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Missie
Member
Username: Missie
Post Number: 53
Registered: 10-2006
| | Posted on Sunday, January 16, 2011 - 10:50 am: | 
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coax is suppose to be 18'now if I want to put a box in line and need a 5' jumper should I subtract that length from the 18'or just ad 5' or?? Thanks. |
Scrapiron63
Advanced Member
Username: Scrapiron63
Post Number: 873
Registered: 12-2001
| | Posted on Sunday, January 16, 2011 - 12:41 pm: |
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I would leave the coax at 18' and add whatever length jumper you need between the amp and radio. |
Starface
Senior Member
Username: Starface
Post Number: 2972
Registered: 1-2005
| | Posted on Sunday, January 16, 2011 - 12:42 pm: |
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just add the jumper |
Tech237
Moderator
Username: Tech237
Post Number: 1401
Registered: 4-2004
| | Posted on Sunday, January 16, 2011 - 6:04 pm: |
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add the jumper, but check the swr right at the base of the antenna afterwards, if it shows any change at the radio.
I suggest this as adding coax length can show an apparent change in SWR at the radio while the actual SWR has not changed at all. This is the reason that 18ft of common on 11m, it is the correct electrical to show the real SWR (I.E. at the antenna)back in the shack.
Tech237
N7AUS
God only made some many perfect head, on the rest he put hair.
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N8fgb
Intermediate Member
Username: N8fgb
Post Number: 102
Registered: 5-2002
| | Posted on Sunday, January 16, 2011 - 5:46 pm: |
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http://www.shadowstorm.com/cb/CB_Myths_Exploded.html |
Hollowpoint445
Senior Member
Username: Hollowpoint445
Post Number: 1487
Registered: 6-2004
| | Posted on Sunday, January 16, 2011 - 6:30 pm: |
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18' is about the right length to reach an antenna on the rear bumper of a car from the CB mounted on a dash - that's the reason for default 18' length.
RF propogates through coax just as it does antennas or air, and at a rate the coax manufacturer is nice enough to let us know - the velocity factor. At half wave intervals, the voltage should be minimum while the current will be maximum, so IDEALLY you would want a half wave multiple from the radio to antenna so you can read the VSWR as accurately as possible on the radio end of the coax. |
Missie
Member
Username: Missie
Post Number: 54
Registered: 10-2006
| | Posted on Monday, January 17, 2011 - 10:35 am: |
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thanks guys |
Tech237
Moderator
Username: Tech237
Post Number: 1405
Registered: 4-2004
| | Posted on Tuesday, January 18, 2011 - 7:40 pm: |
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Missie and Hollowpoint I point you both to the article I wrote about coax lengths in the Subscriber Preview section. If you cannot find it let me know and I cut and paste it as a thread here.
Tech237
N7AUS
God only made some many perfect head, on the rest he put hair.
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Hollowpoint445
Senior Member
Username: Hollowpoint445
Post Number: 1488
Registered: 6-2004
| | Posted on Tuesday, January 18, 2011 - 9:33 pm: |
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I've read your article Simon. It could have been clearer about why to use half wavelengths of coax to present a more accurate VSWR reading at the radio end of the coax.
Perhaps you'd like to correct your earlier statement about the reason for 18' of coax being used on 11m. 18' is perhaps close to an electrical half wave on the outside of the shield, but not inside the coax where the desired RF signal is present.
For coax with a velocity factor of .66 the half wavelength of coax is about 11.25', and about 13.5' with a velocity factor of .78. |
Hollowpoint445
Senior Member
Username: Hollowpoint445
Post Number: 1489
Registered: 6-2004
| | Posted on Tuesday, January 18, 2011 - 10:00 pm: |
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To expand on my earlier post, the reason you want a halfwave interval is to have the minimum voltage of the signal present so any additional voltage caused by a standing wave will be more easily detected. At halfwave intervals the voltage is minimum, so you want a halfwave multiple.
For coax with a velocity factor of .66 the half wavelength of coax is about 11.25', and about 13.5' with a velocity factor of .78. |
Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 25
Registered: 12-2009
| | Posted on Sunday, April 03, 2011 - 10:46 pm: |
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I have found that changing the length of coax affects the impedance and reactance readings demonstrably. Given this, doesn't coax length then matter? Put another way, how do you know which reading is correct with each length? Or is this the same issue as SWR?
From my readings, if the antenna is not a true 50 ohms, the length of the coax becomes part of the impedance match. I'm not exactly sure why the reactance changes however.
Any insight on this would be welcome. I have a LP-100a and this has been a hotly debated topic around my parts since I got it. |
Tech833
Moderator
Username: Tech833
Post Number: 1945
Registered: 8-2002
| | Posted on Monday, April 04, 2011 - 2:01 pm: |
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If your radio and your antenna are exactly 50 ohm, jØ matches, then the 50 ohm coax length or velocity factor does not matter. There won't be voltage and current maximas and minimas.
