Home made Dipole antenna

[IanLaw]

I have made a dipole antenna, following the instructions found on the web. This looks great and I hope it works!

The problem is that it is, of course, by definition twice as long as a more usual 1/4 wave aerial and my Boredom Fighter fuselage can't accommodate the full length, which has to be vertically orientated.

The handiest way I can mount it inside the rear fuselage, just behind the seat bulkhead, results in about 16" of the top element of the aerial sticking out above the turtle deck, into the airflow. The arms of the antenna are made from 3/32"(appx) piano wire and I'm not too confident they will survive the 100mph airflow, without whipping about and thrashing my Boredom Fighter to pieces.

First question is: is this likely?

The second question is: could I safely bend the lower element of the aerial back, at its lower end, such that I could mount the whole thing lower down and keep the entire antenna inside the fuselage? Think I have seen somewhere that these aerials will tolerate a short length being bent back to clear the airframe, without losing much performance. This would also save a little unwanted drag and maybe prevent leaks through the turtle deck.

I plan using this aerial to maximise the efficiency of the small hand held radio I have to use in the open cockpit of the Boredom Fighter.

Look forward to any advice.

Thanks. Ian

[FlyingRon]

I assume this is not a metal aircraft. Bending the legs of an antenna will change the radiation pattern slightly but probably not cause enough problems to get worked up about. You may need to shorten the legs a little bit to keep the antenna resonant (not more than 5%). There's probably enough other metal in the airframe that the dipole isn't perfectly radiating anyhow.

[rwanttaja]

It should be oriented as vertically as possible, but it's not an absolute requirement. I've got a horizontal dipole antenna in my Fly Baby and it's worked fine for 30 years.

What you'll encounter is poor performance when the ends of the antenna are pointed at the other radio. Mine is fore-and-aft, thus it doesn't work as well when I'm flying directly at or directly away from the person I'm talking to.

Ron Wanttaja

[IanLaw]

Many thanks Ron & Ron. Much appreciated and I'll consider bending to fit and see how well it works.
, but maybe make two mounts inside the fuselage ( which isn't covered yet ) so I can move it more easily, if I have to.

Regards.

Ian

[rwanttaja]

Ian, the best person to answer your question is Jim Weir of KITPLANES magazine. He takes questions on the Pilots of America forum page:http://www.pilotsofamerica.com/forum/index.phpYou could join and post yourself, or with your permission, I can re-post your questions there for him. I can then post a link, here, directly to the thread, there. You don't need to be a member to read the responses.Ron Wanttaja

[IanLaw]

That sounds great Ron and I'd be grateful if you could re-post my question on the Pilots of America forum. I look forward to any comments Jim might have and would value his expert opinion.

I guess I need to nail this issue down, so I can carry on and cover the fuselage.

Thank you again!

Ian

[1600vw]

I am going through this myself. Not wanting to build an antenna I have been searching and came across this website. He told me he has everything I need for my wood and fabric airplane.

http://www.theantennafarm.com/catalog/

I was told you cut these to length. They come with instructions. I was told none of these need a ground plane.

http://www.theantennafarm.com/catalog/antennas-191/mobile-antennas-192/field-tunable-whips-206/

[rwanttaja]

That sounds great Ron and I'd be grateful if you could re-post my question on the Pilots of America forum. I look forward to any comments Jim might have and would value his expert opinion.

I've made the post, you should find the thread here:

http://www.pilotsofamerica.com/forum/showthread.php?t=76199

Ron Wanttaja

[IanLaw]

Many thanks 1600vw - interesting web site and I have bookmarked it for future use.

Thanks for re-posting my question Ron and I'm amazed by all the useful comments already received. Thanks to all who have replied and offered suggestions.

To keep current, I fly a group owned Aeronca Chief and it has a handheld radio, separate battery supply and 1/4 wave aerial. This normally works fine, but I do have a problem sometimes with my transmissions not being heard too well when I'm more than a few miles away from some stations on the ground. In an open cockpit, I believe I'll need good reliable signal strength to stand a chance of hearing anything in that noisy environment. Hence my attraction to the dipole, with all the claims that suggest it will function better than any 1/4 wave, if correctly set up.

In my Boredom Fighter ( fuselage now ready for covering - hence urgency to deal with the antenna installation ), the rear fuselage is a mess of cross members, control cables, trim cables and more cable for the locking tail wheel. There has to be a large "V" shaped zippered access panel in the bottom fabric, to allow minimum access to the cables, turnbuckles and pulleys etc. This rules out a ground plane anywhere near that.
The "baggage" compartment behind the headrest area is too small and confined for installation of a ground plane there. If I placed a ground plane there - and it could be made big enough, the aerial would either have to go up or down inside the fuselage. If it went upwards, I would have the same or a similar problem with the aerial sticking up through the turtle deck - so I might as well stick with my existing dipole set up?
If the 1/4 wave alternative were used, pointing downwards, it would conflict with all the cables. The dipole can be fitted offset to one side so the lower element clears the control cables.
There is a "tail heavy" issue with most Boredom Fighters and not much leeway with gross weight ( several modifications required by the UK LAA have added unwanted extra weight already). So I want to keep the antenna & coax etc as far forward as possible. The dipole I have made seems light and otherwise very suitable for my needs - as long as I can get away with bending the bottom 8" or so of the "lower" element back to clear the zipped access panel and avoid having the lower aerial wire sticking out under the fuselage ( that would not be a problem at all - but the zipped access in the fabric would get in the way, again.)

I'm concluding that the way to go is to keep my dipole where I've put it ( not yet permanently) and bend the lower wire element back to clear the lower fuselage.

( The web instructions for my dipole have been taken from the following, by the way :-
A HIGH PERFORMANCE AIRBAND ANTENNA FOR YOUR ULTRALIGHT / LIGHTSPORT AIRCRAFT by Dean A. Scott ©August 9, 2006 (revised January, 2011)

Thanks again. I'll continue to follow the posts in the excellent Pilots of America forum & I'll report how I get on.

Regards to all!

Ian

I

[Al Burgemeister]

"...In my Boredom Fighter ( fuselage now ready for covering - hence urgency to deal with the antenna installation ), the rear fuselage is a mess of cross members, control cables, trim cables and more cable for the locking tail wheel. There has to be a large "V" shaped zippered access panel in the bottom fabric, to allow minimum access to the cables, turnbuckles and pulleys etc. This rules out a ground plane anywhere near that. ... The dipole can be fitted offset to one side so the lower element clears the control cables..."

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.

Al Burgemeister

[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