| Author |
Message |
    
Heavyweight
New member
Username: Heavyweight
Post Number: 9
Registered: 11-2004
| | Posted on Monday, May 30, 2005 - 6:27 pm: | 
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I have always used 18 foot of good coax in all of my mobile's but now im hearing that could be to be to long.Im really dumb when it comes to this stuff so i wanted to know if this is true? |
Bruce
Senior Member
Username: Bruce
Post Number: 2759
Registered: 9-2003
| | Posted on Monday, May 30, 2005 - 7:03 pm: |
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The point at which you have too much is when you can measure loss..... |
Patzerozero
Advanced Member
Username: Patzerozero
Post Number: 844
Registered: 7-2004
| | Posted on Monday, May 30, 2005 - 7:49 pm: |
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1' of 10cent rg58junk is too long. you can measure loss by watching idiot lites do tricks if running more then a few watts. use rg8 high % shield or better, & multiply 1/2 wavelength X velocity factor. OR if what you have now works-DON'T CHANGE IT! |
Crackerjack
Advanced Member
Username: Crackerjack
Post Number: 578
Registered: 7-2004
| | Posted on Monday, May 30, 2005 - 9:15 pm: |
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Akways cut the coax in half wavelengths, even if it makes it a little longer, the loss will be less and the SWR better. Never coil the excess.
Multiples of 18 feet are considered best. That is radio to antenna, you can figure any jumpers into that initial 18 feet. |
Road_warrior
Advanced Member
Username: Road_warrior
Post Number: 612
Registered: 7-2004
| | Posted on Wednesday, June 01, 2005 - 11:19 am: |
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The different lengths of coax is
a debated subject. Some say run it from
point A to point B and use that length.
Some say cut it in multiples.
I have tried both ways and can see no
difference. Just never add or subtract coax
to change your SWR match. Get the SWR correct
at the antenna.
JIM/ PA/ CEF 375 |
1861
Intermediate Member
Username: 1861
Post Number: 218
Registered: 2-2004
| | Posted on Wednesday, June 01, 2005 - 1:25 pm: |
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I use whatever amount will reach , without streching it. |
Road_warrior
Advanced Member
Username: Road_warrior
Post Number: 614
Registered: 7-2004
| | Posted on Wednesday, June 01, 2005 - 1:50 pm: |
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But, no 18 ft is not to long. Many people
use the 18 ft length for mobile installations.
JIM/ PA/ CEF 375 |
Patzerozero
Advanced Member
Username: Patzerozero
Post Number: 853
Registered: 7-2004
| | Posted on Wednesday, June 01, 2005 - 4:46 pm: |
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18' is not the true electrical half wavelength. you must multiply it by the velocity factor of the coax to get the true length. it is shorter then 18'.
as an example, setting up radio-2pill-8pill-swr/wattmweter-antenna by way of mfj259b antenna analyzer, 1 particular installation saw swr's acceptable, under 1.3:1 on diamond sx100, yet higher reflected power(then a 1.3:1 swr would normally infer) on coaxial dynamics meter. putting low pass filter after 2nd amp lowered output dramatically-output contained much harmonics. from the amp? don't know. change of coax to same grade cut to 1/2 wave X velocity factor of .84, along with jumper from driver amp to main amp increased in length to same as from big amp to antenna reduced reflected to under 2% & removal of low pass filter showed no decrease in output.
impedence mismatches between multiple accessories
compounding a tiny problem enough times to make it more serious? possibly. my single amp installation is installed 1/2 wave X velocity factor. my 11m & 6m base antennas are done the same way. most installs i do of amps i do the same way. if a texas star 5 pill or smaller is installed & shortened coax won't reach, i use whatever length is needed to reach.
ALWAYS USE THE BEST GRADE OF COAX POSSIBLE!!! rg58 on stock mobiles for ch 19 conversations is sufficient. for serious communications HIGH QUALITY rg8X at the LEAST! many interference and performance problems can be corrected by BEST QUALITY COAX!!!