With that said, assuming there is a variance of ideal parameter at the antenna feedpoint (even a few ohms), then coax length comes into play because the voltage and currents are unequal at various distances from the transmitter and the feedpoint. Move the feedpoint away from the transmitter (more coax) and the voltage minima moves further down the cable. Opposite for moving the feedpoint closer. The more variance from perfect 50 ohm, jØ feedpoint match, the more critical the varying coax length or velocity factor.
So, the question becomes, if you do not correct the antenna feedpoint issue, what effect does it make to the transmitter when you add things inline?
This is difficult to answer because nothing you add inline will have the same phase shift as your coax alone. A low pass filter will have some phase lag. A high pass filter has some phase advance. An external amplifier (assuming we're talking about on the ham bands here) will have an unpredictable phase shift when in the bypass mode. However, when in the operate mode, the phase shift between the amplifier and feedpoint is the critical factor.
Phase shift between the transceiver and the external amplifier is a product of the amplifier's input tuning, nothing else. Assuming the amplifier's input tuning is adjusted properly, the transceiver should see a 50 ohm, jØ match, regardless of cable length between them. Current and voltage mismatch =Ø. If the amplifier input match is not tuned correctly, then the transceiver will see some mismatch, but it is entirely independent of the antenna feedpoint issue. They are not related.
If your transceiver and your feedpoint are matched, then the only thing various lengths of coax cable between them will do is alter the rotation. That is a non-important element in the analog transmission realm.
Your radio 'Mythbuster' since 1998
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Tech237
Moderator
Username: Tech237
Post Number: 1420
Registered: 4-2004
| | Posted on Monday, April 04, 2011 - 10:40 am: |
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Stacy, yes it does, as an article I wrote a couple of years back describes, if you are not dong your measurements right at the antenna.
To answer Hollowpoint - the 1/2 wave gives an accurate reading because, if you look at the voltage on a length of coax it repeats ever 1/2 wave length. That is if you have a peak V at the start, every half wave length along you'll have V peak. Therefor because it matches exactly the conditions at the end, it will match the true valaues as if you were measuring at the antenna end.
Sorry, but short of going into diagrams, that about as clear as I can describe it, which why I tried to skirt around it a bit in the article, so as not to go in over peoples heads.
Tech237
N7AUS
God only made some many perfect head, on the rest he put hair.
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Stacy_adams
Junior Member
Username: Stacy_adams
Post Number: 27
Registered: 12-2009
| | Posted on Monday, April 04, 2011 - 5:48 pm: |
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Based on this, if one takes adjusts by measurement at the antenna and achieves the best match, as I did with a MFJ, then, if at the end of the coax at the LP-100a load interface one gets a different reading, that says one needs to adjust the length to get the same match as at the antenna. That is what the transmitter is going to see just as if it was connected directly at the antenna as the MFJ.
If this is correct, then I think I understand this issue much better and the need to adjust coax length. |
Tech833
Moderator
Username: Tech833
Post Number: 1949
Registered: 8-2002
| | Posted on Wednesday, April 06, 2011 - 1:34 pm: |
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If you get a different reading at the antenna vs. the end of your coax, then you either do not have the antenna matched 'perfectly', or you have bad coax, or possibly a flat spot in your coax..
Your radio 'Mythbuster' since 1998
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Tech237
Moderator
Username: Tech237
Post Number: 1421
Registered: 4-2004
| | Posted on Thursday, April 07, 2011 - 3:15 pm: |
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Paul, not necessarily true.
Line length can affect apparent SWR, and I emphasise apparent. I used to demo this, when teaching apprentices, by adding extensions to a 1/4 wave (at 156MHz)length of cable and showing that the apparent SWR did change at the radio, but remained constant right at the load. Why does it appear to change?? Because the extra cable length changes Inductive (Xl) and Capacitive (Xc) reactance. And in case you're wondering the extensions had a female connection at one end and a male at the other, to eliminate the need for a joiner.
I also used to use a set of Lecher lines to show the repetiton of conditons voltage and current levels) every half wave along the lines.
Stacy, as I tried to emphasise above, the difference is an apparent, not a real difference. JUst keep in mind adjusting coax length will never change the actual SWR of an antenna.
Tech237
N7AUS
God only made some many perfect head, on the rest he put hair.
