Can I split a RX antenna?

[airmcn_3]

Does anybody know if I can essentially run a "Y" antenna on a standard 36MHz rx? Basically what I want to do is run one side of the Y on the bottom side of the aircraft and run the second side of the Y on the top side of the aircraft. Problem Is I have a carbon fiber flying wing and it seems to shadow the receiving frequency when I have a standard antenna ran down the bottom side of the wing.

Any help would be appreciated.

Chris

[PLMS]

Gee that's an odd one, carbon flying wing...

I can't think of a way to have a Y antenna at 36Mhz either, but my professional field is microwave and UHF radio not HF, so someone in the HF radio field may have an idea.
But one thing I did think of, maybe try running the antenna wire (about a meter long on most 36Mhz systems) half way above the wing and then dive through or around it ,and have half on the other side.
The other way is to it if you can't use a long trailing wire is too use a base loaded antenna (4.7uH inductor) and use thin piano wire trailing off the back. This keeps it short and protected (combat model ?).

On my Zagi I run the wire across the wing, then follow the outline of one side tiplet. This gives some side view form any flying angle.

Martin

[Tomapowa]

got to love double posts..... :rolleyes:

http://www.rcgroups.com/forums/showthread.php?t=1022686

... and yes, it's possible... with some diligence.

http://en.wikipedia.org/wiki/Antenna_diversity

http://www.rf-systems.nl/sp-1.html

[PLMS]

I see I was a bit misled. This is a long range UAV application, not a slope soarer or combat wing.
I agree with the original thread then. Use 2.4Ghz, and Futaba FAAST most likely, as it may cope better with an onboard 2.4Ghz video TX than DSSS (on paper). Use a helper holding a parabolic TX antenna pointed at the UAV if you have too. Works well for WiFi hackers...

[airmcn_3]

got to love double posts..... :rolleyes:

http://www.rcgroups.com/forums/showthread.php?t=1022686

... and yes, it's possible... with some diligence.

http://en.wikipedia.org/wiki/Antenna_diversity

http://www.rf-systems.nl/sp-1.html

This is far from a double post; they are two different questions in two different forum sections. I asked you if I could split an antenna, I asked them what they were doing to work around the RF issues.

At least I finally got an answer. (From you)

Thanks,

Chris

[Tomapowa]

This is far from a double post; they are two different questions in two different forum sections. I asked you if I could split an antenna, I asked them what they were doing to work around the RF issues.

At least I finally got an answer. (From you)

Thanks,

Chris

Chris, sorry... thought it was the same question... (I also meant cross-post, not double post). I stumbled upon your other post doing a google search amazingly... in any case, glad to have helped. Honestly, if shadowing is an issue with your operating freg. & platform, I would lean more towards a dual receiver/antenna option (like the Spektrum 2.Ghz system). Your other option is to use a small dipole mounted (taped) to the flying wing's winglet... if you in fact use one.

More info on that idea is here:
http://www.rcgroups.com/forums/showthread.php?t=1000425

[airmcn_3]

Chris, sorry... thought it was the same question... (I also meant cross-post, not double post). I stumbled upon your other post doing a google search amazingly... in any case, glad to have helped. Honestly, if shadowing is an issue with your operating freg. & platform, I would lean more towards a dual receiver/antenna option (like the Spektrum 2.Ghz system). Your other option is to use a small dipole mounted (taped) to the flying wing's winglet... if you in fact use one.

More info on that idea is here:
http://www.rcgroups.com/forums/showthread.php?t=1000425


No worries, you know the dipole option was mentioned yesterday by a mate of mine, it would be nice if I could have 4 of them total, one on each winglet going vertical and one on each winglet running horizontal. This RF thing I just plain don’t know much about, I am trying to learn as much as I can in this field as its part of my job, I have begged to hire a RF professional but they (work) keeps putting me off.......

Thank you again for the link, I will read up.

Chris

[Ron W3FJW]

You can use 4 dipoles but they would have to be phased properly to maintain a 50 ohm impedance. The length of coax necessary to do so would negate any advantage 4 dipoles might provide. It ould be too heavy for one thing. A 1/2 wave dipole at 36mhz would be 13 feet long.
The long wire supplied with your Rx is supposedly "tuned" to 36 mhz so adding another long wire would halve the impedance the Rx is looking at resulting in a reduction of signal strength, everything else being equal.

But try "Ying" it. Do a proper range test with your Tx antenna collapsed though.

[Tomapowa]

Ooops... I should had noticed the freq. he is using... Yes, for certain a dipole for that freq. (36Mhz) would be huge (each leg = 6-1/2 feet !). A better reason to switch to a much higher freq., ... the higher the freq., the shorter the diople. The one shown in the previous link mounted to a flying wing's winglet is for a 900MHz Maxstream modem/system I've used (6-1/2" total length). On the other winglet, I have an even smaller dipole for the 1.7GHz video system (1-3/4" total length). A cool dipole calc is here: http://bfn.org/~bn589/antenna.html. We've gotten exceptional RF reception using these dipoles... key is to place them in the vertical plane.

[Ron W3FJW]

Also make sure the coax feeding the dipoles is a multiple of (odd IIRC) 1/4 wavelengths at the chosen frequency taking the dielectric constant of the coax into consideration for best results.
Not as important for Rx as it is for Tx, but important nonetheless.

