This is the book that i bought maybe 15 years ago that started to change my mind on so many myths from cb shops and truckdrivers (which i am one). Best 20 something dollars i spent.
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A few comments, amateur radio vs CB
- Thread starter C W Morse
- Start date
This is the book that i bought maybe 15 years ago that started to change my mind on so many myths from cb shops and truckdrivers (which i am one). Best 20 something dollars i spent.
Bioman,
No, you are not doing it wrong.
Some facts.
The length of feed line used can be used for impedance transformation. That's good in that there aren't a zillion 'sizes' of coax to match whatever an antenna's input impedances. It's bad in that that impedance transformation can, and sometimes does, produce voltages/currents that will harm the feed line doing the transformation. It's a very quick-n-dirty way of doing it. Emphases on the 'dirty' part. There really are 'better' ways of doing it. Not simpler, or easier, but better, the idea being to get the most signal to the antenna to be radiated without having a lot of signal lost in the process. Right? Notice I didn't say coaxial feed line? Any feed line can be used for this "adjustable length for impedance transformation" thingy, not just coax, which just happens to be the most common, and certainly easier to use in mobile applications (reason for it's invention, WW2, aircraft).
If the impedance seen at one end of the feed line is the same as on the other end of the feed line then there's no 'restrictions' as to length of that feed line, it'll work just fine. (For practical purposes, if what's 'seen' is at least 'close', it's doing as well as can be expected.) There are particular lengths that have certain characteristics that make them usable for particular things. But that means that those impedances on both ends of that section of coax are not the same. If you can make those impedances the same, then the feed line doesn't have to do something that it wasn't designed to do in the first place and probably can't handle in some circumstances.
What about when those impedances are matched just fine in two separate antenna systems and you want to combine those two systems into one system (dual antennas). Think parallel circuits, usually half the impedance is the result if both of those systems are the same. How about using a "step up" impedance transformer? For coax, since both are unbalanced, and you wanna go to another unbalanced feed line, how about an 'Unun' (unbalanced 'balun' sort of)? Hmm... won't find them on the shelves anywhere, so just use 75 ohm coax as is typically done. Or just make the @#$ thing!
This is getting way off of topic so I'll quit. Sorry 'bout that.
- 'Doc
No, you are not doing it wrong.
Some facts.
The length of feed line used can be used for impedance transformation. That's good in that there aren't a zillion 'sizes' of coax to match whatever an antenna's input impedances. It's bad in that that impedance transformation can, and sometimes does, produce voltages/currents that will harm the feed line doing the transformation. It's a very quick-n-dirty way of doing it. Emphases on the 'dirty' part. There really are 'better' ways of doing it. Not simpler, or easier, but better, the idea being to get the most signal to the antenna to be radiated without having a lot of signal lost in the process. Right? Notice I didn't say coaxial feed line? Any feed line can be used for this "adjustable length for impedance transformation" thingy, not just coax, which just happens to be the most common, and certainly easier to use in mobile applications (reason for it's invention, WW2, aircraft).
If the impedance seen at one end of the feed line is the same as on the other end of the feed line then there's no 'restrictions' as to length of that feed line, it'll work just fine. (For practical purposes, if what's 'seen' is at least 'close', it's doing as well as can be expected.) There are particular lengths that have certain characteristics that make them usable for particular things. But that means that those impedances on both ends of that section of coax are not the same. If you can make those impedances the same, then the feed line doesn't have to do something that it wasn't designed to do in the first place and probably can't handle in some circumstances.
What about when those impedances are matched just fine in two separate antenna systems and you want to combine those two systems into one system (dual antennas). Think parallel circuits, usually half the impedance is the result if both of those systems are the same. How about using a "step up" impedance transformer? For coax, since both are unbalanced, and you wanna go to another unbalanced feed line, how about an 'Unun' (unbalanced 'balun' sort of)? Hmm... won't find them on the shelves anywhere, so just use 75 ohm coax as is typically done. Or just make the @#$ thing!
This is getting way off of topic so I'll quit. Sorry 'bout that.
- 'Doc
CW Morse while trying to understand your original post it is still way over my head. Are you saying regardless of coax length, whether it be 10' or 36', the antenna will still tune the same via the whip length? The SWRssssss would still tune down to the lowest point regardless of the coax length?
What is funny is when I was told to make coax lengths in multiples of 18' for my truck, it didn't matter what length coax I used for my base!! I set my Vector to 131 inches or what ever the length was and it had a perfectly low SWR reading.
What is funny is when I was told to make coax lengths in multiples of 18' for my truck, it didn't matter what length coax I used for my base!! I set my Vector to 131 inches or what ever the length was and it had a perfectly low SWR reading.
