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Author Topic: How long the open wire feed line should be  (Read 3347 times)
SV1KGA
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Posts: 11




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« on: December 05, 2002, 04:11:02 AM »

Dear All,

Could you please inform me how long the length of the open wire feed line should be?
Is there any equation that gives the result?

73 de Kostas
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« Reply #1 on: December 05, 2002, 05:10:04 AM »

If you are using open wire line as nothing more than a transmission line it can be any length you need.  I suspect there is more to your question such as does cutting the line to a certain length make an antenna tuner work better with certain loads???  The most universal rule of thumb is to cut the line so it is not a 1/4 wave or multiple for any freq you use.  This takes some number crunching but is worth it.
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SV1KGA
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« Reply #2 on: December 05, 2002, 05:57:57 AM »

So you think that if I want to build a dipole with open wire feed line feeded with a coaxial cable through a balun the length of the open wire should NOT be 1/4 of the frequency I want to use?
Did i get it?
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AC5E
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Posts: 3585




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« Reply #3 on: December 05, 2002, 10:28:58 AM »

The electrical length of the feedline should NOT be close to 1/4 wave or to any odd, 3-5-7-9 and so on, multiple of a 1/4 wavelength. If possible, it should be close to 1/2 - 1 - 1 1/2 - 2 and so on, wavelengths.

To complicate life, the electrical wavelength is a few percent shorter than the free space wavelength, or the "antenna wavelength," but it's not usually enough to worry about.

73  Pete Allen  AC5E
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AD6JN
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Posts: 172




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« Reply #4 on: December 05, 2002, 11:19:07 AM »

Any length provided you have the power.

450 ohm window line is about 0.15 dB attenuation per 100 feet at 30 Mhz - see the ARRL Antenna book.  

If you are properly matched at the antenna feed point cuting the line in 1/2 wave length increments is ok and does not have any reactance.

1/4 wave length increments add or subtract reactance to aid in feed point matching and tuning.

For multiple band operation this may not be as easy unless you operate on those bands that divide by 2 nicely.

I am assuming you have an antenna tuner with a balanced output.

Bob AD6JN

 
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AC5UP
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Posts: 3875




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« Reply #5 on: December 05, 2002, 11:21:51 AM »

An open wire feeder can be any length if you're using a tuner to work multiple bands on a Doublet or random wire antenna. If your tuner won't match a band segment you're interested in, try adding a meter or two of wire to the feeder. This will shift the reactance a bit and may help the tuner.

If your antenna is resonant on one band (or 40 and 15 Meters on a Dipole), an open wire feeder that's a multiple of 1/2 wave will replicate the antenna impedance at the radio. This means you can feed a 50 or 70 Ohm antenna with 450 Ohm ladder line without any significant loss. The same is true of coax, but with length compensation for the velocity factor. If you're using a mix, let's say a Dipole fed with open wire line up to the shack, then coax indoors, try to make the open wire line a multiple of 1/2 wave and use a 1:1 Balun beteeen the open wire feeder and coax.

Oughta' work...

PS: Whenever you hear the question about using RG-11 or RG-59 (75 Ohm) coax on something like a mono-band antenna, as long as it's a multiple of 1/2 wavelength there will be no mismatch.

- AC5UP
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AC5UP
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« Reply #6 on: December 05, 2002, 12:38:55 PM »

Ooops, shoulda' included the math...

A half wave in free space (FEET) is 492 divided by the Frequency in MHz. For METERS, it's 150 divided by the Frequency in MHz.

So... Let's say we're building a 20 Meter Dipole and feeding it with a half wave of open-wire line.

A half wave is 492 / 14.2 = 34.64' (feet)
A half wave is 150 / 14.2 = 10.563 Meters

To compensate for the velocity factor of open wire line (approximately .95) we take the length of the line and multiply by .95, so 34.64 feet becomes 32.91 feet and 10.563 Meters becomes 10.035 Meters... 95% of the free space length.

A typical solid-dielectric coax like RG-8 or RG-11 has a velocity factor of .66, so a half wave of coax for the same frequency would be 22.86 feet or 6.971 Meters.

In your case, the coax can be any length...

- AC5UP

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WB6BYU
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« Reply #7 on: December 05, 2002, 04:01:47 PM »

The answer will depend a lot on what antenna you plan
to use and on what bands, how you plan to match it
at the other end, etc.  So to give an answer we all
have to make assumptions, which may not apply to your
situation.

If you are putting up a half-wave dipole for one band,
and you want to replace the coax feedline with open
wire line to reduce losses, then the open wire line
should be a multiple of 1/2 wavelength (corrected for
the velocity factor.)  This will cause the input
impedance at the bottom ot be the same as the load
impedance at the antenna:  with a 1 : 1 balun you
should be able to connect it to your coax with a low
SWR.

