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Coax length in multiples of 11?

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greusome1

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Was reading an article online about base stations and home made antennas, swr, and COAX LENGTH. There it stated that if you needed 50' of coax to run from antenna to transmitter, use 55' instead... if you need to add a meter or another component, the shortest to use would be 6' 9' or 12'... but in order to maximize the resonance of your set up.. you would need to trim from your main line whatever length you added for your accessory. Says that if you need 100'... use 110'...... need 10'? use 11' of coax... catch the multiple of 11? I have read through a lot of threads and have never really heard this before... sounds legit since our cb wavelength is 11 meters... Is it true that from ant connection to transceiver, if kept in multiples of 11, (11-22-33-44-55-66-77-etc it will make for a more efficient setup? Actually I found the thread halfway down here: RadioMods coax length page (um can we post links in here?) hope so if not please edit and I won't ever never do it again..OR.. if not too long I can post the info:-------------------------------------------------------------------------------------------------------------------------------------------------------------------

Any person who says that, "coax length has nothing to do with SWR's" has no knowledge or understanding of broadcasting. I have been in commercial and private radio broadcasting for 25 years, and it's time to dispel the myths about SWR and coax.

Your coax is a vital part of the antenna system and must be tuned for proper operation. Installing an SWR meter at the antenna will measure the SWR's (AT) the antenna, but not the SWR's (OF) the antenna. Even with the meter connected at the antenna, any change in coax type or length, will change the SWR reading on the meter. But it doesn't make sense to check SWR's at the
antenna, because the most important component -THE RADIO- does not see the antenna alone. Your radio is reacting to the entire antenna system, including the coax. Even if you were to set the SWR's at the antenna and make them perfect, the SWR will be different at your radio's end of the coax.

Coax not only has resistance, but it also has capacitance which changes with different lengths. Resistance (Impedance), determines how the transmission line and antenna match the radio's requirement to send the greatest amount of signal to the antenna. But capacitance controls which frequency the antenna is best tuned at. Changing the capacitance of the antenna system, will alter the frequency at which your radio sees the best SWR. Any component you add to the coax line, such as a meter, linear, or
even a coax switch box, changes both the resistance and capacitance. My own radio, a Kenwood TS-570DG, has an SWR meter built in to it.

I also use an external set of meters to keep tabs on my radio's performance. When I check the SWR's on the meter in my radio, It shows a perfect SWR at one frequency. When I check the SWR's with the external meters, they show my best SWR at a
frequency almost one full MHz lower. So which meter is correct? They are both correct. My external meter is reading a perfect SWR at Frequency (X). That is because the resistance and capacitance of the coax and antenna is resonant at frequency (X). But
the meter in my radio is seeing the addition of a meter and a 6 foot coax jumper. This changes the resistance and capacitance of the antenna system that my radio is seeing. It also changes the frequency that the antenna system is now resonant at, so the meter in my radio sees a perfect SWR at frequency (Y) 1 MHz higher. Both meters are telling the truth, but the meter in your radio is the most important one to use for SWR. The meter in your radio is reading the SWR closest to your radio's Final output transistors, and that is more important than the SWR at the coax connector on the back of your external meters.

The following is for use ONLY on 11 meters.

For 11 meter operation, use these figures to set up your antenna and coax.

SORRY! I only know the measurements in FEET. Convert them to meters if you need to for better understanding.

