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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