Compact RC antenna design

[PLMS]

Guy's

Dunno if this is covering old ground here but..

I have equipment to test the tuning of antennas here at work, but would like a starting point to begin trying to make my own. I'm interested in 36Mhz, but most simple designs could be retuned to work at other counties frequencies as well.

One important bit of information I need is what is a typical input matching impedence of a single conversion receiver ? I bet it's not 50 ohms.
Anyone done a RC receiver design here, that would have done the maths on the front end ?

Martin

[mem]

Have you looked at Antennas and matching - Microantennas! (http://www.rcgroups.com/forums/showthread.php?p=5816659)


edit: broken link fixed

[vintage1]

There is almost no matching at all. Typically its a 2p7 or 4p7 capacitor somewhere in the vicinity of the mixer..IF there is an RF input coil, it might end up somewhere near there as well.

But the coupling is deliberately weak to preserve the input coil Q at something sensible.

Frankly, if you want a shorter antenna, the answer is to stick a bit of wire into an addon FET RF amplifier..and forget about tuning the antenna altogether.

[XJet]

And the interesting thing is that when it comes to receiver antennas, the strength of the signal you get is strangely proportional to the amount of conductor that intersects the wave -- ie: shorter antennas (no matter what you do with them) will never produce as much range as an optimaly tuned one of the correct length.

Base-loading, top-loading and all that is just smoke and mirrors when it comes to receiver antennas on RC gear. As Vintage1 says, the coupling is so loose that you don't need to worry about impedance mis-matches.

And, if the impedance isn't an issue, the only other thing that loading an antenna achieves is to alter the radiation pattern. When using a vertical whip (for instance) as a transmitter antenna, top-loading it (or capcitively loading it) will produce a pattern that has lobes which are at a greater angle to that whip. That's good for ground-to-ground comms, not so good for ground to air (assuming the whip is held vertical).

Base-loading will produce lobes that are at a lower-angle and therefore work more effectively if the antenna is held vertically.

Of course on a receiver, it doesn't really matter too much -- since the antenna is going to be moving significantly in its orientation to the transmitter. The best you can hope for is a pattern which is as close to a sphere as possible (not possible with a wire antenna though).

So... no matter what you do, cutting that antenna shorter will reduce the range by a (roughly) proportional amount -- whether you add loading coils or not.

[PLMS]

Hi Guy's

Thanks for the replies.

I have to agree with the comments.

Xjet - I think form my old RF training that the effect you mention is antenna 'capture area'.

Mem- That link is broken but I'll try searching on the subject.

Vintage - Thanks. I guess that's why when I've bench tested RX sensitivities the coupling method seems to be quite broad. I've used 50 ohm equipment in the past.

The RF FET idea sounds good, I have a friend who has done that. Have you done that yourself with readily available FET's ?

Martin

[PLMS]

Sifted through the posts in the microantenna thread/s.

This was interesting. Some test data on 35Mhz.

http://www.rcgroups.com/forums/showpost.php?p=5827986&postcount=19

Shows some improvement in a base loaded verses un-loaded micro antenna, but less effective than a full original antenna.
However the 7uH loaded micro antenna shows to be about 3dB (volts) better than the same length without.

Interesting. I might try 7uH and same length on 36Mhz.

My application is on helicopters (t-rex), otherwise I can see that the full original antenna is going to be best.

Martin

[Mr DIY]

Why do you need a shorter antenna on a T-Rex?

Move the antena tube as far forward as it will go. Then the antenna wire will exit the nose and move forward to antenna tube entry point. On the exit end of tube, I have the antenna pass through the horizontal stab and back toward the body again. Never had range issues with a half decent Rx.

Brian

[vintage1]

Hi Guy's

Thanks for the replies.

I have to agree with the comments.

Xjet - I think form my old RF training that the effect you mention is antenna 'capture area'.

Mem- That link is broken but I'll try searching on the subject.

Vintage - Thanks. I guess that's why when I've bench tested RX sensitivities the coupling method seems to be quite broad. I've used 50 ohm equipment in the past.

The RF FET idea sounds good, I have a friend who has done that. Have you done that yourself with readily available FET's ?

Martin

Nope strictly armchair theorist here..but the rationale is worth saying..