If the actual feedpoint impedance of an antenna (mobile or fixed station) is 50 ohms, and your coax has a characteristic impedance of 50 ohms, then you can use any length of coax without problems. Many if not most mobile antennas do NOT have feedpoint impedances at or even near 50 ohms. 25-30 is typical. Here is where the coax length can make the transmitter feel more comfortable.
However, since SWR is the ratio between feedpoint impedance and coax impedance, you'll still have the SWR and the resultant loss ON THE FEEDLINE. The transmitter's happy, even though there may be a significant impedance difference between the feedpoint and the coax.
How to cure it? Get the antenna closer to 50 ohms.
However, since SWR is the ratio between feedpoint impedance and coax impedance, you'll still have the SWR and the resultant loss ON THE FEEDLINE. The transmitter's happy, even though there may be a significant impedance difference between the feedpoint and the coax.
How to cure it? Get the antenna closer to 50 ohms.
The entire thing is, we are making a mountain out of a molehill most of the time! Simply put the antenna ON correctly, following the mfr's instructions. Run whatever length of feed line you need to get to the radio and "fergit" about coax length (assuming you have a conventional metal body and sufficient ground. Next, lengthen or shorten the whip to resonate the antenna to frequency. Like "Doc" said, the impedance of the antenna system is important. This is the "50 ohms" we read so much about. Normally, at 27 MHZ, nothing else is really necessary to get a low SWR (and if you have a way to measure it, such as an antenna analyzer) & a 50 ohm match. If you haven't delved into antenna theory much, we'll leave it at that. 
For those who just INSIST that the way to "tune" antenna is thru a certain length of coax, I offer this:
Antenna theory is the SAME no matter what frequency you are operating on---whether it is 27 MHZ or 3 MHZ. That means, using the flawed and false doctrine of "coax length to tune the "swr'ssssssssss" (It is S W R, or VSWR-voltage standing wave ratio--and NOT SWR'SSSSSSS), we allegedly use 18 feet for 27 MHZ, right? So now if that is TRUE (it's NOT!), then I must use 120 FEET for that 3 MHZ, about 60 FEET for 7 MHZ, 32 FEET for 14 MHZ, and 16,7 FEET for 28 MHZ. Btw, if you use
468 divided by freq, in MHZ, it will eliminate worrying about all that velocity factor stuff And so and so on. Using the CBer's "bible" about coax length, I must have a specific length of coax for EACH frequency I work, right? Let's muddy the water a bit!
In the amateur radio world, there is a mobile HF antenna called a "screwdriver", so named because it (formerly) used a hand-held cordless screwdriver converted to 12V to move a loading coil UP and DOWN within
tight fingers (look it up on the internet). By extending the loading coil UP, the frequency of operation goes DOWN. Retracting the coil DOWN into the mast INCREASES the operating frequency. What this means is, the operator can operate ANY frequency FLUIDLY and without interruption from 28 MHZ down to around 3.5 MHZ with an average SWR of 1.2 to 1.5
on ANY frequency he chooses. BUT WAIT!!!!!!!!!!!! I've gotta have a "certain" length of coax to work them frequencies or my "SWR" will be off, right? There's about 10 bands that hams work (and some have authorizations in other services that require other HF frequencies, etc.) so NOW I've gotta have a coax cut to 120 feet in my MOBILE:drool: for that 3 MHZ!!! AND ANOTHER one at 30 feet for 7 MHZ, and another one at 32 feet for 14 MHZ, YET ANOTHER at 16 feet for 14 MHZ----TEN OR MORE SPECIFIC PIECES OF COAX CUT TO A 'CERTAIN' LENGTH FOR EVERY FREQUENCY I NEED TO WORK!!! Where on EARTH am I going to put 10 or more coaxes in a MOBILE, ALL cut to a certain length:laugh:? Yet, hams and commercial operators operate thousands of frequencies with ONE mobile antenna, ONE coax cut to no particular length (whatever it takes to get from front to rear) and an SWR of 1,2 or less.
Another publication to look for is from Don Johnson, W6AAQ. It may still be available in disc (see "screwdriver antenna, DK3" and related): "40+5 Years of HF Mobileering" AND "What you Forgot to Ask about HF Mobileering". It will open your eyes!