For base stations. Cut your RG-8/U coax in multiples of 11 feet. For example.. if you need 50 feet of coax to reach your antenna, use 55 feet instead. 11 X 5 = 55. Or 11 feet multiplied by 5 multiples equals 55 feet total. If you need 37 feet, use 44 feet instead.
11 X 4 = 44. What ever length you need, go to the next highest length that can be divided evenly by 11. Include all jumper cables in that length. Jumper cables should be cut to 6, 9, or 12 feet. Never less than 6 feet, or more than 12 feet. Example. . . If you need 55 feet of coax to reach your antenna, and you want to add a meter in the line. Use 49 feet of coax from the antenna to your meter, and 6 feet from your meter to your radio. 49 + 6 = 55. If you find that 49 feet is too short from the antenna, then add 11 feet to the coax from the antenna to the meters, making your total length, with jumper cable, 66 feet. 11 X 6 = 66. If later on you decide you want to add another device like a linear in the line, do it this way. Do not just add a jumper for the new device. Where your main coax line from the antenna ends inside your home, cut off 6 feet of the coax, and put antenna connectors on each side that was cut, and add your new equipment at that point. The total length of coax stays the same, and the jumper is also the proper length.
For set up on 11 meters in your automobile, use RG-8X or RG MINI coax. Do the same as I instructed for base station installations, except this time cut your RG-8X to 20 feet minimum. Do not use less than 20 feet. Even if you only need 1/2 of that length, still use 20 feet. The extra coax can be coiled up and put in the trunk or under the seat. If it is possible, try to coil the extra coax as close to the antenna as you can get it. If not, the trunk or under the seat is good. It will not cause any problems. A length shorter than 20 feet can cause RF feedback, because of the antenna being so close to the radio. Also, mobile antennas are usually 1/4 wave or less.
This causes more RF to travel down the coax to your radio. A coax length of 20 feet, and coiling the extra coax under the seat helps to stop some of that RF from feeding back into your radio.

I hope I have helped you better understand SWR and antenna systems.
 

AC9BX

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Assuming you have a common antenna, that is something that does not use the coax for phasing or tuning or impedance matching where the length is critical to perform that task, you should completely ignore all that drivel about how long you coax should be. If you have a typical ground plane antenna or a common mobile vertical with a tuning coil in it somewhere, and changing the length of the coax changes your SWR there is something seriously wrong. The ONLY issue coax length has in a basic system is the longer it is the more loss you have. In fact because SWR is a measurement of reflected power and if you measure at the transmitter output your SWR can look better because there's more cable loss than if it were shorter. It works worse but looks better on the meter. All of that non-sense is wrong and so is their math. There are also issues where your metering will lie to you if the coax is exactly the right multiple of the wavelength.

Any person who says that, "coax length has nothing to do with SWR's" has no knowledge or understanding of broadcasting. I have been in commercial and private radio broadcasting for 25 years, and it's time to dispel the myths about SWR and coax.
...
Coax not only has resistance, but it also has capacitance which changes with different lengths. Resistance (Impedance), determines how the transmission line and antenna match the radio's requirement to send the greatest amount of signal to the antenna. But capacitance controls which frequency the antenna is best tuned at. Changing the capacitance of the antenna system, will alter the frequency at which your radio sees the best SWR..

Coax also has inductance. The idea behind coax having a "characteristic impedance" is that the capacitance and inductance of the line are such that no matter how long it is it will appear to have the same impedance. Both capacitance and inductance create reactance to radio signals and thus an impedance. Then there's something called velocity factor. Without getting in to it too deeply basically the radio signals travel slower in the cable than they do in a vacuum. This velocity varies depending on what the cable is. Its a value that must be calculated into the length to make the cable behave as intended. He didn't even bring that up.

Installing an SWR meter at the antenna will measure the SWR's (AT) the antenna, but not the SWR's (OF) the antenna.

That's the same thing. An SWR meter at the radio will measure the SWR of the system at that point.

That chucklehead just made up a value of 11 feet. Yes, feet is not the same as meters. No wonder so much private and commercial broadcasting performs poorly. I'm not sure what the difference between private and commercial means with regard to broadcasting. Broadcasting in the USofA is both private and commercial at the same time.

Do not use less than 20 feet. Even if you only need 1/2 of that length, still use 20 feet. The extra coax can be coiled up and put in the trunk or under the seat. If it is possible, try to coil the extra coax as close to the antenna as you can get it. If not, the trunk or under the seat is good. It will not cause any problems. A length shorter than 20 feet can cause RF feedback, because of the antenna being so close to the radio.

