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Possum
| | Posted on Thursday, May 30, 2002 - 12:14 am: | 
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I have a ford explorer and I have mounted a dosy meter under the dash so I can monitor my output. The only problem is the amp is in the back. This leaves me with 12' from box to meter and 15' from meter to antenna. When I had the meter in back my swr was okay but cannot adjust below 2.5 with the meter under the dash. I have a second meter I could use to set the swr at the amp but was wondering do I ignore the swr on the meter under the dash? |
BIG FOOT
| | Posted on Thursday, May 30, 2002 - 10:13 am: |
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...not sure of the facts on this ... but I am thinking I was told that a HIGHER swr reading will show an ( inaccurate ) wrong reading on watts, thereby it may be SHOWING that you are transmitting 500 watts but really be doing around 325.... is that what you want ? BIG FOOT |
Possum
| | Posted on Thursday, May 30, 2002 - 5:44 pm: |
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actually I was more interested in why moving the meter up and down the coax gives different swr reading. What is the rule for using the meter to adjust swr and does it really matter where it is place in the line. Does the meter read only from where it is to the antenna or does it take into account the entire coax. Thanks |
ss8541
| | Posted on Thursday, May 30, 2002 - 10:35 pm: |
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are you using a k40 or wilson mag mnt?
it is my experience and understanding that i have drawn from these experiences that the coax length supplied with these antennas act as part of the entire antenna system. yeah, we've all heard the 'coax does not tune an antenna' rule. but all 'rules' depend on certain factors. in this case it seems as though the braided part of the coax acts as a counterpoise, aiding with the capacitive coupling of the magnet to gnd the antenna, thereby helping to tune the antenna system. if you add to or subtract from the supplied coax, you throw this out of whack and your swr/match suffers. the way i see it, if this wasn't the case, and the antenna was a perfect 50ohms without any coax at all, then you should be able to use -any- length of coax(as long as its electrical characteristics are similiar to the rg58 used from the manufacture)without any 'noticeable' change in swr.
now, if you are using a steel whip or other type of 'whip', then the coax rule mentioned above applies(in most of these types). if your swr is changing this much then your antenna is -not- tuned properly and the piece of coax you were using for the 1st measurements are just fooling the meter. the best way to tune it -correctly- is with a 12ft section of rg 58 or 14.5ft section of rg8/mini8 between the meter and antenna. these are electrical half wave lengths of coax and will give your meter, the -true- swr. after you set it with this, you can use any length you want for a permanent set-up and your swr should only change slightly, if there is any change. once you turn your amp on with -any- type of set-up, swr may change dramatically, but that is another story for another day. |
Tech181
| | Posted on Friday, May 31, 2002 - 12:06 am: |
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Possum,
SS8541 made probably the best summation of this topic in his post. The reason you get different readings depending on where you put your meter in-line is because you are moving the meter up or down the voltage wave traveling along the antenna system.
The meter reads voltage VSWR (Voltage Standing Wave Ration) which is the measurement of two opposing voltages along the same feedline (I think that's correct). By changing the meters location you are "fooling" the meter into giving a better, or worse reading of the standing voltage along that line.
Steve
Tech181
Tech181@copperelectronics.com |
bruce
| | Posted on Friday, May 31, 2002 - 7:04 am: |
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thanks steve i've told people this over and over... guys coax does not tune your antenna cuttung coax only fools the meter if you want to tune the antenna it has to be done at the feed point. get a pile if rf beads make a balum choke at the feed point and if your still seeing swr the antenna is not tuned and no amount of coax cutting is going to change that. |
Dmh
| | Posted on Friday, May 31, 2002 - 7:51 am: |
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Now, my question "what about higher SWR readings when using power? ---what is too high?" I've been told higher SWR readings are to be expected when using power. |
101
| | Posted on Friday, May 31, 2002 - 8:44 am: |
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If anyone is interested enough in these topics a good book to read is Reflections II by Walt Maxwell. Coax length can indeed be used to tune your antenna system. The SWR on the line doesn't change with length (ignoring losses) but the input impedence looking into the source end does change with length if the coax isn't terminated with a 50 ohm resistive load.
101 |
Tech181
| | Posted on Friday, May 31, 2002 - 12:02 pm: |
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Dmh,
Higher power will result in more loss especially if you have a lossy system to begin with. If you can maintain a 2.1:1 SWR or lower, generally you will not have any problems. Obviously try for the lowest you can.
