usdefcon1 said:
Can the SWR meters positioning in the circuit affect the reading that much?
It can. (The caveat here is that I work with microwaves, where loss is a very significant problem even in very-high-qualty cable. But low-quality cable can have measurable loss at 27 MHz, where your CB operates.) The thing of it is that you want to tune the antenna so it reflects as little power as possible, because if power is being reflected, it's not being radiated into the air, which is where you want it. Taking your VSWR measurements close to the antenna lets the meter sniff out as much of the reflected power as possible and will let you fine-tune things more accurately. Putting a lot of coax in the middle means that some of the reflected power will be converted to heat, and the meter will see a wider-than-true range where the VSWR looks perfect even though it isn't.
Also if I remember my electronics training the length of coax is a constant to the length of the antenna and the frequency if you what them to match correctly. So does the NGP antenna take advantage of this fact and messing with the length is just screwing yourself or is there something different about NGP antennas?
That's news to me. Some NGP antennas use the shield of the coax as an antenna element. Changing the length of the coax changes the length of that element, which can hose up how well the antenna is matched. In that case, you'll want to take your measurements at the end of the cable supplied by the manufacturer, because it's actually part of the antenna.
Also do you have to take into account that these antennas are designed for large(er) ground planes like on trucks and cars and do you have to make adjustments to counteract that on a motorcycle?
For an antenna that requires a ground plane, the only adjustment you're going to be able to make is to provide one, either in the form of lots of sheet metal, a single counterpoise wire or lots of them arranged in a radial pattern. Ground plane antennas simply won't work well without one.
If your reading is above 3 on all channels doesn't that mean shortening the antenna.
Not necessarily. What it does mean is that your antenna's way out of tune, but "above 3" doesn't tell you which way.
I've attached a chart of VSWR measured on an actual antenna using a network analyzer. (The antenna is one I had around the lab and isn't designed for 27 MHz, but it should get the point across.) This particular antenna has acceptable VSWR through the entire range shown, but for the sake of this example, you can pretend that the upper end of the chart is some large number like 6:1. You'll notice a sweet spot at about 112 MHz where the VSWR dips very close to 1:1. This is where the antenna is resonant and reflects almost no power.
Let's say I wanted to use this antenna in the range of 105-110 MHz. You'll notice that as you go higher in frequency through that range, the VSWR gets lower. That's an indication that the antenna is resonant at a frequency higher than the range where you want to use it, which you can verify by looking at the chart. To make the sweet spot occur at a lower frequecy, the antenna needs to be made longer, either physically or through electrical means such as loading coils.
Similarly, using this antenna in the range of 115-120 MHz has the opposite problem. As you go higher in frequency through that range, the VSWR gets higher, indicating that the resonant point is happening at a lower frequency. Making the antenna shorter will increase its resonant frequency, and just the right amount will land it in the desired range.
To carry that over to a CB antenna: If the VSWR is lower on channel 1 than on 40, then the antenna is resonant at a frequency lower than channel 1. That means it needs to be tuned to be resonant at a higher frequency (shortened). If it's lower on 40 than 1, the resonant frequency is above the band, which you correct by lengthening. (This is a generalization that works for this discussion but may not for others because some antennas are designed to be resonant at several frequencies.)
And if you've shortened the antenna as much as you can and still have above 3 on all channels (Assuming this is the firestick with the non destructable shortening) then there is something else wrong I would think.
If you can't find a length where you get VSWR of 2:1 or less on any channel, you've either got the wrong antenna, a crappy one, a broken one or you're coming up short in the ground plane department.
By the way, after you tune the antenna, it's a good idea to take a couple of VSWR measurements at the transmitter to make sure you don't have any breaks or other nasties in the rest of the cabling that cause energy to be reflected before it even reaches the antenna.
--Mark
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