Coax Lenght..?

[Straightshooter58]

I have been told that the length of the coax cable makes a big difference..to use a minium of 50 feet..if your running a amplifier! Is this True or False? Also the type of coax makes a difference..? Is one coax better than others..will it effect your SWR or output power? I've been out of radio many years..forgot more than I can remember! Thank You.(old gut in east texas)

 

[W4KVW]

I run a variety of lengths of coax & have since the 1960's & have never had an issue with any length.I have heard of this so called issue but have no idea why people have an issue & I never have with so many different antennas & pieces of coax? I never measured the coax just cut them to the needed lengths & go from there.I have jumpers in line now on several radios from 2 ft to 15 ft with Zero issues.I have always thought that if the length of the coax matters to your SWR reading there must be another issue somewhere.

SIX-SHOOTER

 

[kopcicle]

When Abraham Lincoln was asked, "How long do you think a man’s legs should be?" he replied, "Long enough to reach the ground."

In nearly all cases you will encounter at HF frequencies coaxial feedline length makes no difference.

Exceptions

Collins 30L-1.
Collins recommended the use of a 20.5 ft coax on the input of this amp. Read W8JI on this. Call it a field expedient to cover a questionable neutralization scheme.

Combining a 75 ohm matching section of coax with a 50 ohm feedline to effect a coaxial impedance transformer. It's in the handbook.

Unrelated, however try using 1/2 waves of ladder line to feed a resonant or non resonant dipole. heh heh. Okay read this .
http://www.w5dxp.com/notuner/notuner.htm

Remember that losses run both ways in coax. If your feedline losses are 10% of 100 then your reflected wattage at the feed point is 10 watts. It's only 9 watts at the transmitter if measured there. So 9 watts, 10 watts , what's the difference? Try this at 1500 watts and ...

Anything above 30MHZ

Have fun

 

[M0GVZ]

I have been told that the length of the coax cable makes a big difference..to use a minium of 50 feet..if your running a amplifier! Is this True or False?

Utter utter rubbish.

Also the type of coax makes a difference..? Is one coax better than others..will it effect your SWR or output power?

Now this IS valid.
Different coaxes have two characteristics we're concerned with other than impedance. Whilst the differences won't affect the SWR:

1) Losses are different at various frequencies, especially higher ones at VHF and above
2) Power handling capability.

In regards to losses you'll find for example RG213 has half the loss at 11m than RG58 has and you'll find as you go up in frequencies the difference in loss becomes wider. Also RG58 power wise tops out around 400W with a good SWR - the higher the SWR the lower that max power limit is.

This table here gives you an idea of the losses at various frequencies per 100ft.

http://www.w4rp.com/ref/coax.html

 

[M0GVZ]

Remember that losses run both ways in coax. If your feedline losses are 10% of 100 then your reflected wattage at the feed point is 10 watts. It's only 9 watts at the transmitter if measured there. So 9 watts, 10 watts , what's the difference? Try this at 1500 watts and ...

...the differences in dB and therefore S points on the meter of the receiving station are the same, less than a single dB and not noticeable to the receiving station despite it being 150W of loss.

 

[Straightshooter58]

Thank You All for the Input. Respects.73s

 

[rabbiporkchop]

If your antenna isn't a purely resistive 50 ohms then random cable length may act as an impedance transformer. If your antenna system is perfect, then don't worry about cable length.