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Charliebrown
Intermediate Member
Username: Charliebrown
Post Number: 176
Registered: 10-2002
| | Posted on Friday, April 08, 2011 - 7:11 am: |
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With this being said about 50 ohm match then what about those that still use ladder line ( 300 ohm) or ( 75 ohm ) on a max 2000 antenna with a coil ( on the 300 ohm ladder line with coil to match)( 50 ohm coax on the 75 ohm line ) to match it as close as possible at the radio? If I understand this correctly then are we just fooling the radio or swr meter and this is not really a true match? So another question is it better to have the 50 ohm cable than say the 300 ohm or the 75 ohm line? These two line's that I have mention are being used because of less line loss. So matching the ohms verse line loss would it then be better to go to 50 ohm ca 400 cable? The main reason I have put this question out like this is I have to run my cable 500 foot to get out of the valley to the top of the hill. Sorry if this seem's like a silly question. |
Tech833
Moderator
Username: Tech833
Post Number: 1957
Registered: 8-2002
| | Posted on Friday, April 08, 2011 - 10:51 am: |
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Oh, wow.
First of all, there is NOT less loss in using a different impedance coax cable than the radio and antenna expect, and making them happy with transformers. Not even close.
As for ladder line (balanced line), the line itself does have lower losses, assuming the currents are truly balanced, for numerous reasons. This assumes that you are feeding a balanced antenna, like a dipole or doublet, Zepp, etc. from a transmitter with a balanced output. If you're feeding an Imax 2000 through a balun, from your CB with an unbalanced output into another balun, then the law of diminishing returns comes into play.
If your Imax 2000 was 3,000 feet away from your shack, then loss and cost might make open balanced line a good alternative to expensive low-loss Heliax. However, I'm going to guestimate that the diminishing return on just loss is going to occur at somewhere over 1,000 feet.
I have stations that use a balun to connect the unbalanced (coax) transmitter output to open wire balanced line, and it works well. The thing is, the antennas are balanced, and the open wire runs are really long, so it makes sense. For the shorter runs (and lower frequencies) at the same site, they use buried Heliax. It just makes so much more sense.
Your radio 'Mythbuster' since 1998
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Tech237
Moderator
Username: Tech237
Post Number: 1424
Registered: 4-2004
| | Posted on Friday, April 08, 2011 - 11:07 am: |
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CB, this is a whole other can of worms, because if the sections at the ends are properly designed you are actually transforming the antenna and radio impedences to match the different value of the transmission line. There is usually a section of a different feed line, of a calculated length and tyep that does the matching. Is it changing the SWR of the antenna?? NO, but it is allowing the two different impedences to work together with minimum issues.
As for 75 ohm, I have used that on a 50 ohm antenna with no matching devices. If the antenna itself is tuned properly, the worst case is an SWR of 1.5:1. If you have one higher than that, you have an issue with the antenna, i.e. it's faulty or needs tuning properly.
It is because we can do this that the myth about changing feed line length to tune an antenna came about. The two situations are completely different.
Tech237
N7AUS
God only made some many perfect head, on the rest he put hair.
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Frankenstein320
New member
Username: Frankenstein320
Post Number: 1
Registered: 9-2011
| | Posted on Saturday, September 10, 2011 - 11:21 am: |
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468 divided by your frequency x the velocity factor of the coax = a half wave electrical length. To tune an antenna this would be a plus or a full wave. This will give you a true reflection at the antenna. Once it is tuned, feel free to use any length you want. This is for 1 frequency so tune to the channel you use most. And remember any odd multiple of a quarter wave of coax will electrically act as one big quarter wave, even if it is 150 feet long, if it is an odd multiple the coax will act as a big quarter wave antenna.=RFI,TVI.Also do not peel the braid back when doing a coax end. This leaves a quarter inch of inner conductor exposed and allows rf to jump to the coax before it gets to the antenna and increces db loss and causes the braid to radiate rf.Adding a coil to the antenna with the coax does not affect the transmit for the reason of the center conductor is shielded and the magnetic field cannot escape. A coil has to have a field to cut across it or it is useless. However bending a wire will add a little resistants.the coil only takes effect if rf is trying to come back down the coax. The reason for this is that any wire with a current flowing in one direction has a magnetic field flowing in the opposite direction, thuse the magnetic flow butts heads when traveling in a loop several times and it gets stuck in the field at the coil.As far as a 2,3 4 floor rf ground you can slope a 45 degree quarter wave wire at the frequency you talk on right of the back of your radio connector or ground buss.THIS IS IN RESPONS TO COPPER TALK IN THE BOOK VOLUME 41 ISSUE 8...Have a good day MudDuck |
Frankenstein320
New member
Username: Frankenstein320
Post Number: 2
Registered: 9-2011
| | Posted on Sunday, September 11, 2011 - 9:05 pm: |
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I Have heard many people tell me to use a radio that does not have side band in it..Reason for this is that the side band radio uses different mixer frequencies and more mixers then one without side band, and produces more harmonics. Try using a radio without side band if you want to try to ride of tvi or rfi.I have not confirmed this yet but it seems like a easy fix. |
Tech237
Moderator
Username: Tech237
Post Number: 1472
Registered: 4-2004
| | Posted on Monday, September 12, 2011 - 10:47 am: |
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My answer to you frankenstein320 would be TVI and RFI existed way before Sideband did. straight AM radios can still cause these just as much as a multimode radio.
