coax length

[wewjr71]

Ok, on a 5/8 ground plane antenna should the length of coax run in 18ft.increments? or does it matter?

 

[SmackDown]

I guess your answer depends on what antenna you're using. I run an Astroplane and I used Avanti's suggested 12' increments. I got a 1.1:1 before I grounded it, after I attached the ground it went flat..I'm using Times Microwave RG-214 and overall I have 72' from antenna connector to the meter.


73's

 

[wewjr71]

thanks for the reply,I have the I-10K

 

[Captain Kilowatt]

The the length of coax does not matter at all.The ONLY proper length to use is whatever it takes to reach from the antenna connector to the radio connector.Avanti's suggestion of 12 foot increments comes from 12 feet being an electrical 1/2 wave of MOST common types of coaxial cable made at the time.Any 1/2 wave multiple of coax will repeat the true impedance of the antenna and in fact makes tuning the antenna more accurate. If you see SWR change when you change the length of cable then the antenna is not tuned to a true 50 ohm match.

 

[KingCobra_CDX882]

QRN answered it Exactly right

 

[hotrod]

oh.. i thought it was cause theres no way to tune the
astroplane.and the starduster for that matter

 

[Master Chief]

oh.. i thought it was cause theres no way to tune the astroplane and the starduster for that matter

This is not true. . Both antennas are tunable.

Also, changing the length of the coax only changes the reading at the other end of the coax. It does not tune the antenna.

 

[Marconi]

MC is right, they can be tuned a little. Maybe he will give us the details if we ask.

I have never tried to change the AP, but I did the ST'r. I extended the top element about a 1/2" OK, but it is not easy to make them shorter the way they secure the parts using the sheet metal screws. I don't recall the exact result, but I do know I did not like it. This antenna is fed a bit off center and I think that upset that balance.

 

[hotrod]

thank-you guys didnt realize that

 

[Marconi]

wewjr71, how are you coming along with your I-10K install?

 

[dudmuck]

If you see SWR change when you change the length of cable then the antenna is not tuned to a true 50 ohm match.

Well ya.. thats the whole point of VSWR, you get standing waves when your antenna is at a different impedance than your coax and/or transmitter.
The 18-foot thing seems to come about from .66 velocity-factor coax being a 3/4 wave. Apparently some cheap VSWR meters can be fooled by an inverted swr. 1/4 wave is 90-degree phase shift which inverts vswr.

 

[bighammer]

you should use coax is full wave increments taking the velocity factor into play.

 

[Captain Kilowatt]

you should use coax is full wave increments taking the velocity factor into play.

Wrong :roll: This question has been beaten to death many times over and the correct answer is: THERE IS NO CORRECT LENGTH.USE WHATEVER REACHES FROM ANTENNA TO RADIO. if you have a properly tuned antenna,and you should, then length does not matter.That is all I am going to say on this. I am tired of beating a dead horse. :roll: :roll: :roll: :roll: :roll:

 

[Cheech]

That dead horse is turning very rotten :shock:

 

[dudmuck]