[TowerTalk] Importance of feed line length

[Jim Lux]

On 9/9/12 6:05 PM, Justin Whitstine wrote:
> Hi everyone, I've been doing research to determine how important feed
> line length is.  I've been reading up on this off and on for the last
> few weeks.  I'm currently studying to get my ham license and am very
> excited about it.  I built my first antenna this weekend.  A 1/4 wave
> vertical on 2m, just to see what I could hear.  It was a rewarding
> experience!

Very cool..


>
> I've seen much talk on feed line length and found some very
> convincing article, but then I seem to find others that contradict
> them.


Yes, indeed... it's an area of much discussion. There's theory and 
common practice and they differ, with a wide, wide array of explanations 
why.  There's simplified theory and not so simplified theory. There's 
also "practical difference" and "immeasurable difference".

You're just starting out, so the practical advice is "make it work, but 
don't agonize about the last half a dB or even a couple dB".




  I have read lots of posts from this forum and this seems to be
> one of the most knowledgeable places on the internet.  Once I get my
> general license I am planning on setting up an inverted v on 80m.
> Just when I thought I had figured out the feed line length issue, I
> learned about cable velocity factor and how common mode current
> travel at much higher speeds down the feed lines than the actual
> signals going to the antenna.  That really blew my mind!

Sort of.. but that's really a "second order" effect. If you have decent 
RF chokes, then you won't have any common mode current (or, practically, 
it's small enough to not make a BIG difference).




>
> My 80m antenna will not be very high off the ground, so it will be a
> NVIS.

How high is high?  10-20 ft is somewhat of a cloud and worm warmer, but 
hey, it works, and not everyone gets to throw a dipole up a wavelength 
above ground..



   I will be using a common mode choke as per Jim Brown's
> document "A Ham's Guide to RFI, Ferrites, Baluns, and Audio
> Interfacing".


Most excellent.

  Which was the most informative document on the
> internet about chokes, and what led me here.  I will go with a 50 ohm
> feed line because my antenna will be between 8 & 15 feet off the
> ground, and probably an inverted v.  I know 50 ohms will match an
> inverted v better.
>

Maybe, maybe not.. but it really doesn't matter all that much.  A low 
antenna has strong interactions with the soil, and who knows what 
properties the soil (and any trees, plants, house, small children, 
grass, etc) have.  You just take what you get.  ANd because 50 ohm coax 
is common, you might as well use it.  75 ohm coax is also common, and I 
doubt you'd notice a huge difference either way.

When you get to running a kilowatt, or ekeing out the last dB, agonizing 
about perfect match is important.  But for the start, 50 or 75 work 
equally well.

CHoke it at the antenna with a couple ferrite cores.. Choke it where it 
comes into the shack with a couple cores.  That's what you want..

Then either tune by pruning, or get a tuner at the shack end, and get on 
the air.

the whole pruning thing requires a decent test set up and the ability to 
raise/lower the (ends) of the antenna.  You don't actually have to cut 
the wire. Just wad it up at the end to shorten it, or double it back and 
take a turn of PVC tape around it to hold it. (that way, you can 
lengthen it again)

If you're reasonably in the ballpark, a tuner at the shack end (maybe in 
your radio, or an autotuner) will match it up nicely.  Sure, there's 
some loss in the coax from reflected power going back and forth, but 
unless it's truly hideous, it will still work.




> I'm trying my best here to not sound totally ignorant, but if any one
> knows the answer I would appreciate it greatly!  I look forward to
> your answer! Thank you, Justin
>
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