[RadioRay]

The radios are not illegal so long as you use them in the amateur service.
They are illegal to be used ON CB, used as "dual-service' for both CB and ham, or used by unlicensed persons.

Now the coax length thing is STILL horse hockey. Here is a simple way to PROVE it----AND as a licensed ham, you will understand where I am coming from better than those who work ONLY with CB radio. They have been told this myth from time immemorial, and it is still just that: a myth.

SO! Here goes. I build AND run on my vehicle the screwdriver antenna which, of course, covers 80-10 Meters and, thru the auspices of harmonic tuning, 6 Meters (tho it does not do this as well as a dedicated 6 Meter antenna. Now. Antenna theory is antenna theory, and that means what is good for the goose is also good for the gander! That means that IF I must have the CB "gospel" of 18 feet for 27 MHZ (Or some smaller multiple thereof) of coax, then I must also have a "certain" length of coax for 3.9 MHZ. Assuming that we are using one HALF wave antennas, then
using that CB "gospel" myth, it comes out to 17.333333333333 out to infinity. Or 18 feet. At 3.9 MHZ, it is over 120 feet! :shock: Now, it gets complicated! Since the screwdriver covers ALL HF bands from 1.8 to 30 MHZ (plus 6 Meters) we have to, again, according to the coax length myth, have about TEN lengths of coax in the vehicle, EACH one ranging from that 120 feet (80M), 64 feet for 40 Meters (7.2 MHZ), then 32 feet for 20 Meters-----then so on and so and so on. QUESTION: WHERE can I put TEN different feedlines, EACH cut at a specific length, some as long as 120 FEET for a total of over 300 feet!!!!! MY GOSH, man! WHERE can I stash 300 feet of coax in an S-10 truck?

Yet, I work ALL bands from 3.6 to 30 MHZ (I don't actually use the screwdriver for 6 Meters; I've got a dedicated 6 Meter antenna I MADE from a 2 Meter antenna) with an average SWR of 1.3 to 1 on ALL bands!!! :shock: Been doing it for years. Did it with a Texas bugcatcher 15 years ago. Did it with a Bandspanner over 25 years ago. DID it with homebrew mobile HF antennas I made in the 60's! AND did it with ONE coax of NO particular length. Mine is around 7 feet. a RANDOM length chosen for one reason: the distance between the radio and antenna! In fact I never even HEARD of "coax length" until 1975 or so, and I'm like WHAAAAAAT????????????????????????? Ya gotta be kidding!!!!!! I was setting up HF mobiles in 1966. I hadn't ever heard of such nonsense, and I have worked the world many times over in the last 41 years.
Not only THAT, that "velocity factor" is absolutely USELESS. It is a WASTE of time and a smoke and mirror tactic to try to impress CBers out at Joe's CB Sales & Hack Shop. There is no NEED to do all that formula mumbo jumbo with 492-VF/frequency in mhz to arrive at a "certain" coax length.
If you are determined to find a "certain" length of coax or the length of a dipole, simply divide 468/frequency in mhz! It ELIMINATES all that mumbo jumbo. What is velocity factor? Simply THIS: the speed at which a signal progresses thru a solid or semi-solid medium. That is ALL. It does not affect the quality of the signal, nor does it make for better "SWR-zzzzzzzzzzzzzzzz" (WHEN did SWR-sssssssssssss become PLURAL? To tune an antenna to resonance, tune the ANTENNA, not the coax! If you have an impedance mismatch, then that can be handled at the feedpoint with silver mica caps, resistors, toroids, or an inductor. Coax length was born on CB and LIVES on CB, and remains alive on CB

I have NEVER figured a 'certain" length of coax, never had any problem getting an antenna to tune. The ONLY way that coax length is a factor is when an antenna uses coax as part of the ground or counterpoise, or the body is not metal. The lack of metal is what is causing the mismatch because it is HALF of the antenna [/u]SYSTEM[/u]!

The screwdriver being a MULTI-BAND, motorized HF antenna that moves fluidly thru a WIDE range of frequencies does NOT have to have a "certain' length of coax. It WILL tune up using the proper feedpoint matching device no matter if you use one foot, 10 feet, or 30 feet, or NO coax at all. I will take any challenges (at my own home, of course) and demonstrate this gladly.

