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Hitec 2.4 ghz attenna mod death of an easystar

PostPosted: Thu May 13, 2010 10:27 pm
by sleepercivic88
So, I broke off my original antenna that came with the Hitec 2.4ghz module from removing it repeatedly to use my 72mhz modules for other planes. The wire got fatigued where it enters the antenna portion.

So, instead of claiming defeat I decided to put a new antenna on the module. I live about 10 min away from sparkfun here in colorado and put in a order for a local pickup. Here are the parts I got.

I did a low power range check with the antenna and was able to walk about across the full size soccer field until I started to notice slight glitching. I thought this was pretty impressive. Counted it to be about 150 paces or so. So I threwh up my easystar and flew it around normally and was watching for cutouts and so forth. I began a high climb feeling pretty confident and on pure coincidence the wingeron mod i did about a year ago broke off the easystar and the plane lawn darted towards the earth. Actually, to be honest I dont know what happened for sure. Its possible that the prop broke mid flight. It basically looked like my plane got shot with a shotgun out of the air. Welp, Im betting no one is ever going to want to do this mod after reading this post.

Do you think its possible that the 50ohm impediance of the antenna I bought is different than the stock hitec antenna. From what I has seen 50 ohms is pretty univeral for the 2.4 ghz antennas. Im going to test my set up more once i get a partner to tell me if its still working from a mile away.


Re: Hitec 2.4 ghz attenna mod death of an easystar

PostPosted: Thu May 13, 2010 10:53 pm
by RCModelReviews
A couple of things...

First, you've got a 5-6dB antenna there which will have a much bigger null-point at the tip.

You should have got a 2dB antenna which provides a far more even radiation pattern.

The reason that the larger antenna has higher gain is because it is designed to be used in situations where it's mounted vertically and the systems it is talking to are positioned at pretty much the same height. To get the gain, these antennas redirect energy that would otherwise be radiated at a high angle (ie: towards the tip of the antenna) to a lower angle -- ie: out the sides of the antenna. If you think of the radiation patter as a ball -- as you increase the gain it's like someone sitting on the ball and flattening it into a disk. The higher the gain, the flatter the disk and the more energy goes out sideways rather than upwards, along the axis of the antenna.

However, having said that, the Hitec system has a lot of headroom so I wouldn't have expected the lower signal being radiated towards the tip of the antenna to be a problem -- you'd still get more than enough range for a small model.

The other thing is the UFL connector which connects the antenna wire to the circuit board.

These are frail little things and when you're removing the old one you have to be *very* careful to lift it evenly and not just pry up one edge with a screwdriver. If you don't lift them evenly then you can damage them and that may produce a reduced output.

Even when inserted and removed correctly, these connectors are only good for 10-20 insert/remove cycles before they start to get flaky.

The first thing I'd do however, is find yourself a lower-gain antenna -- a shorter dipole with 2dB of gain and repeat your range check.

Chances are that the problem wasn't related to the RF connection at all but at least you'll have more peace of mind that way.

Re: Hitec 2.4 ghz attenna mod death of an easystar

PostPosted: Thu May 13, 2010 11:32 pm
by sleepercivic88
Yeah I am well aware of the dead spot you can get with a higher gain antenna. I looked at the data sheets for both the 2.2dBi and the 5dBi antenna. The data sheet shows the negative effects on the E plane of the antenna really are not that much different between the two. The 2.2dbi was a bit broader on the E plane but I would not call it significant. I have some extra small wifi antennas laying around i might do some range checks with. Im pretty convinced it was not a fault with the radio system because the plane went down extremely violently. It appeared to be shot out of the air by a gun. The wing sheared off mid flight right after the spar. If it was a radio problem the failsafe would have just let the easy star glide. Possibly the prop broke. On the sparkfun site they have datasheets for the antennas.

Those connectors seem pretty fragile. I did a range check with no antenna attached to make sure the connection was true. The system powered down and no antenna got about 25 meters or so before faulting out.

Re: Hitec 2.4 ghz attenna mod death of an easystar

PostPosted: Thu May 13, 2010 11:58 pm
by RCModelReviews
You should never run the transmitter module without an antenna.

When you have no antenna there's nowhere for the RF power to go, so it is reflected back into the RF output stage and can cause both overheating and excessively high voltage swings. Both of these can cause damage to the devices used in this part of the system.

Although we're only talking about 100mW of power, the devices concerned are *very* small so the heating effect alone can be significant.

So long as your system still range-tests okay you'll probably be alright but I'd avoid any no-antenna operation in future.