Length of coaxial cable in transmission line

[SV1JRT]

Hello folks,
 There is a debate among my friends, on the length of the coaxial cable in radio amateur use. (band 144 - 146 Mhz and 430 - 440 MHz). Me and some of my friends, believe we should cut the coaxial cable in a specific length, determined by the wavelength multiplied by VF ( Lcoax = (300 / F) * VF in meters). The opposite team believe that this is irrelevant and the coaxial cable can be cut in any random length, without affecting the SWR or the matching of the antenna.
 Which one is correct ? If possible, i whould l;ike to give me a proof of the correct answer.
 Thank you for your time.

[scrat]

I'm not a ham radio expert, just studied some time ago e.m. fields with interest, and theory says that it depends on how the matching is done.
If the impedance at the antenna terminals is matched to the cable, and the radio device is matched to the cable, cable length is not relevant.
On the other hand, if the something is not matched, impedance will vary according to the Smith diagram along the cable length, so you must choose the right length or add some matching network...

[ElectroIrradiator]

The VSWR will not change depending on the length of the cable, but scrat is correct that the matching at the transmitter will.

However the cable losses are highly dependent on the VSWR, so at VHF and above you really shouldn't use the cable length as a tuning device.

Firstly, it can be hard to get the length right, unless you have the correct test gear to get everything tuned correctly.
Secondly there is no guarantee you will be able to get an acceptable match in every case just by varying the cable length. This is in fact quite unlikely though theoretically possible.
Thirdly, even in the rare cases where you can find a suitable match, you will still suffer the high cable losses associated with a high VSWR. This is true even though the transmitter is now seeing a somewhat reasonable load impedance at its output.

So the bottom line is: Match the antenna to the feed cable at the antenna's feed point, then use as short a cable length as possible to limit cable losses.

[SV1JRT]

 I see. What happens if we use an dummy load in the place of the antenna and "tune" the coax before installing it ??
 Is it better to first cut off the coax in the correct length (using the VF parameter) and then match the antenna to the cable or it doesn't matter ?
 What i want, is to tune each part correct. Not to use the coax as "tuning device". So, is it better to tune the coax or not ?
We are always talking on V/U band.
 

[G7PSK]

I have never used an exact length on an aerial transmission lead except for balanced arrays but the instruction sheet for my dummy load power meter says that for VHF that an exact half length is essential, i attach a copy pdf of the page.

[ElectroIrradiator]

If the load, be it an antenna or a dummy load, is correctly matched to the cable and transmitter impedance, then the cable length doesn't matter at all. If there is no impedance mismatch, then there is nothing to tune.

So you won't be able to tune anything with a proper dummy load at the far end of the cable.

If VSWR is less than 1.5 or so, then just attach transmitter and go. If not, then tweak the impedance matching at the antenna until it is. Once it is, ignore the precise length but use as short a cable as you can to limit losses in the cable.

The reason why the cable length matters for the dummy load, is that if the cable isn't precisely 50 ohm at the frequency of interest, then the mismatch created will make the power reading erroneous. A precise half wavelength (including VF) makes a nearly perfect 1:1 impedance transformer, regardless of the cable impedance. Only factor you'd then have to correct for would be cable losses.

[SV1JRT]

OK. I got it. Thank you all for the info.
 My Coax will be about 4 meters long. My tranceiver (Yaesu FT857) will be in an metallic electrical box, attached to the antenna (Diamond X510) base. The console of the FT857 will be in my shack with a 15 meter serial cable.
 So, the losses in a 4 meter H2000 coax cable should be minimum.

[AG6QR]

Just to put a nail in this one...

Excellent information.  But I still can't resist adding one more blow on that last nail.

Statements 2a and 2b above are only exactly true for lossless coax.  As the length of lossy coax increases, the impedance at the source end starts to be influenced, and eventually dominated, by the coax losses.  The impedance at the load end becomes less "visible" to the source.

If there's a mismatch at the antenna end of the feed line, there will be reflected power in the coax, which increases the current carried by the coax.  If the coax is nearly ideal, then the statements made about matching the length are all you need to know.  But real world coax is never completely lossless, and at higher frequencies, losses may be significant.  Those figures the manufacture publishes about losses of "4.5 dB/100m at 150MHz" are only accurate for a good match at the antenna feed point.  If you have a mismatch at the antenna, resulting in lots of circulating current, losses will be worse than the tables would predict.

That's a roundabout way of saying that if you want to radiate as much signal as possible, you really should tune the antenna for the best match practical at the antenna feed point.  Perfection isn't required, but a poor match will increase losses in the coax, and this can't be completely overcome by using a tuner at the radio end of the coax, regardless of the precise length of the coax.

Since SV1JRT is talking about a short length of low-loss cable, connected to a well-matched antenna, this detail is unimportant for practical purposes in his particular situation.  But in the general debate of whether coax length matters in theory, it may be something to consider.

[ElectroIrradiator]

Looks like w2aew's post went missing before I had a chance to read it... 

[w2aew]

Looks like w2aew's post went missing before I had a chance to read it... 

Hmmm...
What I said was...
If the load matches the transmission line impedance, then the line length is unimportant except for losses as described above.

If the load does not match the line impedance, then the line length will affect the impedance seen at the input.   There are two special cases...

For half wavelength (or any even-multiple of quarter wavelengths), the input impedance will match the load impedance, regardless of line impedance.

For quarter wavelength or any odd-multiple of quarter wavelength, the impedance seen at the input will be the dual of the load impedance. More specifically, the relationship is:
Zin/Z0 = Z0/Zload

Interesting features of this last property...  A quarter wavelength line that is shorted at the far end will look like an open circuit. And, if the far end is open, it will look like a short at the input.

[KJDS]

Interesting features of this last property...  A quarter wavelength line that is shorted at the far end will look like an open circuit. And, if the far end is open, it will look like a short at the input.

It's that feature that is often used to bias narrow band amplifiers with the aid of a good decoupling capacitor to make an RF short.

[SV1JRT]

 Thank you all folks.
 Great info.