Posted by
Bob K6UJ on
Jan 01, 2012; 8:37pm
URL: http://elecraft.85.s1.nabble.com/Single-Band-40-Meter-Dipole-tp7141541p7142536.html
Phil,
LADDERLINE VS COAX
I would use coax feed and a 1:1 balun right where the coax connects to the dipole.
I have used window line and 450 ohm home made ladder line on various antennas over the years and grew tired of working on
feedline issues. "Theoretically" with balanced feedline there will be no radiation from the feedline but the surroundings affect the balance
from the antenna down to the shack. I have used a 1:1 balun as you mentioned at the end of the ladder line and then into the shack with
coax and could solve the RF in the shack issue but the toughest issue to solve was the feedline reradiating electrical noise. The "not so balanced"
feedline attracts noise like a big magnet and reradiates it. Not so good when you are trying to copy weak DX and you have an additional s unit or so of electrical noise.
By experimenting with additional RF choking and grounding as learned from Jim K9YC in his excellent
webpage I could eventually reduce the reradiated noise down to a very low level, however not as low as coax feed with a 1:1 balun at the antenna.
Each new ladder line fed antenna required different RF choking measures. Its not just a hobby, its an adventure, hihihi.
FAN DIPOLE OR SINGLES
I have had very good results with a fan dipole for 80 and 40. The trick is to have enough spacing between the legs of the dipole. My last configuration
had 3 feet separation between the end of the 40 meter dipole and the 80 meter dipole. Much better than lossy traps ! I have tried fan dipoles with
other bands in addition to 80 and 40 and never could get tuned right, one band affects the others, etc.
LOOP ANTENNA ?
I am a big fan of the delta loop antenna. A full size delta for 80 can be used on all bands (80 thru 10) very well with an antenna tuner and depending on the feed point can provide either horizontal or vertical polarization. It needs only a single center support of 50 feet or so and about 100 feet end to end. The lower leg can be zig-zagged to reduce the lower leg dimension without affecting the performance. It is basically a triangle with the apex at the top. It can be "flattened" so the bottom horizontal leg is longer than the other two legs without impairing the effectiveness, actually a flattened delta provides almost a perfect 50 ohm match. It also can be sloped, doesn't have to be strung exactly vertical. The horizontal wire across the bottom can be very low, mine is 6 feet high across the top of a wood fence on the side of my property. A delta gives a nice low radiation angle for DX and has gain over a dipole. It can be fed with either ladder line or coax, gu
ess which one I use ? ( hihi. ) With an antenna tuner an 80 meter delta will cover all bands and will out perform dipoles, particularly on 80 and 40 meters where you would have to have 100 feet or higher dipoles to give a low radiation angle. Comparing the delta to individual dipoles I have found it to be a quieter receive antenna and on 40 and 80 just outstanding, I can copy DX signals on the delta which can not even be heard on the dipole. On the higher bands, above 80/40, unless you have a 70 foot or so tower, the delta will outperform the dipoles. With just a single center support if you have the room for the lower leg it is a great all band consideration.
REFERENCES
I highly recommend these two sources for antenna design and RFI / matching information.
(My ON4UN book is very dog eared. It is my bible on antennas. Read the chapter on loop antennas and you will never be the same. hihi. )
1. Low Band DXing by ON4UN
2. A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing by Jim Brown
available on his webpage
http://audiosystemsgroup.com/RFI-Ham.pdf73,
Bob
K6UJ
On Dec 31, 2011, at 9:14 PM, Phil Hystad wrote:
> OK, lots of discussion on open wire feeders and other ideas so I thought I would throw out a question to soak up some of the Elecraft community knowledge and wisdom.
>
> Given a 40-meter dipole antenna, up about 50 feet, what is the best feed configuration for single band usage to minimize loss and minimize common mode currents and radiation from the feed line?
>
> I am thinking of building such an antenna and I would have no plans for using it on any other band but I would like to use it on the full spectrum of 40 meters with primary focus on the low end for CW.
>
> My current idea is to cut the antenna for 7.1 MHz but I am not sure about the best plan to feed it. I am thinking of 450 ohm window line (aka ladder line) to a 1:1 balun and the coax the rest of the way into the shack. Or, how about 300 ohm twin lead into a 6:1 balun with coax the rest of the way into the shack? Would this be a significant difference? Or, some other combination?
>
> Or, maybe a 1:1 balun right at the antenna feed point with coax the remaining distance to the shack?
>
> Any comments or suggestions?
>
> I am thinking of also raising up two other dipoles. I have room to put up 80, 40, and 30 as separate dipole antennas -- it is actually a little bit more awkward to do a fan dipole so I am leaning on not doing that.
>
> 73, phil, K7PEH
>
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