Steve
Tech181
Tech181@copperelectronics.com |
bruce
| | Posted on Friday, May 31, 2002 - 1:01 pm: |
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101 That is correct you CAN tune a antenna using coax but this is NOT a smart thing to do. A coax is a infinte amount of L-C filters and any change in input/output inp will screw it up but useing your coax to " Trick " a radio is not a good way to acheve engery transfure. Why would you use the coax to cover a defective antenna?? Im diebetic it would be like me increasing my medication because i wanted to go on a sugar binge. There are multi band antennas which use a blanced feed hooked through a antenna tunner to acheve enough bandwidth / VSWR but look at what they can do ... yes they work well but NOT as well as a well tunned antenna on a good feed line. Fokes DONT cut your coax FIX your antenna make it right
Bruce |
Possum
| | Posted on Friday, May 31, 2002 - 2:43 pm: |
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I appreciate everyones input and let me add some specifics to the equation. I have a roof top mounted monkeymade(big) antenna with plenty of ground. I am running a davemade 2 pill driver with a straight six for a big box and plenty of juice 200 amp plus deep cycle batteries. I would like the meter in the front to read what the current status is. This would require at least a 12' coax from the 2nd box to meter and about 15' from meter to antenna. Should I measure initially from the box 2nd box with a seperate meter or does it not matter where the meter is placed and tune the system that way. Is the reading of the meter in the front reliable for tuning or should I live with it at the second box. I have a total of radio , 2 boxes, meter, and antenna. can I set the swr at the radio and leave it or measure it at each insertion point?(ie each piece of equiptment). Can the meter be reliable in seeing total coax or only forward from where it is inserted. I know I have repeated some questions but at a 1300.00 investment you can see why I am concerned about everything. Thanks |
101
| | Posted on Friday, May 31, 2002 - 4:50 pm: |
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bruce:
Do you actually believe because an antenna has a high SWR it is "broke"? After all, a dummy load will give you a near perfect SWR but it's not a great antenna. An antenna doesn't have to be resonant to be efficient. How many AM broadcasting stations have resonant antennas? - none!
Tell me which one of the following conditions your radio will like better:
1> Resonant antenna that presents a pure 50 ohm resistive load to the radio.
or
2> Non-resonant antenna of 36 ohm resistive - 45 ohm inductive that presents a pure 50 ohm resistive load to the radio after adjusting the coax length (to get the 50 ohm resistive load looking into it from the source end).
I would bet that using even the most expensive equipment you could not discern a .01% difference between the two in field strength.
Pick up a copy of Reflections II, understand it, and dispose of all those coax and power transfer myths.
101 |
BIG FOOT
| | Posted on Friday, May 31, 2002 - 6:25 pm: |
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.... $1300.00 on a MOBILE cb station ?? guess he forgot that a 4 watt cb 3 miles away can plug his recieving extremely well.....oh well... he can talk but he may not hear... BIG FOOT |
BIG FOOT
| | Posted on Friday, May 31, 2002 - 6:28 pm: |
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... theory and real life are two different things... what will work in one vehicle may not work in an identical vehicle sitting next to it... BIG FOOT |
Possum
| | Posted on Friday, May 31, 2002 - 6:51 pm: |
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Im not using the coax to "tune" my antenna. I am only interested in those who have placed a meter in the front of the vehicle which requires lengths of coax which I myself would not normally use. Given that in order to due this it requires 12' coax fdrom amp to meter and 15' from meter to antenna. My question is with this much caox and the meter in the center approximately, can I rely on the swr readings I get while I am tuning the antenna with this setup. Ideally you would have a 2' coax from the amp to the meter and tune it that way. All I want is to have the meter up front as a constant reference source and how accurate(relative) can I assume those readings to be. Maybe I misled those who believe tuning is done with coax, When all that I am really after is convenience and information from those who have a similar setup. Thanks Possum |
Dx431
| | Posted on Friday, May 31, 2002 - 7:21 pm: |
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LOL Good one Big Foot! |
Possum
| | Posted on Friday, May 31, 2002 - 8:30 pm: |
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All things dealing with technology started with theory. As you must have missed in my post BIGFOOT was that I was not asking for theory only those who had a similar setup. Also as far as what might work in one vehicle may not work in another I guess Henry Fords theory of mass production was just a foolish thought. However I do not believe Henry Ford got it right the first time either. By asking if anyone had a similar setup would be considered research. I ask for information from those who have tried this and what were some of the issues they might have encountered. I did not see in your post anything relavent to this type of setup. As far as a 3 watt radio interfering with my recieve you are correct but those I talk to on the radio take turns so "earplugging" is not an issue, and those that attempt it do not last long on our home channell as they are usually embarrassed after awhile any way. If any has attempted using a setup as I have mentioned before please respond, if you have not thank you for your thoughts on theory but I am beyond theory and just want to get my rig setup as to my personal preference. I will repeat my request of placing my meter up front and the issue surround the swr readings while attempting to tune it. Thanks again Possum |
bruce
| | Posted on Friday, May 31, 2002 - 10:08 pm: |
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i would think a 12 foot hunk of coax then tune the ANTENNA for best match should work Ive been puting up antennas for 40 years YES ive used the coax to tune it but that was because the ANTENNA was WAY too short to work by its self. As for a 36 ohm -45 ohm inductive on a smith chart that is why some antennas HAVE A STUB AT THE FEED POINT on vhf/uhf that stub is only 1-6 inches long. Ed a engeeneer i worked for at the GE nuke plant spent years designing antennas the STUB and the old smith chart was his baby i still have several of his designs nice ideas even after 20 years. But again this is getting away from my point a antenna made CORECTLY for cb a 27 MHZ ant should be a NO BRAINER. |
ss8541
| | Posted on Friday, May 31, 2002 - 10:39 pm: |
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possum, your question has been answered. i told you how to get the readings you want. in this case, your monkey made needs to be tuned with either a 12ft section of coax for poly or 14.5ft section of foam coax. after that you can use -any-lengths you want for your set-up with fairly reliable swr readings. if your 15ft section is foam, it is probably showing your -true- swr (if just radio and no amp is used).