73
CWM
For those who just INSIST that the way to "tune" antenna is thru a certain length of coax, I offer this:
Antenna theory is the SAME no matter what frequency you are operating on---whether it is 27 MHZ or 3 MHZ. That means, using the flawed and false doctrine of "coax length to tune the "swr'ssssssssss" (It is S W R, or VSWR-voltage standing wave ratio--and NOT SWR'SSSSSSS), we allegedly use 18 feet for 27 MHZ, right? So now if that is TRUE (it's NOT!), then I must use 120 FEET for that 3 MHZ, about 60 FEET for 7 MHZ, 32 FEET for 14 MHZ, and 16,7 FEET for 28 MHZ. Btw, if you use
468 divided by freq, in MHZ, it will eliminate worrying about all that velocity factor stuff
And so and so on. Using the CBer's "bible" about coax length, I must have a specific length of coax for EACH frequency I work, right? Let's muddy the water a bit!In the amateur radio world, there is a mobile HF antenna called a "screwdriver", so named because it (formerly) used a hand-held cordless screwdriver converted to 12V to move a loading coil UP and DOWN within
tight fingers (look it up on the internet). By extending the loading coil UP, the frequency of operation goes DOWN. Retracting the coil DOWN into the mast INCREASES the operating frequency. What this means is, the operator can operate ANY frequency FLUIDLY and without interruption from 28 MHZ down to around 3.5 MHZ with an average SWR of 1.2 to 1.5
on ANY frequency he chooses. BUT WAIT!!!!!!!!!!!! I've gotta have a "certain" length of coax to work them frequencies or my "SWR" will be off, right? There's about 10 bands that hams work (and some have authorizations in other services that require other HF frequencies, etc.) so NOW I've gotta have a coax cut to 120 feet in my MOBILE:drool: for that 3 MHZ!!! AND ANOTHER one at 30 feet for 7 MHZ, and another one at 32 feet for 14 MHZ, YET ANOTHER at 16 feet for 14 MHZ----TEN OR MORE SPECIFIC PIECES OF COAX CUT TO A 'CERTAIN' LENGTH FOR EVERY FREQUENCY I NEED TO WORK!!! Where on EARTH am I going to put 10 or more coaxes in a MOBILE, ALL cut to a certain length:laugh:? Yet, hams and commercial operators operate thousands of frequencies with ONE mobile antenna, ONE coax cut to no particular length (whatever it takes to get from front to rear) and an SWR of 1,2 or less.
Another publication to look for is from Don Johnson, W6AAQ. It may still be available in disc (see "screwdriver antenna, DK3" and related): "40+5 Years of HF Mobileering" AND "What you Forgot to Ask about HF Mobileering". It will open your eyes!
73
CWM
So would this antenna setup fall under the radiator or counterpoise category.
I'm not sure what you are asking
The "counterpoise" for this antenna IS the truck body AND the other "half" of the antenna system. If you remove this antenna and try to get it to work alone, it won't. This "counterpoise" would then have to be provided by radials or wires on the ground, or "base" side of the antenna. Counterpoise is not part of the radiating (or + side) of the antenna. This is also why those silly little things we see UNDER the coil of some CB antennas are NOT "counterpoise", but actually a part of the radiating element. IN OTW,
these do absolutely NOTHING and are one of the "snake oil" solutions that people often fall for out at the local CB shop.
CWM
Well that rig does 1kw avg. power with miniscule relfected wattage, somewhere around .5 watt so I'd say that the coax length and counterpoise are in total harmony from 26.555 to 27.900 in AM mode.I'm not sure what you are asking
The "counterpoise" for this antenna IS the truck body AND the other "half" of the antenna system. If you remove this antenna and try to get it to work alone, it won't. This "counterpoise" would then have to be provided by radials or wires on the ground, or "base" side of the antenna. Counterpoise is not part of the radiating (or + side) of the antenna. This is also why those silly little things we see UNDER the coil of some CB antennas are NOT "counterpoise", but actually a part of the radiating element. IN OTW,
these do absolutely NOTHING and are one of the "snake oil" solutions that people often fall for out at the local CB shop.
CWM
C W Morse - I can't stand his whole attitude. Never could read all the way through one of his posts.
Coax length is very important when dealing with phasing lines.It introduces the proper delay,or none at all and just transforms impedance, when dealing with multi antenna arrays. Coax length from the radio to the common point of the phasing lines does not matter in the slightest. Using coax length to "tune" an antenna system is a bandaid solution at best.
Hey, "holy one", :tongue:, what's you opinion on Art Bells big loop.
I have googled it, but is it static that made him have 400V, pretty much constant, on his huge antenna. He seemed to have no real answer..
