Short 40m dipole antenna, part 2

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 Comments on “Short 40m dipole antenna, part 2”

  1. 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 http://www.dx-wire.de, Type A wire; which is polyurethane coated copperclad steel.
    It becomes invisible at a distance of only 5mtrs
    ’73 Mark,PA5MW
    http://pa5mw.blogspot.com

  2. 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

  3. 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
    9V1JP

    1. 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…

  4. 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

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

Comments are closed.