coaxial jumpers

[295 antenna]

So Again i have been reading about jumper lengths and formulas. My question is there a formula for jumper length between transceiver and amp ? Now i have always used a three foot jumper but i understand there is a lot of discussion on this. Has anybody tried something different and had better results ?
I have tried shorter and it worked ok and i have tried a formula i read about so my jumper ended up at 9 feet no real change i could tell.
Thought ? experiences ? Formulas?

[Grinder74]

I use whatever length fits. Only reason I can see length matter is a co-phase antenna setup.

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[295 antenna]

I agree Grinder its just i watched this you tube guy and he made a big deal out of it and i want to find out if anybody else had different ideas/

[MDYoungblood]

I make my jumpers long enough to reach from what it is connected to to what it needs to be connected to with maybe a couple inches of slack.

3's

Greg

[Wildrat]

Same here, length is dictated by what I needed.

[Deleted User 30222]

That's good to know, as i'll probably end up making my own as well. Maybe i'll even cut my 3 footers in half. Hmm... or not

[Bobcat]

I agree with that "little bit of slack" in case you ever need to change a connector. But, if everything is 50 ohms.... whatever length it takes to do the job....

[jessejamesdallas]

I have jumpers anywhere from 2" to 5 1/2", to 9', all work perfect...like said...whatever it takes to get from A to B plus a couple extra just for good measure...

[Deleted User 30222]

I have jumpers anywhere from 2" to 5 1/2", to 9', all work perfect...like said...whatever it takes to get from A to B plus a couple extra just for good measure...

Once again, thanks Jesse. Nice to know i can cut cable to any length i want. Will probably be doing that soon.

[De_Wildfire]

I use jumpers "just enough" with a little extra to do the job. I use LMR 400 Ultra FLEX. The other hard LMR 400 you have to be very careful about sharp bends.

[KP68]

Please correct me if I am wrong, but if you are looking for a formula here it is. To get exact electrical wave length for a specific coax you need its velocity factor or velocity of propagation. For example, Times Microwave LMR-600 VF is 87 (87% of speed of light I believe). So, the formula is (492 X 0.87) divided by transmit frequency in Mhz. An example for CH28, (492 X 0.87)/ 27.285=15.68 ft. For the general length everyone goes by with out specific velocity factor 100%, (492 X 1)/ 27.285=18.03 ft. Both half wavelengths obviously. Or I could be full of crap lol.

[TNT1450]

Unless you're connecting phased antennas with coax, the coax length matters not one bit. I still occasionally see that old chestnut of "Your coax has to be a multiple of 3 foot length between radio & antenna or else it won't work right." But 3 ft. doesn't even factor in to 1/4 wave at 27 MHZ. 1/4 wave on channel 20, center of the band, is roughly 8.5 feet.

No, the coax only has to be as long as it needs to be to get from radio to antenna, or radio to amp. And obviously you want to keep it as short as possible due to losses.

[Bluerunner]

If changing your coax length makes a significant difference in SWR you probably have CMC (Common Mode Currents) flowing on the surface of the coax.

Just put a coax choke in the line and then you have no more worries about coax length / SWR related issues.

[Rock]

Depends on the ground plane of the antenna.
I have chokes on both ends of jumper from radio to the amplifier. Yet the radio likes the 1/4 wavelength jumper the best.
It is a impedance at the port issue rather than
Swr.

[51]

Transmission line length absolutely does not matter. Its a cb myth.

[MDYoungblood]

Depends on the ground plane of the antenna.
I have chokes on both ends of jumper from radio to the amplifier. Yet the radio likes the 1/4 wavelength jumper the best.
It is a impedance at the port issue rather than
Swr.

Transmission line length absolutely does not matter. Its a cb myth.

You are both right to a point, there are some antennas that are reliant to coax as their counterpoise, one example is the Saturn 100B (goes by other names as well) or the FireStik NGP. I personally have never went by any of the recommended "lengths" as most of my runs of coax are usually way over a full wavelength at any frequency.

3's

Greg

[TNT1450]

If an antenna has a good ground plane, it shouldn't have to rely on using the coax as a counterpoise.

[MDYoungblood]

If an antenna has a good ground plane, it shouldn't have to rely on using the coax as a counterpoise.

The antennas I mentioned don't have a "per say" ground plane, including the ones that have ground plane kit as an afterthought. If you check out antenna instruction of the past, antennas like the Shakespeare Big Stick say they work best if the coax is cut to multiples of 18ft, HyGain had a few with extremely short radials that had a spec for coax type and length.
The topic was about "jumpers" and those if they change anything in the SWR there is another issue to attend to, Bluerunner's post said it best.

3's

Greg

[Rock]

Transformation of impedance occurs with different lengths of coax Not swr.

[Rock]

Transformation of impedance occurs with different lengths of coax Not swr.

Theory and Perfect world states length does not matter .... If everything is 50 ohms.
In the real world and in the field practice we all know , Or should know CB equipment amps etc..
Is Not 50 ohms, This is why when we change length jumpers between the radio and amp we get
a better match at the radio . This is why it is a Impedance at the port issue.

[mod=MDY] repaired quote[/mod]

[arcone]

Any length is fine if the back of the radio is 50 ohms and the input impedance to the amp is 50 ohms, give or take. I think this is what has kept the patch lead myth going. For instance, one guy will swear it's 6ft and another 9ft. And their watt meter tells them they're getting more power out and the radio is happier. But what's happening is that the patch lead is acting somewhat like a matching stub and since no two amps are the same, especially home built, different lengths work better for others, and the input impedance could be out by a long way. In the past I have run a half wavelength of coax (taking in the velocity factor, coaxes with 66% helps) because I suspected the input impedance was out. This is a long lead, but at least it takes the coax impedance out of the equation. The impedance of the coax will be ignored as long as you cut it accurately and don't go too far from the design frequency, however you are still going to get reflected power.
Someone mentioned common mode current which is right too, and this is going to affect your swr readings with most meters, most meters lie anyway. The use of coaxial chokes is good a measure, I prefer to wind the coax through several ferrite cores of 43 mix.
So if you could measure the complex input impedance of your box (and it's stable), in theory you could cut a jumper as a coaxial transformer. The ARRL antenna handbook has a formula, but I think that only applies to quarter wave stubs. Here's a link that should cover it: [Please login or register to view this link]
Good luck.
Jeremy.

[arcone]

And sorry for not acknowledging the two replies previous to mine, I should read all the replies first, I basically repeated what they said. I have seen this question asked on forums many times and am glad to see that people are finally putting it to bed.