99.9% of the time COAX LENGTH MAKES NO DIFFERENCE.
thankyou and
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Kj7gs
Junior Member
Username: Kj7gs
Post Number: 25
Registered: 12-2004
| | Posted on Thursday, June 02, 2005 - 12:45 am: |
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Heavyweight, are you confused enough yet? Where the "too long" might be coming from is the formula for 1/2 wavelength of coax at 27 MHz, which comes to about 12 feet. 18 feet is simply 3/4 wavelength of transmission line, and I believe the ARRL Antenna Book makes mention of odd 1/4-wavelength multiples as helping to reduce common mode currents. But the illustration that goes along with that shows a horizontal dipole, not a vertical 1/4-wave antenna, and you might not have common mode currents to worry about anyway in a mobile antenna installation.
Transmission line (notice I didn't say coax) measurements become critical if you're using other than 50-ohm coax with a 50-ohm antenna. Yes, that means you can use zip cord or 300-ohm TV twin lead as transmission line (and antenna) if you measure the line correctly for the frequency you're using. Length of line is also critical for phased installations, where the "18-foot rule" would probably come in handy -- but again, not in a mobile situation in due to the separation requirements for the antennas.
Did that help at all? |
Heavyweight
Junior Member
Username: Heavyweight
Post Number: 12
Registered: 11-2004
| | Posted on Sunday, June 12, 2005 - 6:35 pm: |
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Yah im pertty much confused,lol.Thanks for your help everyone. |
Patzerozero
Advanced Member
Username: Patzerozero
Post Number: 925
Registered: 7-2004
| | Posted on Sunday, June 12, 2005 - 11:20 pm: |
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ARRL antenna book i have says odd multiples of 1/2 wavelength. 1 wavelength at 27 mhz is approx 36 feet, therefore 1/2 wavelength is approx 18 feet. velocity factor of coax is usually between 66 and 84%, depending on type. 9913 i use is 84% velocity factor. using the better coax gets higher V/F and true electrical 1/2 wavelength is a bit longer that way, to help reach in mobile installations, & not have too much leftover.
got that? |
Couchpotatoe
Junior Member
Username: Couchpotatoe
Post Number: 22
Registered: 6-2005
| | Posted on Tuesday, June 14, 2005 - 9:16 pm: |
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AH, BUT GRASSHOPPERS, YOU ARE FORGETTING THAT RG-8,RG-8X,RG-58,RG-213 ARE ALL 50 OHM COAX UNDER RF LOAD. NO MATTER WHAT LENGTH YOU RUN THE ONLY PROBLEM IS HOW MUCH LOSS IN dB YOU CAN STAND TO LOSE.
HALF-WAVELENGTHS ARE GOOD FOR BALUNS AND MAKING GOOD COAX ANTENNAS, BUT YOU DON'T NEED 18FT OF ANY 50 OHM COAX TO TERMINATE AT YOUR ANTENNA. ONLY WHAT IT TAKES TO GET THERE. READ THE ARRL HAND BOOK FOR RADIO AMATEURS. |
Patzerozero
Advanced Member
Username: Patzerozero
Post Number: 939
Registered: 7-2004
| | Posted on Wednesday, June 15, 2005 - 5:22 pm: |
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i guess the arrl handbook & arrl antenna book say different things. knew i shouldn't have trusted a bunch of hams |
Kj7gs
Junior Member
Username: Kj7gs
Post Number: 27
Registered: 12-2004
| | Posted on Saturday, June 25, 2005 - 10:26 am: |
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Patzerozero, my 20th edition Antenna book shows 1/2-wave multiples dressed symmetrically away from the feedpoint provide worst case scenarios and 1/4 wave multiples minimize common-mode currents. "Modeling in this case produces no difference in response between the dipole with unbalanced feedline and the reference dipole with no feedline" (ARRL Antenna Book, 20th edition, p. 26-18). Other than that, the dB loss that the operator needs to be concerned about would be length of the feedline. A mile of it with any load at the far end would give you perfect SWR, but the loss would be so great that you may as well use a dummy load at the transmitter output. |
Hollowpoint445
Intermediate Member
Username: Hollowpoint445
Post Number: 447
Registered: 6-2004
| | Posted on Saturday, June 25, 2005 - 11:56 am: |
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I think Patzerozero is talking about the propogation of the signal through the coax rather than common mode currents traveling on the shield - which is why he's talking about velocity factor.
