----- Original Message -----
From: "Gene Fuller" <w2lu@rochester.rr.com>
To: "Jim Lux" <jimlux@earthlink.net>
Sent: Monday, September 10, 2012 9:33 AM
Subject: Re: [TowerTalk] Importance of feed line length
Another way to appease your transmitter, if the transmission line SWR is
too high, is to vary the transmission line length by fractions of a
wavelength. This doesn't actually improve the SWR on the line but will
change the impedance where it terminates into the transmitter. I built a
box with 2,4, and 8 foot (physical, not electrical) lengths of RG-213
that, with relays,can be selected in 2 foot increments from 2 to 14 feet
that I use for various applications, including "line flattening" and phase
adjusting (e.g. with phased arrays). The approach of changing line length
can also be helpful if the SWR is high enough so it's outside the range
of an "antenna tuner". The lengths I use at too short to be much help on
the low frequency bands but good for 10-20 maters.(If your radio has a
built in tuner and the 80 meter SWR is too high you could try inserting
additional coax in 10-20 foot increments.)
Gene / W2LU
----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: <towertalk@contesting.com>
Sent: Sunday, September 09, 2012 11:20 PM
Subject: Re: [TowerTalk] Importance of feed line length
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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