Your turn (Coax length--------------------------Horse HOCKEY!!!)

[kc0iv]

Originally Posted by Bandit102

Because radio waves are tuned wavelengths of energy, we have to take into account the coax cable length. A typical 11-meter signal has a basic wavelength of 36 feet/wave. "Tuning" the coax for the exact full wavelength tends to throw off the SWR meter by not allowing any standing waves to return to the meter. Excess RF on the coax has been given an ideal medium by which to "hide" electrically from your SWR meter. That is not to say that the excess RF is not returning to the radio, you just can't see it on your meter.

What we want to do is create an environment where any excess RF (standing waves) are rendered as visible as possible to the meter. This is effectively done by using multiples of the 1/2-wavelength of the radiated signal. One half wave for the 11-meter band is 18 feet. :shock: :shock: However, this is not the length that you will cut your coax. There is another factor that affects the length. This is Velocity Factor. The velocity factor is basically a term for how fast the signal moves through the coax. This factor affects the overall electrical performance of the coax and thus needs to be accounted for when determining the true half wave length

Yeah, most people read it in a cb repair book somewhere.

But, what do I know? I'm just a truck driver.

Yes, Ray, I am a licensed Ham and I just like a good pissin match. The 2 radios I run in my truck, however, are FCC illegal. I have a Superstar 3900 and a Ranger 2950. I like them very much, as they have frequency range I desire for mobile. The FT1100 is just too big for the dash.

Bandit102,
Your theory of "Velocity Factor" is some what correct. The correct term is Velocity of Propagation (VoP). Quoting from wikipedia:

Velocity of Propagation (VoP) or velocity factor is a parameter that characterizes the speed at which an electrical signal (e.g. radio or audio) passes through a medium. Expressed as a percentage, it is the ratio of a signal's transmission speed compared to the speed of light in vacuum or the speed of sound. Thus, transmission in a vacuum would have a VoP of 1 (100%). VoP equals the reciprocal of the square root of the dielectric constant of the material through which that signal passes.

Your theory of "hiding" it from the SWR (correct term is VSWR or ISWR) is way off base. When a load is present that does not match the coax impedance a reflected impedance will occur at the radio. That reflected impedance is what is being shown on the VSWR meter. Since the standard VSWR meter does not show phase angle it does not show if the phase angle is positive or negative. What the VSWR will display is the ratio between the load and the free space VoP. As that ratio is reduced the VSWR will be less. What a VSWR meter will not show is what the phase angle is. And in most cases (as when matching an antenna to a radio) it can be ignored. Where it does become is when you are matching two or more antennas in an array.

There are many ways to match a antenna to a radio. The use of coax is just one of them. Another ways is the use LC circuits.

The subject of antenna matching is quite complex and well beyond this simple discussion.

kc0iv

[countryhorseman]

Wow, I am enjoying this thread! An actual conversation about radios without a pissing match. Good job all, I actually learned something, I think, in reading this thread.

Would make sense about the 18' rule, otherwise this old 11 meter Radio Shack HT, that dates back to the '80s would not work at all, no coax. , just a rubber duck.

[kc0iv]

Originally Posted by RadioRay

The radios are not illegal so long as you use them in the amateur service.
They are illegal to be used ON CB, used as "dual-service' for both CB and ham, or used by unlicensed persons.

Now the coax length thing is STILL horse hockey. Here is a simple way to PROVE it----AND as a licensed ham, you will understand where I am coming from better than those who work ONLY with CB radio. They have been told this myth from time immemorial, and it is still just that: a myth.

