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Getting an HF antenna to work (Part 2)

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KK4CGM
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Getting an HF antenna to work (Part 2)

Here is Part 2 of Ralph Bilal's email to me from www.isotronantennas.com

Your antenna is mounted and the coax is installed. What are you looking for to see if it works?

 

Resonance.

 

What is that? You can liken it to a piano wire. Physically the size of the wire determines how fast the wire oscillates back and forth per second. That value is the resonant point of the wire. When it resonates you hear the tone it produces.

 

With radio waves there are no moving parts. However depending on the frequency you chose to operate, the electrical energy oscillates at that value. Take 7 Mhz, the current in the wire moves in one direction, weakens to the point of no current at all, then reverses in the opposite direction. This is done at 7 million times per second.

 

In order for that energy to leave your radio and coax it needs a resonant device at the end, the antenna. What is in the antenna to make it a resonant device?

 

Coming articles will go into this more. However to start, we can use a measurement most are familiar with, the Standing Wave Ratio or SWR.

 

We usually measure SWR at the radio. What is it?

 

You can liken it to a calm body of water. Now throw that HT in the water that did not work. The object makes waves coming to shore. The waves look like they are traveling in one direction. Once it hits the shore the wave can not go further so it returns along the same path. When this happens the wave going out and coming in meet and it will look like the wave now is standing still, just going up and down in place. This also happens electrically when we have a Standing Wave on our coax line.

 

The cause of the standing wave is due to an antenna that is not resonant. It also involves the resistance of the antenna, but that is for later.

Therefore if you scan your band you are operating on and observe the SWR dip to a minimum then rise again, you had just past the resonant point of the antenna system. Many times this is all you need to do to know if your antenna is working and if you need to raise or lower the resonant point.

This can be done with an Antenna analyzer or your radio. Using the radio, set it to a low power, use CW or FM, (not ssb) and slowly scan the band you are using. If the antenna is working you should see a rise and fall in SWR value. Where the SWR is minimum is the resonant point of your antenna system.

What if you need to know more to get it working? More will be covered in the next articles.

73,

Ralph WD0EJA

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