Smaller antennae means less signal. Less signal means also less interference signal..so no harm to use a booster.

The typical sensitivities I have seen quoted are around 2-100uV..now thermal noise figures on a FET input at 5Khz bandwidth should be one or two orders below that, so plenty of room to improve..

Most SC receivers have no RF stage at all, so not problems with too much RF gain etc..i.e. no instability.

With smaller antenna no issues with too MUCH signal overloading mixer..so RF stage looks good as well.

I'd say a simple pair of input and output LC filters and an RF FET and a bit of loose coupling into the input is all you need.

Heres something someone posted up that I kept..See the AGC in the front end amp..and how the antenna is coupled in. Uses a grounded gate config in the front end FET.. I'd probly go for grounded source, and a looser antenna coupling meself..

[XJet]

The big problem will be preserving the linearity of the mixer (and suppressing IM products) when you put a relatively wide-band amp on the front.

Although the AGC will turn down the gain for strong in-(IF)-band signals, that amp will still be bosting adjacent channels if your own transmitter's signal strength is low -- and that significantly increases the chances that the mixer will be pushed beyond its linear operating region so as to produce intermod/crossmod products that fall within the IF bandpass.

In the RC world, a good dynamic range and resistance to strong in-band and out-of-band signals is more important than outright sensitivity. In fact, all you really want is *adequate* sensitivity. So long as you can fly to the limits of visibility with full control, any additional sensitivity is simply placing an unnecessary burden on your mixer design and IF filters.

[vintage1]

The big problem will be preserving the linearity of the mixer (and suppressing IM products) when you put a relatively wide-band amp on the front.

Although the AGC will turn down the gain for strong in-(IF)-band signals, that amp will still be bosting adjacent channels if your own transmitter's signal strength is low -- and that significantly increases the chances that the mixer will be pushed beyond its linear operating region so as to produce intermod/crossmod products that fall within the IF bandpass.

In the RC world, a good dynamic range and resistance to strong in-band and out-of-band signals is more important than outright sensitivity. In fact, all you really want is *adequate* sensitivity. So long as you can fly to the limits of visibility with full control, any additional sensitivity is simply placing an unnecessary burden on your mixer design and IF filters.


Thats why I said it was good to have a small antenna. Plus a coil in front to get rid of MOST of the OOB stuff. The a further RF stage selects only inband to apply to the mixer.. The AGC is really only FOR inband stuff..anything else will get dumped by the input stage tuned circuits

[PLMS]

Mr Diy - Fair comment, but my vanity doesn't allow me to use the antenna tube. It destroys the right look of the model IMHO. I hate seeing an obvious antenna on any of my planes also, most have the antenna wire built into the wing while constructing them. Hence my quest for an adequate compact antenna for the Rex. Ultimately I'm going to fit a scale body to the Rex and never 3D it.

I'm going to try some a range comparison of a 7uH base loaded 11" antenna verses the slightly cut down one I use now. If I can successfully use a 11" base loaded one then it can easily run along one tail brace and across the vert stab. I do like the idea of an active antenna too, although range is not an issue in the case of a Heli.

[Kwok_Yu]

Azarr "RadioActive" Antenna
http://helicopter.geek.nz/space/node/6

I have been out of the loop for the last year, but I remeber Azarr had this new active antenna out. I can't find any info any where, expect the above review. So they must have stopped producing it. I remeber it actually need power from an used rx slot, so it probably worked on the amp principle mentioned by Vintage1.

OK, found the announcement thread which has some pictures of it. I would think you could get a custom one made up by contacting Azarr.
http://www.rcgroups.com/forums/showthread.php?t=375190

-Kwok

[z-matrix]

hi guys,

what antennas should be used ground to air?
so an aeroplane can have same ranges at different orientations relative to the transmitter antenna?

[Mr.RC-CAM]

I can't find any info any where, expect the above review. Last I heard, Azarr canceled the amplified antenna product. I suppose putting a wide band amp in the Rx's front end, without passband filters, turned out to be problematic. But I never heard the exact "why" so that is just my personal speculation.

[XJet]

You're probably right, RC receivers have a hard enough job as it is -- dealing with massive changes in signal-strength, very powerful adjacent-channel signals, and a raft of cross-mod/intermod products that can occur on a busy flightline.

