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Mr.RC-Cam

Does R/C Rx Antenna Length Really Matter?

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On the various R/C forums there have been many discussions about the effects of antenna length changes. Some folks cut the wire aerial shorter, some compact them by winding them up on straw, and others wonder if making them longer will offer increased range.

The opinions on these things varies all over the map. I decided to perform some basic measurements using a popular park flyer 72Mhz R/C Rx. The techniques and measurement results are shared here: http://www.rc-cam.com/ant_exp.htm

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Thanks Thomas for an excellent article.

"Whip me"... too funny.

Bill

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You're welcome. It was just something I recently decided to whip up. :)

What is so surprising to me, is that for as long as the endless dialogs have gone on about R/C Rx antennas, I could not find any evidence that someone actually published measurements. If there is data out there, it is certainly well hidden.

I am also surprised how the collective R/C market (us) have made no demands of the R/C manufacturers to publish real performance specifications for their transmitters and receivers. We demand to know everything about batteries, motors, servos, and such, but we totally ignore the radio system's basic RF performance. And, vendors that sell those short Rx antennas should be supplying useful specs too. But even if we asked for this information, which we don't, they do not have it.

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Thanks a lot!

Concerning details concerning R/C systems, maybe it's simply because RF data is not as easy to understand as the speed or torque of a servo, or the capacity of a battery...

And not all R/C'ers are as interested in the technical aspects than the people who are here... It's still very easy to find people who don't even know how to calculate a charging current/charge time for a battery on flying fields around here...

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I agree that few people are interested in the technical data for R/C gear. But, the sort of data that I'm after appears in the manuals for my TV, stereo, cordless phone, etc.

All the R/C mfg's have to do is publish it somewere so that those that care can review it. And as I mentioned, even if you ask an R/C mfg for such specs, they usually do not offer it. I wonder if even they know.

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Yes that's right. I've even never found precise data about the RF power of a R/C TX. Read once that it should legally be around 500mW...

I hope they know, otherwise I would consider designing my own R/C :lol:

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Interesting experiment, Thomas.

I've always taken the "father knows best" philosophy with r/c radios, but it's good to see someone taking their own look. I guess that r/c radios have a certain "black box mystique" so folks are not clamoring to know more.

--Bill

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Mr. RC-CAM,

One interesting conclusion from your measuremnts is that whip antenna (8 inches of antenna plus another 8 inches of connecting wire for total of 16 inches) at -8dB is worse that 18 inches of straight wire at -5dB. This sounds like inductive elements in the whip antenna has no effect, or even negative effect.

I have another configuration to test, if you don't mind.

I've noticed that many heli pilots (including me) run their antenna parallel to the helicopter frame and sort of along the tailboom. The antenna configuration uses the original wire and RC receiver normally sits on front tray. The last 6-12 inches of antenna wire are floating in the air, but the rest goes parallel to the tail boom in some sort of wire guide or attached to the tail fin. I hope you can picture the setup.

I wonder how much signal is lost due to the fact that at least half of the antena length is very close to the body of the helicopter.

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...whip antenna (8 inches of antenna plus another 8 inches of connecting wire for total of 16 inches) at -8dB is worse that 18 inches of straight wire at -5dB.

Yes, it seems that a 18" antenna (cut stock antenna in two) would be better if you can mount it straight and away from everything. I would expect that for a model heli, this is not always possible. The base loaded antenna, if mounted at the front of the heli canopy, next to the Rx, and far from interfering sources, would probably do better in the long run. But, the variables make it a tough call.

I've noticed that many heli pilots (including me) run their antenna parallel to the helicopter frame and sort of along the tailboom.

I see that alot too. I suppose it is a good thing that most heli pilots don't spec out their model while flying. I also noticed that many of these fellows are flying PCM, so antenna performance problems would be hard to recognize until it was too late.

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Thanks very much Mr. RC-CAM. I've been wondering why I see my Deans base loaded antenna got range reduced significantly. I start seeing glitches at around 200 - 300 feet. And the original wire of the RX was on a straw and the end of it connected to the Deans one. I did not know it was reduced up to -12db in that configuration. Thanks very much for the invaluable information.

Thanh

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Great article.

Just one question - does the type of wire used for the antenna make a difference as well? Is there a special type used?

