Help for a Newbee

[wayne]

Hello,

I just bought a new AIM 4170C and am beginning to learn how to operate the device.

For practice, I connected the 4170 directly to the feed point of a 15M vertical. The vertical is made by Comtek Systems and I bought it and all of the 40, ¼ wavelength radials from DX Engineering. The vertical’s length is exactly 11 feet, which the manual says should put the center frequency at 21.300mHz.

I first selected the 15M Band on the AIM program, and ran a scan from 20.9mHz to 21.65mHz. To my surprise, all of the graphs were essentially straight lines that sloped upward slightly. The program did not find a resonant frequency and the min. SWR of 1.77 was at 20.9mHz.

Next, I broadened the scan frequencies to 15mHz to 25mHz. This scan found 2 resonant frequencies: 15.34mHz and 18.81mHz. The min. SWR was 3.12 at 17.3mHz.

Then, I shortened the vertical 1 inch at a time. According to the vertical manual, 1 inch should change the resonant frequency by .15mHz. I then ran scans from 17 to 26 mHz. No resonant frequency was found. The min. SWR was 2.63 @ 22.26mHz.

I would really appreciate your help from a newbee!

Wayne

[Harold]

Wayne,
Welcome to the group. You are perhaps only looking for a minimum
SWR reading. The real information that you wish is the impedance of
the antenna. You will find that a vertical antenna should never give a
50 ohm match unless there is losses unaccounted for.
Look again at the resistance and impedance plots and let us in on
those results. The antenna may have been working just fine.
Harold

[smokchsr]

Wayne,
the first thing to do is to get familiar with the device. That would include testing loads and other things to get used to what you are going to see.
When measuring an antenna, many factors come into play, the most important one is your calibration. If you are wanting to measure the antenna alone be sure to calibrate to the end of what ever cable you are using to connect to it. If you ever have any doubts about your measurements a quick indicator is to connect your calibration load at the test point. If it doesn't read very close to 50 ohms with 0 reactance across the screen, then you know your calibration is not (or no longer) valid.

[kc0ws]

Wayne,

Vertical antennas are quite sensetive to their environment. The manufacturer tests the antenna in the open, away from any buildings. Very few of us can install a vertical that way. Everything within at least a couple of wavelengths is going to change the impedance of the antenna, its drive impedance, and its resonance.

If you have set up the test equipment properly, you will see what you are handing your transmitter as a load. Your transmitter is probably not overly pleased with you.

In a real-world, cluttered environment, there is no such thing as setting up a vertical "by the book" and having the drive impedance be close to what the manufacturer measured, because your antenna is not "in the clear". Welcome to real-world antennas.

Ted