1/2 wave multiples are good in that the impedance from the antenna is repeated at 1/2 wavelength multiples throughout the coax length. If you want to read the antenna's actual VSWR on the radio end of the coax you need to have a 1/2 wavelength multiple. |
Kj7gs
Junior Member
Username: Kj7gs
Post Number: 30
Registered: 12-2004
| | Posted on Thursday, July 14, 2005 - 5:46 pm: |
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The reason I mentioned common mode currents is because this is the only significance that I can see for 18 feet of 50 ohm coax. A half wavelength though, is 18 feet for antennas, but not for coax, which must be multiplied by a velocity factor, commonly 0.66, bringing the length to 12 feet, which is the length to use for any impedance measurements. But except for a few special circumstances, and of course common mode current considerations, the "18 foot rule" isn't a rule. The 50 ohm antenna that the coax "sees" at the load, since it's the same impedance as the transmission line, is an infinite continuation of that line, meaning run as much coax as you need to your antenna -- unless 18' -- or 12' for that matter -- makes you feel better. Hopefully I didn't repeat too many other posts here... |
Dale
Intermediate Member
Username: Dale
Post Number: 141
Registered: 12-2002
| | Posted on Thursday, July 14, 2005 - 6:15 pm: |
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i never worry about lenth if your swr is good i wouldnt mess with it.from what i read cutting coax just confuses your meter.imho if your swr changes with different lenth of coax you got problems with coax.now if ya use 75 ohm coax then yes ive heard of having too use certain lenths but not on 50ohm coax.ive always use times lmr 400 50 ohm coax very low loss coax ,very expensive also lol |
Jon666
Intermediate Member
Username: Jon666
Post Number: 361
Registered: 11-2003
| | Posted on Friday, July 15, 2005 - 6:48 pm: |
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if it works dont fix it
really does it matter
i think not much |
Twowatt
Junior Member
Username: Twowatt
Post Number: 29
Registered: 7-2005
| | Posted on Saturday, July 16, 2005 - 4:21 am: |
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heavyweight,
I think most everyone is basically correct, but just saying it in a different way, whether they are using voltage nodes, current nodes, leading currents, lagging voltages, shield nodes, velocity factor etc etc.
Here are the basics: (assuming 50 ohm coax, single antenna installation)
The length of the coax DOES NOT MATTER!!! (see exception below). The coax is just the conduit to get the signal from the radio to the antenna (and vice-versa).
exception: (except for being way too long because of unacceptable voltage/current losses. That will not occur in a mobile installation if the coax is correctly sized to match the rf output)
What does matter is "where" to place the swr meter inline along the coax to get the correct reading while adjusting a "shorter than 1/4 wavelength antenna". (for simplicity, let's stick to 11 meters)
Before i continue, let me expand on the last sentence. If we use an ~9' steel whip (or a 96" (8')fiberglass anenna) {why the different lengths - see below} on a minimum (depending upon what metal is being used) 201 square inch good metallic surface, with a good metal to metal contact for both dc & rf gound, u DO NOT EVEN NEED a swr meter to check the standing wave, because the swr will be 1.0:1; (on 1 frequency only, but not more than 1.01:1 on the band edge: i.e. if the antenna is sized to channel 19 for 1.0:1 swr, then channels 1 and 40 will have a maximum 1.01:1 swr no matter what the length of coax).
