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dalbert02

antenna design for 50Mhz

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I would like to extend my range when flying my RC airplane. I am currently using the 50Mhz ham band and as curious what antenna would be both omnidirectional and offer the most gain. Currently, I am using a smiley power duck antenna. I have been reading many websites looking for my answer and the more I learn, the more I am amazed that our RC transmitter/reciever systems work at all. For example, for the most part our tx antenna is vertically polarized while the Rx antenna is most likely in in the horizontal position. The Rx circuitry must be very sensitive to recieve out incorrectly polarized transmissions. Also, it seems that most people in the ham world use at least a 1/4 wave antenna for DX'ing which would make a 50Mhz antenna about 5 feet long. It has been suggested that I take a CB antenna and cut it down to 56 inches, then construct a ground plane antenna using 4 horizontal metal rods. Finally, this antenna should be mounted on a tripod at least 5 feet above the ground. This seems a bit bulky and offers 0dB gain. Hence, I come to you all seeking knowledge. Any suggestions?

Thanks in advance.

-dave

KI4HVT

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(1) With your stock setup, if you performed a careful range test, what would you get?

(2) What is your goal?

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I'm also starting to get frustrated by R/C range... I thought about something else, that is adding a preamp before the R/C RX. I've just ordered a little 22dB preamp kit, wonder if that will be of any use but I thought it was worth a try. I'll keep you tuned when I receive it :)

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If the Rx preamp is very low noise then you should see a remarkable increase in range. While you are at it, an optimized Rx antenna would be good too.

My opinion is that the Rx end is where the attention should be if more range is needed.

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(1) With your stock setup, if you performed a careful range test, what would you get?

(2) What is your goal?

With stock setup, ie. rubber ducky antenna from smiley antenna, and a number of 50Mhz Rx's I get some glitches at a distance of a little greater then three soccer fields. Rx's used include Futaba R149DP PCM, FMA M5vII, Berg 6 DSP G III, and FMA FS8 co-pilot. The co-pilot will even tell me the number of glitches. I have also used HiTech electron on 72Mhz. While the glitches are often minor, such as a sudden amount of aileron or throttle change with the HiTec electron, the other rx's just refuse to acknowledge commands for a few seconds (that seems like eternity). With foamies, I can live with that, but with a $200 video camera, $200 video tx, $100 gps, $100 motor, $400 li-po batteries, $200 ESC, and a $200 plane, $400 overlay board, opto isolation board, etc, I want to ensure glitches never happen.

My Goal? Glitch free operation beyond visual range. Before one starts about not being AMA legal, this will be used by a local government that will not operate under AMA guidelines.

Forgot to add:

I might try this antenna since it is so cheap, any thoughts?

http://www.mfjenterprises.com/products.php...rodid=MFJ-1806T

Edited by dalbert02

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With stock setup, ie. rubber ducky antenna from smiley antenna, and a number of 50Mhz Rx's I get some glitches at a distance of a little greater then three soccer fields. .... My Goal? Glitch free operation beyond visual range.

Just so that we can understand what range improvement you need, what relative magnitude increase in range are you looking for: 2X, 3X, ... 10X?

Before one starts about not being AMA legal, this will be used by a local government that will not operate under AMA guidelines.

Keep in mind that a USA Gov Agency or commercial venue is not normally allowed to use the Ham Band (there are exceptions, like for RACES/Civil-Defense or approved military recreation). If it is a personal hobby activity then you are good to go.

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While you are at it, an optimized Rx antenna would be good too.

I'm getting a bit off course here, should I base myself on your test report to adjust lengths, or would you see other ways of improving things?

I can't think of something really better than wire considering the size of a "real" antenna for these freqs would be on a plane... and omnidirectionality is obviously mandatory... :unsure:

My opinion is that the Rx end is where the attention should be if more range is needed.

Definitely, like always :)

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I'm getting a bit off course here, should I base myself on your test report to adjust lengths, or would you see other ways of improving things?

Well, this info is probably what I would have eventually posted here, so it is not off course at all.

I suggest that you install the longest Rx antenna element that you can accommodate, up to a 1/2 wavelength. Mount it so that its hung path offers the largest RF profile possible. Perhaps have it go from the model's fuselage to a wingtip, then straight back to the tail.

For extra credit, and if you have the room on the model, install a counterpoised antenna, of similar length, to the Rx's RF ground (any vast grounded copper area in the Rx's front end). The counterpoise should be mounted opposite of the main antenna element (sort of mirrored). Cyber-flyer did this long ago, so maybe he will chime in to say if it was worth the effort for him.