SO! Here goes. I build AND run on my vehicle the screwdriver antenna which, of course, covers 80-10 Meters and, thru the auspices of harmonic tuning, 6 Meters (tho it does not do this as well as a dedicated 6 Meter antenna. Now. Antenna theory is antenna theory, and that means what is good for the goose is also good for the gander! That means that IF I must have the CB "gospel" of 18 feet for 27 MHZ (Or some smaller multiple thereof) of coax, then I must also have a "certain" length of coax for 3.9 MHZ. Assuming that we are using one HALF wave antennas, then
using that CB "gospel" myth, it comes out to 17.333333333333 out to infinity. Or 18 feet. At 3.9 MHZ, it is over 120 feet! :shock: Now, it gets complicated! Since the screwdriver covers ALL HF bands from 1.8 to 30 MHZ (plus 6 Meters) we have to, again, according to the coax length myth, have about TEN lengths of coax in the vehicle, EACH one ranging from that 120 feet (80M), 64 feet for 40 Meters (7.2 MHZ), then 32 feet for 20 Meters-----then so on and so and so on. QUESTION: WHERE can I put TEN different feedlines, EACH cut at a specific length, some as long as 120 FEET for a total of over 300 feet!!!!! MY GOSH, man! WHERE can I stash 300 feet of coax in an S-10 truck?

Yet, I work ALL bands from 3.6 to 30 MHZ (I don't actually use the screwdriver for 6 Meters; I've got a dedicated 6 Meter antenna I MADE from a 2 Meter antenna) with an average SWR of 1.3 to 1 on ALL bands!!! :shock: Been doing it for years. Did it with a Texas bugcatcher 15 years ago. Did it with a Bandspanner over 25 years ago. DID it with homebrew mobile HF antennas I made in the 60's! AND did it with ONE coax of NO particular length. Mine is around 7 feet. a RANDOM length chosen for one reason: the distance between the radio and antenna! In fact I never even HEARD of "coax length" until 1975 or so, and I'm like WHAAAAAAT????????????????????????? Ya gotta be kidding!!!!!! I was setting up HF mobiles in 1966. I hadn't ever heard of such nonsense, and I have worked the world many times over in the last 41 years.
Not only THAT, that "velocity factor" is absolutely USELESS. It is a WASTE of time and a smoke and mirror tactic to try to impress CBers out at Joe's CB Sales & Hack Shop. There is no NEED to do all that formula mumbo jumbo with 492-VF/frequency in mhz to arrive at a "certain" coax length.
If you are determined to find a "certain" length of coax or the length of a dipole, simply divide 468/frequency in mhz! It ELIMINATES all that mumbo jumbo. What is velocity factor? Simply THIS: the speed at which a signal progresses thru a solid or semi-solid medium. That is ALL. It does not affect the quality of the signal, nor does it make for better "SWR-zzzzzzzzzzzzzzzz" (WHEN did SWR-sssssssssssss become PLURAL? To tune an antenna to resonance, tune the ANTENNA, not the coax! If you have an impedance mismatch, then that can be handled at the feedpoint with silver mica caps, resistors, toroids, or an inductor. Coax length was born on CB and LIVES on CB, and remains alive on CB

I have NEVER figured a 'certain" length of coax, never had any problem getting an antenna to tune. The ONLY way that coax length is a factor is when an antenna uses coax as part of the ground or counterpoise, or the body is not metal. The lack of metal is what is causing the mismatch because it is HALF of the antenna [/u]SYSTEM[/u]!

The screwdriver being a MULTI-BAND, motorized HF antenna that moves fluidly thru a WIDE range of frequencies does NOT have to have a "certain' length of coax. It WILL tune up using the proper feedpoint matching device no matter if you use one foot, 10 feet, or 30 feet, or NO coax at all. I will take any challenges (at my own home, of course) and demonstrate this gladly.

Your turn (Coax length--------------------------Horse HOCKEY!!!)

RadioRay,

Coax length is NOT (using your term) horse hockey. There are valid reasons to use a certain length coax between an antenna and the radio. Anytime the antenna impedance does not match the impedance of the radio (which it won't unless you use 36 ohm coax (for a 1/4 wave)) some form of matching network should be used. That matching network can take the form of LC circuit or a length of coax. The problem with using a length of coax is it's limited range of matching.

The correct formula to use for a 1/2 wave antenna is 492/f (where f is in Megahertz (MHz) and the length is in feet). Your number of 468/f (where f is in megahertz (MHz)) is for an average antenna mounted 1/4 wave above ground over average ground and feed with 75 feed impedance. Many things will affect your 468/f formula. Such things as proximity to ground, trees, etc. will affect the length.