{why the different antenna lengths - the fiberglass has a copper braid inside of it, and since copper is a better conductor than steel, the rf travels quicker on the surface of the copper {"skin effect"} and therefore reaches it's 1/4 wavelegnth point sooner and is therefore shorter than steel's {"skin effect"}length of travel. There is also some effect from the dielectric, but that's for another thread}
O.K., now for the 'easiest' (imho) meter placement to adj the antenna. To get a correct reading of the ANTENNA'S SWR -(ONLY THE ANTENNA CAN BE MIS-MATCHED AS THE COAX IS ALREADY @ 50 OHMS), place the meter at a 1/2 wavelength point (from your radio) along your coax - the easiest way to do that is by using a 1/2 wavelegnth jumper to connect the swr meter to the radio. (see Patzerozero's posts above for the correct physical length of coax.) Now adjust the ANTENNA for the lowest reading on your swr meter.
By the way, the BEST way to check swr is to attach the swr meter "directly" (no jumper) to the feedpoint/base of the antenna and take readings there, (since that's where the ANTENNA is at), but that is not usually practical; but this scenario proves that the length of the coax does not matter!
Another simple way to prove my hypothesis is by using some good (95% shield or better) coax, and using a 1/2 wavelength jumper, and a good shielded 100 ohm dummy load, and a good swr meter. Place the 1/2 wavelength jumper between the radio and swr meter; next use whatever length coax jumper or multiples thereof and attach the dummy load to the end of the jumper(s). The swr will stay 2:1. Now atach the 100 ohm dummy load to any jumper other than the 1/2 wavelength jumper, and attach this jumper to the antenna connection on the swr meter, and using different lengths of coax from the radio to the meter, notice the DIFFERENT swr readings. (: [i've done this before at my shop to prove this point]
hope this helps clear up some mis-understandings. |
Tech237
Moderator
Username: Tech237
Post Number: 165
Registered: 4-2004
| | Posted on Wednesday, July 20, 2005 - 9:45 am: |
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Different lengths of coax (properly terminated) can cause a different SWR to be shown at the radio than what would be shown at the antenna. This is because at legnths other than 1/2 wavelength the coax does not act just as a pipe for the RF but also as a sort of transformer.
In all honesty if you want to see the true SWR of your antenna system check it right at the antenna. And yes, even antenna anylisers can give an inaccurate reading if the coax is of a length that transforms the impedence rather than reflecting. Admittedly not as often as a simple SWR meter. (This is from experience and not just reading dry theory). |
Hollowpoint445
Advanced Member
Username: Hollowpoint445
Post Number: 570
Registered: 6-2004
| | Posted on Wednesday, July 20, 2005 - 7:27 pm: |
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Twowatt - a 102" whip won't show a perfect VSWR when mounted on a car. The feedpoint impedance will be about 36 ohms - not 50. So the VSWR will be slightly higher.
For a 1/4 wave antenna to have a 50 ohm feedpoint impedance the counterpoise radials would need to slant down about 45 degrees from perpendicular to the radiator. A car body doesn't do that.
When I ran |
Bigbob
Senior Member
Username: Bigbob
Post Number: 2058
Registered: 12-2001
| | Posted on Wednesday, July 20, 2005 - 8:58 pm: |
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But you can mimick a sloping ground.My pickup does give about a 1.4 to 1 swr on channel 20 with no cap(plain aluminum)on the bed,but if I install the cap with foam tape between cap and bed rails(capacitively coupled to bed),the swr will hold 1.2 channel 1 minus 10 to channel 40 plus 10,with just a needle wiggle above 1 to 1 on channel 20,at least on my truck anyhow.The antenna is mounted on cab with 4-mag mount and 8 gauge ground strap.Bigbob |
Twowatt
Junior Member
Username: Twowatt
Post Number: 44
Registered: 7-2005
| | Posted on Wednesday, July 20, 2005 - 10:40 pm: |
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thanks tech237 for explaining why the different lengths of coax give different swr readings.( i wanted to go into current higher/lower and voltage higher/lower at various points along the coax, but was needing a chart, & the post was getting long anyway. thanks again for simplifying)
aw, that IS right hollowpoint, i had forgotten. that's why the old groundplane base antennas were called 'droopers'.
i stand corrected. thanks, |