I noticed my receiver antenna wire is a multicore (maybe 24 strands or so), does a wire with less strands change anything? does the insulation on wire affect it?

thanks

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Basically, antenna wire can be nearly any standard hookup wire. In our R/C application, the choice would most likely be made on how well it would hold up under the punishment we give it vs. how bulky it is.

Insulation increases strength and prevents fuel and oil from getting into the conductors. Solid wire is usually not the best choice; Stranded wire reduces failures from fatigue and other abuse (more strands = more flexible). However, on my park flyers, where weight is important, I transition the stock stranded wire to a insulated 30AWG solid, which is protected under the covering of the wing or fuselage (clear tape works great on a foamie).

FWIW, on a technical level, larger diameter antenna elements have broader RF bandwidths. But in our R/C application, this contributes too little to matter. The areas that matter are where and how you string the antenna across the model, as discussed in the article.

Lastly, (and contrary to the feedback I have received), the article does not claim that short antennas or loaded whips should not be used. The article only offers some data on what happens when you alter the stock antenna arrangement.

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I am not certain if this will be seen by anyone with the know-how to answer, if not I'll make a new thread. Please forgive me if this has been answered elsewhere, but I cannot find the answer anywhere.

I am trying to make a base loaded antenna for a receiver for a small model (purely to not have the antenna dangling out the back) and measuring at the the RSSI output on the receiver to find the best of the less ideal setups.

My question is regarding the "normalized" RSSI value as mentioned in the articel. How is it normalized (The actual math and values behind it?) I used to know what this meant but the reasoning has since escaped me.

Also would it be reasonable to assume that the length where my base loaded antenna gives me the greatest RSSI value would theoretically be the best choice to start with for actual range testing?

Thank you in advance.

Regards,

Bob

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My question is regarding the "normalized" RSSI value as mentioned in the articel. How is it normalized (The actual math and values behind it?)

The original data I posted showed the RSSI normalized to the 0dBm reference. Later I updated it with the hard measured RSSI values so that other folks could easily relate it to what they measured. I forgot to change the table titles. Now fixed.

The exact RSSI voltages will vary with different R/C Rx designs and test setup. So, the absolute values you measure will not map the same as the posted data (if it does, then you are ready to play the state lottery). You will want to check your values with the IC's data sheet as a sanity test, then perform the testing by observing the relative RSSI trends.

Also would it be reasonable to assume that the length where my base loaded antenna gives me the greatest RSSI value would theoretically be the best choice to start with for actual range testing?

Assuming that the Rx's RSSI feature is only responding to the RF from the R/C Tx, then yes, the best RSSI would mean the strongest RF signal. Some RSSI's are inverted, where the strongest signal is represented by the lowest RSSI voltage. So, consult the IC's data sheet.

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Thank you for your reply. I appreciate the prompt response!

On the 3 RXs I am playing with (GWS Pico, Futaba R156 and Berg Micro Stamp) I have seen the same numbered chip: 31136, but I do not know the manufacturer of the specific ICs. There is a voltage that changes relative to the transmitters distance from the RX on pin 12 on all three RXs so I am assuming it is the RSSI output.

I will go try some different setups now and get testing.

Thank you again.

Regards,

Bob

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Fine writeup, but I'd like to note that with any Rx that uses an AGC approach the water gets rather muddy. When a weak signal is received it'll increase the gain, hence you gain in signal strength, but the noise also goes up, so it is up to the subsequent circuitry to dissect the signal from the noise, so you could end up trading one disadvantage for another.

Cheers,

Sander.

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I'd like to note that with any Rx that uses an AGC approach the water gets rather muddy

I agree. Here is what was written in the antenna article: "I decided to use a 72Mhz GWS Pico Rx as the test subject. It uses the TDA31136. It does not have AGC (automatic gain control), so it simplifies our setup. We would need to disable the AGC feature if it was present."

I understand that the R/C Rx's AGC is implemented to attenuate the strong RF sources, rather than amplify the weak ones. If true, this would not significantly impact the noise floor (versus a full gain Rx without AGC). But, it's presence could affect the RSSI method used in the antenna tests. Fortunately AGC is not a standard design feature in our R/C Rx's. I would expect it in the high-end PCM Rx's since those are offered at a premium price. As we work down the R/C food chain, the typical PPM or DSP Rx would {generally speaking} probably not have it, but YMMV.

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