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 
for 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 
The 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.
Entries (RSS)
May 22nd, 2008 at 12:06
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
May 22nd, 2008 at 17:59
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
January 2nd, 2009 at 09:14
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
January 2nd, 2009 at 15:51
No not yet. But do you really mean 20m ?
And with current temperatures of -7 ° Celcius I’d rather stays indoors…
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