Re: Antenna System Multi-band


Paul Christensen
 

I modeled the ZS6BKW using NEC 4.2 using the specified antenna and line lengths, and assumed a 30 ft flattop height over average ground, which is probably attainable in many portable conditions.  Generic 450-ohm windowed line was also assumed.

 

The 41 ft line length works reasonably well on 80/40/20/10m but SWR at the input end of the windowed line is approx. 20:1 in the middle of 75m.  Reducing line length to 33 ft brings SWR under 2:1 at 3.8 MHz.

 

On 15m, SWR is also near 20:1 at 21 MHz. but reducing line length to 31.5 ft brings SWR to less than 1.5:1. 

 

On 30m, SWR is over 100:1 but can be brought down to less than 1.5:1 with 65 ft of line. 

 

So – if I were to use the ZS6BKW antenna, 30m would be discarded and I would cut the main line length to 31 ft instead of 41 ft.  Then, windowed line jumper sections of 1, 2, 4, and 6 ft. are made with sockets/plugs on the end.  Once the antenna is up in the air, make note of the lowest SWR at the transceiver by trying different line jumper sections.  It might take 15 minutes or so, but as long as the result is recorded on paper, switching in/out sections for band changes should be easy.

 

17m/12m modeling shows that SWR can be brought to under 1.5:1 with the line section jumpers. 

 

The closer the input end of the windowed line is to 50+j0 ohms, then the SWR at the input end of 50-ohm coax section will be minimized with varying length.  

 

Given the hassle involved for true all band operation, I would lean more toward using an efficient ATU at the input end of the windowed line that can manage high SWR.  The windowed line can be trimmed in length to find a reasonable complex impedance within the range of the ATU.  If efficiency and antenna field strength isn’t of utmost concern, then consider the EFHW or OCFD. 

 

Paul, W9AC

 

From: QRPLabs@groups.io <QRPLabs@groups.io> On Behalf Of flatpickn via groups.io
Sent: Sunday, May 29, 2022 10:31 AM
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] Antenna System Multi-band

 

This is a better description of the ZS6BKW

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