That whole coax length thing...

[Tallman]

Apparently he has moved to Dime Box, Texas, is no longer on the internet, has no landline and has changed his cell phone number.

The only option is to make the pilgrimage to Dime Box and try and persuade one of the locals to tell you where he lives.

You probably could catch him at the old dance hall out there on Friday or Saturday night.
I used to go out there from Bastrop, TX as a teenager.

 

[Riverman]

You probably could catch him at the old dance hall out there on Friday or Saturday night.
I used to go out there from Bastrop, TX as a teenager.

Cool. I was afraid most would think I was pulling their leg about the town's name. And the dance hall is a great idea! Ol' Ben had some moves back in the day. Put Archie Bell and the Drells on the jukebox and he could really Tighten Up they say.

 

[Tallman]

Cool. I was afraid most would think I was pulling their leg about the town's name. And the dance hall is a great idea! Ol' Ben had some moves back in the day. Put Archie Bell and the Drells on the jukebox and he could really Tighten Up they say.

Do you live in Dimebox or near it? The last time I was in Texas the closest I got to Dimebox was Giddings, TX.

 

[Riverman]

Do you live in Dimebox or near it? The last time I was in Texas the closest I got to Dimebox was Giddings, TX.

Nope. If I did, maybe Ol' Ben would take me on as a 64-year-old apprentice.

 

[codeman]

Nope. If I did, maybe Ol' Ben would take me on as a 64-year-old apprentice.

I know a knucklehead who drives for a small trucking company out of Giddings,TX.

 

[Riverman]

I know a knucklehead who drives for a small trucking company out of Giddings,TX.

Ben's easy to spot. Been wearing the same thing for years and years. Old Astros cap and Indian motorcycle jacket with a NASA patch on front. (Favorite team, favorite bike and previous employer).

 

[br549]

since this in in the CB section and rarely does any CB'er use exotic antenna designs that need particular feedline lengths ...... it really doesn't matter . you're only tuned perfectly to one frequency to start with . focus on having a antenna SYSTEM with usable bandwidth for the radio you have in front of it or the band you want to use it on .

 

[DainBramage]

... it really doesn't matter . you're only tuned perfectly to one frequency to start with... .

... and for many, they don't even do that.

Yeah, lets just spread this coil and watch it swiiiiing.

 

[Bryan G]

The SWR AS SEEN BY THE RADIO will change HOWEVER THE SWR AT THE ANTENNA WILL NOT CHANGE.

Some people say that coax lenght does not matter, other people say it does!

The coax does not change your SWR, your antenna does!

However the SWR AT your antenna stays the same, if you change your coax it can fool your meter into having a different SWR.

There is so much back and forth on this. I keep seeing that the only thing that matters is the vswr at the antenna. While I agree that is important for transmitting, getting your signal out, but isn't the other end important too?

Won't a really high swr burn up the radio...

Also, how can you check the swr at the antenna? Just being there to read the meter your body will change the swr and most don't have the money for a remote meter...

I've read many posts on many forums where the swr changed at the radio by changing the length of the coax... I would think finding the right length of coax to get a good swr at the radio would be important, not for the antenna or transmitting purpose, but to protect the radio.

 

[rabbiporkchop]

https://electronicsclub.info/impedance.htm

Impedance can be split into two parts:

• Resistance R (the part which is constant regardless of frequency)
• Reactance X (the part which varies with frequency due to capacitance and inductance)

The capacitance and inductance cause a phase shift (see note) between the current and voltage which means that the resistance and reactance cannot be simply added up to give impedance. Instead they must be added as vectors with reactance at right angles to resistance as shown in the diagram.

Four electrical quantities determine the impedance (Z) of a circuit: resistance (R), capacitance (C), inductance (L) and frequency (f).

The following section on reactance explains how capacitance, inductance and frequency affect impedance.

What does 'phase shift' mean?
Phase shift means that the current and voltage are out of step with each other. Think of charging a capacitor. When the voltage across the capacitor is zero, the current is at a maximum; when the capacitor has charged and the voltage is at a maximum, the current is at a minimum. The charging and discharging occur continually with AC and the current reaches its maximum shortly before the voltage reaches its maximum: so we say the current leads the voltage.

 

[Captain Kilowatt]

There is so much back and forth on this. I keep seeing that the only thing that matters is the vswr at the antenna. While I agree that is important for transmitting, getting your signal out, but isn't the other end important too?

Won't a really high swr burn up the radio...

Also, how can you check the swr at the antenna? Just being there to read the meter your body will change the swr and most don't have the money for a remote meter...

I've read many posts on many forums where the swr changed at the radio by changing the length of the coax... I would think finding the right length of coax to get a good swr at the radio would be important, not for the antenna or transmitting purpose, but to protect the radio.

