| Author |
Message |
    
MikeB
| | Posted on Monday, May 28, 2001 - 12:28 pm: | 
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I have noticed that coax supplied with CB antennas is generally 18ft in length. Is there harm in shortening the coax to a more manageable length? THANKS, MikeB |
307 (307)
| | Posted on Monday, May 28, 2001 - 1:52 pm: |
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In theory , it should stay at 18 feet to keep the impedance at 52 ohms. If all antennas were really designed to be terminated at 52 ohms it really would not matter , however they are not because of cost and so it is made up in cable length. |
MikeB
| | Posted on Monday, May 28, 2001 - 3:22 pm: |
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Thanks, 307. MikeB |
vernonott
| | Posted on Monday, May 28, 2001 - 5:09 pm: |
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I've been playing around with coax jumpers for four or five years with good success.On some setups I can gain up to fifty watts by changing jumper length between the radio and amp and between the amp and antenna.Of course I have to keep checking impedance to make sure I stay in the 50 ohm range.Also these could be harmonic ghost watts which is helping nothing , but I enjoy trying to gain some free power. |
FlaGator66
| | Posted on Wednesday, August 15, 2001 - 6:23 pm: |
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18 ft? Well Most off the shelf coax works in 6 ft or very close to 6ft multiples due the velocity factor of the coax thats where the 18ft myth comes from You might find that if you take that 18 ft piece of coax and Put a meter to it You should get the same readings at 18ft, 12ft, and 6ft Once youve determined the shortest length For example 6ft the Impedance should stay the same if you add 6 more feet of coax Velocity factor of the Coax determines the Length it needs to be |
bruce
| | Posted on Thursday, August 16, 2001 - 7:20 am: |
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FlaGatot66 You are just saying what i said the reason you are getting thoes readings are your meter is a Voltage Standing Wave Meter and will respond every 1/4 wave with a peak or null if your antenna is correctly tuned there will be no reading or very little if you are using crappy coax that is not consistent then you will get readings no mater what you do to your load but leaving hunderds of coax clippings all over your floor is not going to improve your match one bit. Read any good ant book ( willam Orr, ARRL RSGB ect )and no where will you find cutting a 50 ohm coax will help a match now if you used a 75 ohm coax to match a non 50 ohm ant that would be a diffrent story. |
bullet
| | Posted on Thursday, August 16, 2001 - 4:37 am: |
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mikeb,
all you need is enuff to go from point a to point b and thats all,and thats it. you dont measure coax by phyical length anyway.if your wanting to cut it for jumpers for watt meters
and swr meters to read more acurate it would be cut at a 1/2 electrical wavelength or a odd multiple thereof. hook a mfj 259b to a coax and use a 50 load at the other end and also do the same with a very small jumper and tell me what differance you find! if ya got good coax they will read the same! |
vernonott
| | Posted on Thursday, August 16, 2001 - 3:35 pm: |
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Bullet : I am not a rocket scientist or a professional radio tech , therefore I have to go by what I see with my own eyes.Last week I checked a friends mobile antenna system with my MFJ 259B and found the coax ohms at 39 with a swr at 3.1. To see if I could help it I added a connector and a three foot jumper .The new reading now was 51 ohms at 1.2 swr.I rechecked the reading several times .Now I have no idea why the reading changed.All I know is a 51 ohm reading at 1.2 swr looks good ,my friend feels better about his system,and I was glad to do it for him. |
bruce
| | Posted on Friday, August 17, 2001 - 9:50 am: |
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a 39 ohm reading will not produce a 3:1 reading by its self if you have doubts about a peace of coax put a 50 ohm resistor on one end and a low power transmitter on the other to swr meter should read near 1.1:1. Now 39 ohms should have read about 1.5:1 good coax will read around 50 ohms the cheap stuff o well you get what you pay for. |
vernonott
| | Posted on Friday, August 17, 2001 - 1:09 pm: |
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I guess the MFJ 259 isn't what I thought it was.I can only go by what the digital read out says.The End. |
bruce
| | Posted on Friday, August 17, 2001 - 2:01 pm: |
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vernonott there is nothing wrong with that device but a 3:1 missmatch would require a 15 ohm or 150 ohm load not 39. I do not own one of thoes i use a bird meter or my drake there is something other than the coax involved here. Get a 50 ohm and a 39 ohm resistor and a lenght of coax and try my test for youeself. If your meter is working your readings will be about what i said if you still get 3:1 i ve got to see how your doing it. |
anonymous tech
| | Posted on Saturday, August 18, 2001 - 12:09 am: |
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i have to agree with bruce. a 39ohm reading would equate to a 1.28:1 swr. (50/39=1.28) even with the reactance present, you still shouldn't be much further off from this. |
