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Author Topic: Help me remember. . .  (Read 1189 times)
N4NOO
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Posts: 106




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« on: September 18, 2008, 09:05:23 PM »

I think I remember that coax jumpers and also complete coax runs will transfer maximum power if they are a multiple of a 1/4 wave including, the velosity factor and the conector, at the desired frequency.

 Sound right?
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N3OX
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« Reply #1 on: September 18, 2008, 09:11:11 PM »

In a matched system, it doesn't matter.

You might be thinking of forcing current for phasing verticals?

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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N4NOO
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« Reply #2 on: September 18, 2008, 09:17:26 PM »

Thank you for your reply but please define a matched system.  
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N3OX
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« Reply #3 on: September 18, 2008, 09:46:32 PM »

A matched system is just where the feedline is terminated in its characteristic impedance.  In such a system, the line length doesn't matter except that line has some loss.

If you are running, say, 50 ohm line to a 50 ohm load the correct length of coax that provides the least loss, (and so gives you the most power transfer) is 0.00ft ;-)  The longer it gets, the less power is transferred to the 50 ohm load.


73,
Dan







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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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Posts: 13477




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« Reply #4 on: September 18, 2008, 10:19:27 PM »

True.  If you are using 50 ohm coax in a nominal 50 ohm system, there
is no magic length that works better than something else.  For most
efficient transfer use low loss coax and make it as short as practical.

On HF having 10 (or even 50) extra feet of coax coiled up may not make
much difference.  On UHF and above you really want to keep it as
short as you can to minimize loss.

There are a few certain cases where a particular length of feedline
is used for impedance matching, in which case you might use a quarter
or half wavelength line (or some odd fraction).  This isn't a case of
loss, but of providing a lower SWR at the transmitter.  For example,
a quad antenna with a feedpoint impedance of 112 ohms can be
matched to 50 ohms using 1/4 wave of 75 ohm coax.  But you can't
generalize that to say any specific length is always the best choice.
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WB6BYU
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Posts: 13477




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« Reply #5 on: September 18, 2008, 10:19:27 PM »

True.  If you are using 50 ohm coax in a nominal 50 ohm system, there
is no magic length that works better than something else.  For most
efficient transfer use low loss coax and make it as short as practical.

On HF having 10 (or even 50) extra feet of coax coiled up may not make
much difference.  On UHF and above you really want to keep it as
short as you can to minimize loss.

There are a few certain cases where a particular length of feedline
is used for impedance matching, in which case you might use a quarter
or half wavelength line (or some odd fraction).  This isn't a case of
loss, but of providing a lower SWR at the transmitter.  For example,
a quad antenna with a feedpoint impedance of 112 ohms can be
matched to 50 ohms using 1/4 wave of 75 ohm coax.  But you can't
generalize that to say any specific length is always the best choice.
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W7ETA
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Posts: 2527




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« Reply #6 on: September 18, 2008, 11:09:27 PM »

See if it makes sense to you, a jumper for 80 meters would be 20 meters long?

You'll find a lot of interesting information in any old ARRL HandBook; they are cheap on eBay and qualify for cheap mailing rates.

I like the ones based upon simple components, like from the 50s.

73
Bob
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VK1OD
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Posts: 1697




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« Reply #7 on: September 19, 2008, 12:06:36 AM »


N4NOO,

That is not a necessary condition for "maximum power transfer", nor will it, of itself, assure "maximum power transfer".

Owen
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AD4U
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Posts: 2179




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« Reply #8 on: September 19, 2008, 05:25:26 AM »

To put what N3OX said another way.....

If you have to trim your coax jumpers and / or your coax feedline to multiples of 1/4 wavelength in order to achieve a good match or to make your system "work", then something is wrong with your system.  Correct the problem.  Don't put a bandaid on it by fiddling with coax length.  

Dick  AD4U
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W8JI
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« Reply #9 on: September 19, 2008, 06:57:08 AM »

 think I remember that coax jumpers and also complete coax runs will transfer maximum power if they are a multiple of a 1/4 wave including, the velosity factor and the conector, at the desired frequency.

Sound right? >>>>>

NO, sounds like nonsense.

Better you just forget that.

If you are trimming or adjusting the length of 50 ohm coax and you are seeing SWR change it can only mean one of three things:

1.) The cable is NOT really 50 ohm cable

2.) The SWR meter is not really a 50 ohm meter

3.) You antenna system has a major problem like an open shield or common mode current on the outside of the shield, and the cable is radiating

Mostly CBers come up with goofy things like that, but sometimes Hams do. Where ever it came from, bury it.

73 Tom






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W8JI
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« Reply #10 on: September 19, 2008, 06:59:40 AM »

N3OX,

In an UNmatched system it doesn't matter. SWR (except for a gradual change because of additional loss in very long cables) is unaffected by length of the cable....matched or not.

If I have a 3:1 SWR on a cable, the SWR is 3:1 no matter how long I make the cable except for the gradual change from losses as a cable gets longer and longer.

73 Tom
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N5LRZ
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Posts: 0




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« Reply #11 on: September 19, 2008, 07:18:16 AM »

RE line loss....

Charts showing line loss per type of coax per 100 feet for different frequencies can be found in the ARRL Handbook and in the ARRL Antenna book.  Probably under the heading of Transmission Lines.

The loss is,if I remember correctly, expressed in DBs per 100 feet.

So if say 8x mini has X db loss per 100 feet at the frequency you intend to use and you run 150 feet of coax your loss will be 1.5 x X for the run of coax

Loss works both ways aka on both outgoing as well as incomming signals.



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N5LRZ
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Posts: 0




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« Reply #12 on: September 19, 2008, 07:57:01 AM »

OX is correct

In re to SWR, the SWR is independant of the length of coax if the coax is not defective.  SWR is a ratio of power not materials.  

IF you get a significant increase or decrease in SWR by changing the length of coax you have a bigger problem then you think.  Your coax just might be defective or your soldering of the coax connectors may be bad.
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N6AJR
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Posts: 9921




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« Reply #13 on: September 19, 2008, 01:59:56 PM »

I remember in the tech school for the Air Force in Biloxi, we were taught something similar.  



If you match a sine wave to a full wave length of transmission line it reflects open, inductive, shorted, capacitive and open again at the !/4 wave marks.



So we used the 1/2 wave point to build a wave guide by usingn a run of 1/2 wave  pieces in side by side mounts to produce a wave guide, ( or something like that, )


 Now I don't recall the exact phrasing, that was back in 1966 so my memory can be faulted...
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N6AJR
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« Reply #14 on: September 19, 2008, 02:01:20 PM »

and that was frequency specific for each wavelenth.
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