but what would probably -show- you a good swr without any tuning is a 18ft section of coax from the meter to antenna. this goes right in line with what 101 is saying, and is true. but depending on power level you are using and type of coax, you may be running the risk of dielectric breakdown(coax burning up) at spots on the line where the voltage has 'standing waves'. |
Possum
| | Posted on Friday, May 31, 2002 - 11:07 pm: |
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Thanks for the info Bruce. Its not that I choose to tune the antenna using coax its just a neccessary evil to obtain my ultimate goal of having the meter up front in a convenient location. In order to do this long sections of coax must be used. From what I gathered 12' from the amp to the meter should work. Im not really concerned with what the power shows as long as the swr is low I know the power will be there. Maybe Bruce you can answer the questions more directly as I lay them out since you seem to have some experience in this or anyone who has direct knowledge.
1) radio--3'--driver amp---12'---2nd amp---12'---meter---15'----antenna. Should I check swr at multiple points.
2)Will the meter only "see" whats ahead of it to the antenna?
3) I can insert multiple meters if neccessary I have plenty on hand. I also have one that measures relative forward and reflective only by M.C. Davis Co. Is decreasing reflective power better than swr ratio.
4) Using 1500 watts as a standard, I know swr is NOT going to be 1.1. The most I am looking for is 1.5 - 1.7 vswr and how much power can I expect to loose.
5) I also have a dummply load rated 1500 watt(can) that maybe I should put in place of the antenna just to verify coax issues.
Thanks Possum |
101
| | Posted on Friday, May 31, 2002 - 11:25 pm: |
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bruce:
I was confused by your earlier post when you said that changing coax length cannot tune your antenna. Then you later contradicted yourself and said that coax can be used to tune your antenna but it isn't a good idea.
What would be wrong with using coax length to tune your 108" steel whip from its approx. 36 ohm impedance to get a better match to your radio's 50 ohm impedance? There is nothing defective with the steel whip - the 36 ohms is its characteristic impedance (which varies greatly in proximity to metal objects). The lengths of coax that we are talking about will have negligible loss at even high SWR.
101 |
ss8541
| | Posted on Friday, May 31, 2002 - 11:54 pm: |
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oh boy, i see this turning into a goat rope on you possum. i'm backing out now since it looks as though your question is no longer 'the issue'. |
bruce
| | Posted on Saturday, June 01, 2002 - 12:11 am: |
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I realy dont know what your tring to prove. 36/50 is what 1.3 or so to one???? at 29pf per foot and a few Uh that would disapair in a 18 foot run of cable just because tha cable is not a resistor in the real world. No you cannot tune a antenna you CAN use the cable if you so desire to play games but why? Im realy puzzled as to what your tring to prove??? The average cb/ham has a stinking VSWR meter not a inp bridge nor is he tracking things on a smith chart. People acuse me of nipicking things to death but in 40 years ive never had to do anything but provide a decent ground plane and set the length of any antenna ive used from 160 khz to 1296 mhz.... so 27 os no problem. Right now im using a 7 foot wip on 10 meters and it was determined after 5 others to be the best using a signal sourse and a signal anz with a 1db grade. Like i said before i could measure about 3db change between a 50 and 86 inch wip capture could be but change never the less.
So go ahead incourage people to cut coax if it makes you feel good ME i dont have the need to wast my time o by the way my 7 foot wip OUT of the box 1.2: 1 at 28.5 and when i go to 29.6 its under 2:1 if i dont like it i drag out my ANTENNA TUNNER
enough said
bruce |
Possum
| | Posted on Saturday, June 01, 2002 - 6:10 am: |
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Finally Saturday Morning has come and I will be able to attempt to set my swr with my meter in front. I didnt realize this would be a national debate about swr tuning when it really was only a question of whether someone had ever mounted a meter in front instead of back. If I can adjust the antenna only (not coax) and achieve a low swr reading with the meter BETWEEN approximately 2 12 to 15' pieces of coax mission accomplished except no has said this is a bad place for the meter it MUST be 2-3 feet from the source or it DOESNT matter where ypu put the meter or how much coax you use to place the meter where it is most convenient visually as long as you can adjust the whip to get a low swr. Please let the subject die after this post as even I am confused now and will use the trial and error approach for this install Thanks again Possum. P.S. ss8541 you are right!!!! |
101
| | Posted on Saturday, June 01, 2002 - 8:06 am: |
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bruce:
I was just calling you on your original statement that you cannot tune your antenna system by varying the length of coax.
101 |
BIG FOOT
| | Posted on Sunday, June 02, 2002 - 11:22 am: |
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...yes BRUCE is right !! and 99 % of those of us reading this forum know it ! BIG FOOT |