If you are using a full wave center-fed dipole fed with
open wire line, then making the feedline an odd
multiple of 1/4 wavelength will transform the input
impedance down a low impedance that will be closer to
that of the coax cable.  Then either a 4 : 1 or 1 : 1
balun should allow you to feed it with coax cable with
an SWR that will be within the range of many tuners.

For other antenna lengths, you can usually find a
feedline length that will give a reasonably low SWR
at the point were it attaches to the coax, but this
will depend on the antenna you wish to use.

However, these situations are for an antenna used on
only one band.  On another frequency, the input
impedance of the antenna and the effective length of
the feedline (in wavelengths) will both change.  For
multiband use, you either need a wide-range ATU, or
you will need to do some calculations to find a good
compromise for the desired bands.  You can estimate the
input impedance at the bottom of the open wire line if
you know the length of the dipole and the feedline:
when the feedline length plus the length of one side of
the dipole is an even multiple of a quarter wavelength,
the impedance will be high.  When this distance is an
odd multiple of a quarter wavelength, the impedance
will be low.  You will get the best match to coax cable
when the impedance is closer to "low" than it is to
"high".

If you have a specific application in mind, please tell
us the details, and we may be able to provide more
detailed suggestions.
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« Reply #8 on: December 05, 2002, 07:32:03 PM »

The notion that a half wave transmission line will reflect the impedance at the other end is false.  This will happen only when the load end is the same resistance as the open wire line.  Notice I said resistance not impedance.  When tuned feeders are used as is assumed here,  any multiple of 1/4 wave (even or odd) will be undesireable.  I wish those who write text in handbooks knew what they were talking about when they write watered down technical theory.
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N6AJR
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« Reply #9 on: December 06, 2002, 12:29:27 AM »

I would make it just long enough from the rig to the antenna...!!!
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K5DVW
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Posts: 2193




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« Reply #10 on: December 06, 2002, 08:30:17 AM »

"The notion that a half wave transmission line will reflect the impedance at the other end is false."

I'd like for you to explain this with some non watered down technical theory. Unless you are talking about loss effects, I can't see what you're getting at here? According to any classical transmission line theory, you can take ANY impedance transmission line of 1/2 wavelength, and ANY termination impedance (yes, even reactive) and at the other end of the line you will see that exact impedance. This is the whole basis of the Smith Chart and it seems to work out that way in the measurement lab. So, I'm lost on this comment. Please elaborate!
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« Reply #11 on: December 06, 2002, 08:34:00 PM »

Mr. k5dvw,  I don't have all day.  Get a copy of a good electromagnetic fields and wave text like "Time-Harmonic Electromagnetic fields" by Harrington.  If you want a hint,  e-field and h-field mode changes.
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AC5UP
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Posts: 3875




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« Reply #12 on: December 07, 2002, 09:42:25 AM »

Awwwwwwwww... C'mon now, Mr. WA8KJP... This is a golden opportunity to educate us, and I doubt it would take all day! Can't you spare just a little time to share a small portion of your wealth of knowledge?

I was intrigued by your earlier comment:

"When tuned feeders are used as is assumed here, any multiple of 1/4 wave (even or odd) will be undesireable."

If we were talking about metallic guy wire segments, I'd agree... But... The classic All-Band Doublet (and other) well-proven antenna designs like a full-wave loop use 1/4 wave tuned feeders to great advantage, and multi-element arrays like those in my NAB Engineering Handbook (1974 Edition, wouldn't want to introduce any radical new ideas here) show radiating elements attached to a pair of parallel feed lines and stacked <gasp!> 1/2 wave apart... Do you mean to say we've been doing it wrong all these years Huh

- AC5UP
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K5DVW
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Posts: 2193




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« Reply #13 on: December 09, 2002, 08:07:54 AM »

WA8KJP

I studied emag for four years, which is considerably longer than all day. I'd appreciated it if you'd make up for my educational deficiency on the topic because obviously I didn't get my money's worth at Texas A&M. And, it shouldnt take you more than 5 minutes to explain it.

Mode changes? In coaxial cable? Hummm all these years I thought it supported a homogenous TEM field within it's operating bandwidth. Lay some physics on me, big guy!
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N0XE
Member

Posts: 197




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« Reply #14 on: December 28, 2002, 09:44:23 AM »

A simple question, and it leads to confrontation, hate to see this happen, my advice, get some books and do some reading, The ARRL antenna book is a good place to start. I have been using center fed dipoles for years fed with open line. I never really worried too much about the feedline length, as long as you have a good outboard tuner with a wide matching range you should be fine. Even the length of the dipole is not critical and you can usually work pretty well even if you have to have it shorter then a halfwave for the lowest frequency you plan to operate. I could get in to all the theory stuff with you but I think in your case you are probably confused enough, remember they call some of this stuff theory for a good reason. Get on the air and enjoy ham radio and you can never read too much, get some good books on antennas and enjoy the hobby.
73 N0XE
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