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Antenna design

Design requirements

 

 -  Simple construction should allow fast and easy deployment

 -  Support most relevant for portable DXing bands

 -  Seamless band switching, loose at less as possible operating time on the summit

 -  Good DX performance, radiation under low angles and lowest loss possible

 -  Antenna should handle 100W TX

  

Principal of operation

 

To satisfy these requirements I decided to settle on vertical antenna design. There are multiple approaches to build multiple band vertical, but I decided to follow trapped approach to minimize amounts of antenna wires needed and simplify matching at the feed-point. Antenna is classical ¼ wave length vertical where the radiator wire is spited electrically by the traps.

 

To achieve lowest losses in a ground plane system and to maximize radiation under the low angles the feed point and radials were elevated. Additional benefit of such approach is that there is no need in a large amount of radials, antenna is just has two of them. 

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I settled on well known by SOTA/POTA community dx-wire/sotabeams 9.6m long “travel mast”, but antenna can be successfully used with another similar 9.5-10m long glass-fiber mast. Additional advantages of vertical antenna comparing to dipole used with the same mast is that feed point is placed lower and shorter coaxial cable can be used. It’s possible to utilize the full length of the light duty mast as thin top antenna sections are supporting just lightweight wire wrapped around. It’s possible to redesign antenna for the  usage with the shorter 5.5-6m long mast and get even wider bandwidth due to higher losses in a ground-plane system :), but I decided to prioritize the performance and selected longer but not most light weight option.

Two versions of antenna were simulated in MMANA-GAL for 15-17-20 and 17-20-30 meters bands and two very first prototype antennas were built during the spring 2022 in a preparation for JW SOTA DXpedion in May 2022. For 17-20-30m antenna radial system is located closer to the ground and experiments showed that I could achieve better SWR at 30m band by using 4 radials instead of 2. Personally I consider 15-17-20m antenna design more successful.

Measured SWR

SWR measurements were taken just after the antenna were tuned with SARK110 antenna analyzer.

Some observations

Antenna requires some ground under the radials, so it’s not suitable for usage on the top of the high tower, SWR would be way off.

Traps make antenna electrically shortened on the band with a lowest frequency and this makes bandwidth of antenna narrower.  Normally SWR is under 2 at all three bands, but not much extra bandwidth left beyond the band edges at 20m band. In some environments (wet, uneven terrain, nearby trees) SWR curve can be shifted up or down and SWR can slightly exceed 2 at the band edge. For 15-17-20m version I observed this at some summits at 17m and 20m bands, no such issue at 15m. At the same time the same could be observed with multiple different antennas in the field.

Experience

Antenna was successfully used during Svalbard DXpedion and during multiple summer/winter activations at home area at Norway. Antenna brought me and some my friends multiple exciting DX contacts on the SOTA summits and I decided to make antenna design manufacturable and mechanically durable and share experience with SOTA and portable radio enthusiasts community.

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