It's been a while since I have contributed to the forum. I have been spending quite a bit of time working bugs out of my Tricopter build but have learned much in the process. What I wanted to share with you was my revision of the "IB Crazy Cloverleaf Antenna for 5.8ghz." Here is what I did:
At these frequencies many things are critical to the correct performance of an antenna. Ghost capacities are a real problem so you have to watch what you do. Certain materials also cause resonance problems so you need to be mindful of these. When in doubt Silver plate all conductors. It will stop a mountain of problems with SWR. Gold is even better if you have it but that is out of the scope of most modelers bags.
What I decided is that, while I liked the IB Crazy Cloverleaf, I never have liked antennas that can flop around on the end of a coax. I prefer a rigid mount and coax is not that. The option exists to make your coax very short but then you are dealing with working in very small areas. Again...not for me. So, I decided to make a rigid coax out of some RG-174 that I had on hand. Here is how I did it.
What I did was get a package of KS brand 3/32 copper tubing with a .014 wall thickness at the local hobby shop. The internal dimension of this cable is the exact outer dimension of RG-174. I cut about a 3 inch section of this to make my antenna.
Next I took apart a screw together SMA connector and looked at the inner parts. If you look at the coax shield retainer you will see that it is built like a long tapered cone. This is important because it holds the outer shield in a bind to make up the ground. In my case I had to solder the copper tube to this and I wanted a good, solid, joint. What I did was to take an automatic (push type) center punch and flare the copper tube to fit this taper. A few good snaps and it fits like a glove. I suppose you could also turn down a piece of steel rod in a drill press and do the same thing. Then use the drill press to press the rod down into the copper. Either way will work. It is a good idea to polish the copper after you are done and silver plate it.
Once you have the copper tube swaged and Silver plated it is time to solder it to the SMA shield cone. I held mine in a small vise and set the SMA cone on top. (taper fits into the taper on the Copper tube) Use some good quality flux when you do this and electronics grade solder. Once done you should have a really solid antenna base (ground). This will be your new shield for the RG-174.
Now you want to strip out the center conductor from the RG-174. Make it long enough to stick an inch past the end of the tube. Make up the SMA connector like you would do for a coax assembly. Don't forget to use the Teflon insert or your antenna won't work! I also put heat shrink on over the copper tubing once I assembled the connector. I ran one piece of clear down to the jam nut and then put white on over the whole connector and the copper tube. You will need to expand the shrink a little with pliers on the connector end before you slide it on. If you pick a size that just fits the connector it may not shrink enough. Most only shrinks 50%.
After all of this just make up the IB Crazy Cloverleaf as it is described in his text. It's an easy build and if you follow the directions fairly foolproof. Personally I would hedge away from using MIG wire as he suggests. The Iron in the wire can and will detune things just slightly and you will get a much better match with copper or Silver Plated Brass. (this is what I used) It is much stiffer than Copper and resists bending out of shape.
Now...Here is my second change to the design: Go to the hobby shop and pick up some white plastic sheet. (Note: you can also use an old Clorox bottle) Cut this up into confetti and put it in a small glass jar with a lid. Next, go to the hardware store and pick up some Methyl Ethyl Ketone and pour it over the plastic chips. (This stuff is nasty and is a Carcinogen so be careful with it.) It is advisable to do this out in the open air. Let this mixture sit for a few hours and it will dissolve into paintable plastic. (This was used as a cement for the original Dubro Whirlybird body).
Use this mixture to paint the exposed surfaces of the antenna. You can do this with a normal model brush as the mixture is fairly thin to apply and you will need to put on several coats. Let the antenna dry between applications and you will have a "Factory Finish" on your antenna that will make it strong and durable for years of flying. (See example)
Very nice, thanks for that. I like that plastic painting idea, a lot of my aerials are looking a bit shabby these days.
Mmm i thought this, so does this mean that very pretty antenna above just doesn't work very well, or was made slightly larger/smaller to compensate for the coating.
No, It will still work but you just can't coat the elements. The hardline can be coated and all of the solder joints. That wouldn't be a problem. It is just that ghost capacities affect the matching of the antenna at microwave (in fact any) frequency. The problem is that these affects are greatly magnified in the GHZ range.
I don't have a SWR meter that works in that range to verify how much that affect may be so it is better to be safe than sorry and it is all about radiated power here. Even though most of these new 5.8Ghz transmitters come with a built in cooler for the final amp they still don't tolerate an SWR >2 for very long.
I'll know more after I try a few of these.
Actually, I thought about that when I was painting the plastic on the antenna. Not having an SWR meter for that frequency spooked me so I removed the coating from the elements as you suggested. I did leave the hardline coated since I knew that wouldn't affect the match. However I did remove all of the plastic from the center conductor as well. No point in burning up a perfectly new transmitter.
After these repairs I moved on to creating a 5.8 ghz helix antenna. I did remember seeing a quad array in an old version of the Radio Amateur's UHF Handbook years ago for the 432 MC band. I have been looking for information on how the matching network is made but haven't been able the find much.
Thanks for the catch on the antenna.
Yes, It turned out really nice. It almost made me cry to strip the plastic back off the elements but I think Joe (above) was right. It would detune the antenna at this frequency. It looks OK with the silver coating anyway.
I also thought about using an egg beater antenna for the aircraft antenna. I thought maybe an inverted ground plane would hold the signal down to the ground. I know we used those on a lot of military applications at UHF and they seemed to work well as an omni antenna. However, I wasn't too impressed when I read the specs on the gain from online websites.