Coax

[thetnhillbilly]

I'm hoping some of you can help me cut throught the misinformation that is out there. The way I'll do this is I'll tell you what I've gathered and then let ya'll correct where its wrong. Remember- don't jump down my throat if something I type is wrong, I'm here asking so as to get it right.

The length of 50 ohm coax using a single antenna doesn't (barring a short) affect SWR, however the length of the coax can affect the reading on a meter. The lengths that will give accurate readings are the odd halves (1/2, 3/2, 5/2, etc). If I go with a length in between those, how can I accurately tune my antenna? Put the meter on the antenna end of the coax instead of the radio end? If so, what about the jumper in between the meter and antenna, it is not a 1/2 wavelength?

The reason I'm asking is because I have my radio in my car and the antenna mounted on the trunk. It takes every bit of 18' coax to get in between (it is a big car- crown vic), and that is running straight back down the transmission tunnel. I want to run it to the side of the car and back, but there is just not enough coax. 50' would be too much, hence the question about tuning lengths in between. If I need to go with 50', what do I do with the extra? Putting into a coil isn't good, right?

Shoot me straight, I know you will. Remember, you were ignorant too, before you knew what you know.

 

[Hamin' X]

In all truth, the best length to use is enough to get to the antenna. If varying the length of the coax changes your SWR reading, then your antenna is not 50 ohms.

That being said, by using electrical 1/2 wave multiples, you can make your finals happy with almost any antenna and all of the power that you transmit will be radiated (minus a minute amount due to cable loss).

When figuring electrical length, it is imperative to use brand name coax. No name, or house brands have velocity factors that vary all over the place. Rule of thumb is to go with the largest velocity factor that you can get. Larger velocity factor=less loss. .95 vf is better than .66 vf.

Multiply your free space 1/2 length by the velocity factor to find the electrical 1/2 wave length for your coax. 468/27.2MHz=17.21ft*.85vf=14.63ft. This means that you would need 29.26 feet of coax that has a velocity factor of .85.

Hope this helps.

Rich

 

[Captain Kilowatt]

Velocity factor has nothing to do with loss.Velocity factor is a function of dilelectric material for the most part.RG8X has a Vf of around 82% with it's foam core but has more loss than RG-8 with a Vf of 66% with it's polyethelyne core. When tuning an antenna you do not have to use ODD 1/2 wave multiples,ANY half wave multiple will work.What I have done for the ham bands is made up lengths of cable that are 1/2 wavelength long for each band.When I want to tune an antenna I use the cable for whatever band I want and after tuning I run whatever length is required to get from point A to point B.Works like a charm every time.

 

[Hamin' X]

Let me clarify my statement. I was referring to different velocity factors among similar cable types. Higher velocity factor indicates more efficient dielectric properties, resulting in lower loss.

Good catch on the odd multiples. Notice that in my example, I used an even multiple. I failed to correct his use of odd multiples. Odd multiples of 1/4 wave lengths are used are used to transform impedances. Co-phase harnesses are a good example.

Rich

 

[K8PG]

Velocity of Propagation

The energy travels along the line at the velocity of light.The actual velocity is very close to that of light only in lines in which the insulation between conductors is air.The presence of
dielectrics other than air reduces the velocity.Current does not flow at the speed of light in any medium other than a vacuum.Therefore,the time required for a signal of a given frequency to travel down a length of transmission line is LONGER than the time required for the same signal to travel the same distance in free space Because of this propagation delay,360deg.of a given wave exists in a physically shorter distance on a given transmission line in free space.The exact delay for a given transmission line is a function of the properties of the line,including the dielectric constant of the insulating material between the conductors.This delay is expressed in terms of the speed of light( percentage or decimal fraction) and is refered to VELOCITY FACTOR.The physical length corresponding to a electrical 1/2 wavelength on a given line is expressed as:

1/2 wavelength in ft =491.8 divided by f times the VF
F= FREQ IN MHZ. VF = VELOCITY FACTOR OF LINE

DE-K8PG Paul CW LIVES :usa

 

[Hamin' X]

Re: Velocity of Propagation

1/2 wavelength in ft =491.8 divided by f times the VF
F= FREQ IN MHZ. VF = VELOCITY FACTOR OF LINE

DE-K8PG Paul CW LIVES :usa

Yeah, your right, Paul. The 468 that I used was the number normally used for 1/2 wave dipole wire antennas. It assumes a VF of .95 for the average 14 gauge, non-insulated wire. 491.8 is a better number.

