Cobra 29ltd

the coax is one part of the antenna system.

 

Again, I've simply presented the facts. Provide a reputable source that says otherwise.

 

Ram has it pretty well nailed here. With a pair of "co-phased" antennas, simply putting two 50 ohm loads in parallel with 50 (or 52) ohm coax will give a 25 ohm impedance, and the best SWR you can hope for is 2.0:1 (50/25 = 2). By using 72 ohm coax (RG-59), the mismatch is lessened (SWR is better).
For a minute, though, forget about twin antennas. Ideally, the antenna should present a 50 ohm load to the transmitted signal....f'r instance, put your SWR meter right at the base of the antenna, not at the radio. Using a half-wavelength of coax (and allowing for the velocity factor, which varies from manufacturer to manufacturer and the actual materials and dimensions of the coax) will show the exact SWR at the antenna, but it won't correct anything by changing its length. If the antenna is cut wrong, or has an insufficient conductor attached to the coax's shield (metal ground plane, or an equal 50 ohm load on the bracket), the half-wave of 50 ohm coax will show the SWR at the antenna feedpoint. If the antenna is only 15 ohms, a half-wave of coax will show a 15 ohm impedance at the radio end of the coax, and the SWR will be roughly 3.3:1 (50/15). Changing the length of your coax won't change the actual SWR at the antenna; it'll still be, in this case, 15 ohms, and will perform poorly.
OK, back to the co-phased antenna array. What's needed to change that impedance of 25 ohms (remember, 50 ohms paralleled with 50 ohms) is an impedance transforming device. By happy coincidence (or good engineering, who knows?) it turns out that a "mismatched" piece of coax (72 ohms) is reasonably close to 50 ohms at the load (antenna) end. Since it is not the same impedance, the length can be adjusted so the mismatch can be "tuned out" and made tolerable. Then, bringing two pieces of 72 (or 75) ohm coax together at the single connector for the co-phase harness (in parallel at the plug) is around 37 or 38 ohms, which is substantially closer to the 50 ohms your radio wants to see than 25 ohms. The length of those two legs of 75 ohm coax is precisely cut so that the SWR as seen at the connector is about 1.3:1. At that point, *any length* of 50 ohm coax is acceptable. And changing the length of 50 ohm coax needed to reach the harness's connector and the radio, if any, will not affect the SWR of the system.
On a related note, the proper length of coax needed to run from a radio is precisely the same as the length of a piece of string needed to make that same reach. In actual practice, I leave about two feet of slack to allow for the inevitable wearing out of connectors being put on and taken off of various radios or antenna mounts, or corrosion on a mirror mount with some leakage into the braid, so I can cut off a couple of inches and not have to run a whole new line. It also gives me room to get my Bird wattmeter or MFJ antenna analyzer conveniently close the the radio and the radio end of the coax to take readings without having to stick my face down into the defrost vents or develop a third arm.
So...... I dunno if I helped the discussion or hindered it........ again.
Cheers,
-- Handlebar --
diddly dahdidah

 

Nah you helped. Long explanation for my simple one is all.

Simpler explanation:

For cophasing:

Cut two pieces of RG59 (V factor of .79) to about 14.5 feet. Run them to a harness, run a single piece of RG8X from harness to radio. Tune antennae to match.