coax length

RG8X Mini foam is going to be the best coax for a truck, The lengh of the coax should not be a factor if you properly tune the antenna.

 

it's best to use at least a 9ft piece of coax....if you have alot left over fold it end over end and tie wrap the excess, DON"T coil it! it will act like an antenna and throw off you SWR's, maybe even damage your radio

 

How many "SWR'ZZZZZZZZZZZZZZZ are there in ONE feedline?


Tuning antennas with coax length is BUNK, a CB radio myth! IF the truck body is METAL, the way to set up an antenna is by LENGTHENING or SHORTENING the WHIP. The shorter the whip, the higher the frequency. The LONGER, the LOWER the frequency. Now. Phased antenna are a different animal and even the cable is different. So the length is important.
A single antenna can also be set up with TWO whips; one on TOP of the mirror, another one GROUNDED out and pointed DOWNWARDS. This also eliminates the need for coiling up coax (*in the way!). NOBODY else in the commercial, military, or amateur services messes with coax to "tune" antennas.


Gadfly

 

An old timer named Doctor John (God rest his soul) here in Oregon taught me
how to resonate coax with a dummy load and a T back in 1975, and I've been
doing it ever since. He was the smartest cottenpicker I ever met and he
taught me all of his tricks.

What you do is use an SWR bridge/meter and a short jumper between it and the
transmitter. A 4 watt radio is used as a source, transmitting in the center
of your usable range. The T-connector must be installed right at the output
of the meter and a small precision dummy load must be connected right to the
"T". Once the test apparatus is ready, you connect your random length of
coax to the other side of the T, open circuited at the far end and check the
match. Start trimming the coax slowly until you see a flat SWR. That's it.

One thing I learned right away was that all brands of coax are NOT created
equal.
RG-213U for example is supposed to have a velocity factor of 66% yet if you
take several brands of 213 you will find that each brand actually resonates
to a different length, telling you that just using the math and the velocity
factor won't get you exactly at an electrical half wave every time. It makes
it much more accurate to construct phasing lines and they consistently work
better if you use this method of tuning. It only works for half wave or
multiple thereof. For making a quarter wave hunk, just tune a piece for the
first half wavelength and cut it in half.

The only thing I can figure as to why this worked so well for TVI was being
that the current and voltage lead and lag each other at different points on
the cable, making the coax a half wave or multiple thereof puts the current
and voltage in the most desirable phase relationship at the feedpoint.

I use 15 feet 2 inches of lmr-240

 

you do understand the relationship between phase angle and perceived impedance do you not? there is a lot more at stake than just swr. how about zero reactance, and a perfect 50 ohm match?

 

EVERYBODY else in the commercial, military, or amateur services does not mess with coax to "tune" antennas because they couldn't care less about efficiency when they use a "tuner" to fool the radio into seeing a perfect 50 ohm match. (which results in loss by the way)

 

changing coax length will not affect actual swr when using an mfj-259, but it will change impedance which will fool a cheap swr meter into giving a false reading.

 

There is no " cut " coax here.