Once all this is in place, and the servo/Rx/Bat wiring is where it will remain, tweak the antenna matching coil in the Rx for the very best range. This coil is not in the IF strip -- it is the one nearest the antenna wire (antenna usually connects to it via a matching network). You should do the tweaking with the proper tool (correct size & non-inductive), in an open area that is free of interference and other annoyances. The Rx's RSSI output can be used to help in this effort.

Lastly, compare the new results to what you had before. I predict you will see much better range.

BTW, if you add the preamp later then the antenna matching coil will need to be tweaked again. Also, the preamp is probably going to be designed for 50 ohm input impedance, so a full 1/4 wave aerial is ideal if you wish to achieve perfection. Keep in mind that a preamp can overload the Rx at close distances or from extraneous sources, so try to use a Rx with AGC (many don't have this feature).

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If I could get a five mile range from my starting point, that would be awesome.

Somthing like,

5mi

|

|

|

5mi-----C-----5mi

|

|

|

5mi

with C=center, I would have a 100 sq mi big box to fly in.

I am starting to think that a tracking system would make the most sense. This would increase the range of my Tx as I could use a higher gain directions yagi and also improve the Rx of the video signal.

This is only a proof-of-concept vehicle to be used in a USAR (Urban Search & Rescue) application. Once I can prove it will work and can get some grant money, then I will worry about getting things on the proper freqs. At this point, it is an out of my pocket expense, an experiment basically.

-dave

KI4HVT

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A reliable 100 sq mile model R/C application is a tough row to hoe. If it is for search and rescue then the altitude will probably be fairly low too (which does not help at all). I'm not sure what to recommend that would offer a safe and reliable solution. For sure, this app will need some sort of autopilot failsafe as a backup.

For licensed 6-meter R/C you are allowed 1W output from the Tx. So, a high gain Tx antenna, like a Yagi, is in order (something with about 10dBi of gain). It would help to properly adjust the Tx's PA for highest output power (up to 1W) with the new antenna. Then incorporate Kilrah's idea of using a low noise preamp and optimized Rx antenna. In the end, your practical range will depend on the operating environment and luck.

Edited by Mr.RC-Cam

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I don't think you'll be able to get reliable 5 mile range with the licensed (<1W output) 50mhz system. You can expect 2, may be 3 miles but 5 miles will be pushing it. And I don't believe the preamp will help but I am happy to be proven otherwise. Carefull receiver antenna arrangement and tuning is crucial to get most of the RC range. Make sure that the electrical noise is reduced to minimum - it can severly degrade the range. Your point on polarization missmatch is correct.

Tracking antenna will help but not on 50mhz (6 m antenna size). Try new 2.4 Ghz DSS systems with compact tracking antenna.

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Obviously a yagi would necessitate a tracking sytem as I mentioned above. I am not sure I have the technical expertise to construct such a system but I am certainly willing to try. It seems as though there are only a handfull of people who have made their own home-brew systems so I guess I'll start by contacting them. I remember reading that our stock systems are supposedly good for 2 miles. If I could get 2 miles 100% glitch free, even that would be a tremendous improvement. Do you think using omni directional ground plane antennas as used by hams on the 6 meter band would accomplish this?

Thanks for your thoughts and opinions, I really appreciate the feedback.

-dave

Add: Didn't see CyberFlyers response. The 2.4Ghz spread spectrum idea is certainly worth considering. However, I have read that the 2.4Ghz video systems we use can conflict. It seems that RCCAM and CyberFlyer agree on modifying the Rx rather then concentrating on the Tx. Kilrah, I can't wait for your results! Please let us know what happens. I wait with eager anticipation!

Edited by dalbert02

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An old man I used to work for said, "One carefully controlled experiment is often times worth more than a thousand expert opinions". Today I tried to place an order for the MFJ 6m antenna (MFJ-1806T) but they did not have any and don't expect them for a number of weeks. :o Good thing the local ham festival is here next month!

