Where to buy a 1/2 wave pre made Coax with ends?

[Galaxy 959]

Where can I get a 1/2 wave electrical pre cut coax with ends already on it? I need one to run with my MFJ259 analyzer to tune antennas. I guess it don't really matter what kind as long as its good coax. Used for tuning antennas only. Not going to be running it on a radio. Thanks

 

[KD2GOE]

You can make one easy enough just cut and add ends if you have a radio shack that is still alive. or look on E-bay there is afew people that will make you whatever length you want..

 

[Galaxy 959]

You can make one easy enough just cut and add ends if you have a radio shack that is still alive. or look on E-bay there is afew people that will make you whatever length you want..

Is RG8x good enough to do this with or would you recommend something else?

 

[Tallman]

Where can I get a 1/2 wave electrical pre cut coax with ends already on it?

Why half wave? If you are dealing with 160 meters you would have to mortgage your house. Remember that with coax a half wave length is less than a half wave due to the velocity factor of coax.

 

[Galaxy 959]

Why half wave? If you are dealing with 160 meters you would have to mortgage your house. Remember that with coax a half wave length is less than a half wave due to the velocity factor of coax.

I thought you have to use a half wave length coax to tune a antenna with a MFJ Analyzer. There is a equation to figure in the velocity factor.

 

[Galaxy 959]

Why half wave? If you are dealing with 160 meters you would have to mortgage your house. Remember that with coax a half wave length is less than a half wave due to the velocity factor of coax.

Like this. This is with RG8x with velocity factor of .82 Tuning for middle Freq of 11 meter.
Cut a piece of coax using this formula for making a 1/2 wave tuned coax jumper. 492 x VF of coax, divided by frequency. So for 27.205 we use 492 x .82 / 27.205 which = 14.82ft.

 

[222DBFL]

Let me rephrase it. An electrically tuned 1/2 wave coax jumper cut for the tuning freq of the antenna. Just like posted above.
You can also go straight to the feedpoint as well and not have to worry about the coax. But to tune antenna correctly you need an electrically tuned 1/2 wave coax jumper cut for the desired tuning frequency. This info was passed along to me from other members here. I've yet to confirm this first hand but the sources are reputable ones and I beleive what they are telling me is true. As listed above. Hope this helps. And this is only for use with an antenna analyzer as well.

 

[Road Squawker]

I thought you have to use a half wave length coax to tune a antenna with a MFJ Analyzer.....

thats not correct,......... an electrical 1/2 wl (or any multiple of 1/2 wl) coax will REPEAT the VSWR at the feedpoint, making it easier to SEE the VSWR from a remote location,... you don't have to climb your tower.

from the MFJ manual:

Another common misconception is that changing a feedlines length will change SWR. If the impedance of a feedline is 50 ohms and the load impedance is 25 ohms the SWR will remain 2:1 as the feedline length changes. If line loss is low it is perfectly acceptable to make SWR measurements at the transmitter end of the feedline.
The feedline does not have to be any particular length. However, as line loss increases, and as SWR increases, more error is introduced into the SWR reading. The error causes the measured SWR reading to appear better than the actual SWR at the antenna. Refer to the section on estimating the line loss on page 10.

 

[Galaxy 959]

Let me rephrase it. An electrically tuned 1/2 wave coax jumper cut for the tuning freq of the antenna. Just like posted above.
You can also go straight to the feedpoint as well and not have to worry about the coax. But to tune antenna correctly you need an electrically tuned 1/2 wave coax jumper cut for the desired tuning frequency. This info was passed along to me from other members here. I've yet to confirm this first hand but the sources are reputable ones and I beleive what they are telling me is true. As listed above. Hope this helps. And this is only for use with an antenna analyzer as well.

When you say hooked directly to feed line do you mean like with just a foot or 3' jumper?

 

[Galaxy 959]

thats not correct,......... an electrical 1/2 wl (or any multiple of 1/2 wl) coax will REPEAT the VSWR at the feedpoint, making it easier to SEE the VSWR from a remote location,... you don't have to climb your tower.

from the MFJ manual:

Another common misconception is that changing a feedlines length will change SWR. If the impedance of a feedline is 50 ohms and the load impedance is 25 ohms the SWR will remain 2:1 as the feedline length changes. If line loss is low it is perfectly acceptable to make SWR measurements at the transmitter end of the feedline.
The feedline does not have to be any particular length. However, as line loss increases, and as SWR increases, more error is introduced into the SWR reading. The error causes the measured SWR reading to appear better than the actual SWR at the antenna. Refer to the section on estimating the line loss on page 10.

So I have a 50' length of RG8U ran from my Sirio Tornado to the back of my radio. Can I use this MFJ to tune my antenna by unhooking coax off my radio and hooking it directly to my coax or will it show false readings since coax is not electronically correct length?

 

[Tallman]

Like this. This is with RG8x with velocity factor of .82 Tuning for middle Freq of 11 meter.
Cut a piece of coax using this formula for making a 1/2 wave tuned coax jumper. 492 x VF of coax, divided by frequency. So for 27.205 we use 492 x .82 / 27.205 which = 14.82ft.

The problem is when you have someone else build it for you are giving away precision for convenience. Most would not even measure out the cable that precisely or even check the velocity factor of the cable they are going to assemble for you. The best way is for you to assemble it yourself.
There has been lots of discussion about whether the lowest SWR gets you the best radiating efficiency. Also if you tune with a particular length of coax you are taking into account the capacitance the cable is adding to the antenna circuit. Change the cable to something else a different type or length and the setup has changed.

 

[The DB]

When you say hooked directly to feed line do you mean like with just a foot or 3' jumper?

No, what you want is a coupler. A coax jumper will throw the X and R values off some which would defeat the purpose of measuring at the antenna itself. If measuring at the antennas feed point you want as little as possible between the analyzer and the antenna, which is a male to male coupler. If you can get that coax jumper under a foot in length it will be very close, however, if you have direct access to the antenna's feed point it is easier and cheaper to just buy a coupler...

Something like this.

And just a note on what Road Squawker said, SWR (or VSWR for one of the long forms) doesn't change with coax length as stated in the manual he quoted. I am clarifying this as he essentially said two very different things in his post. What repeats is the R and X readings, and they repeat every electrical half wavelength, not physical half wavelength, which is why you need to factor in the velocity factor, and why the coax used will be be physically shorter than a physical half wavelength.


The DB

 

[binrat]

So I have a 50' length of RG8U ran from my Sirio Tornado to the back of my radio. Can I use this MFJ to tune my antenna by unhooking coax off my radio and hooking it directly to my coax or will it show false readings since coax is not electronically correct length?

That is correct, you can use that coax, most people do.

 

[222DBFL]

Thanks The DB. You always seem to have a way with words!! Now listen to what is being said by The DB. I am sorry if I confused you Galaxy 959. And the length of this coax will also be determined by its type as in rg8, rg8x, lmr400 and so on as you need to know the velocity factor for the coax being used to get the length correct.
This was the reason for me suggesting just like the DB did to go directly to the feedpoint with a pl259 to pl259 barrel connector.

 

[Captain Kilowatt]

I used my MFJ analyzer to determine the proper 1/2 wavelengths and cut coax for each of the amateur bands. Well actually two 10m lengths together for 20m and a 10m and 6m together was close enough for 15m.