Coax, null or not?

[Cable Guy]

I have been inundated recently with antenna and feed line questions that I can't answer, and there seems to be as many opinions as days of the year. Do you trim your coax to fractional wavelength null? Is it a myth? Is there merit based on compromise in mobile installation or CMC?

 

[Shadetree Mechanic]

In theory, it sounds great. Has anyone ever been able to show that the wrong length actually shows a loss in some way?

It might help someone who is driving Chinese transistors on the edge of failure in order to make reflected power low as possible?

 

[19wrc333]

The "Screwdriver's guide" from Franklin has a whole section discussing this.
In principle it's a myth, but there's some specific cases (IIRC when the coax is intended to be part of the antenna system) when it does matter.

 

[Crawdad]

From a very smart fellow that knows mobile installs inside and out, minus the myths:

www.k0bg.com/myths.html#length

 

[Cable Guy]

From a very smart fellow that knows mobile installs inside and out, minus the myths:

www.k0bg.com/myths.html#length

Lots of good things in there, but you can't tell some folks these things because it contradicts what they have always heard from 5 different cb shops who all agree with the myth. I have said again and again that if coax length "fixes" your problem, then your antenna and/or mounting is your real problem. But what do I know? Very little on the subject, really, just spouting things more intelligent folks have said and I accept as gospel. I keep getting brow beaten every time I say that to the average user, who believe swr measurements are king and that tuning the coax seems logical since the radio and antenna can be and are often tuned. I was starting to doubt myself.

 

[Crawdad]

Lots of good things in there, but you can't tell some folks these things because it contradicts what they have always heard from 5 different cb shops who all agree with the myth. I have said again and again that if coax length "fixes" your problem, then your antenna and/or mounting is your real problem. But what do I know? Very little on the subject, really, just spouting things more intelligent folks have said and I accept as gospel. I keep getting brow beaten every time I say that to the average user, who believe swr measurements are king and that tuning the coax seems logical since the radio and antenna can be and are often tuned. I was starting to doubt myself.

You are absolutely right CG, don't doubt yourself. The myths are damn near impossible to kill off since most find it easier to believe the myth rather than learn anything new. Tis a shame.

7 3

 

[brandon7861]

If a shop can make a buck by cutting your cable to a specific length, they will try to convince you to do it. The scam is what keeps these myths alive.

In my opinion, there are only two cases where coax length matters. When you want to transform a non-50Ω impedance to something else non-50Ω, or when you want to reduce common-mode current ~ the impedance looking down the shield. Both of those cases are situation-dependent, and the shop isn't going to know what you need.

There is a little piece of proof you can provide to people in doubt, the transmission line equation that relates input and output impedance based on characteristic impedance, length, frequency and loss. All they gotta do is learn some complex math solve it.

I remember back when I was in the slammer with nothing but the ARRL handbook to keep my mind busy, I had to figure out how to calculate complex numbers on a calculator that didn't even have trig functions. I had trig tables and log tables, series-parallel conversions and I forget what else - but I did it. If a person is not willing to dive into a little coax math, then let them believe what they want.

 

[sp5it]

Read this: https://www.w4wb.com/Reflections_III.pdf

 

[Cable Guy]

All they gotta do is learn some complex math solve it.

Haha, I won't say they can't, but they won't.

 

[M0GVZ]

In theory, it sounds great. Has anyone ever been able to show that the wrong length actually shows a loss in some way?

There is no such thing as a wrong length. There is only one correct length, the length needed to get from radio to antenna by the route you want.

If altering the length of the coax, coiling it up, putting in a RF choke or touching it alters the SWR then this is proof that the antenna system is using your coax to compensate for a poor RF ground, making it part of the antenna which we don't want.

There are only three exceptions to this:

1) Co-phasing antennas where you need each leg from the T piece to be an electrical quarter wave.

2) Mirroring the feedpoint impedance and reactance so you can measure the values at the antenna feedpoint which isn't possible to reach where it will be installed. You would use a length of coax that is an electrical half wavelength or multiples of half wave length.

3) It's an antenna design that deliberately uses some of the coax to form part of the antenna. A Carolina Windom is such an antenna.

 

[wavrider]

Time and place for everything?
Very general open ended question/ statement.
YES trimming coax has benefits, especially in LDMOS applications from pa board to filter board, size matters. vhf, uhf? yes cable loss
In an HF installation or use,, it does not matter.

 

[Greg T]

I have always said that if you have a quality antenna with a quality install and proper grounding, the coax should be virtually transparent. I still get laughed at with that response, but I have been playing radio since 1964 when my older brother got me interested, and I have so many installs I can't count them. My current rig with 955HPC+, meter, Amp, meter, Sirio 2008 is the same way. I have used many different jumper lengths testing my thinking and even going from 3' to 6' to 12' and many others, I have no SWR changes and no one can tell the difference while receiving. My take is this; Who cares if there IS a difference? No one on the receiving end is going to hear it. Currently, most of my jumpers are 1' or less. FLAT SWR meter with ZERO reflect.