How to make a led flashing antenna cap

[kaos]

My Sky9x now has two tx modules that can drive 16 ch or 8 ch. I put a 4 position switch to use either DJT or DHT or both with either one as primary. The DJT/other modular antenna always has an antenna attached to. But the DHT on top, when i am not using it, I only put a rubber cap on it.
To avoid accidental flip the switch to the wrong position while the DHT does not have an antenna attached and burn up the DHT. I would like to attach an antenna cap with led/flashing on it as a safety measure.
something like this: http://www.mtrenz.com/details.php?p=619&pid=13700
I had something like that with my old cell phone before. I think the power is from the antenna itself.

any one knows where can I get one for a 2.4 GHz antenna attachment or DIY one?

[huginen]

Have a look here
http://www.555-timer-circuits.com/flashing-led.html

Skickat från min GT-I9505 med Tapatalk

[kaos]

I see that uses a 9V power source. Does the 2.4GHz antenna SMA outputs 9V? what i like to do is that little attachment draw the power from the antenna and can be left on there for extended period of time while not burning up the DHT even if the DHT is left on.

[ShowMaster]

My comments are to be taken as an educational reply. Good question to ask and hopefully my answers have a workaround for us all.

The module final rf stage wants to see a 50 ohm load. A led is not the correct load. Your best option is to find a screw in connector that you can solder a 50 ohm carbon 1/4 watt resistor across and perhaps pot it in a blob of hot glue or silicon.
I'm not sure the DJT module outputs enough rf to actually light a led to start with.
Powering any of your modules with a short or open on the antenna connector should blow the output stage. The led is a diode like device and once the junction voltage is reached the led will light and be a short to one half the AC wave if there is no current limiting. Adding a series resistor limits current but then the led may not light due to the low output power to start with and the added resistance. If this isn't a 50 ohm termination the rf output stage will hopefully shutdown to protect itself or just burn out trying.
You may have bought one from a mall cellphone shop but I'll bet they never warned you it could damage your cell phone. There job was to sell pretty blinking antennas and send you on your way.
Rf is not as easy to deal with as a battery, a switch, and a led. It's a science that many spend years learning about.
Comments and working circuits welcome. We learn from it.

[kaos]

I think it won't be a simple led in there. I just wonder how those blinking led antenna is made and if it can be applied to our RF module. I know I have used those blinking antenna (and it is an active antenna, which I had used on a regular basis. It derives the power from the antenna. I just don't know how it is made.). There may be a 50 ohm load included in those caps (or whatever a cell phone rf load is needed.) That is why this thread. If I just want a 50 ohm cap, there won't be this topic here. just put a RP-SMA with 24 mm of RG316 would be easier.
One possible source of power I can think of would be from a built in inductor type of thing. I think there may be engineers around here may come out with something.

[ShowMaster]

I found this
http://www.candlepowerforums.com/vb/sho ... ne-antenna
Basically it's saying the diode action can or does create harmonics that are radiated. That's probably why they disappeared off the market? That and the fact that external cell phone antennas are no more. The led also takes the power it need to light from the available output power. This alone will decrease the tx power out but still radiates the power left.
I don't think 100mw or less from our TXs is going to light anything. Cell phones are capable of much higher power out and it's controlled by the RSSI the cell tower feeds back basically.
The basic circuit is a UHF diode across a led.
The blinking antenna tip actually started with CB antennas 30 years ago. Those 3 watt boosted up to 100w radios had power to waste.

I did find this on Utube
Large UHF diode and dipole antenna but not built small into a screw on cap. The UHF diode it too large.
It does show it can work.

http://m.youtube.com/watch?v=CNJVhzOjWg ... autoplay=1

[kaos]

the way it works is pretty close to what huginen's post. only it uses a capacitor and transistor/timer to charge/discharge the led. the hard part is to find a micro circuitry that small. But I think I have a potential way of doing this. I will try it out. I have some flashing led has a built in micro chip (has to see it with 20x magnification) probably is doing the same thing. the question really is when this is added, how that affect the load on the tx module and whether it is going to damage the module. Sure you are going to have a reduced RF output. energy will not coming out of no where. But the purpose of this cap is not to blink during real usage of the module. It is a reminder / safety feature that the module is on so I won't burn out the module without an antenna by accident. With the plastic cap I have on there now, if it is turned on by accident, there is no way for me to tell till smoke comes out. The led of DHT is mounted on the top of the Tx surface. But it is too dim under day light. It can be missed easily (been there, done that).
People may ask why not just put an antenna on there and leave it there. Because most of the time I only need the removable module at the back. when this DHT is used, more than likely a 5-7 dB antenna and/or a wifi booster will be added. With most flying only needing the back module, and leave a 7 db antenna on this is just a hassle .
I will come back with the test result. I do have a shelfed 2.4GHz cheap tx for me to fool around.