It is still important to use name brand coax, with published VF numbers, or it will not make much difference which number you use. It will just be a crap shoot.

Rich

 

[thetnhillbilly]

First, thanks for the quick and informative replies.

I was going to use Times LMR 240 with a VF of 83. So thirty feet would be really close to what I need, right? That would get me to my antenna and give accurate SWR readings. If I get to my atenna and still have some coax left, what is the best way to store it (coiled, accordian style, etc)?

 

[Toll_Free]

First, thanks for the quick and informative replies.

I was going to use Times LMR 240 with a VF of 83. So thirty feet would be really close to what I need, right? That would get me to my antenna and give accurate SWR readings. If I get to my atenna and still have some coax left, what is the best way to store it (coiled, accordian style, etc)?

Here's an idea. Use the length of coax you need. When installing, take all your SWR measurements AT THE ANTENNA (use as short a jumper as possible). Then, when it's flat there, it will also be flat (really flat, not a lie from a complex impedance) at the xmitter end. Then, if it jumps up, you know something is amiss, but as long as it stays flat, your in happy-land.

I use a remote measuring SWR / Wattmeter at my antenna feedpoint (the switchbox, actually) to get the most accurate measurement possible. I feed that with the electrically correct length from the swr meter sensor to the antenna, and don't worry about the length of coax to get from the xmitter to the swr meter / switchbox.

Not a fancy ham way of doing it, but works for this dumbo CBer



--Toll_Free

 

[DXman]

First, thanks for the quick and informative replies.

I was going to use Times LMR 240 with a VF of 83. So thirty feet would be really close to what I need, right?

30' will work just fine - 1 full wave length.

 

[667 chicago mobile]

I have a Crown Vic also. I run and Wilson 5000 truck mount. I ran my coax just like you did to the radio. You should have reach the radio. Just remeber your TUNING THE ANTENNA. Like one member said as long as you reach the radio from point A to point B.
Tom

 

[thetnhillbilly]

It takes every bit of 18' coax to get in between (it is a big car- crown vic), and that is running straight back down the transmission tunnel. I want to run it to the side of the car and back, but there is just not enough coax.

It did reach the radio if I ran it down the transmission tunnel, but that was not how I wanted it. I wanted to run it over to and down the side of the car, under the door frame's trim, and then back over to the radio. There wasn't enough for that. I wanted to do that because there is something about running parallel and close to the transmission that I don't like, so I wanted it farther away from the trans.

I ended up getting 30' of LMR 240, it is a little longer than I needed, but gives me more flexibility in routing.

Thanks again to everyone for their patience and advice.

 

[billy the kid]

i would use 36 foot this is just what i have been told and it works great in mobils and base stations ive all ways used 18 foot in pick up it was all i needed but in your case use 36ft the way u look at is 18,36,54 and so own u probably done no all this use 3,6,9ft jumpers u just divide 18,36,54 and they all will come out to a even number using these jumpers except on 18f a 6ft dont come out and on 54ft a 6ft dont come out they will work but swr may be off all my jumpers and coax are really close to the T a 16th of a inch or a 8th of a inch off of the lenth you want will mess with your swr's just my thoughts 73's god bless

 

[DXman]

billy the kid,
If what your thinking that coax length changes the SWR....this is not so.

If your antenna is set at say, 1:2 swr then no matter what length of coax you use, the swr will not change.
If is does change, then you have a problem either with the ground or maybe how the antenna is mounted.

 

[KingCobra_CDX882]

Hey there..

To start...

use Quality coax ( LMR-240 is good )
use the shortest posible length

these odd or even lengths are rubbish
no such thing if you are using true 50 ohm coax
(LMR-240 is availible true 50 ohm)

borrow an antenna analizer if you have problems in getting proper SWR reading....if you can not borrow one then do as TollFree said and use a short jumper at the antenna itself to tune as low SWR as posible...

length whether shorter or longer does not affect SWR
verosity factors are of little consern given how short a coax you are using..

 

[billy the kid]

belive what u want to belive ant tryin to start nothin ive talked skip nearly all my life talked to a lot of old timers that stay on the maul all the time hard to compeit with so just goin buy what advice some big rigers give me and bowevil my cousion he was hard to handle when he was in there so 73's god bless ole billy workin this home made 3 element and ts 500 is quite