Low Power / Short Range Transmitter Antenna

Choosing an RF Module? What modules work? What is compatible with the 9x?
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aesmith
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Low Power / Short Range Transmitter Antenna

Post by aesmith » Mon Dec 03, 2012 1:54 pm

Hi,

I'm planning to modify my 9X to make it switch-able between my normal internal Frsky, and whatever module is installed (initially Flysky, future maybe DSM). The purpose of the module would only be to support short range indoor of back garden models, like the little helicopter I've just got.

Is there a different sort of aerial (antenna) that I could use, maybe less obtrusive than the normal 2.4 one? Could I just use a tail like used on receivers, maybe even keeping that internal to the module housing?

Thanks, Tony S
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Tony S


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ShowMaster
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Re: Low Power / Short Range Transmitter Antenna

Post by ShowMaster » Mon Dec 03, 2012 5:14 pm

I think the short answer is yes on the short tail as you call it antenna. You may not find one with the reverse sma type connector on it but you could make one out of an extension coax. You want to cut the shield off at the end leaving the same length of insulated center wire as your RX has.
Another way is to buy a spare 2.4 TX antenna and just remove the internal coax. The main issue is putting a little distance between the antenna tip and your module so you don't get any RF back into the module I'm thinking.
There is also a dummy load type antennas. A 50 ohm CARBON 1/4 watt resistor across the output connector on your module will make the module happy and still radiate some RF if in the open and not a shielded metal cavity of any kind.
As for keeping the antenna internal,
Yes you could just wrap it around the upper case half. I do however think it could cause some RF issues? Most of the newer TX's being showcased with no external antenna but it's all in a large molded case handle.

I must ask?
Why not use the full 50Mw power out?
Being digital it should play nice with most modern devices.
SM

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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Mon Dec 03, 2012 7:31 pm

ShowMaster wrote:I must ask?
Why not use the full 50Mw power out?
Being digital it should play nice with most modern devices.
SM
Thanks. It's really just for tidiness, not having to have an antenna mounted on the back sticking out of the module housing. I deliberately did my Frsky install using the Hack module and mounting the Frsky antenna in place of the original Turnigy. Since I'm going to be working with a disassembled module to start with, I thought I'd look at alternatives.
----------------------------
Tony S

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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Thu Dec 06, 2012 7:37 pm

The first picture on this post shows what I DON'T want sticking out of the back of my transmitter.
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Tony S

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ShowMaster
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Re: Low Power / Short Range Transmitter Antenna

Post by ShowMaster » Fri Dec 07, 2012 4:02 pm

Seems like only yesterday and for 50 + years before that we all had 37 in antennas sticking out of out of our tx and had a 37 in wire trailing behind our planes. Now we worry about a 30cm RX antenna and a 6 in "Rubber Duckie" antenna sticking out of our tx.
Progress?
SM



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aesmith
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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Sat Dec 08, 2012 10:08 am

ShowMaster wrote:Seems like only yesterday and for 50 + years before that we all had 37 in antennas sticking out of out of our tx and had a 37 in wire trailing behind our planes. Now we worry about a 30cm RX antenna and a 6 in "Rubber Duckie" antenna sticking out of our tx.
Progress?
SM
My 35MHz aerials used to stick out of the top of the transmitter, and could be telescoped down when not in use. They didn't stick out of the back making it impossible to put the set down without the weight coming on the connector. I'm not sure why you ridicule my desire to avoid that, I've seen plenty of previous criticism of that design for example on the early Frsky Futaba modules.

On my 9X I put the Frsky aerial on the top which is fine. I can't use that location for interchangeable modules that I want to have available.
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Tony S

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Re: Low Power / Short Range Transmitter Antenna

Post by ShowMaster » Sat Dec 08, 2012 4:12 pm

Sorry I upset you. I've always used a tx folding stand attached to the tx handle. I've never laid my case back on the table or ground. It's always almost vertical.
I also use Frsky DJT modules that have a nice design to avoid antenna damage if the tx back is flush with a surface and stock handle.
As for self contained replaceable modules, there's not many antenna placement options. We criticize Turnjgy for their tx mounted antenna setup but it does keep the antenna off the back. It also makes using other modules difficult.
Being creative however is why we're all here and mounting or relocating the 2.4 tx antenna to suit ones needs is of course do able.
All the necessary antenna parts and pre made coax cables with critical 50 ohm impedance are available to do your own thing.
I can see where the orange DSM modules antenna placement can be a problem when the tx lies on its back and stock handle.
I believe my after market tx fold out stand still protects the antenna when the tx is sitting on the bench but I'll recheck it today.
As I first said, I'm sorry I upset you. Being a ham radio operator I deal with so many "large" antennas even on handi talkies I never gave our 2.4 a second thought. Please post your antenna modification pictures of your final configuration that you like. We can all learn something.
SM





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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Sat Dec 08, 2012 4:43 pm

ShowMaster wrote:Sorry I upset you.
Hey no problem! You gave some info about possible configurations in your earlier post before we got squabbling about why I feel the need to do this. Anyway I don't have the bits yet, but I'll post whatever I come up with whether good, bad or ugly.
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Tony S

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Re: Low Power / Short Range Transmitter Antenna

