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Tuning jumpers and coax length

Does the length matter or not

  • Yes tunning length and coax length make a diffrence

    Votes: 123 90.4%

  • No the length will not afect the tunning

    Votes: 13 9.6%
  • Total voters
    136
no, unfortunately we're not........

"reflection loss resulting from Zl being something other than Zo represents ONLY the unavailability of power to the load because the load absorbs and radiates all the power the generator makes available to the line, regardless of the value of Zl."
 
Dang! Ok Well I think Im starting to see how the hole sytem works. Guess I need to get some thing and play around maybe? Seem to me that if everyone who says coax length dont matter I could change the length out from my antron and swr would be the same. That I understand. But what about tunning and fooling swr meters, is the whole six and 3 foot cable deal what should be used and this will stop any incorect meter readings? Maybe and swr meter that shows reflected power would be a good investment.
 
freecell said:
no, unfortunately we're not........

"reflection loss resulting from Zl being something other than Zo represents ONLY the unavailability of power to the load because the load absorbs and radiates all the power the generator makes available to the line, regardless of the value of Zl."
Click to expand...
If it is unavailable to the load (antenna), it is loss.

And yes, we are done. At least, I am. It's just not worth it to continue. So, for what it's worth, "Freecell is 100% right about everything and you only need his input. My input is no longer valid."

There, now I can go and play radio, with a very efficient, low loss antenna system (I-10K) and any length coax I want. :D

Master Chief is requesting permission to go ashore. DING DING, DING DING, Master Chief, departing..........
 
s

Without arguing the point (actually, since I have never measured a 'certain' length of coax in ANY system I have every run) what's all the fuss anyway?

But the coax length question seems to be most prevelent on CB forums--9 feet, or 18 feet, 3 foot jumpers, ad infinitum. But we CAN agree that if that is true for 27 MHZ, it would be true for ALL frequencies, right? :) So how can a system effectively cover ALL frequencies from 3.5 MHZ thru 54 MHz usually with an SWR of 1.5 to 1 and do so with ONE random length of coax? That is what a screwdriver mobile antenna does! It covers all those "channels" with an average SWR of 1.2 x 1 AND with a random length of coax. Mine happens to be about 7 feet long and the criteria for it was whatever it took to get from the back bumper to the radio. I have run the SAME antenna in a different vehicle with a coax run of 8 inches with absolutely NO effect on SWR or the performance of the radio.

Yet, by what the coax length folks say, I would have to have a length of coax of around 60 feet for 3.5 MHZ, another at 33 feet for 7.2 MHZ, another of 16.5 feet for 14 MHZ, another at 13 feet for 18 MHZ, another at 11 feet for 21 MHZ, 9.3 feet for 24 MHZ, and around 8.3 feet for 28 MHZ---AND a whole bunch of others for non-ham frequencies I am authorized in the HF bands in between. SO, if the coax "rule" was hard and fast, and you gotta have "9" feet, "18" feet and so on for 27 MHZ, then I would have to have over 12 runs of coax in my vehicle of various lengths in order for my antenna to work! Where would I put all those coaxes in an S-10 pickup? :p However, without worrying about formulas, bothering with Velocity Factors, I work the entire world with a nice mobile signal on ALL those frequencies with ONE antenna that is RESONANT at each frequency I visit with ONE coax that is about 7 feet long--and it is 7 feet long simply because THAT is the length equal to that required to reach from the radio to the antenna.

None of this is meant to argue. If you wish to fiddle with coax length, fidget with formulas, fuss with velocity factors (which simply means ONE thing: the speed at which a radio signal travels thru a solid medium) it is perfectly OK. Me? I will choose the appropriate antenna, play out the coax needed to reach, and shorten or lengthen the antenna itself to resonate it, and achieve the lowest possible SWR-the same way I did it before anyone ever told me that I had to have a "certain" length of coax over 35 years ago. :p

73 to all,

CWM
 
O

Oh, I get it! RF theory only applies to 27 MHZ, right? :p All I am saying is, what's good for the goose is good for the gander.
So regardless of what antenna it is, IF I *must* have a "certain" length of coax for 27 MHZ, then I must have a "certain" length of coax for 3.5 MHZ---which would be about
60 feet. That would be awfully awkward for a compact pickup truck. Where would I PUT 60 feet of coax in a little Chevy?
The type of antenna I have happens to be, if properly installed,
RESONANT at each and every frequency I visit, not "tuned" via "tuners" or coax length with ONE feedline of about 7 feet chosen simply because it was was required to REACH! :D .
But what you are saying is, if I build a center-loaded antenna for 28 MHZ I would have to have a "certain" length of coax and if I build a center-loaded antenna for 3.8 MHZ, the coax for EACH dedicated mobile antenna would have to be a "certain" length. And, again, if I make an antenna for 7.2 MHZ, it would have to be a "certain" length--and so on and on. Thus, if I wished to change to another band, I would not ONLY have to swap out the antenna on the car, I would also have to either swap out the coax, OR keep about 12 different coaxes somewhere in the truck all cut to a "certain" length! THAT IS what you are saying! And again, if RF theory applies to 27 MHZ, it also applies to 3.5 MHZ, or 146 MHZ, or 1.8 MHZ! But in actual practice, the theories are a waste of time because the results of figuring and fussing over the "correct" length of coax are not measurable. When it comes to SWR, it is actually funny to see someone strain for hours to get their SWR down flat when 1.5 is perfectly acceptable: the difference in the precieved signal at the other end would not be detectable! However, again, I cover an infinite range of HF frequencies with ONE antenna and ONE feedline---including CB--and do it just fine without worrying if my coax is the "right" :p length!