Okay, it's possible if an antenna is too close to the radio it may cause local interference. But that's not a function of cable length. It is only a function of signal strength versus proximity.

Also, mobile antennas are usually 1/4 wave or less.

He got that right.

This causes more RF to travel down the coax to your radio. A coax length of 20 feet, and coiling the extra coax under the seat helps to stop some of that RF from feeding back into your radio.

Wrong. Assuming "down" means back toward the radio happens in 2 ways. 1 is reflected power. Only an absolutely perfect antenna will not reflect any power back. Anything other than perfect which is all antennas will reflect some. This reflection is energy that the antenna cannot accept (phasing or impedance mismatch or other reasons) and it travels back toward the transmitter on the cable in the same manner as it went forward. The ratio of the 2 is the SWR. A shorter length will not prevent that. The 2nd is common-mode current which happens when there's any unbalance. Coax is a balanced line. It expects a balanced antenna. Most are not. Any imbalance causes current to flow on the outside of the coax braid. A ground plane antenna acts as a balanced antenna although it is far from perfect. There will be some common mode current. This current radiates from the cable instead of being trapped inside. A little isn't much of a problem. A lot is a lot of a problem. Just because the antenna is less than 1/4 wavelength won't cause reflected power. Even if it did coax length won't make that go away. Antenna length affects the impedance. A mobile antenna most always has a loading coil or helical section which "tunes" the antenna back to the intended impedance. If the impedance at this point is correct coax length would have no bearing. This sort of antenna is less efficient. There is loss in this tuning.

A coil of coax by the way can form a common-mode choke which is used to prevent this common-mode current. He fixed the common mode current issue without knowing it.
 
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mmckenna

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11 feet ≠ 11 meters, as I'm sure you know. When talking wavelength, you can't substitute feet for meters without doing the full conversion, as I'm sure you know.

Also, there is some minor truth to what he is saying. But, in reality, using the least amount of coax needed is still the best.
You can "hide" issues by using the proper length of coax, but it doesn't fix anything. 1/4 wavelength stubs can do all kinds of neat things, even act as lightning arrestors.

Where he's missing something is he doesn't seem to be figuring in velocity of propagation, and that will vary depending on the type of coax.

And I've worked in radio for years, I've never, not once, seen someone cut coax to exact wavelengths for connecting to an antenna. That includes cell sites, land mobile radio, repeaters, etc.

I wouldn't necessarily trust a "pirate broadcaster" as being a fount of accurate information.
 

greusome1

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11 meters and feet

Yes... I knew something was up when he used feet.... 11 meter band.. .so use coax 11 feet in length? come on.. yes I am smarter than that.... if your going to use coax in multiples of 11.. and he uses feet.. .. heck .. ill make sure my coax is 11 inches, OR 11 centimeters, OR 11 mm's.would be the same... increments of 11.. total bs to me too!. (lol).. yes.. this is why I double checked here first... had nothing but good advice and info from you guys.... thanks for calling this dude out on his bull... never heard of the 11 foot increment before (reason for posting).. glad I checked here before I bought 55' of coax.. lol.... thanks again!
 

KC4RAF

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You'll find good and bad advice anywhere you go.

Generally here at RR, there's quite a few who ARE or were in the communications field and know what they are talking about. Yes, there are some here who THINK they are experts, but don't know their arse from a hole in the ground!!! lol But those who have posted so far know quite a bit of what they write. After you've been on RR for a while, you'll get to know those that are knowledgeable (yes, we do have some experts here). When you see the aforementioned posters, you can rest comfortably knowing you're getting correct answers or information.
HTH
73s

edit: as an after thought; just because there's an amateur badge on some of our posts, doesn't mean we know it all, or in some cases, any of it! lol
me included.
 

prcguy

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This topic comes up a lot in CB forums and the so called broadcast guy claiming you have to tune your coax is giving a blanket and mostly wrong answer to a very complicated question.

