Times Microwave Lmr 400uf

[Highway Man]

What would be the proper length of this coax to be an electrically correct half wave (repeating) being the velocity factor is 86 I'm thinking. Kinda thinking center frequency channel 20 is what most calculate at.
better yet where is the formula to calculate this. teach a man to fish thing.
thank you in advance

 

[TheRealPorkchop]

http://n1gnn.com/webData/CoaxVelocityFactorLength.html

 

[Highway Man]

chop , thanks I'll check that out

 

[Highway Man]

well i was wrong, velocity on speck sheet says 83. wondering if different connectors figure in.
well i had ordered 15 foot. may be short by an inch depending on channel.
that calculator was easy if i done it right. good stuff.

 

[Highway Man]

so now I'm confused. Even if i calculate for different coax with velocity of .66 a half wave repeating is right at 12 foot. So my confusion is how does this 18 foot stuff figure in? This isn't free space.

 

[TheRealPorkchop]

I could drop my 10 cents here but I won’t, my thoughts would get lost in translation. The basic answers you’ll get here will be :

(In no correct order)

1) it’s a myth

2) it doesn’t matter

3) you need whatever length it takes to make the run

4) use 18 feet

5) use 15 feet

6) use 12 feet

7) connectors give a 1 dB loss

8) no they don’t, you’re crazy

9) yes they do

You get the idea?

I’d suggest what the calculator said for that coax which was what, about 15.5 feet? And then in the same breath I’d say just use whatever it takes to make it from point A to point B.

This is a can of worms.

 

[Highway Man]

yeah. I'm gathering that. also those saying a 108" whip and spring with 1.5" stud are going end up sawing off a lot.
go ahead and lay your 10 cents down for me. I'm all ears.

 

[Highway Man]

i think 15 foot will ball park it because we don't know what end of the spool the coax was made from.
I'll have to use an analyzer to figure it out

 

[fourstringburn]

I'm gathering that. also those saying a 108" whip and spring with 1.5" stud

I always heard it as a 102 whip and a 6 inch spring makes a 108 whip.

 

[loosecannon]

i think something went wrong with your calculations.

1/2 wavelength @.83 vf is 15.017 feet.
1/2 wavelength @.66 vf is 11.941 feet.

when it comes to coax being an impedance repeater or an impedance inverter, it's really about avoiding the exact 1/4 wave multiples, not trying to get an exact electrical wavelength.
that applies to coax being used to connect an antenna to a radio system.
phasing lines and using the coax as part of an antenna is different.

so, if your goal is to not have an impedance inverter length of coax going from your radio to your antenna, the lengths to avoid for 27mhz with a coax vf of .83 are:

1/4=7.5 feet
3/4=22.53 feet
5/4=37.55 feet
7/4=52.56 feet
9/4=67.58 feet
11/4=82.6 feet
13/4=97.62 feet

staying clear of these lengths will keep you from having a different SWR reading at the antenna end of the coax than you do at the radio end.
LC

 

[TheRealPorkchop]

That’s sorta my 10 cents there, LC summed it up.

And I’m with the 102” thing myself, though I haven’t been able to acquire 6” springs, used 4-4.5” and all was well.

 

[Highway Man]

right. the hustler ss spring is 4"..
ok. im grasping this better but don't know what an impedance inverter is. I'll have to Google that. if a box were in line would you want the same 15' all through or is the proverbial 3 footer alright?

 

[Highway Man]

LooseCannon my goal is to build a new mobile right with good materials. so if I followed you correctly I'm connecting a radio to an antenna with things in the middle.

 

[TheRealPorkchop]

Well, I’m fixin’ to get smacked down...

I’d do the run from the amp to the antenna at 1/2 wave which I believe it comes out in that calculator 15 and some odd feet. I’d then do my jumper from the radio to the amp at 1/4 wave cause I don’t want 15 feet there, that figures to what, about 8 foot?

I’ve set up many big trucks this way with absolutely no issue at all but I’m not saying it’s the absolute correct way. LC, chime in on this cause you’re better with the wording than I am.

And then you’ve got the run what’cha brung theory where you use whatever reaches and honestly I think that’s fine too.

 

[Captain Kilowatt]

Unless you are trying to match impedances or make up phasing lines coax length really does not matter. If it did then hams would have a bitch of a time using multiband antennas.