Home made Dipole antenna

[rwanttaja]

Ian, the challenge I think you may have is that all those cross members (if they are conductors) and cables is that they form a distributed, but messy, ground plane, whether you want them or not. Running one leg of your dipole down among this "mess" will affect the performance of your antenna, both the loading that affects how much power gets out and the antenna pattern.

Jim Weir may have some other suggestions but in my opinion you might want to consider constructing a ground plane for a 1/4-wave antenna in your turtle deck and take the drag hit of an exposed antenna. Also, you mentioned constructing the antenna of piano wire. The bandwidth (tuning range) of an antenna is affected by the diameter of the radiating element. A piano wire antenna will work well at one frequency and less well if you tune to other frequencies in the VHF aircraft band.

I actually did what Al suggested, a while back. I fly a Fly Baby, an all-wood airplane similar to the Boredom Fighter. It had the horizontal dipoles I mentioned, then I built a quarter-wave antenna that bolted-on under the baggage shelf.

http://www.bowersflybaby.com/stories/antenna.htm

Cost was about $20. This worked pretty good until last summer, when it didn't. Plan on building another one.

Also...are the interplane struts on a Boredom Fighter wood or steel? If they're wood, they might work for the antenna.

BTW, a friend used to have a Boredom Fighter...here's a video we shot of our two airplanes:

https://www.youtube.com/watch?v=zf3QEsHWkjA

Ron Wanttaja

[IanLaw]

Many thanks for your advice, video and comments, Al and Ron. I suspected that the cables would likely not help ( rats!).

The inter plane struts are laminated spruce "I" section, but aren't long enough to accommodate the dipole, unfortunately, - this had crossed my mind. They are pretty complicated to make and I haven't got further than making a ply template, as per drawings. I have some good spruce ready for when I pluck up courage enough to start work on them. I'm sticking with these because I prefer the look of them.

Thanks again and I'll keep following the forum(s) for any more comments.

Ian

[IanLaw]

Forgot to confirm that the cross members are spruce, not metal and they only represent a small physical "nuisance" to the actual location of the antenna. If I were to drop the antenna low enough to install it completely inside the fuselage and have it a little offset, with a bend in the lower 8" then it would fit & clear most of the cables. Fitting the aerial inside would also be more appropriate to the style of the aeroplane.

The author of the web page whose instructions I've been following installed his near the tail of his Weedhopper ( tubular metal, of course ) and it appears that worked very well, but I guess there are so many variables, it is difficult to be certain.

Ultimately, the U.K. LAA "Inspector" who is overseeing my project will have to be happy with the installation, but it will be useful to have all the information given to me here - thanks again -, to convince him that I've put some thought into the whole thing!

Ian

[rv7charlie]

A quarter wave with ground plane might be much more compact, and easy to make. The ground plane can be 3 or 4 equally spaced wire 'whiskers' radiating from the base of the antenna. Half the height, and width slightly less than the length of a dipole.FWIW,Charlie

[IanLaw]

Thanks Charlie and I guess I'll have to discuss all options with my LAA Inspector, bearing in mind all the useful suggestions.

I suspect the dipole will perform best, from all the reports I've read of actual usage but if I can't squeeze it inside the airframe, the 1/4 wave would have to be the next choice. I think I'll try the dipole first, with a bend in the lower "arm" to clear the bottom of the fuselage, but make up a 1/4 wave/ground plane and all the fittings needed to change over, as easily as possible, with fabric in place, so I can swop the aerials later, if I have to.

Thanks again!

Ian

[Jim Heffelfinger]

Might try this source [url]http://www.rst-engr.com/ [url] Added to my vertical stab and along the "decking".help want to attach a pic.....

[IanLaw]

Thanks for the link Jim - much appreciated.

Ian