An antenna can and should be an independent part of the "antenna system" where its properly decoupled from the transmission line and matched to the characteristic impedance of the transmission line. In this case it doesn't matter what length the coax is and changing it will have no effect except adding loss with more coax. Think of a dummy load on the end of coax that is well matched to the impedance of the coax. You can insert any length of coax between the SWR meter and dummy load and it will read a perfect match at any point along the coax.

When the antenna is not properly decoupled from the transmission line, RF will flow on the outside of the coax and the coax becomes a radiating part of the antenna system. In this case there will be certain lengths of coax that will make the SWR meter show more or less reflected power and you can tune the length of coax to make the meter read better. But your not really fixing the actual problem.

Some common CB base antennas like the Antron 99 have poor decoupling to the coax and the length of the mast its mounted on and the coax length can be a problem with these antennas. If you were to isolate an Antron 99 from the metal mast and use an effective common mode choke at the antenna and tune the antenna for a perfect match, then the coax length will not matter.

Antennas with inadequate ground plane like a CB mag mount or mirror mount will have lots of RF currents flowing on the coax and the coax will radiate. Changing the length of coax on these types of antennas can also change what the SWR meter reads because the coax is a part of the radiating antenna.

When dealing with problem antennas where the length of coax changes things there can be two different fractions of a wavelength in coax to consider depending on the antenna problem. For a poorly decoupled antenna where the coax is lit up with RF, a total length of 1/2 wavelength or multiples of 1/2 wavelength in coax might give the lowest SWR reading, because the feedpoint of the antenna will usually be a low impedance and low voltage point and every half wavelength will be another low voltage point on the outside of the coax which will show up as a lower SWR point.

For an antenna that is tuned to resonance but way off in impedance, then certain lengths of coax will transform the wrong impedance to the desired impedance and there are formulas for this if you know the actual impedance of the antenna. In this case you would calculate the needed length of coax in fractions of a wavelength then multiply by the velocity factor of the coax.

The goal for tuning your "antenna system" in most cases should be to make sure the antenna is adequately decoupled from the coax so coax length is never an issue, then tune your antenna for the best match. This way adding meters or jumpers or changing the coax will have little or no effect on the SWR.

If your tuning an antenna and things are not going well you can usually grab the coax and run your hand up and down some of its length while transmitting to see if that has any effect on the SWR. If it does then there is RF traveling on the outside of the coax, the coax is a hot radiator and you need to decouple it before going any further. Mr so called broadcast guy mentioned in the first post would have you trimming the coax fool the SWR meter and the operator. I say fix the actual problem and enjoy a properly working antenna.
prcguy
 

greusome1

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so by decouple.. you mean to get the lowest swr reading possible? and it would then have low rf on shield of coax... hope I got it!
 

Project25_MASTR

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Anyone with a ton of system field experience carries coax jumpers in odd quarter lengths (1/4 wave VHF is 3/4 wave on UHF) for the following. You can have acceptable SWR but still have a mismatch in the feed line. if adding/removing an odd quarter wave worth of jumper changes the SWR…there is a feed line mismatch. Now you generally only see this as an issue with stations as once installed it's quite a bit of a task to retune the antenna. On a mobile installation, you tune the antenna to the feed line in the tuning process…so it really doesn't matter.
 

prcguy

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Decoupling the feedline from the antenna would be insuring the antenna and feedline are separate entities and they play together nice without one affecting the other. Some antennas need extra consideration like a 1:1 choke balun near the antenna or an increase in ground plane size or other trick to keep the RF currents on the antenna and off the coax.

Tuning the antenna for lowest SWR is a separate function and if your coax is hot with RF it will be difficult to find the lowest SWR or the SWR will change when you take out the SWR meter and jumper cable.
prcguy

so by decouple.. you mean to get the lowest swr reading possible? and it would then have low rf on shield of coax... hope I got it!
 

