• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
  • Notice: You may not use this site to facilitate the unauthorized selling of copyrighted material including, but not limited to, in the public dxForums, dxGallery, dxChat and private messsages. If you get caught doing this, you will be permanently banned with no second chances. A notice will be placed on the site of why you were banned and any information collected by this site about you will be made known to the copyright holder.

Does coax length really matter when tuning?

53 bayous

Member
Jun 4, 2005
17
0
11
Louisiana
My first 10k tuned at a 1.1 SWR with 18 feet of coax. If use 17 feet on other vehicle to tune another antenna, will this stop me from getting the same reading? or is coax length just a myth once you get the lowest SWR achievable at any length?
 

High Miler

Active Member
May 30, 2005
298
3
26
Pennsylvania
They say for Mobile work that Coax length is optimum... I always stayed with 18' ... Some people swear up and down it's true, some people say it don't matter...

Ultimately no matter what type of installation... It's always good to use that SWR Meter to make sure...
 

53 bayous

Member
Jun 4, 2005
17
0
11
Louisiana
Yes, its true

On my Dodge Ram I used 18 feet and a bolted down mount. 1.1 SWR on the 22 inch shaft 10k. I was just concerned that Im about to use 17 feet of coax on a quad mag mount tuning the new 12 inch SW shaft on top of my Expedition. I really don't believe I'll have a problem tuning it down.
 

2at124

Active Member
Apr 23, 2005
239
0
26
43
Cape May NJ
coax length

1 foot shouldnt affect it really too much it might make it a 1.2 but that coyl be adjusted at the stinger in the antenna
 

High Miler

Active Member
May 30, 2005
298
3
26
Pennsylvania
It won't be bad... I was just trying to make the point that no matter what you make for installation... Always check the SWR...

A guy in another forum took out a piece of Coax and put a much longer in thinking it would do nothing... Until it blew his amp up.. He stated that since his SWR was perfect he didn't feel the need till it was too late.

Me personally, I feel that Coax does affect it, I mean afterall I have built Coax antennas for the ham bands.. You just don't tie off a piece of coax and call it good... You measure everything up and trim if need be.

But going from 18' to 17' will be minimal and you may not see any at all as far as change in SWR is concerned...
 

FL Native

Registered User
Apr 3, 2005
1,385
31
58
50
Down the street and around the corner.
the guy that blew up his amp had nothing to do with coax length. they say use around 18 feet cause that is a 1/2 wave multiple on 11 meters. the coax lenght will only make you see incorect swr readings if your antenna is not tuned and your not using a 1/2 wave multiple of coax.

now 18 feet is only getting close or around a 1/2 wave multiple. actualy it depends on coax type. meaning it depends on the veolicty factor of the coax to determine what a 1/2 of coax is on any given frequency.

heres a link to explain the coax thoery to maybe help displell some of the myths on coax length.

www.freewebs.com/skunkworks/coax1.htm
 

DXman

Yes, that's 3100 degrees F. Nine yrs of hard work.
Apr 5, 2005
1,673
954
123
75
West Coast - Washington
53 bayous,
Here is what took place with my coax.
10K - 22" shaft
using MFJ 259, 17' of coax
SWR - 1.1:1
R=48, X=2

Using 14.8' (measured for electrical length) of same brand.
SWR - 1.1:1
R=50, X=1

So there was very little change in going to 2.2' less of coax.
I have been using this length of 14.8' now for a year and it has perform excellent for me.

When going to a different vehicle things can change, may not be much or it could be alot.
--------------
DXman
 

High Miler

Active Member
May 30, 2005
298
3
26
Pennsylvania
FL Native said:
the guy that blew up his amp had nothing to do with coax length. they say use around 18 feet cause that is a 1/2 wave multiple on 11 meters. the coax lenght will only make you see incorect swr readings if your antenna is not tuned and your not using a 1/2 wave multiple of coax.

now 18 feet is only getting close or around a 1/2 wave multiple. actualy it depends on coax type. meaning it depends on the veolicty factor of the coax to determine what a 1/2 of coax is on any given frequency.

heres a link to explain the coax thoery to maybe help displell some of the myths on coax length.

www.freewebs.com/skunkworks/coax1.htm


Ok... :LOL: I guess he SENT me a PM bout it 'cause he had nothing better do... It had to do with switching out coax and having not checked it... Anytime you make a change in your system it's always best to check.
 

paws264

Active Member
Apr 6, 2005
647
5
28
53 bayous said:
My first 10k tuned at a 1.1 SWR with 18 feet of coax. If use 17 feet on other vehicle to tune another antenna, will this stop me from getting the same reading? or is coax length just a myth once you get the lowest SWR achievable at any length?

The 18 ft actually equals out to be 3/4 wave length (figure in the velocity factor for RG-8 ) add to that a !/4 wave length antenna and....

BINGO! ONE complete Wave Length from antenna jack of the radio to the tip of the antenna.

What more could a radio signal want?


.
 

53 bayous

Member
Jun 4, 2005
17
0
11
Louisiana
Well, it worked

well, I just tuned Kale's new 12in dual coil SW series, and tuned it out perfectly with 17 feet of coax and a quad mag mount, and I didn't even have to cut anything (just pulled it out a few inches), and SWRs fell flat!! I then adjusted it to a 1.1 Never had one go that well before in just a few minutes time.
 

W5LZ

Crotchety Old Bastard
Apr 8, 2005
6,832
823
173
Oklahoma
53 bayous,
That's a yes and no question. It can make a difference if the antenna isn't near a 50 ohm impedance match, but not after the antenna is tuned for resonance and impedance matching.
To start with, since the antenna and feed line are not of the same impedance value the combination can give some odd value of impedance, or the correct value of impedance even though the thing isn't really tuned. If the feed line is something close to an electrical half wave during the tuning process then the mismatch while still apparent, will be compensated for to some extent (like the feed line not being there at all, sort of a direct connection to the radio). Once the antenna is close to a good 50 ohm impedance match, the the length of the feed line doesn't really matter (as long as it's the same/close as the impedances of the antenna and radio).
So, there you go. If you are aware of the mismatchs during the tuning process and can compensate for that with out the electrical half wave of feed line, then it isn't necessary. But in general it does make things easier...
- 'Doc
 

dxChat
Help Users
  • No one is chatting at the moment.
  • @ BJ radionut:
    ButtFuzz: Good Morning. Feeling pretty good (y) :) :) :) :)
  • @ BJ radionut:
    6m open to 5 Land from Indiana....hope it goes wide open FD
  • @ f4joc:
    is this amp still available and where is it shipping from??
  • @ ButtFuzz:
    " f4joc: is this amp still available and where is it shipping from??" Antecedent, please...
  • @ ButtFuzz:
    Meaning: There's a whole lot of "this amp"