Post by ShowMaster » Sat Dec 08, 2012 6:31 pm

aesmith wrote:
ShowMaster wrote:Sorry I upset you.
Hey no problem! You gave some info about possible configurations in your earlier post before we got squabbling about why I feel the need to do this. Anyway I don't have the bits yet, but I'll post whatever I come up with whether good, bad or ugly.
I did buy a few right angle "elbow" 50 ohm adapters made by Frsky so I could redirect the antenna direction.
They however stick out more but will keep the antenna from touching a surface.
What I've done for club members that I've done the Turnigy tx er9x mods for in order for them to reuse the stock module, is to mount it in an attached plastic box on the modules back. The module ends up looking like the Frsky module with the antenna connector pointing flush with the modules rear surface. The antenna is also flush and can be angled in several directions. When laid down, the added antenna box rests on the surface and not the antenna connector, very clean and mechanically safe.
My main objective is to not have extra coax connectors that can become lose or fatigued and affect the 50 ohm match to the antenna system, thus reducing power out efficiency because of SWR.
A common failure at my field of radios using this type of clip on antenna attached to a screw on coax connector (Spektrum DSM module).
Of course the stock antenna itself can have the same issue if lose.
What ever we do we must remember that we're only starting with 50-60mw of power. In a perfect 50 ohm rf world all power from the tx IC will get to the antenna and be radiated in a 360deg donut shaped pattern. The most common antenna systems pattern. The donut hole of dead rf will be the antenna tip.
Any and all modifications we make to the antenna systems 50 ohm balance will reduce the power out or radiation pattern. Even too tight a coax bend( kink), or a crushed coax will reduce power out.
Every connection to the antenna/coax path, connectors, coax with different coax velocity ratings, will affect the actual radiation or worse, the antenna length. The ~30cm tip length discussed so much.
In the end it all works pretty well at 100 meters. At 300 or more meters is where it gets exciting and really tests the antenna systems design.
RF is magic and most of the time we can pull the rabbit out of the hat, and then again... gravity wins.
Have fun, always use a junker plane for R&D. Thanks to R&D many of my planes have become junkers so I have many choices now it seems.
SM






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aesmith
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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Sat Dec 08, 2012 6:43 pm

ShowMaster wrote:In the end it all works pretty well at 100 meters.
That'll do me, I don't need anything like that range. My modules will only be for indoor helicopters, my current one that needs Flysky and my planned upgrade which will need DSM.
----------------------------
Tony S

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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Mon Dec 10, 2012 7:11 pm

Right, I know what I'm working with for the first option. I have a Turnigy/Flysky module replaced by a discerning owner presumably switching to Frsky. The coax has been cut leaving me around 50mm still soldered onto the board. How do you think that would work if I stripped back the last 30mm of that coax? The active bit could be aligned so it's horizontal across the inside of the module case. Worth a try? I must say I'm holding myself back from just trying it as it is, but I need to add a switch to change between my internal Frsky and the module slot before I can really try it.
Image
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Kilrah
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Re: Low Power / Short Range Transmitter Antenna

Post by Kilrah » Mon Dec 10, 2012 7:30 pm

Yep, should work. Strip the outer insulation and ground mesh of course, leaving the inner insulation.

aesmith
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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Mon Dec 10, 2012 7:36 pm

Kilrah wrote:Yep, should work. Strip the outer insulation and ground mesh of course, leaving the inner insulation.
Thanks. I'll report back once I've tried.
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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Wed Dec 12, 2012 7:22 am

Kilrah wrote:Yep, should work. Strip the outer insulation and ground mesh of course, leaving the inner insulation.
That seems to work OK. I've only tested it indoors so far, but I'll try a range test when I get a chance.
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Re: Low Power / Short Range Transmitter Antenna

Post by ShowMaster » Wed Dec 12, 2012 4:16 pm

Because of the close proximity to the tx with your mounting scheme you may not have the max long distance range but line of sight may still be fine. Anything close to the antenna may detune it enough to reduce power out due to SWR. Your hands or wiring can do that and that's why the antenna tip is usually on the end if a 6" rubber antenna, to keep it In the clear. Radiated RF can also get into the main board circuits and mess with them.
Since this is a 2 way system one path may be reduced? RF is magic and when in doubt give it a try.
Polarization is not that critical for line of sight. Not cutting the antenna 23 to 30 cm range and not tuning it could and usually does decrease the power radiated out. Again do a range test.
I think many TXs in the future will have the antennas mounted in an exaggerated handle and maybe one horiz and one vert. No more antenna sticking up. I see so many broken off with the bare tx antenna waving in the breezes or in a ball of tape! New look for RC it seems ha ha.
Give it a try after a good range check and you'll know your good to go.
SM

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Re: Low Power / Short Range Transmitter Antenna

Post by aesmith » Wed Dec 26, 2012 4:25 pm

I've range tested up to around 120 feet, that's as far as I can reasonably go with my tiny model and still see whether it's under control or not. A cruder check shows the model's bind light goes off at around 90m with the transmitter left standing on the ground, and holding the model at shoulder height.

That looks like more than sufficient range for a very small and light indoor model, but I would like to see if there's scope to optimise further while still keeping the convenience. Is there any benefit on this sort of aerial in having a 31mm whisker connected to the screen, and extended at 180degrees to the core whisker? An arrangement like that could be led through the front of the module and aligned horizontally, pretty much flush to the back of the transmitter.

Ironically I'm probably going to be upgrading to a collective pitch helicopter, and that will almost certainly need DSM2, so I'll have to do this all over again. However that will be short range as well, anything full range is going to be getting Frsky.
----------------------------
Tony S


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