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K3 SWR

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Jay Krishna

K3 SWR

Just completed assembling my new K3/100; I am completely new to this rig, and just getting used to it. In initial transmitter power calibration into a dummy load, I saw an SWR of ~1:1. However, into my 20M mono-band dipole, I see it is ~1.8:1. I am wondering if anybody can help answer these questions:

1. Is it safe to operate the K3 with an SWR as high as 1.8:1 (just eager to test the new K3 on the air)?
2. Do I need to check if there is a problem with my dipole antenna (wet coax, etc.)?

Thanks,

Jay (KD6AMA)
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alsopb

Re: K3 SWR

Jay,

The K3 protects itself against too high SWR's by cutting back power.
An SWR of 1.8:1 is quite acceptable.

As to why the SWR is 1.8 , there could be many reasons.
1) Antenna too long or too short. Run an SWR curve vs frequency to see
which.
2) Antenna height. Depending upon height the impedance of the antenna
can be very low at low heights and as much as ~100 ohms at some heights.
From the antenna handbook for a half wave dipole for 20 M:
Height in wavelengths Feedpoint Impedance
.1 (~7 feet) 20 ohms (SWR =2.5)
.17(~11 feet) 50 (SWR = 1)
.34 (~22 feet) 98 (SWR =2)
beyond this point the impedance decreases and oscillates about a value
of 72 ohms (SWR =1.44)
3) With any feedpoint impedance other than 50 ohms, the SWR seen at the
shack end of the coax is highly dependent on coax length.

If you bought the coax new, the last thing to consider is the coax
itself - provided it hasn't been abused or is very old.

It is common practice to trim the length of the antenna at a given
height to attempt to adjust the SWR favorably.

73 de Brian/K3KO

On 12/9/2012 2:16 AM, Jay Krishna wrote:

> Just completed assembling my new K3/100; I am completely new to this rig, and just getting used to it. In initial transmitter power calibration into a dummy load, I saw an SWR of ~1:1. However, into my 20M mono-band dipole, I see it is ~1.8:1. I am wondering if anybody can help answer these questions:
>
> 1. Is it safe to operate the K3 with an SWR as high as 1.8:1 (just eager to test the new K3 on the air)?
> 2. Do I need to check if there is a problem with my dipole antenna (wet coax, etc.)?
>
> Thanks,
>
> Jay (KD6AMA)
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
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Vic Rosenthal

Re: K3 SWR

In reply to this post by Jay Krishna

It is perfectly safe.

The K3 cuts back power when it sees the equivalent reflected power of a 2:1 SWR at 100
watts. So it will operate normally at a 1.8:1 SWR. If the LOWEST SWR is 1.8:1, then it
probably exceeds 2:1 somewhere in the band, where you will not be able to get a full 100
watts.

I suggest you check the SWR at various points (an antenna analyzer is helpful, but you can
just transmit at different frequencies and record the results). Once you find the point of
lowest SWR, you can determine how much to lengthen or shorten the antenna to move this
point to the center of the band or your favorite spot.

As another writer said, it is unlikely that you will get a 1:1 SWR from a coax-fed dipole
under most circumstances. The K3 will be happy as long as it is under 2:1 (and you can
still operate with reduced power if it is over 2:1).

On 12/8/2012 11:16 PM, Jay Krishna wrote:

--
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/

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