SCPD

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When tuning an antenna, one should use an electrical 1/2 wave tuned coax jumper cut for desired frequency, and an antenna analyzer. After the antenna has been tuned, whatever reaches from point A to B will work just fine. You don't need specific coax lengths for an antenna that has been tuned either directly at the antenna feed point or by use of a short jumper of 12" or less, or a electrically tuned 1/2 wave coax jumper cut for desired frequency. After this has been done, again, what reaches from point A to B will work just fine. Anything else that has been said is just gibberish. Too many people fall for the myths surrounding coax.
 

AC9BX

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Sorry, I meant to say coax is unbalanced line and expects an unbalanced antenna. Most antennas are balanced. The ground plane behaves as unbalanced. I wrote that backwards. That's what happens when you go too fast.
 

AC9BX

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Lowest SWR does not mean best performance. It is common for the resonant frequency where ideally the antenna radiates the best to not be an exact match for the feed and radio and have some SWR.
 

prcguy

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If the feedline has a lot of common mode RF on it then different lengths of coax will show different SWR at different points along the coax. Tuning an antenna like this with 1/2 wavelength in coax between the antenna and analyzer will tune the antenna to that specific piece of coax and removing it will change the tuning of the "antenna system", which includes the coax in this case.

If the antenna has RF on the coax you need to fix it before tuning the antenna, then the length of coax between the meter and antenna will usually have no effect on VSWR. Using a 1/2 wavelength in coax between the antenna and analyzer will let the meter see what is happening at the other end since every 1/2 wavelength in coax the impedance and other parameters repeat. But if the antenna is properly decoupled and tuned to a good 50 ohm match its a moot point.
prcguy


When tuning an antenna, one should use an electrical 1/2 wave tuned coax jumper cut for desired frequency, and an antenna analyzer. After the antenna has been tuned, whatever reaches from point A to B will work just fine. You don't need specific coax lengths for an antenna that has been tuned either directly at the antenna feed point or by use of a short jumper of 12" or less, or a electrically tuned 1/2 wave coax jumper cut for desired frequency. After this has been done, again, what reaches from point A to B will work just fine. Anything else that has been said is just gibberish. Too many people fall for the myths surrounding coax.
 

SCPD

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To the poster read up on how to properly tune an antenna (base or mobile). To prcguy.
I don't get where you are getting common mode issues on the coax from as this was not the question asked. But what I said is correct and is not moot. Please stop trolling and get a life man!! My last post again on this thread as it appears as if I have a stalker!! I will be deleting my account anyhow as I am not going to get into a pissing match with someone that obviously has to be right about anything to make someone else seems wrong. To the poster of the thread, read up on how to tune an antenna properly. If you for some reason experience common mode current on the coax, then use some ferrite chokes at the feedpoint of the antenna or a 1:1 isolation balun. Good day and 73 and God Bless.
 

wyShack

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For the most part, a coax is thought of similar to a wire-that is we tend to idealize it. In a similar vein, we tend to over rate SWR (actually measured as VSWR). Any transmission line is much more complex than the ideal and when we 'push' the model things can get strange. For the most part, the longer a coax the more loss-although for most CB installations coax loss is negligible. Most VSWR meters measure at the point in the line they are inserted and VSWR actually changes along the line with readings repeating every half wavelength. Add to above the fact that wavelength changes when frequency does and you will realize an 'exact' wavelength (or multiple thereof) is at a single frequency.

VSWR meters are like any other tool-they tell part of the story and their readings are affected by where and how they are used. The higher voltages and currents resulting form high SWR can damage the finials so manufactures have designed radios to protect themselves by lowering power when a high SWR is sensed. This protection usually starts above a 2:1 VSWR.

In a typical single antenna installation using commercially available radios and antennas, I would suggest keeping the coax runs as short as feasible (don't eliminate drip loops) and minimize VSWR to 1.5:1 or less. Effort beyond that is subject to diminishing returns. Keep in mind that an S-unit represents a 400% increase in received signal -a few percent improvement is not noticeable.