A high SWR is not good for a radio as we all know. A high SWR at the antenna feedpoint is a indicator of inefficiencies in the antenna and not all the power from the radio is getting radiated therefore a high SWR will also reduce your signal. That is why it is a good idea to have a good SWR throughout the entire antenna SYSTEM from the radio to the antenna. Transmission line can act as an impedance transformer if it is connecting two different impedances and a certain length will match those impedances exactly and this is called a conjugate match. Using this method to obtain an acceptable SWR at the radio when in fact the antenna is far from 50 ohms is akin to using an antenna tuner and simply compensates for a bad situation that should be cured rather than band-aided.
One way to check the actual IMPEDANCE at the antenna feedpoint is to use exact multiples of 1/2 electrical wavelengths of coax. This length repeats the impedance at the antenna back to the transmitter end without any impedance transformation and will allow an accurate measurement of the actual antenna impedance. Note I the use of the word IMPEDANCE here and not SWR. In truth the SWR does not vary along a transmission line but the impedance does. There is an almost infinite number of resistance/capacitance/inductance combinations that will give the same SWR reading on a meter and changing the coax line length will alter these R/L/C ratios HOWEVER the SWR will remain the same so you should be able to use the meter in the shack to tune the antenna with any length of cable. The only time the meter reading SWR should change is if you have common mode currents of the outside of the coax cable and that itself is an indicator of something else wrong and is usually related to grounding or mounting.

 

[rabbiporkchop]

It should also be noted that many SWR meters do not measure SWR hence the confusion.

Antenna analyzers are always the way to go since you can't fool them into showing different SWR readings with different cables.

 

[bob85]

There is enough info on here that nobody should be getting confused about vswr & coax length,

When you do not have common mode current on the outside surface of the coax braid vswr measures lower the further from the load/antenna you place the meter due to loss in the coax,

This is why you can calculate the vswr at the antenna if you know how far from the antenna the meter is & what the attenuation per foot your coax has,

When changing coax length a few feet does notably change vswr readings It is not fooling the vswr meter or the analyser, you cannot fool the meter

They are both reporting vswr on the coax resulting from the combined load/antenna impedance,

The common mode impedance of the coax outer braid is seen in parallel with the load/antenna impedance,

when you have common mode current on the outside surface of the coax braid changing coax length changes the common mode impedance in parallel with the load/antenna & changes vswr on the coax,

MFJ tell you this in their analyser user manual.

 

[silver devil]

Length? From the shack to the antenna is what matters to me...

 

[silver devil]

I'm one of those who believes coax length is a bunch of ^&%#. I NEVER even consider "coax length" in an installation, and I've been doing installs since 1965! Never even HEARD of such a thing.......until I heard it from a CBer. And I rolled in the floor when I DID hear it. Sure, there are some installs, like where there are ground issues, or where the antenna itself requires a certain length of coax as part of its design. But for the usual CB installation, FORGET about the coax length bull****, FORGET about velocity factor bull****, DONT WORRY about VF when setting up your station! IT IS ALL C R A P when it comes to the USUAL CB mobile install that STARTED on CB, it LIVES in its ignorance on CB, it continues to THRIVE on CB, and will continue (apparently) to live ON CB ad infinitum. Not to make any comparisons, but I almost NEVER hear about "coax length" in any other radio media, or outlet---not even the hams spend so much time obcessing about the length of their coax! And at the risk of length and making folks mad, I'm gonna to burst the coax length guru's bubble right now!:tongue:

As a ham, CBer and as user of certain other commercial and military frequencies (USAF and FEMA, etc) I run on my mobile what is known as a "screwdriver" antenna. It uses a 12V motor to move a 3" diameter coil UP and DOWN within a hollow mast (making it longer and shorter as needed) to operate HF frequencies between 3.5 and 30 MHZ. The antenna is resonant on EACH frequency of interest as it actually uses ONLY the number of turns that are OUT-side the top of the mast. By pressing a button on the dash, I can move this coil UP or DOWN to "tune to any of these frequencies.

NOW! Radio antenna theory is the VERY SAME for ANY band or frequency. CB radio is NOT "special" or requires some unique arrangement of coax length as if all other radio is exempt from the SAME rules. SO! IF what the CB coax length gurus say is true, and it MUST have 18 feet, or 9 feet, or some other ridiculous nonsense to work (to get them 'SWR'zzzzzzz' right), then to work 3.923 MHZ, I must have........OH, about 60 FEET to work that 3 MHZ on that multi-band screwdriver! Then when I go to 7 MHZ, then I gots to have about 33 feet. Then at 14 MHZ, another at 16 feet, then another at about 14 feet. Then there's 21 MHZ at 11 feet, and then there's the 9 feet for 27-28 MHZ. IOW, according to the CB theory of coax length, I would have to have AT LEAST 10 DIFFERENT COAXES IN MY LITTLE PICKUP so as to work all those various bands, and the screwdriver antenna could NOT work!!!! WHERE would I PUT all those coaxes in a small pickup truck?

Yet, I work ALL those bands from 3 to 30 MHZ (including CB, but actually I have a separate CB antenna for that to remain legal and use a legal CB set) with ONE antenna, and ONE coax of about 7 feet. The amp (when used--it's out of the truck right now: don't need it) has a jumper of about 10" long! The SWR is about 1.2 to 1 on ALL bands on that ONE coax and ONE "length" which is equal to that needed to REACH from the bumper to the radio!

Coax length and Velocity Factor used to tune antennas is bull**** and a lot of hocus pocus designed to make people think they know more than the other folks about radio. Coax length is that length that is directly proportional to the distance between the radio and the antenna. And THAT's the bottom line!!!!!! If I upset somebody, I'm sorry; it's just the way I feel about it! NObody has ever been able to answer this with a true and accurate rebuttal because they CAN'T!:laugh:

CWM

Mic drop....... Well said sir!!