As to your "screwdriver antenna." In it's design it adjust the length (radiating portion) of the antenna by adjust the matching network to the surrounding environment. This can be proven by doing the following:

(1) Using a random length of coax set your "screwdriver antenna" to the lowest VSWR reading as read on the meter.
(2) Now change the coax to a different impedance.
(3) Without changing the length (radiating portion) of the antenna read the new VSWR on the meter.
(4) Try this with several different lenght of coax and impedances.

You will see as the impedance changes the VSWR meter gives a different reading.

The use of using coax as a matching network has been around long before CB became popular. In the old days most "hams" were using some form of ladder line where the losses were low and the matching network could be in the shack. Once coax become became popular more concern was placed on matching the feed impedance of the antenna to the coax impedance. Plus correcting the balance to unbalance condition.

RadioRay what are you doing when you adjust the length of the radiating portion of the antenna? Your changing the length to match the impedance of the coax. As I showed in above changing the coax length or impedance requires a change of the radiating portion of the antenna. Think I'm wrong? Cut your antenna to your formula of 234 (1/4 wave length) then without adjusting the length tell me what your VSWR is.

Another major factor is the "Q" of the antenna. The shorter the antenna the higher the "Q." The "Q" is why you have to change your "screwdriver" on 80 meters than you do on 10 meters.

kc0iv

[kc0iv]

Originally Posted by countryhorseman

Wow, I am enjoying this thread! An actual conversation about radios without a pissing match. Good job all, I actually learned something, I think, in reading this thread.

Would make sense about the 18' rule, otherwise this old 11 meter Radio Shack HT, that dates back to the '80s would not work at all, no coax. , just a rubber duck.

countryhorseman,

That Radio Shack HT as with any transceiver that use a rubber duck or any shorten antenna functions by adjusting the L/C network of the final circuits. It also correct the balance to unbalance condition of the antenna.

The concept of using a 18' length of coax comes about as an attempt to reflect an impedance from the antenna to the radio. Needless to say that length only works at certain frequencies.

kc0iv

[countryhorseman]

Thanks, I remember reading something like that, but the total technical aspects of things escape me sometime. Like I mentioned to you before, I am trying to learn as quick as I can, and appreciate that correction.

Originally Posted by "kc0iv[color=green

countryhorseman,

That Radio Shack HT as with any transceiver that use a rubber duck or any shorten antenna functions by adjusting the L/C network of the final circuits. It also correct the balance to unbalance condition of the antenna.

The concept of using a 18' length of coax comes about as an attempt to reflect an impedance from the antenna to the radio. Needless to say that length only works at certain frequencies.

kc0iv[/color]

[Fredog]

Originally Posted by countryhorseman

Thanks, I remember reading something like that, but the total technical aspects of things escape me sometime. Like I mentioned to you before, I am trying to learn as quick as I can, and appreciate that correction.

all of this technical mumbo jumbo can be avoided if you just join Useless and me on the tin-can radio network

[countryhorseman]

[quote="Fredog
all of this technical mumbo jumbo can be avoided if you just join Useless and me on the tin-can radio network[/quote]

I am afraid my horses and dogs my get tangled up in the strings.

[golfhobo]

all of this technical mumbo jumbo can be avoided if you just join Useless and me on the tin-can radio network

Man.... I've been TRYING to call you all week, Jed!! But, I keep getting a message saying, "the party you are trying to reach is 'on the can!'" :lol:

Don't y'all have call waiting??? :shock:

[Bandit102]

That text about velocity factor was quoted out of a book printed in 1973...Antenna something or other for dummies. My point was to illustrate where the 18 foot myth comes from, largely anyway.

I do run 18 feet in my freightliner with my Predator antenna and get a flat SWR at 25.6 and about 1.2 at 28.5.

Now, on the Moonraker IV at home, I just bought 2 100' rolls of the old RG-8 (Big fat stuff) and get a varying SWR all over many bands but never over 1.85 or so - with a perfect 50 ohms on both the horizontal and the vertical.