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And apparently some "ham technicians" as well... I was honestly shocked when I went to some websites of manufactors of antennas and the like, and they pushed this as well.

I'm glad I'm not alone... And it just proves, "a technicians license" doesn't mean they ARE technicians.
It's been a while since I took my tests but I think that antennas and transmission lines are only covered at a basic level on the technician and general tests. The discussion starts to get serious on the Extra exam.
 

mmckenna

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Exactly. My wife has a technician amateur license, but she passed the test in one of those all day cram sessions and then quick take the test before you forget the right answers things. She knows how to work a radio, knows her call sign, but that's about it. A technician license just means you memorized enough of the question pool to squeak by. She is my soul mate, and I love her, but radio tech she ain't.

You can use tuned lengths of coax to hide antenna problems (phasing harnesses). You can use a tuned 1/4 wave stub grounded at the end as a lightning protector. So yes, an exactly correct length of cable can cause some issues but that is rare. You are much better served by cutting the cable to the length you need an then doing a good job of putting the right connector on.

If the "must use 18 feet or whatever" thing was true, it would depend on what exact channel you were using. Anyway, do you really think that some assembly line worker is sitting there with a high precision measuring tool measuring out the coax for your $40 "good buddy" brand CB antenna? Answer = NO. They cut it to the length they were taught to using the mark on the bench and that old worn pair of cutters.

I've installed likely 100 or so antennas of one sort or another over my career, and I've always cut the coax to the correct length of the specific installation and I have never had an issue. Every tech I've ever worked with has done the same thing.
 

mmckenna

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I'll add that there are a lot of variables here, and the 18 foot, or whatever thing completely ignores the variables. Velocity of Propagation for Coaxial cable varies by design, and that would affect the length of the coax if this was really true and really and issue. I doubt most CB'ers would know that, so I think they are just repeating what they heard. Time to ignore the talk and BS and get down to radios!
 

majoco

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The cr@p about feedlines being a multiple of one wavelength only applies if you have a mismatched antenna (ie, one that gives a high VSWR) and a tuner at the the transceiver. Then the VSWR that you see at the tuner is the real VSWR and not something changed by not having a multiple of the wavelength in the feeder - thus by adjusting the tuner you can "tune" the antenna. BUT because there is really a high VSWR, there may be some high voltage peaks in the coax resulting int the odd spark or two. Best solution is to tune the antenna with a short bit of coax of the right impedance and a good VSWR bridge, then reconnect with the same impedance coax back to where you want the rig. Attenuation in the coax hides the VSWR.

Cheers - Martin ZL2MC - a professional radio engineer since the mid 1960's! :)
 

j0nnyb1aze

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So you're vote is for "shorter =better" not the "needs to be 18' or fractions there-of (like 3') " to be a partial wavelength. (that's what I think) but this guy said "you need to use 18' from the antenna, and anytime you insert something , it needs to be a fraction (3', 6' or 9')"
Yes. Somehow I totally missed the poll at the top, must have been a long day. If changing your coax length affects your SWR then it(the coax) is radiating, potentially into your shack/car. This will possibly lead to RFI(interference) issues.
 

LtDoc

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The only sure thing changing feed line length affecting the SWR can tell you is that the load on the end of that feed line, the antenna, isn't tuned very well. Which means that it isn't near the same impedance as that feed line (and transmitter's output). It doesn't necessarily mean that the feed line is radiating. It might be, but it's certainly not a sure thing.
- 'Doc
 

SCPD

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Most CB VSWR/Power meters aren't actually power meters but volt meters, so inserting the meter at different lengths of the coax will give you different readings only because the voltage at different points varies. It has no relation to the actual power delivered to the antenna. The only time coax lengths are important are in phasing networks such as those on stacked dipole antennas, duplexers, etc. or in the case of tuned stubs, as mmckenna mentioned.
 
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estefan2020

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I have tried to do research (on the web) with regards to a question who's answer seems obvious to me; but I have been known to be wrong. Yesterday, at a gas station; got to talking to a guy with a ham rig in his car -- and we got around to discussing cb's.

It seems I can go to one site and read one way of thinking; and read another site and get a different way of thinking -- both ways are mutually exclusive. Even from vendor websites, it seems to vary.

I remember a psych class lesson that said as long as "hysteria" doesn't enter into it; collectives usually are right (collective as in a group of people will usually come up with the right answer to a given problem).

So -- Which is right? If you have math/physics/electrical laws to back it up; please explain.

I think it's pretty obvious; but maybe the guy with a "technicians license" does know something I don't. When it got down to "doing the math" he suddenly had to leave.
Cable length, use as little cable as can be, but also use the correct cable. long cable runs 30'ft or shorter can use rg-8x and be ok, though that cable can be better, lmr-100 lmr-400 lmr-600 for examples you can google the rg and lmr and see the resistance per 100ft, the less resistance in longer cable runs the more signal at the antenna. buy the best cable you can afford.

You can make your own tripod/6pod out of 2 10ft emt conduit and a use it for a ground plane / counter posie (I did) and that and really cut the swr down, hardly even moves. without the 6pod I made the swr was high.
 
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