[Kilrah]

Those cell phone LED thingys are usually contactless, there's just a coil that picks up the RF energy sent out by the antenna. They're not physically connected to the antenna port.
It works because the power on a phone is much higher (typically 2W) than on an RC radio.

[kaos]

yes, I did see some of the antenna picture has a coil in it. So it is contactless? interesting.
Do you know what is the voltage across a 2.4 GHz antenna?
While we are on this topic. in a whip/dipole antenna the center core and gnd shield is separated, no contact. while in a CL antenna the gnd and the center core IS connected via the 'leaves'. How does that work with the same 50 ohm impedance?

[Flaps 30]

Do you know what is the voltage across a 2.4 GHz antenna?

Here you go ----> http://www.rfcafe.com/references/electr ... 2volts.htm

While we are on this topic. in a whip/dipole antenna the center core and gnd shield is separated, no contact. while in a CL antenna the gnd and the center core IS connected via the 'leaves'. How does that work with the same 50 ohm impedance?

Ahhh... That is Voodoo. Just accept that it works by the magic of matching.

[ShowMaster]

Kaos is getting closer to being able to get his ham license with these question! By the time he has the answers to his questions and reads up on all the links he's pretty much here.
Just saying? Antenna theory, understanding resonance and the high voltage it can create across a tuned circuit, or what impedance is vs DC resistance. I think he's pretty much been
studying without admitting it based on his questions so far.
Bravo Kaos.

[thomas9x]

While we are on this topic. in a whip/dipole antenna the center core and gnd shield is separated, no contact. while in a CL antenna the gnd and the center core IS connected via the 'leaves'. How does that work with the same 50 ohm impedance?

A specialty antenna like the Cloverleaf (which is DiY 3-lobe Skew Planar Wheel developed by ibcrazy) is probably not a good place to begin learning about the impedance of loop shaped antennas. Instead, start your research with a folded dipole. That is a dipole with the element ends connected together {somewhat like the Cloverleaf}.
http://www.antenna-theory.com/antennas/foldeddipole.php

And then move on to loop antennas in general:
http://www.mhprofessional.com/downloads ... chap05.pdf

Once you digest all that then move on to the Skew Planar Wheel. According to this article the calculations are discussed in the 1964 ARRL Antenna Book:
http://www.ve3byt.com/SkewPlanarAntenna/

Ahhh... That is Voodoo. Just accept that it works by the magic of matching.

Amen.

- Thomas

[kaos]

I knew all this is voodoo from the get go.
Thx for all the links, looks like I will have something to read for a while. So may be I will be quiet for a while.
Then exploded with more voodoo questions.

and yes, I did see that loop dipole ant while I was making the tiny dipole for the Q-bot micro.
to get a quick answer, advantage/disadvantage for a loop dipole from a simple whip dipole?

Just checked the table form FLap30, looks like the voltage will be around 3+- volt for the Frysky module, perfect for a led. Just took apart the shelfed 2.4GHz Tx, unfortunately, the ant broke of at the UFL connector (can't believe this cheap 10+ dollar tx has a UFL connector). Looks like I need to sacrifice a Frysky ant (with UFL connector) to try this led thing out. thx for the link, it may take me a yr to find that one.

[kaos]

Once you digest all that then move on to the Skew Planar Wheel. According to this article the calculations are discussed in the 1964 ARRL Antenna Book:
http://www.ve3byt.com/SkewPlanarAntenna/
- Thomas

I jump to the SPW after glancing through previous articale (which made my head dizzy/spinning ). It is interesting about the 4 1/2% increase on the half wave length segment and you can stack two SPW together and increase gain. What I did not get is when you stack them together the horizontal elements are lined up together on the same plane or alternating in the middle of each other (45 degree to each other)?