Does coax need to be in multiples of 3 feet? I see it sold in 25 foot length with pl259 on each end.

No, for general use it can be whatever length you need.

Where are you looking to buy?  Some pre-made ones are better than others and you don't want the junk ones.

No, it needs to be a multiple of 1/2 wavelength of the lowest freq you will use it.

This.

Impedance repeats every half wavelength.

I already have 60 feet of LMR400 just need to cut it and put PL259 connectors on. I just measured from my antenna to the lighting protector and it is 25 feet. I could do 24 feet if I rout the coax a little different. I only want to cut once.

Two quarter wavelengths is a half wavelength.  Four quarters is two halves... all even multiples of 1/4 wavelength are OK, impedance repeats
for coax or any other type of transmission line.

A quarter wavelength or odd multiples are shorted stubs.  

Experiment, measure the impedance (with an analyzer) of 100' of RG-8X at 3.900 mhz, 200', etc.

Repeat the experiment with 50', add on 50' and measure again.  Add 50' measure again, etc.

You will measure at 50' very close to 0 ohms, perhas 5 or 6 ohms.  You will measure at 100' 50 ohms.

You can put 70', 75', 80', 90', 110', 120', no problem.  Don't put exactly 50', 150', 250'.

Actually, the 1/4 wavelength, considering velocity factor, of RG-8X at 3.900 mhz is (from memory) about 49.5' or so.

I would refer you to page 10 of "The Fifty Ohm Enigma" by Bill Lieske, Sr. (SK) founder of EMR Corp.  His son
Bill, Jr. now runs EMR.  This paper can be found here.  Everything you wanted to know about coax impedance.

http://www.emrcorp.com/techinfo

Go to the home page, http://www.emrcorp.com/.  EMR is well known in the commercial comms industry.

Your wife or girlfriend may lie to you, but the math shows that length does matter.

Not.

For general use, you should be using reasonably matched loads, and transmission line length will be completely irrelevant.

I'm not sure where the "1/2 wave at lowest freq" advice came from but it makes no sense.

Line length IS important if you are using a section of line as an impedance transformer, to make a phasing harness or something like that.  It's also important in the special case of using mismatched feedline, as an example using 75 ohm cable in a 50 ohm system - but such a feedline would then be frequency (band at least) -specific.


If you need to cut the feedline for your installation do so. If you don't need or want to, don't. This idea of 3 foot multiples is utter hogwash. As the most base example, if you're not changing the "3 foot" rule for the velocity factor of your feedline the whole thing fails.

The video on understanding impedance was just posted not long ago, some didn't watch or absorb it it seems like.

If you combine these two ham/CB myths, you would have a section of LMR400 only suitable for use around 128MHz (and harmonics thereof).  That's obviously not that case.

OP, what antenna do you plan to use?

I love ya man, but that's wrong :(

Use whatever length of coax you need (as Gamma said above).  If you are using an in-shack tuner, the length can be tweeked a bit to provide an impedance that may be more favorable for the tuner (i.e. lower loss match, higher power handling capability, etc...)

Tues I'll post a new episode...MPD Digital Coax Assembly Mfg...they go through it all...

TLWL (2 lazy won't listen) use the coax calculator on the Times-Microwave site, enter the highest freq you plan to use, along with the max power you anticipate, include length of run and voila, it tells you what would work best.

https://mpddigital.us/about-mpd-digital-rf-cable/

http://www.usacoax.com/

same co 2 different sites.

Good solid peeps! YMMV

They do make a good USA cable and I've bought several from them.  I think the topic of this thread is a little different and more complex than attenuation and power capacity though.

ETA:  Edited for clarity.

I cut the coax to 25 feet and soldered my PL259 connectors on and it works fine. I put up a Comet GP-6.

deleted

ar-jedi

Old school here, I don't worry about coax length. I just cut it a little more than needed in case I move something or have to replace a connector.

The SWR at the shack end of my dipole coax is insanely high on 10 Meters, yet I was running the band during Field Day with the radio pumping out 50 Watts.

When I got my license, there was one SWR meter in town. It belonged to the two way radio repair shop and he was not about to loan it out. We calculated length, cut it, and worked the world in our blissful ignorance.

[oldfart]Today's hams are spoiled.[/oldfart]

It's just because we are 'green'.

I am I am
a no-code ham.
I must use tools to see my bands,
and then obsess about those scans,
I do not like high SWR,
I do not like it, ham I are.

I'll tell you what, once you use a good analyzer, whatever you've done in the past seems like ancient history.

I was putting together our field day antennas and tuning them with my Rig Expert meter, right at the antenna. Had a couple guys standing around who were just flabbergasted, as I was adjusting the antenna and the graph on the screen moved, and I just put it where I wanted it. Somewhat incredulously one asks... "and that's showing you the SWR?".

Yup this.

I used a rig expert last year at field day for the first time. I had been using my MFJ269 but the graph is amazing.

Now I use my mini VNA and cell phone. If Fox comes along he can confirm it is even better :)

My AA-54 arrived a couple of weeks ago and it makes analyzing the antenna system so much easier than the old MFJ. What I really like about it is that it can be normalized to 72 Ohms instead of 50 since I use RG-11 for my dipoles. It also makes sorting inductors very easy.

A little article on transmission lines and SWR that contains a couple of eye-openers regarding what your SWR meter is reading and actual line SWR: http://www.angelfire.com/electronic/funwithtubes/T-Lines.html

More SWR info -- scroll donw to Section VI: http://www.hamuniverse.com/n4jaantennabook.html

Graphical illustration of  SWR: http://www.astrosurf.com/luxorion/qsl-transmission-line2.htm

I have been giving my AA-54 a real workout the last few weeks.  REALLY helpful!

I need to figure out how to download to my computer, make pictures and such.

It's very easy to do.

You should have gotten a CD and a software manual with the analyzer. If not, they are both available as downloads at the RigExpert website. If you don't have the USB cable, PM me and I'll mail you one.

I can mail you one too.  

I'm offering to mail him the one you mailed me.