Which is the feed-point

[Metal Head]

If I put a amp in line in my mobile does the amp then become the feed-point to the ant.? If so how long should it be from the radio to the amp? I'm tring to figure out my coax length before I get into the intallation. I'm slowly getting this, but I keep reading differant things. In my installation I can't put the amp next to the radio. I need to put it somewhere in the middle of the coax if possable. How do the guys with the keydown Suburbans run theirs?

 

[Moleculo]

The "feedpoint" is where the coax hooks to the antenna. It has nothing to do with the amp. How long should the coax be from radio to amp? As long as necessary to get there.

 

[Captain Kilowatt]

Man I wish I had a dollar for every time someone asked how long the coax should be.I would have $$$$$$$$$$$$$$$$$$$$. :shock: My answer is :measure and find out.

 

[Metal Head]

The "feedpoint" is where the coax hooks to the antenna. It has nothing to do with the amp. How long should the coax be from radio to amp? As long as necessary to get there.

From what I read I thought the feed point was the radio or source of input.

 

[Metal Head]

My answer is :measure and find out.

That's what I'm tring to do. :roll:

 

[Captain Kilowatt]

As Mole said the feedpoint is the point at which the feedline (coax) connects to the antenna.Antennas have feedpoints,radio amps etc. do not.Also what I meant about measure and find out was simple.Decide where you want the radio.Decide where you want the amp.Decide where you want the antenna.Take a tape measurement and find out how long the cable has to be to reach.There is no "magic" length that is supposed to work.Don't base where you want to mount something on how much cable it should take to reach it.Mount where convieniant and use whatever length fits.

Edit: I just realized I have finally made the 1000+ post crowd.

 

[Metal Head]

What about using the correct length for the wavelength?

 

[Captain Kilowatt]

What about using the correct length for the wavelength?

Unless you are designing phasing sections or impedance transformers then the length of the line WILL NOT MATTER regardless of the operating frequency.If you NEED a certain length of line to make something work there is something wrong and all you are doing is masking the problem.

 

[Metal Head]

Please read this link and you'll see what I'm referring to.
http://www.radiomods.co.nz/coaxlength.html

 

[Captain Kilowatt]

OK I read it but I must say that there is some truth and some untruths on that site.The last comment by "Dave" the so-called broadcast tech,(I am one BTW with 22 years of AM and FM commercial broadcast engineering) is just plain wrong in several cases.A proper system has the antenna impedance equal to the line impedance which in turn is equal to the output impedance of the radio.In that case line length has NO EFFECT whatsoever on the SWR reading regardless of where the meter may be placed along the line.I am not quite sure what you are really asking.Are you trying to determine the "proper" line length to make sure that the actual antenna impedance is what is presented at the Meter? If that is the case an even number of 1/2 waves will accuratly reflect the true SWR of the antenna and for RG-8 cable that would be about 11"10".Again I am just not sure of what you are looking for.

 

[Metal Head]

Ok, If I need 1/2 wave length from the radio to the ant. what happen when I put an amp in line? should I use two 1/2 wave coax, one from the radio to the amp and the other from the amp to the ant. or can I split the 1/2 wave. I know I can play with the coax untill I get it close I've done it for years but I'm tring to figure out the "proper" way.

 

[DXman]

Metal Head,
If you can get to your AMP with just 6' from your radio then go for it - you don't need the 1/2 wave figure here.

Like QRN was saying..just measure what you need to get to antenna from the box....15' 16' 18' or what ever.
---------------
DXman

 

[FL Native]

also if you antenna is allready "tuned" the coax length wont matter you will get the correct readings regarless of coax length. if you antenna is not tuned and you try using different length to ajust swr. all you will be doing is masking the problem.

swr problems can also be from bad coax or an untuned antenna.

the best way to go about this is either buy or find someone who has a antenna anylizer to check your antenna at the feed point of the antenna.(were you plug the coax in) get 50 ohm match and lowest swr at the middle freq of what ever band you talk on. say channel 20 on 11 meter cb band. then you can use what ever length of coax you want and know for a fact if you have bad swr readings on your meter near the radio end of the coax that is is a coax problem or a mismatch of input or output tuning in your radio or amp.

hope that helps explain the coax theory a little better.

and why "they" say the corect length is the amount needed to go from one device to the next.


PS: the only tim eyou would have to worry about using 1/2 wave multiples is when 1: you dont have an antenna anylizer to check/set swr/antenna resonate freq at the feed point. or 2:
your by yourself in base set type of situation and cant read the anylizer out side and at the radio at the same time. so in case number 2 to get an acurate reading at the radio end of the coax you would need to use 1/2 wave multiples to get a acurate reading at the radio end of coax.

pheeeewwww that made my head hurt.

 

[Metal Head]

The tuned ant. part kind of gets me. I'm using a 102" whip. I don't have an ant. anylizer to check it. The rest of what you said make sence...Thanks!!!!!

 

[FL Native]

The tuned ant. part kind of gets me. I'm using a 102" whip. I don't have an ant. anylizer to check it. The rest of what you said make sence...Thanks!!!!!

102" whip is a 1/4 wave antenna and will be bellow 1.5 swr if grounded corectly but usualy you have to ground real good and add 4 inches to the antenna most people use a 4inch sping or just buy a 108 inch antenna. so really not much tunning to do with a 102inch whip. just mount it and make sure its got a good ground.