Art Bell is an idiot and likes to make more of something than it really is. Haven't you noticed that? Let's see, he has 3200 feet of wire up 75 feet in the air and lives in Nevada. Any high school kid can tell you what happens to that wire when the wind blows across it in that dry environment.I isolated a 170 foot high AM broadcast tower from ground in order to do some tests.It was cold and dry as it was in early December. Stupid me forgot to ground out the tower before I reconnected it to the tuning system. I got knocked to my knees from the static buildup. Art's antenna is no differant,just bigger.
If it is a single band dipole there is no truth to that. If it is multiband then some weird things can happen on certain bands with certain lengths of coax but it is nearly impossible to fins a "proper" length that suits all bands.
B
BOOTY MONSTER
Guest
so the "492/fmhz. X %vf = length in feet" formula to get the coax to show the truest swr measurement (with cb equipment on cb channels) is nonsense ?
Booty',
No, it isn't nonsense. There are some "but's" in that though. The formula is correct, the use of the whole thing typically isn't.
The formula produces a 'neutral' feed line for a particular frequency of use. Means it acts like it just isn't there, sort of a direct connection to the antenna. That characteristic can be used in a number of ways. It can also be used incorrectly in a number of ways. The exact same results can be had by making sure that the input impedance of the antenna matches the feed line and radio. In that instance, the length of the feed line makes no difference at all. The biggest draw back with using coaxial feed line is that it has certain limits (voltage/current capacity). When using the feed line for impedance matching the voltage (biggy) and current (not so 'biggy') carrying capacity of the feed line has a huge importance. Since the two can be very much more than expected, and since coax just wasn't ever intended to do this sort of thing, why take the chance? As long as things are kept within reasonable bounds, have at it, it works. Not the best way of doing it, in my opinion, but so what. Something to take a look at when you get really bored is any feed line that's used for impedance matching. It means destroying that line so keep that in mind. Strip the outer insulation off a matching line. Take a good look at the discoloration of the braid of that line. The more the discoloration, the more the line was 'stressed'. It'll be in varying amounts, but all matching/phasing line will show that discoloration (high voltage points/areas). It contributes to losses. May not be much, but those losses are still there, and if done to the extreme, it melts the dielectric.
More than you ever wanted to know, huh?
- 'Doc
No, it isn't nonsense. There are some "but's" in that though. The formula is correct, the use of the whole thing typically isn't.
The formula produces a 'neutral' feed line for a particular frequency of use. Means it acts like it just isn't there, sort of a direct connection to the antenna. That characteristic can be used in a number of ways. It can also be used incorrectly in a number of ways. The exact same results can be had by making sure that the input impedance of the antenna matches the feed line and radio. In that instance, the length of the feed line makes no difference at all. The biggest draw back with using coaxial feed line is that it has certain limits (voltage/current capacity). When using the feed line for impedance matching the voltage (biggy) and current (not so 'biggy') carrying capacity of the feed line has a huge importance. Since the two can be very much more than expected, and since coax just wasn't ever intended to do this sort of thing, why take the chance? As long as things are kept within reasonable bounds, have at it, it works. Not the best way of doing it, in my opinion, but so what. Something to take a look at when you get really bored is any feed line that's used for impedance matching. It means destroying that line so keep that in mind. Strip the outer insulation off a matching line. Take a good look at the discoloration of the braid of that line. The more the discoloration, the more the line was 'stressed'. It'll be in varying amounts, but all matching/phasing line will show that discoloration (high voltage points/areas). It contributes to losses. May not be much, but those losses are still there, and if done to the extreme, it melts the dielectric.
More than you ever wanted to know, huh?
- 'Doc
That is the dumbest thing I have EVER heard!:headbang Umm, lets see, Art ran a very sucessful radio show, retired like three or four times, is a Popular ham operator, had that popular loop, was a popular talk show host, married like three women, went the Philippines and got a beauty after Romona flopped. What I understand Art just got back from a Cruise, so um yeah an idiot? I think not.:headbang
If you had your coax length cut to precisely the length in your formula, and you were right on frequency, the reflection would appear the same as at the antenna. (minus the tiny amount of coax loss).
Your coax length there gives a 180 degree phase shift. If you shorted together one end of the coax, you would see 0 ohms also at the other end at precisely the right frequency. (and btw halfway down you would see an open circuit)
The trouble is that cheap vswr meters really arent up to the task of giving accurate readings over the full range of coax lengths. Its not such a trivial thing to measure, and its why antenna analyzers cost several hundered dollars.
On CB we can take shortcuts because we only operate on such a narrow frequency range.
And running only 4 watts, if power is dumping somewhere instead of radiating, it rarely causes problems, other than being a mud-duck.
This is a good explaination on VSWR Moonraker Antenna Systems
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