[vintage1]

At 36Mhz any arrangement of longish wires will work.

You have no hope of really tuning it at all, and if you do, chances are it will get very directional.

[airmcn_3]

Thank you guys for all the help, it is very much appreciated. I will run through all the options and see what comes up. Currently I am back on 2.4 with 5.8 vid so I can get the autopilot tuned.

Thanks,

Chris

[mjsas]

I agree with Vintage1, at 36 Mhz the antenna wire is too short to be anything other then a hunk of wire. You can do just about anything with it.

[Tomapowa]

I agree with Vintage1, at 36 Mhz the antenna wire is too short to be anything other then a hunk of wire. You can do just about anything with it.
Too short? Didn't you mean too long?

airmcn_3, curious,... which AP you trying to tune? Also... what CF wing are you using?

[Ron W3FJW]

I gotta agree with mjsas & Vintage Toma. Too short. Not even a 1/8 wavelength.
Even so, I would imagine it is sort of a "tuned" length of wire (depending upon the input stage) and shortening or lengthening it MAY affect the range somewhat.

[Tomapowa]

I gotta agree with mjsas & Vintage Toma. Too short. Not even a 1/8 wavelength.
Even so, I would imagine it is sort of a "tuned" length of wire (depending upon the input stage) and shortening or lengthening it MAY affect the range somewhat.

huh? :confused: What's short?

[Ron W3FJW]

I agree with Vintage1, at 36 Mhz the antenna wire is too short to be anything other then a hunk of wire. You can do just about anything with it.

This short.

[Tomapowa]

This short.
:confused: :confused: :rolleyes: :rolleyes: :confused: :confused:

Again, I think he (mjsas) made a mistake... nothing "short" about a 13 foot 1/2 wave dipole which we were discussing... plus vintage mentioned "longish"... the opposite of "short".

I gotta agree with mjsas & Vintage Toma. Too short. Not even a 1/8 wavelength.
Even so, I would imagine it is sort of a "tuned" length of wire (depending upon the input stage) and shortening or lengthening it MAY affect the range somewhat.
What do you mean by 1/8 wavelength?
Antennas should always be tuned/matched, and shortening/lengthening the antenna will certainly detune it...

[vintage1]

:
Antennas should always be tuned/matched, and shortening/lengthening the antenna will certainly detune it...

!/. No, they shouldn't always be.

2/. Because any random lack if straightness, bundling., flapping in the breeze, being near metallic materials, will certainly detune it anyway.

Which is why, on a SW rx, its neither tuned not matched to any high degree.

2.4Ghz stuff is though.
.
And so are our HF transmitters

[PLMS]

Comes back to what I said some posts ago I feel, weave the original RX wire through the wing somewhere along it... Partly on top, and partly below.
The best option is still an open trailing wire though IMHO.

Comment about an earlier post mentioning 50 ohms, the impedance match of the RC RX and TX antennas is far from 50 ohms I've found, so conventional RF splitters etc won't work as expected. I worked that out through tuning experiments on my 36Mhz gear some years back. Tuning the TX into a 50 ohm power meter or spectrum analyzer gave poor power once connected to the real antennas again.
The results pointed to the actual impedance being far higher than 50 ohms.

I agree with other comments about 2.4Ghz. You'd need a main and a remote receiver, one on top, one on the bottom (AR7000). Carbon fiber is a good shield at higher frequencies. Also surprisingly electrically conductive, I've had stray filaments of airborne carbon strand get inside my sheds wall heater and blow the mains fuse !

[Tomapowa]

!/. No, they shouldn't always be.

2/. Because any random lack if straightness, bundling., flapping in the breeze, being near metallic materials, will certainly detune it anyway.

Which is why, on a SW rx, its neither tuned not matched to any high degree.

2.4Ghz stuff is though.
.
And so are our HF transmitters

In R/C (single freq) the staighter the antenna (in one plane) the better... period. Folding it onto itself is not a good idea unless you really know what you are doing (folded dipoles are typically used for broader bandwidth apps, i.e TV). And always trying to match/tune your antenna to your RF output section is always a good idea (from efficiency standpoint), granted, some times it's not that easy due to factors beyond your control. I've never had any issues tuning our 2.4GHz antennas... pretty simple when you have the right eqpt (http://www.flycom.co.uk/antenna_swr.htm).

Here's a few links to some good antenna basics...
http://www.electronics-tutorials.com/antennas/antenna-basics.htm
http://www.microwaves101.com/encyclopedia/Navy%20handbook/3.1%20Antennas.pdf

[mjsas]

A meter long length of wire that is used for an antenna at 36 MHz is not matched, tuned or anything else. It is just a hunk of wire taped to the side of the airplane.

[Ron W3FJW]

That depends on the radio. Some have had the antenna tapped a few turns up from ground on a rf input coil. When the coil was tuned it was tuned with the antenna connected to the Rx, so the length of wire was tuned as well..
The Heatkit RC equipment was tuned up like that as I recall and I think the crystaled Corona Rx are that type as well.
Reducing or lengthening the wire had the effect of detuning the front end.
A long wire antenna is usually considered a High Z antenna.

[airmcn_3]

Too short? Didn't you mean too long?

airmcn_3, curious,... which AP you trying to tune? Also... what CF wing are you using?


Its a new Beta AP developed in AU, the wing is our own design, nothing special just efficent.

Thanks,

Chris