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coolbreeze
06-16-2012, 02:34 PM
Time and time again I have made a comment about how coax length does matter.
Booty had very often argued the issue. But in this post he sucked up to the guy...
I disagree with fact 4 though. 1-3 shows why fact 4 is bull shit

this was posted by some other tool bag

The electrical half wave length of LMR-400 at 27.205Mhz (middle of the CB band) is:

15.365 feet (15' 4 5/8")

6 electrical half wave lengths would be 92.2' (92' 2 1/2")
7 electrical half wave lengths would be 107.5' (107' 6")

FACT #1:
This takes into consideration that the LMR-400 has a perfect velocity factor of 85%. It may, or it may not. In reality, the velocity factor of a piece of coax can vary across the same exact reel of cable! Coax is not perfect ! This is why phasing cable manufacturers will use a network analyzer and trim the coax down to the perfect length for that piece of coax.

FACT#2:
That electrical half wave length of coax is tuned specifically for the frequency you choose. It will be too short for channel 1 and too long for channel 40. The length you have will work fine for any frequency just as a 100' piece of coax works fine for any frequency. After all, 100' is an electrical half wave length of some frequency, and people use that all the time, right?!

FACT#3:
The ONLY advantages of using electrical half wavelengths of coax are twofold;

* Your reading at one end of the coax is IDENTICAL as the reading at the other end of the coax, but only on the frequency that cable was cut for. If you build your antenna for channel 20 and have it in the air, its easier to stand on the ground to take your SWR reading then hanging from under the antenna 60' up.
* For phasing two or more antennas.

I have an electrical half wave length of coax I use when building I-10K antennas cut for 27.205MHz. This allows me to stand on the ground, away from the antenna, and test the feedpoint for R-50 and X-0 on 27.205. As the analyzer teaches us, as we move away from that frequency, the readings will change. But it will be dead nuts on 27.205! I've tuned a couple up at 27.025 and 26.915, but only used a very short jumper for those. Again, the reality is that its close enough.

FACT#4:
The correct length of coax you should run from your antenna to you shack is; The shortest length that will reach! Again, look at the two examples in FACT 3 above for using an electrical half wave length of cable. If these are not relevant to your operations, don't worry about them! Why do I say, "the shortest"? The longer the cable the more the loss!

I use electrical half wave lengths for testing purposes and phasing antennas. I use what ever I have on hand for everything else. I hope this helps.

BOOTY MONSTER
06-16-2012, 02:57 PM
LOL , still making yourself look stupid ..............
some things never change .

Mr.5150cbrn
06-16-2012, 03:13 PM
LOL , still making yourself look stupid ..............
some things never change . yep sure is ! you cant handle the truth!

BOOTY MONSTER
06-16-2012, 03:20 PM
so you say ..
and we all know how much value what you say has ...................

Mr.5150cbrn
06-16-2012, 03:27 PM
yea like you can do better ?





















on









your own , with out macaroni &cheeze

NY714
02-11-2013, 04:21 PM
Some people say multiples of 3 are better like 25,50 or 75. But not how sure this really is? I've known alot that didn't use this rule of thumb and still got good swr's. Same with jumpers supposed to be in mulitiple of 3's. I use 2-4 footers and notice no change.

High Voltage Mobile NJ
02-11-2013, 06:12 PM
Some people say multiples of 3 are better like 25,50 or 75. But not how sure this really is? I've known alot that didn't use this rule of thumb and still got good swr's. Same with jumpers supposed to be in mulitiple of 3's. I use 2-4 footers and notice no change.


you better look at you radio !!!!! i bet the swr between your radio and your driver box is a 2 swr or higher . alot of you guys are clueless about tuning period . capacitance plays a very important role in your feeline. do some experiments and you will find out what we are talking about . :sleep:

NY714
02-11-2013, 07:09 PM
I am afraid not my swr is flat barefoot and at times 1.3 at highest, come on that is just common sense. My height makes up for all the bs you talk about! It's all about location and height, you should even have the common sense to know that. Lol !!!

NY714
02-11-2013, 07:10 PM
Remember i am a base with no tvi to neighbors !!!!

High Voltage Mobile NJ
02-12-2013, 03:02 PM
i will not use anything smaller than a 6 foot jumper between the driver and the big box . from the radio to the first driver box is at least a 12 footer , 18 works a tab better . :300 (230):

NY714
02-12-2013, 03:39 PM
yes at times i do agree, but it depends on set-up !!!!

Moonchild 747
05-04-2013, 11:14 PM
Breaker breaker...coax length does play a roll when factoring your common mode current coming back down the coax from an improperly constructed (matched) antenna system...if you have a sufficient amount of reflected energy making your coax shield radiate like part of your antenna system then adjusting the length of the feed coax to match the antenna will affect any reading whether testing for lowest SWR or even plugging in your MFJ 259B or equivelent antenna analyser...key is to ensure your antenna is the proper length for the frequency your TXing on and ensuring that your impedence matches what your radio is designed for (50ohms), then the length of your coax really doesn't matter except for loses. Velocity factors only play a role when your trying to determine the length of your antenna when transmitting through that material, some people use coax as an antenna so they factor in the velocity factor of the coax coating and shorten the length depending on the percentage...so both of y'all are correct depending on how you look at it, if your antenna is not built correctly then length does matter but if it is then loses are pretty much all that your going to need to worry about with increased length of any measurement.

scooter71nc
05-06-2013, 01:57 PM
I agree with moonchild 100%