Let's simplify this, shall we?
Hi,
kb2vxa has it right, but bombarded some of us with technical stuff.
He is right though.
For measuring your SWR you should do it at the antenna. This is because an SWR meter is a "dumb" device. It measures current. It only measures current that is present on the coax where it is attached. The currents
vary depending on the location on a given length of coax, even if the antenna is perfectly tuned. The meter doesn't know it is not at the antenna or instead at a certain point on a piece of cable, it just indicates the current where it is.
At 8 feet away from the antenna on 27mHz it
may show a very high reading, even if the antenna is tuned. At 16 feet it
may show a low reading for the same antenna. (notice I said "may".) The currents can be different at different points on the cable. The meter will show only these currents. It doesn't know where it is on the cable, ergo the term "dumb device".
Now ... do not confuse this with the performance of the system. If the antenna is tuned, the length of coax has no effect on the systems performance (radio, coax & antenna) in radiating the energy at the antenna. It also has no effect on the tuning of the antenna itself, contrary to many opinions. It only effects the meter reading (the measurement), depending on where the meter is on the length of the coax.
So tune the antenna with the SWR meter as close to the antenna as practically possible. This eliminates any chance of a false reading due to varying currents on a cable. A tiny piece of coax cable, like 6", is good. Even better, if you have the space, one of those male-to-male UHF adapters are great, if you can get the meter in near the feedpoint like that.
Oh and try to step away from the antenna when making measurements, it can throw off the tuning by you being in close proximity.
Tuning the antenna this way you know it is tuned, and you are not reading currents on the cable. This way you know you can use
any length of coax and the system will radiate the energy properly.
BTW, this effect of varying currents at different points on a coax cable is why people think the length of coax effects tuning. It doesn't. They just assume it is, because they are being just as dumb as the meter. They do not understand
the measurement (the false reading) and are making bad assumptions based on incomplete knowledge. Tuning is not usually affected by coax length. Measurements of current always are. The two are not the same thing.
Was that simple enough ... I don't know.
John LeVasseur, W2WDX