Adding a relatively untuned amp to the front is only going to agrivate *any* linearity issues with the receiver's front-end/mixer.

Given the small profits involved from such a device, I don't think *I'd* want to be facing the possible liabilities :-)

[Kwok_Yu]

Regardless of any possible technical issue, the $20 price tag is pretty tough to swallow. At close to 50% of the cost of an RX, he probably wasn't sell too many. That would be my guess.

[vintage1]

hi guys,

what antennas should be used ground to air?
so an aeroplane can have same ranges at different orientations relative to the transmitter antenna?


Probably a spherical balloon of conductive material tied to the antenna input?

One way I will do the wingtip to wingtip and tail to center section wire thing and use that. Plus a bit of vertical wire.

[Pinecone]

My question is, are these antenans really tuned antennas of are they just working as a long wire. Typcially 1/4 wave tuned antennes need a counter poise or ground plane to work properly.

And tuning has been more of a consideration in transmitting than in receiving.

[PLMS]

While we're talking about wierd antennas.... Is there any issue with me using piano wire for the antenna on my Heli, mounted so it 'hangs' out below and under the skids of the Heli at 45 degrees rearwards ? I'm thinking that I'd use a 7uH loading inductor and the approximate 11" of wire. If it's hanging back at 45 degrees I should be able to land the heli without breaking or snagging the wire. It will be earthing out on the grass though..... Hmm.

Martin

[Mr DIY]

Is there any issue with me using piano wire for the antenna on my Heli

I have used a flexible piano wire antenna on an electric pylon racer in order to reduce drag that results from the conventional antenna flapping behind. It worked well without any issues. If the piano wire is thick though, some crappy receivers (like a JR610, JR700) front ends may become untuned as a result of the extra metal.

The antenna touching ‘earth’ can induce glitching on ‘basic’ receivers. A decent receiver that has some sort of processing (like a DSP based receiver or PCM receiver) should help prevent issues. If you are using one of these receivers I would give it a try.

Brian

[PLMS]

The antenna touching ‘earth’ can induce glitching on ‘basic’ receivers. A decent receiver that has some sort of processing (like a DSP based receiver or PCM receiver) should help prevent issues. If you are using one of these receivers I would give it a try.

Brian

Thanks,
I'm using a Futaba PCM so I have DSP. Did you use a loading coil/inductor in your app ?

[Mr DIY]

Did you use a loading coil/inductor in your app

No.

Brian

[macquen]

Guy's

Dunno if this is covering old ground here but..

I have equipment to test the tuning of antennas here at work, but would like a starting point to begin trying to make my own. I'm interested in 36Mhz, but most simple designs could be retuned to work at other counties frequencies as well.

One important bit of information I need is what is a typical input matching impedence of a single conversion receiver ? I bet it's not 50 ohms.
Anyone done a RC receiver design here, that would have done the maths on the front end ?

Martin
Yes in several micro model posts - but to your question on shortened antennas which by the way is necessary for 1/72" scale RC airplanes. I never could understand why some flyers ruin the apperance of a fine scale model by dangeling an antenna out of the airplane when this technology is available, and has been for over 35 plus years.
Attached is the equation used to calculate base loaded antennas, I have calculated a sample antenna for use on 72MHz how ever this equation can be used to calculate base loaded antennas for any freq. I uaually fly on 6 meters or 72MHz, and using this equation I successfully designed, used base loaded antennas on 54MHz, and 72MHz for several years with no noticable adverse effects. The range was greater then 500'.
73 - Steve - WB2TMR

[Pinecone]

Here: http://www.uoguelph.ca/~antoon/gadgets/flywire.htm

[PLMS]

Attached is the equation used to calculate base loaded antennas, I have calculated a sample antenna for use on 72MHz how ever this equation can be used to calculate base loaded antennas for any freq. 73 - Steve - WB2TMR

I calculated on a 0.4m antenna and 36Mhz. I got a choke of 5uH as a result.
I think 4.7uH will be the closest available value.

[vintage1]

That feels about right. I suspect you will get modest gain from using the inductor, but not a huge gain. Maybe 50% more range or so. Nothing is as good as a long piece of wire, but a modestly tuned piece of wire helps.