Author Topic: "tuning" a VHF antenna with just a SWR meter  (Read 1596 times)

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GreekMan

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"tuning" a VHF antenna with just a SWR meter
« on: December 10, 2016, 11:05:50 AM »
Ok gere goes another question of mine.

How can one "tune" an antenna with just a SWR meter?

Typically it cannot be done cos there are coax & connector losses that decrease returned power.
But what if oen factors them in?

in example, a dipole is expected to have a 1,5 SWR. That means about 2 Watts for every 50 watts of power.
Now if we are to expect 1 watt of losses by the cable length, we start cuting down the length of the antenna until the SWR meter shows us 1watt of reflected power.
How about this idea?

spacecase0

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #1 on: December 11, 2016, 12:56:25 AM »
you tune an antenna with geometry of the antenna,
I look for the lowest SWR when doing that
also I look at resonance, so i try to make changes that are likely to be useful
if you have near resonance of your coax cable, then I might think that changing the antenna length to fix that would be misleading
if I am not changing the cable to the antenna, the the losses there are fixed,
looking for the lowest SWR is likely the best answer,
then I do know people that seek the highest radiated power out and not SWR...
I am not sure I care about math that much to care what one is better by some very small margin

Tevin

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #2 on: December 11, 2016, 07:26:28 PM »
Your idea of tuning for lowest reflected wattage would work, but there is no advantage to it over tuning for lowest SWR. It's a difference without a distinction.

Coax, connectors, and other stuff in the feed line, if properly working, will introduce loss but will not effect SWR. To put it another way, SWR effects loss but loss does not necessarily effect SWR.   

spacecase0

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #3 on: December 12, 2016, 10:01:37 AM »
I am now reminded of something that was told to me

someone with one SWR meter knows what is going on, someone with 2 or more SWR meters know they have no idea what is going on.

try putting SWR meters at various points, one at the radio, one at the antenna, and one somewhere else along the path,
and run them all at the same time.
you will be sure to have lost faith in the idea that you know what is going on by the time you really start thinking about what you see.
only exception to this is if you have perfect SWR with your antenna

ghrit

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #4 on: December 12, 2016, 11:27:06 AM »
I am now reminded of something that was told to me

someone with one SWR meter knows what is going on, someone with 2 or more SWR meters know they have no idea what is going on.

try putting SWR meters at various points, one at the radio, one at the antenna, and one somewhere else along the path,
and run them all at the same time.
you will be sure to have lost faith in the idea that you know what is going on by the time you really start thinking about what you see.
only exception to this is if you have perfect SWR with your antenna
I submit that the only SWR that counts is the one the xmitter sees.
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Tempstar

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #5 on: December 13, 2016, 04:51:39 AM »
To expand on what Ghrit correctly stated, tune the antenna with the SWR meter close to the radio, and include the feedline and all connectors just as it would be operated. Then if the SWR will not come down you can begin eliminating pieces until the culprit is found. I learned this the hard way when a 1:1 tuned antenna returned >3:1 installed. Finally traced to the whip from the hardline to the antenna. Since then, I tune in place.
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jdh

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #6 on: December 26, 2016, 11:15:08 AM »
My first step is to put a dummy load on the antenna end of the feed line then check the SWR or reflected power power at the transmitter end.  This tells me my feed line does not have major problem that would cause me to chase my tail trying to get the antenna to match.  Then the dummy load comes off, antenna goes on, readings taken.  Move up in frequency, check again.  Move down below the original frequency, check again.  This gives me a good idea of how much and in which direction the antenna needs to be moved.  Make changes, rinse and repeat until it is as good as I can get it, meaning lowest SWR or Reflected power.

As ghrit said, the most important thing is what the transmitter sees is the most important.  This is why it is important to put the meter as close as possible to the transmitter antenna connector using the shortest possible length of cable.  I use a double male adapter instead of a jumper if at all possible. 

spacecase0

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #7 on: December 26, 2016, 07:40:01 PM »
I have seen old or water soaked coax have perfect SWR while absorbing 97% the signal on 440
don't trust that perfect SWR at the transmitter means that anything is getting to your antenna

Tempstar

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #8 on: December 27, 2016, 07:12:20 AM »
I've seen waterlogged waveguide do the same, no reflected power but no output either.
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Lamewolf

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #9 on: January 05, 2017, 12:05:31 PM »
To expand on what Ghrit correctly stated, tune the antenna with the SWR meter close to the radio, and include the feedline and all connectors just as it would be operated. Then if the SWR will not come down you can begin eliminating pieces until the culprit is found. I learned this the hard way when a 1:1 tuned antenna returned >3:1 installed. Finally traced to the whip from the hardline to the antenna. Since then, I tune in place.

Any antenna should be tuned in the "installed" position.  Not doing so can cause huge changes once it is installed and installing it first is easy with VHF and UHF antenna - especially mobile antennas !

Tempstar

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #10 on: January 06, 2017, 05:44:56 AM »
Any antenna should be tuned in the "installed" position.  Not doing so can cause huge changes once it is installed and installing it first is easy with VHF and UHF antenna - especially mobile antennas !

This particular one was at 800' and we tried to shortcut and tune it on the ground then install it. A crappy whip bit us in the **s and cost us another half day.
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PeterJohn48115

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #11 on: May 08, 2017, 01:01:03 PM »
Hi...i am a new user here. In my case I look for the lowest SWR when doing that. Also I look at resonance, so i try to make changes that are likely to be useful. If you have near resonance of your coax cable, then I might think that changing the antenna length to fix that would be misleading. If I am not changing the cable to the antenna, the the losses there are fixed,looking for the lowest SWR is likely the best answer,then I do know people that seek the highest radiated power out and not SWR...

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« Last Edit: June 30, 2017, 09:55:14 AM by PeterJohn48115 »

Tevin

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #12 on: May 08, 2017, 06:26:13 PM »
Hi...i am a new user here. In my case I look for the lowest SWR when doing that. Also I look at resonance, so i try to make changes that are likely to be useful. If you have near resonance of your coax cable, then I might think that changing the antenna length to fix that would be misleading. If I am not changing the cable to the antenna, the the losses there are fixed,looking for the lowest SWR is likely the best answer,then I do know people that seek the highest radiated power out and not SWR...

Thanks for jumping in, John. Welcome.

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Re: "tuning" a VHF antenna with just a SWR meter
« Reply #13 on: May 08, 2017, 07:14:25 PM »
Yea JohnPeter48115

Thanks for jumping in and commenting.

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