So without being disagreeable, I will continue to install radio systems without EVER consulting formulas (yes, they are there and can be used at times), and do it with an average SWR of 1.2 from 28 to 3.5 MHZ.

Have a happy day all!

CWM
 
Ready for this?

The thing that differentiates all of the preceding 'stuff' is where you do the impedance matching. Using various lengths of coax can work. Or, use an inductive/capacitive device to match impedances (wire on a torroid). What's the big deal? Just in ~how~ it's done, not ~if~ it's done. The difference between the two methods is primarily that a coaxial matching system tends to have more losses, or a greater degree of potential loss, as in catastrophic. Coax just isn't as 'forgiving' as other matching systems. How 'forgiving' a torroidal system or some kind of other impedance matching system is depends on how it's built. Or in not abusing it by running gobs of power it wasn't designed for. Same thing holds true for using coax, but coax just doesn't have the physical characteristics to stand up to the abuse like other ones do.
Which should you use? Whichever turns you on, I guess. Wanna use coax? Fine, then you will need to understand how all this matching stuff works so you can select the right length. Wanna use some other means? That's fine too. You still need to understand how all that stuff works so you can select the proper sizes of components. There's almost always more than one way of skinning the cat. Some tend to be 'better' than others. It's your cat, skin it how ever you want! But don't think that there's only the one way.
- 'Doc

PS - Tried the 'screwdriver' antennas, went back to the BugCatcher. Didn't more around in frequencie all that much, and mounting the @#$ screwdriver was a royal pain. Also, being naturally 'cheap', I wouldn't pay what they're asking for them now.
Now if I can figure out where the @#$$ to mount a radio in my truck, I'd still use the BugCatcher. Still 'cheap'... or 'frugal'...
 
"THAT IS what YOU are saying!"

no that's not what i'm saying and if you bothered to actually read and understand anything i've been discussing here you would know that. on the other hand, most of your posts in this thread read more like advertisements for your CC antenna. don't you have some truck drivers to chase off of the 10M band or something?

"the theories are a waste of time because the results of figuring and fussing over the "correct" length of coax are not measurable."

i find the above statement pretty damn funny, especially when followed (or preceded) by this one........

"since I have never measured a 'certain' length of coax in ANY system I have every run."

yes they are measurable....here's a sample.

RG-8M, 0.78 VF.
Zo = 50 ohms
Zs = 50 ohms
Frequency: 27.185
Load Resistance: 33 Ohms
Load Reactance: 0 Ohms
with given length from
load to source....
line input Z = :

50.001 ohms @ 3.497'....1/8 wave....0.125 wl @ 45 deg.

75.758 ohms @ 6.994'....1/4 wave....0.250 wl @ 90 deg.

49.997 ohms @ 10.491'...3/8 wave....0.375 wl @ 135 deg.

33.000 ohms @ 13.9875'..1/2 wave....0.500 wl @ 180 deg.

49.997 ohms @ 17.484'...5/8 wave....0.625 wl @ 225 deg.

75.758 ohms @ 20.981'...3/4 wave....0.750 wl @ 270 deg.

50.001 ohms @ 24.478'...7/8 wave....0.875 wl @ 315 deg.

33.000 ohms @ 27.975'...ONE wave....1.000 wl @ 360 deg.

look over the listing above while refreshing your memory
with this post form thread #3.

"that's because when the load impedance is greater than the characteristic impedance of the feedline a maximum impedance point occurs every half wavelength from the antenna and a minimum impedance point occurs at every odd quarter wavelength point between. when the load impedance is less than the characteristic impedance, the maximum and minimum points swap places. at all points between the odd quarter wavelengths and the even half wavelengths exists an entire range of impedances in between the upper and lower limits. here's something else interesting. in the example presented here you will also find that at every odd 1/8 wavelength down the line away from the load that approximately 50 ohms can be located. this 50 ohm figure can be found approximately every 42.375 inches along the line away from the feedpoint. this figure can and will vary slightly from one sample to the next."

"EVERY LENGTH OF COAX OF ANY CHARACTERISTIC IMPEDANCE (Zo) IS AN IMPEDANCE TRANSFORMER." (L.B. Cebik)
 
y

Yes, freecell, I HAVE read it. And all I am saying is it simply isn't necessary to have a "certain" length of coax. If formulas float yer boat, it's all good. And what Doc is saying is all good, too. I don't have a thing to prove nor do I really care about it. I can ignore all those formulas (and, yes, I do know what they are for), select a dedicated antenna for a particular band--or a multi-bander such as the SD, measure enough coax to reach from the antenna to the radio, adjust the radiating elements of the antenna itself (are we getting a bit defensive here?), and get a perfectly acceptable SWR! And, like Doc says, much impedance matching can be done with capacitors, toroids and small coils jumpered to ground---especially for those frequencies below 8 MHZ.

As to selling antennas, I had a person inquire about the CC antenna for his CB set. I told him that that type of antenna was "overkill" and a waste of good money--hardly an indication of trying to "advertise" any product. RF theory applies to any
antenna, any set of frequencies. So, again, if I have to have a "certain" length for 27 MHZ, then I have to have a "certain" length for 3.8 MHZ (where, yes, the IMPEDANCE matching is done at the feedpoint via a small coil to ground-my preferred method). So regardless of the KIND of antenna, the same formulas IF needed (and they mostly aren't) apply to ALL.

So I now bow out of this topic and let you "win" cuz 'frankly I don't give a damn'. I've been installing HF radio systems since 1966, so I'll just keep doing it the same "dumb" way I've done it for 39 years, OK? This is simply not worth the effort. :D

73 to all,

CWM
 
Mc on a previous post you said it changes the readings on the meter but does not correct anything. So to me you are saying the bird is being fooled I was just wondering how it changes the readings without fixing the problem.
 

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