Tuning jumpers and coax length

[6.0 Liter]

Good thoughts wavrider, My question would probably be aimed more at a mobile application, a scenario where the antenna would be hardmounted to the roof of the vehicle. The feedline being connected with lug terminals, a little more difficult. I guess in this situation a balun would work fine? What I have done before is cut a piece of coax at approx 18 ft and swept it with a analyzer, clipping it till it reads 0 reactance, and 1.1. This way the analyzer dont see the coax, mabe this is not the best, but seems to work ok

 

[Captain Kilowatt]

So..........."checking the impedance of an antenna, at the feedpoint, is never easy." I believe that statement has been quoted many times. My question is what is the easiest way to accomplish this using an analyzer, such as the MFJ 259B? How about some ideas on how some of you guys do that. Is a very short piece of coax used, or are there other ways to conect the device that are more effective?

What I do is tune the antenna using an electrical half wavelength of coax cable between the antenna and the antenna analyzer. This assure the true antenna impedance is what is being read by the analyzer. After tuning is done I remove the half wavelength coax and insert whatever length is necessary to get from the antenna to the radio. In doing so I have never had the SWR change when swapping cables.I just used some cheap RG-8X and made up some tuning cables for each of the ham bands for 80m and up.Actually I made lengths up for 6m, two for 10m (when joined together they work for 20m). One 10m and the 6m work well for 15m as it's close enough. I also have one for 40m and another for 80m. I bought a couple 500 foot reels of RG-8X coax from Radio Shack on a close-out sale for five cents a foot so I couldn't go wrong.

 

[Marconi]

What I do is tune the antenna using an electrical half wavelength of coax cable between the antenna and the antenna analyzer. This assure the true antenna impedance is what is being read by the analyzer. After tuning is done I remove the half wavelength coax and insert whatever length is necessary to get from the antenna to the radio. In doing so I have never had the SWR change when swapping cables.I just used some cheap RG-8X and made up some tuning cables for each of the ham bands for 80m and up.Actually I made lengths up for 6m, two for 10m (when joined together they work for 20m). One 10m and the 6m work well for 15m as it's close enough. I also have one for 40m and another for 80m. I bought a couple 500 foot reels of RG-8X coax from Radio Shack on a close-out sale for five cents a foot so I couldn't go wrong.

Heck CK, I thought you were going to tell us to do like Beetle does and get your antenna already tuned up and working like a toy in a Cracker Jacks box. I know he must have been alseep in CB radio class the day they discussed tuned jumpers. I missed a guitar class the day they talked about tuning the guitar and I never did learn how to play the thing good.

 

[wavrider]

What I do is tune the antenna using an electrical half wavelength of coax cable between the antenna and the antenna analyzer. This assure the true antenna impedance is what is being read by the analyzer. After tuning is done I remove the half wavelength coax and insert whatever length is necessary to get from the antenna to the radio. In doing so I have never had the SWR change when swapping cables.I just used some cheap RG-8X and made up some tuning cables for each of the ham bands for 80m and up.Actually I made lengths up for 6m, two for 10m (when joined together they work for 20m). One 10m and the 6m work well for 15m as it's close enough. I also have one for 40m and another for 80m. I bought a couple 500 foot reels of RG-8X coax from Radio Shack on a close-out sale for five cents a foot so I couldn't go wrong.

Great answer CW,

I also like Marconi's reply

Reminds me of an old R&R album, I forgot who put it out.

"You can tune a piano but you can't tune a fish"

Mobile installs can be tricky, CW method leaves no room for doubt with the tuned 1/2 wl jumpers to look at the feed point.

 

[6.0 Liter]

Good info guys

 

[BOOTY MONSTER]

 

[6.0 Liter]

And here all along, thats all we needed.............Geez!

 

[dudmuck]

haha. that swr cream is great. Apply liberally!

Anyways, that MFJ analyzer is mediocre but has a good instruction manual for the "antenna hints" section.
No value in pasting it here, just reat it.
It says that if swr changes with coax length, then the coax is radiating (common mode currents), or is not a 50ohm line or is lossy.
But comparing to cheap swr meters, the other item mentioned in there is how a non-1/4wave length piece of coax changes the apparent reactance of antenna on frequencies its not resonant.
This is a probable cause of why coax length change causes an apparent change in swr.