PA0FBK 2m/70cm coaxial antenna
I have been looking for a design for a portable dual band 2m/70cm antenna and found the PA0FBK coaxial dual band antenna. I followed the design exactly and double checked all dimensions but it hasn't worked out as well as expected. Another web page was found with alternative dimentions which I will be trying. Below are some images from the construction and the analysis using a nanoVNA.
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Coaxial stub
Coaxial stub
Coaxial stub protected with heatshrink after an insulator was placed between the shield and inner conductors. I used thin strips of a bare section of PCB.
One of the sections with heatshrink for weather protection
Top of antenna with loop to support it from.
Antenna best swr at VHF
Antenna best swr at UHF
VHF frequency sweep for swr
UK 2m frequency sweep showing FM calling frequency.
I'm not so happy with this.
UK 70cm frequency sweep showing FM calling frequency.
I'm happy with this.
A RAYNET group in the UK have done some experiments with this antenna and have different dimensions, a shorter top section, so there will be a version 2.
The antenna was acceptable at 70cm but 2m the s.w.r. was too high. Looking at the RAYNET dimentions, on this page, only section A was significantly different and so I decided to reduce this.
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Reducing section A to 345 mm changed the s.w.r. at 145.500 MHz from 2.69 to 2.72. and minimum s.w.r. appeared to be at 141.400 MHz rather than 142.200 MHz so reducing the antenna appeared to have the opposite effect to that expected.
VHF sweep from 130 to 150 MHz
PA3FBK - minimum 142.200 swr 1.11 - spot 145.5 swr 2.69 - IC-7100 radio 145.425 2w swr 1.5 - 1 turn click on ferrite
RAYNET - minimum 135.400 swr 1.09 - spot 145.5 swr 1.65 - IC-7100 radio 145.425 2w swr 2.0 - 1 turn click on ferrite
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UHF sweep from 430 to 440 MHz
PA3FBK - minimum 440.200 swr 1.24 - spot 433.500 swr 1.83 - IC-7100 radio 433.550 2w swr 1.0 - 1 turn click on ferrite
RAYNET - minimum 427.800 swr 1.06 - spot 433.500 swr 1.20 - IC-7100 radio 433.550 2w swr 1.3 - 1 turn click on ferrite
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I'm not having much luck with this antenna. Web pages that I have found say it is easy to make but following the dimensions closely just doesn't work for me. I need to do some more research into this design to fully understand the principles and decide what to adjust.