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Author Topic: Anyone Know the approx. SWR of a 3/8L 20 meter vertical?  (Read 2169 times)
KF6OCI
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« on: May 27, 2013, 11:39:51 AM »

Thinking of a 30 meter vertical and also using it on 20 meters, which
is close to 3/8L. Like to know the approx. SWR of a 3/8L 20 meter
vertical, generally speaking, know some variances.

Thanks,

   Dan
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W5DXP
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« Reply #1 on: May 27, 2013, 12:04:50 PM »

SWR of a 3/8WL 20 meter vertical, ...

EZNEC says the SWR will be about 11:1 unless you neutralize the inductive reactance with a -j225 ohm series capacitor in which case it would be about 2.5:1. Personally, I would just put an autotuner at the base and use it on 40m-10m.

http://www.w5dxp.com/vert4010.htm
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
KF6OCI
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« Reply #2 on: May 27, 2013, 01:26:45 PM »



  Thank you,

     Dan
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WB6BYU
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« Reply #3 on: May 28, 2013, 06:56:22 AM »

The antenna itself should work well.  You do have to watch the losses in the
coax if you are feeding it with coax to a tuner in the shack.

I built a similar antenna some years ago, using a top hat to make it resonant
on 30m.  I designed a simple matching network for the base that would allow
it to work on both 20m and 30m with a low SWR.  It's possible that I still have
the notes somewhere, otherwise I'll have to invent it from scratch again.  And
I might actually have an application for such an antenna soon...

Usually it isn't too difficult to design a matching network for just two bands,
especially when the antenna is resonant on one of them and the operating
bandwidth is fairly narrow.
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K3VAT
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« Reply #4 on: May 28, 2013, 02:52:59 PM »


EZNEC says the SWR will be about 11:1 unless you neutralize the inductive reactance with a -j225 ohm series capacitor in which case it would be about 2.5:1. Personally, I would just put an autotuner at the base and use it on 40m-10m.


Cecil, I got a different X[L] ... closer to +j400 ohm; what did you use for the vertical radiator diameter?  What did EZNEC say was the Z feedpoint equation (both R and X)?

thanks, Rich, K3VAT
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RFRY
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« Reply #5 on: May 28, 2013, 04:53:36 PM »

EZNEC says the SWR will be about 11:1 unless you neutralize the inductive reactance with a -j225 ohm series capacitor in which case it would be about 2.5:1.

Just wondering... what was the reference used for the SWR value that was reported when the inductive reactance of that radiator was neutralized?

Normally when an antenna system has zero reactance at its feedpoint it is considered resonant, even if that resonance required an external reactance to produce it (not that such externally-produced resonance always produces the same radiation efficiency as possible from a radiator that was intrinsically resonant).

Whether or not the non-reactive term of that antenna system input impedance matches the Zo of the transmission line at its feedpoint is another issue.
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W5DXP
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« Reply #6 on: May 28, 2013, 07:20:43 PM »

what did you use for the vertical radiator diameter?  What did EZNEC say was the Z feedpoint equation (both R and X)?

I used the EZNEC VERT1.EZ and changed the length to 23 ft. It is a one inch radiator over mininec ground. The feedpoint on 30m was about 35 ohms and the feedpoint on 20m was about 125+j225 ohms.

... what was the reference used for the SWR value that was reported when the inductive reactance of that radiator was neutralized?

It was the above 125+j225 ohms on a 50 ohm SWR scale.
« Last Edit: May 28, 2013, 07:23:47 PM by W5DXP » Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
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