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Have tried measuring antenna wire using formula 234/frequency for 1/4 wave vertical but this gives high swr. How do I lower swr on a wire vertical antenna. Do I Cut or Add? wire'

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If you're only measuring SWR at one frequency, or in a narrow range like the ham bands, you won't know until you cut it.

Mount the antenna in its final location as the SWR and resonant frequency will change a bit depending on the environment.

Because cutting is permanent, I recommend first extending the wire by 5% of its length, perhaps with a bit of aluminium foil and tape, or for lower frequencies and larger antennas, a crocodile clip lead. See if the SWR in your band goes up or down.

If it goes up, that's good, you can start trimming the antenna. Go slowly!

If it goes down, sorry, you've already cut it too short. You can extend it by soldering some wire to the tip. Fortunately the mechanical stress is low there so it doesn't matter if you use copper wire.

If you can measure over a wider band, like with a nanoVNA, then look for the minimum SWR - if it's at a lower frequency than you need, then cut the antenna slowly. If it's higher, extend it. (and if you're going to build any more antennas, I strongly recommend some sort of nanoVNA for your experiments).

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The first consideration is that a 1/4 wavelength antenna has a wide bandwidth, it is not a high Q or high gain antenna. Don't worry about getting SWR as low as you can, it's not critically important and an SWR of 2:1 is just fine and won't affect your signal one iota. A VNA will give you the true picture of your antenna. If this is a VHF antenna, the best approach is to adjust the length of the whip, but take it easy!

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Should you lengthen or shorten a radiating element to lower VSWR? Well, one answer is to simply check your VSWR by transmitting on a higher frequency, if your VSWR goes down, you need to lengthen your radiating element for the frequency you want a lower VSWR for, conversely, if you get a lower VSWR when transmitting on a lower frequency than the frequency you wnat to use the antenna on, then you must shhorten it for the higher frequency you want to transmit on. You can even use your formula to determine how much to cut or add:

Current Antenna Length - (234/Best VSWR Frequency) = If negative cut, if positive, lengthen.

IF you can't get a satisfactory VSWR anywhere near where you test, you have another issue that is not simply antenna length.

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