shortened 35mhz ariel rssi voltage

[dirtyharry]

I've started a wee experiment with a base loaded 35mhz ariel.

The idea is to make a tunned short length ariel so it fits my tricopter and hopefully increases range.

So far I've modified an old corona reciever for RSSI , attached a 75mm ariel , then a inductor and 190mm of pianno wire.

RSSi voltage peaks when I cut the ariel down to 177mm . I'm getting 1.9v with the transmitter in my garage with the ariel up and me in the house . With the old wire ariel I was getting 1.8v but it was more consistant , the new base loaded ariel dips to 1.65v if you move it about the room.

Are these good results or am I better doing it in open space?

also whats the safest minimum RSSI , i'm going to modifie a hobbyking super simple OSD to display it , so I need to know what the safe flying limit is and scale it to suit the osd.

[phibin]

what is the max rssi voltage you can get?
and what is the lowest rssi voltage you get down too before it starts glitching?

that would be a safe limit?


the antenna sounds very interesting, i have thought about something similar in the past.
micron rc sell a 'small' 35mhz rx antenna, but they say its for short range so..

iv never modded a corona for rssi, how did you find it?

simon.

[Coyote]

Interesting project, as far as I know base loaded decreases range not extends it, but I watch with interest.

RSSI values vary with each unit so there is no set figure for best, you have to trim it until you record a drop in value from your highest point, then remake the antenna to the previous length that gave you the highest figure

[BigMouse]

Your basically using an electrically shorter aerial which will always perform worse (ie have lower gain) than a full quarter length whip. It almost certainly modifies the polar pattern as well (No doubt for the worse) which would require some signal strength tests done at the distances expected to fly away from the Tx, and at varying altitude.

Also don't forget that the original is already electrically short! It must be already using some sort of base loading as it is, being that it's connected to an 1/8th wavelength whip. Ie: An end fed whip is a natural 1/4 wave resonator. The purpose of the coil is to cancel out the capacitive reactance of the short aerial so to keep it in resonance.

A 75mm long aerial at 35 mHz would be 1/114th wavelength. Any very close range signal strength tests will be misleading and practically of no use (ie you could probably get similar readings with no aerial at close range).

Whats wrong with letting the aerial dangle below! Ok if no aerobatics are planned...

[dirtyharry]

Hello bigmouse
I know in theory a shorter ariel should give less range however I realy don't want to have a meter of ariel trailing from my tri ,  I know it will get broken on a regular basis and looks terrible in my opinion.

As I currently have the standard 104cm ariel doubled over and its straight from the factory without being "tunned" i'm hoping to improve my existing range , if I get 1000m it will be all I need from a multicopter.

I'm also thinking that if I use shielded wire from my reciever so I can place the active part of the ariel away from motors/esc's etc it might be an improvement on the standard wire which has to come from the centre and be right next to all the noisy stuff, what do you think?



Simon
Modifying the coronas for RSSI was easy , take apart , lift white block that covers the reciever chip and solder a 12.5k resistor to the wire jumper on the top of the chip. This gives you your +RSSI  without pulling it down when you conect to a voltmeter/osd etc.
http://www.rcgroups.com/forums/showthread.php?t=1227487.

[electrotor]

... i'm hoping to improve my existing range , if I get 1000m it will be all I need from a multicopter.

How do you hope to maintain unaided direct visual contact with a multicopter at 1km?
If you settled for something a bit more realistic then you might not have such a problem with a shortened antenna wire.

[dirtyharry]

well tried it out today "on the ground"

with the new base loaded antenna I get pretty much the same result as a 102cm standard ariel.

I taped the TX to the back of a chair (ariel fully extended)  and walked away with the tri running and fpv stuff on , once I got down to 8v rssi on my osd I stopped . My osd is a hacked hobbyking one so the output is scaled up,  at about 7.5v it glitches a fair bit and below 7v the failsafe kicks in.

Got to 460m with both antennas , my video got to 300m.

Anyone know how this would work out with it up in the air a bit ? if I held the tri above my head the rssi went up to 8.2v with the whip but stayed at 8v with the dangling 102cm wire.

Too windy here at the moment to do experimenting in the air and having lost it once don't want a repeat performance.

[phibin]

12.5k resistor ?

whats that for again?

i think you can normally expect your ground range X3 in the air...?

so 460m x3 = 1380m.... you would get your 1000m your after..?
1600m is a mile near enough..

simon.

[dirtyharry]

well heres hoping , sometimes you just have to give it a go.

Ariel did have to be tuned to within a couple of MM so could understand why commercial versions may not work very well. Also tried it with a different inductor and had to make ariel 221mm??

The 12.5k resistor is just to make sure the OSD can't drag the RSSi voltage down.

I used 12.5k as I had some left over , but anything from 10k up should do.

hopefully get a chance to go for a fly tomorrow , no office work for a couple of weeks which is a bonus.

pics of new ariel below

[Coyote]

Looking forward to seeing you progress with this, the results so far are promising

[Devonian]

An experiment conducted by Thomas (Mr RC_CAM).

http://www.rc-cam.com/ant_exp.htm

Nigel.

[dirtyharry]

latest update

have tried it out airborn and its beyond my video range of 600m , definately worth doing if your  doing multicopter as it really is very tidy compared to a 1m trailing lead, can't comment on distances beyond that .

[hexo]

How did you make it DH? Just a diode you say?

[dirtyharry]

10 uh inductor and piano wire (think it was size 4 ??) is what you need

75mm  servo type wire soldered onto reciever where the ariel was
stick bullet connector on the end of the wire
inductor connected to other bullet
pianno wire soldered to other end of inductor (you need to sand the piano wire end to solder it as its coated)
then you need to cut it down to get maximum range with your tx ariel down  (piano wire will be between 160-230mm)

once you've done that
put some hot glue on the inductor to give it some support , also blob on end to stop u poking your eye out.
heat shrink over inductor and piano wire

http://www.ebay.co.uk/itm/Qty-5-Ferrite-Cored-RF-Coil-Leaded-RFC-Inductor-Inductance-Range-1uH-to-82uH-/120852350775?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item1c235c8737

http://www.ebay.co.uk/itm/Piano-Wire-Roslou-3m-long-9ft-10-Extra-Thin-Pianos-Autoharp-Zither-Strings-etc-/261083583160?pt=UK_Musical_Instruments_Sting_Instruments&hash=item3cc9cb52b8

hope that helps

[MarkLincs]

A lot of glider pilots were using 35mhz whip aerials

Would they do the job ?