It was a long weekend and the weather was fine. Sunny and warm. Time to test the 40m antenna.
Note for myself : build or buy an antenna analyzer. Try not to dismantle the whole shack for antenna testing again.

First thing I did at my parents is to solder all amp connections. Then assembled everything together and started searching 40m-short-inverted-V-1for a deasant pole. The mount for the pole is fence wire (correct word ?) waiting to be used (See the pictures).
The first readings between the 2:1 SWR points are 6319 and 6372 kHz. That is about 53 kHz usable bandwidth. Keep in mind that the antenna is mounted near/on the ground and has metal close by.

After tweaking the outer elements I managed to get these outer limits : 7012 and 7065 with the center around 7038. Near the center of the PSK activity. I am not fan of voice communications, I like PSK31 more, good enough for me 🙂

40m-short-inverted-V-3The total length of the antenna is about 8 meter. Will the bandwidth raise if I make the antenna longer ?
Back to the K7MEM site and start modelling. The inner elements are now 2,85 meter. I added 65cm on each inner element. It now has a bandwidth of 60 kHz. We just won 7 kHz….
Not much compared to the length we added. I’ll stick to the first length of 2,85m.
It has a practical use : the mounting pole can be smaller.

Part 3 of this project will be written if the antenna is installed on the roof, probably in june or july.
I’ll first have to build a remote antenna switchbox because I will not run 5 or 6 coax cables in a small ventilation shaft.

6 Replies to “Short 40m dipole antenna, part 2”

  • Hi!

    I have had some similar trapped dipoles in the past and found:
    – small bandwidth
    – outside the SWR 2.0 range the efficiency drops quite substantial
    – height above ground/roof & other structures nearby have a great impact on SWR and bandwidth(again)

    My 2x 12m95 copper wire + 450Ohm twinlead +balanced ATU worked soooo much better.

    Currently I use DX-Wire from, Type A wire; which is polyurethane coated copperclad steel.
    It becomes invisible at a distance of only 5mtrs
    ’73 Mark,PA5MW

  • Hi Mark,

    I agree with your findings, the problem is that I don’t have that much room. Another idea is to make a magnetic loop 3m in diameter. Still a compromise I know. But I like to play with different antenna designs.

    73 Joost ON3JT

  • Hi Joost,
    Have you tried the short dipole on 20m? I live in an apartment at 16th floor (concrete with reinforce steel bar buidling) and have problems with proper RF ground and might have space to put out a dipole not exceeding 9m (end-to-end) and about 0.8 meters away from the wall.
    Best regards
    James Puah

    • No not yet. But do you really mean 20m ?
      I have a friend that has built a shortened 40m rotary dipole and from what he’s says I can assume that I works pretty good. The problem at the moment is that I first have to finish my remote antenna switch and install the necesarry cables. I have now just 1 cable for V/UHF and one for HF. I cannot get fast on the roof to switch antenna’s 🙂 And with current temperatures of -7 ° Celcius I’d rather stays indoors…

  • Hi Joost,

    Do you use a balun or you feed directly the antenna with coaxial ? I have an article suggesting that the antenna needs o 2:1 balun because the feed point has matching impedance of 25Ω.

    73 Dimitri

  • hi joost..i wanna know about your loading coil for this antena..and about the feed point, is it use a balun?

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