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GETTING AN HF ANTENNAS TO WORK.(Part 7)

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Barry
GETTING AN HF ANTENNAS TO WORK.(Part 7)

 

Last article mentioned 4 characteristics or values that an antenna has. XL-INDUCTIVE REACTANCE, XC-CAPACITIVE REACTANCE, RADIATION RESISTANCE AND  R-NONREACTIVE RESISTANCE.

If you are using a SWR meter at the radio through a length of coax, you are limited as to what you can determine. You can scan a section of frequency and find the minimum SWR. This will tell you the resonant point of antenna and coax combined. Not too bad, but sometimes we need more data to make the antenna match closer to 1:1.

If you are building an antenna, many have purchased some kind of analyzer. This device has measurements for XL, XC, Z and R. How do we use it?

The instrument needs to be connected at the antenna. On HF a short coax jumper will work.

Now scan the area of frequency you are working with. If you are building an antenna for 40 meters, then scan the frequencies in the 7 Mhz area. What are you looking for?

Zero Reactance. Be sure your analyzer is set to reading reactance. When you have found 0 you are at the resonant point of the antenna. This will also be where the minimum SWR is.

What if the minimum SWR is greater than 1:1? Now change the setting on the analyzer to the Resistance setting. In some units you have a resistance meter. You should hold your thumb over that meter so not to see it until you find where the Reactance's are at 0. Now take your thumb away and see what you have. How close are you to 50 ohms? At any other point this value does little to help you.

Now you know if the antenna is above or below 50 ohms. Depending on the design of the antenna, R value can be adjusted (possibly) to measure needed 50 ohms.

Now you are ready to connect your coax and operate. Wrong! Once you connect the coax the values will most likely change. The coax is also a component of the antenna. It does not radiate, but it does change the readings you just obtained.

Now what do you do?

The next article will start to address this.

73,

Ralph WD0EJA

NOVEMBER 2013

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