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Broadband (multiband) antenna by 9A4ZZ

For QRP is very important to have good antenna. Good antenna, in most cases, is big antenna. If you have limited space good solution is Inverted V. With my QRP is possible to work all mode at 40m band, so, I needed antenna with low SWR at first 100kHz at least.

At 9A4ZZ site you can find plans for broadband 80m band coaxial dipole (Croatian language only). I decide to build one but for 40m band. Antenna is very simple for realization. All you need is coaxial cable RG58/U, copper wire and thee pcs of insulators.

Calculation and realization

Lenght of copper wire is calculated using this formula where is k=0,97. For central frequency set to 7030kHz, length is Ld=20,7m. Using same formula and k=0,66, length of coaxial part is Lk=14,08m. I place antenna central point 10m high and copper wire ends at only 1m from the ground.

After testing in my conditions, I needed to extend copper wire about 20cm on each side to reach resonant frequency at 7030kHz. SWR was in range 1:1 to 1:1,1 in first 100kHz of 40m band. 9A4ZZ in his manual sad that is possible to work with this antenna on higher bands. In small part of 15m band SWR was about 1:1,3 and growing fast with changing frequency.

Antenna is very quiet in compare with simple dipol.

 
 

9A4ZZ broadband dipol (Inverted V) fo 40m band

 

After some time I try little experiment. I extended copper wire to full length of 42m. On each end I put about 10,5m of same wire but at only 1m from the ground (see sketch). After little length trimming I set central frequency to 3580kHz and SWR to 1:1. And again, antenna was very broadband.