From my own experiences (35 yeasr in the trade) as well designed, properl;y tuned SSB radio is less inclined to cause issues, because a decently designed one has better filtering to remove unwated frequencies. About the only time where this may not be the case is with simple front-end overload..
Tech237
N7AUS
God only made some many perfect head, on the rest he put hair.
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Tech833
Moderator
Username: Tech833
Post Number: 2042
Registered: 8-2002
| | Posted on Monday, September 12, 2011 - 1:08 pm: |
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Lots to cover here, so will break the posts down line by line with my answer and correct information.
"To tune an antenna this would be a plus or a full wave. This will give you a true reflection at the antenna. Once it is tuned, feel free to use any length you want."
Coax length does not matter if an antenna is tuned correctly. The best way to tune an antenna is with no coax at all. If that isn't practical, just install the antenna and whatever length of coax cable you will be using permanently, and tune the antenna with the random length of cable. If it tunes to 1:1, you're done. Who cares how long the coax is? If it doesn't tune well, then retune the antenna until it does. Simple.
"if it is an odd multiple the coax will act as a big quarter wave antenna.=RFI,TVI."
No way. Having a quarter wave, or odd quarter wave of coax does not, and cannot create TVI or RFI. They are completely unrelated in every way. For instance, TVI is the transmission of spurious frequencies or harmonics that fall into the receiver passband of a television receiver. There is no way that changing a coax length to any length will ever make coax become a transmitter and create its own unique frequencies(s). That is simple physics.
"Also do not peel the braid back when doing a coax end. This leaves a quarter inch of inner conductor exposed and allows rf to jump to the coax before it gets to the antenna and increces db loss and causes the braid to radiate rf."
That is untrue. Assuming the outside of the connector is soldered to the braid properly, there is no additional loss, and no way the braid will suddenly "radiate RF". Also, this is a good time to point out that RF will not "jump the coax". Cannot happen.
"Adding a coil to the antenna with the coax does not affect the transmit for the reason of the center conductor is shielded and the magnetic field cannot escape. "
If that theory were true, then a "Shileded Loop" antenna would not work. And, as we all know, a shielded loop is a great antenna, particularly on LW and MW bands. But, the main point is, adding turns of coax can act as a common mode choke if done properly.
"The reason for this is that any wire with a current flowing in one direction has a magnetic field flowing in the opposite direction, thuse the magnetic flow butts heads when traveling in a loop several times and it gets stuck in the field at the coil."
No, that is also incorrect. Current flowing through, or over a wire (in the case of RF) creates a magnetic field simply because of its flow along a conductor. A magnetic field does not "butt heads" and disappear. It either adds to or nulls a field. However, even in the case of a null, it still adds in some direction or another. You can only null in one bi-polar direction (bi-directional null). That is why a magnetic loop antenna has such deep nulls in two directions, and sensitive lobes 90 degrees to those other two. An easy experiment to show this is to use two small magnets in your hand (speaker magnets will do). Hold them near each other and rotate just one of them. Notice that as the one magnet is rotated, the field changes from a "push away" to an "attract" state. That's as simple as it gets. All megnetic fields behave the same, so you can use this simple experiment to show how radiated magnetic fields work too.
And, by the way, you can indeed use multiple turns in a magnetic loop antenna.
"I Have heard many people tell me to use a radio that does not have side band in it..Reason for this is that the side band radio uses different mixer frequencies and more mixers then one without side band, and produces more harmonics. Try using a radio without side band if you want to try to ride of tvi or rfi.I have not confirmed this yet but it seems like a easy fix."
Simon's reply to this is correct. Using a radio with SSB capability is actually better than one without, not only for the reason Simon stated, but also generally an SSB radio is a higher price point item, and more engineering has gone into it. Engineering the filters for an SSB transmitter to work on SSB adds great benefits to the AM operation as well regarding spurs and harmonics.
And, to expand on what Simon said even further, RFI (often just called TVI) predates both SSB and AM operation. RFI was actually around before the first vacuum tube. Look up "rotary spark gap transmitter" on your favorite search engine, and see if you can study how much noise was created worldwide by the Marconi Wireless company with his worldwide network of spark gap wireless stations around the same time Wilbur and Orville were experimenting with flight. The Alexanderson alternators began to replace the Marconi spark gap transmitters in the 1920's because they were more spectrually pure and produced cleaner signals all around. Look up SAQ Grimeton for some great info on this.
Regards,
833
Your radio 'Mythbuster' since 1998
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