After nearly 40 years in two way radio of one form or another, the only place things work out to two decimal places is when buying parts and/or equipent-don't sweat the small stuff
 

prcguy

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Sorry if you think I'm a stalker but I'm only trying to point out for the OP that antennas are very complicated and your advise on using 1/2 wavelength in coax with some antennas is correct some of the time. Other times its not and common mode currents are one of the things that will change the VSWR tuning game.

For a little background, in the mid 70s early 80s I worked for a two way radio, CB, car stereo joint and my job was repairing anything that came in the door, managing and training installers and doing a great number of installs myself. The shop offered free SWR testing and tuning for a couple of $$ if needed. It seems every CBer within a few hundred miles came to our shop and I probably had my hands on 6 to 12 of these antennas a day, 6 days a week for about 6 years since it was the CB heyday. I probably measured SWR on maybe 10,000 antennas in that time, tuning about half and rebuilding a good number of screwed up installs.

Later in life it was interesting learning more in depth on the technical side of RF and antennas where I already experienced and validated much of what I was taught before schooling. Now I have a few patents on antennas and there is a good chance you have one on your roof. Most people go to school then get their hands on whatever they learned about, which is opposite of my path.

The point I'm trying to make with this silly info is I have a lot of hands on experience, some schooling and can hopefully help others that may not have as much experience with antennas. Maybe I come across as a condescending hole and its not intentional, sorry about that. I just like to see the whole picture explained when sometimes only a portion of it is posted and that could send the OP off on the wrong course.

Back in my hole but with my antennas sticking out.
prcguy


To the poster read up on how to properly tune an antenna (base or mobile). To prcguy.
I don't get where you are getting common mode issues on the coax from as this was not the question asked. But what I said is correct and is not moot. Please stop trolling and get a life man!! My last post again on this thread as it appears as if I have a stalker!! I will be deleting my account anyhow as I am not going to get into a pissing match with someone that obviously has to be right about anything to make someone else seems wrong. To the poster of the thread, read up on how to tune an antenna properly. If you for some reason experience common mode current on the coax, then use some ferrite chokes at the feedpoint of the antenna or a 1:1 isolation balun. Good day and 73 and God Bless.
 

JayMojave

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Moderators please delete my.

Yo222DBFL: Don't run off, this subject of coax lengths and such has been, is, and will be talked about for many more years to come. I don't believe you were being stalked or singled out.

If you have a idea on the coax lengths and stuff that's great, it may be technically incorrect, but the good thing you have an idea and can go with it, you may even prove your self right or wrong with hands on testing.

The 11 feet multi lengths I will have to disagree with. I have been corrected many times in my views, its all about learning this stuff, and the academic world of books and such isn't the best on these subjects.

If the coax multi length thing was fact, tables and such would have been posted in amateur, commercial, and military technical books. I am open to learning new stuff, and seeing measured results is a big help to understanding.

Jay in the Great Mojave Desert
 

Project25_MASTR

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Yo222DBFL: Don't run off, this subject of coax lengths and such has been, is, and will be talked about for many more years to come. I don't believe you were being stalked or singled out.



If you have a idea on the coax lengths and stuff that's great, it may be technically incorrect, but the good thing you have an idea and can go with it, you may even prove your self right or wrong with hands on testing.



The 11 feet multi lengths I will have to disagree with. I have been corrected many times in my views, its all about learning this stuff, and the academic world of books and such isn't the best on these subjects.



If the coax multi length thing was fact, tables and such would have been posted in amateur, commercial, and military technical books. I am open to learning new stuff, and seeing measured results is a big help to understanding.



Jay in the Great Mojave Desert



It has been posted in such books jay. However, we are talking about books that were published before many of us were born back when 15 kHz was the deviation in use.

Lots to do with smith charts and feed line characteristics.


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