While on the MFJ website, I cam across the following:

With fixed HF vertical antennas the efficiency is greatly improved when using some form of ground plane. VHF and UHF handheld radios and their associated whips work well allowing you to wander around the countryside, or down the street, enjoying ham radio communications. However, stop and think how this antenna actually works. Consider a simple quarter wave vertical on a 2m VHF handheld. Like any quarter wave vertical it needs a ground plane, in this

case around 19” in length. This is easily provided by the copper on the radio’s circuit board and aided by the operator’s hand capacity. Therefore, when operating VHF or UHF, do not consider the missing quarter wave “counterpoise”. There is enough stray capacity and inductance to take care of this. However, on HF it is completely different. There is no way that the radio can offer a sufficient

ground plane for an HF vertical antenna to work. In fact, you can easily prove the point for yourself. Attach a resonant antenna, then switch to the appropriate band and listen to a signal. Then, grasp the radio firmly with both hands and hear how the signal rises. Place the radio on a metal surface such as an office filing cabinet and hear the same effect. This shows the importance of a ground plane. It is crucial to provide the missing quarter wave ground plane in order to achieve efficient operation. To provide the missing length of wire, you need to attach

a length of flex to the earth terminal and run it along the ground or throw it out a

window. Length is not absolutely critical but you should aim for the lengths mentioned on the next page. For the low frequency bands, you could save yourself some length by adding an inductor in the wire, but you will have to experiment with values and lengths. To improve signals even more, carry a short copper stake to go into the ground and attach this to your radio and the

ground plane. The gauge of the flex is not important and it does not matter whether you use bare copper or plastic covered flex.

Length of telescopic is bottom end of each band, with ground plane wire fixed

to earth connection of radio.

LENGTH OF GROUND PLANE WIRE

2m, 70cm, and dual band antennas do not need a ground wire.

MFJ-1806T 6m.............109.22cm (3’ -7”)

MFJ-1810T 10m...........195.58cm (6’ -5”)

MFJ-1812T 12m...........220.98cm (7’ -3”)

MFJ-1815T 15m...........261.62cm (8’ -7”)

MFJ-1817T 17m...........304.8cm (10’ -0”)

MFJ-1820T 20m...........391.16cm (12’ -10”)

MFJ-1830T 30m...........548.64cm (18’ -0”)

MFJ-1840T 40m...........784.86cm (25’ -9”)

MFJ-1880T 80m...........1569.72cm (51’ -6”)

Ground plane lengths given for lowest frequency in each band. Longer lengths

of wire are needed the lower the band.

The funny thing I noticed was that inside my radio, Futaba 9C, there are two wires coming from the Rf amp. One goes to the center of the BNC connector for the smiley rubber duck, but the other is attached to the plastic case of the Tx. I know this is to 'ground' the system, but I can't imagine the plastic case being much of a conductor. Perhaps attaching this ground wire to a 1/8" jack and physically grounding the radio to a copper earth ground would help? Perhaps a small copper stake from the local home depot I could put in the ground?

Thanks.

-dave

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Obviously a yagi would necessitate a tracking system as I mentioned above.

Frankly, if you had a helper, the Tx antenna could be aimed by hand. A human would do a fine job -- just aim in the general direction of where the model is flying. At moderate distances, the beamspread becomes very wide, so laser like aim is not needed. But I agree with Cyber-Flyer that the required range is beyond practical use.

Keep in mind that the tracking system you are speaking of would work opposite of what folks like cyber-flyer have done. For example, his moves a Rx antenna, which has some inherent advantages. In your case, the Tx antenna is what needs to be moved around. So, to work via automation, the model would need to transmit its coordinates back to the ground station so that your Tx antenna would know where to point. Or you could do the obvious thing and build a traditional Video Rx signal tracker and mount the big 6-meter Tx antenna on it.

EDIT (saw your new post after entering this one):

One goes to the center of the BNC connector for the smiley rubber duck, but the other is attached to the plastic case of the Tx. I know this is to 'ground' the system, but I can't imagine the plastic case being much of a conductor.

Get rid of the rubber duck if you want long range. The duck is costing you -5dB.

The human body counterpoise on your RC Tx is capacitively coupled to the RF deck. It is not very efficient, but works fine for the "beyond visual range" performance that the mfg's claim. Adding a real RF ground to Earth will help a lot, but should be done in concert with a re-tweak of the R/C RF deck's final PA (antenna matching).

The gauge of the flex is not important ...

Although anything is better than nothing, that vendor's statement is not entirely true for ideal performance. A good ground wire is more than just a low gauge fellow connected to a short stake in the ground. You need a beefy RF ground wire/braid and the copper ground rod must be driven several feet into earth, perhaps with some radial members; sometimes a metal water service pipe is a good workaround.

And I don't believe the preamp will help but I am happy to be proven otherwise.

Cyber-Flyer, from what I have read, most R/C Rx's have RF sensitivity that is greater than 2uV. That suggests that a good preamp (and a Rx with AGC), should help out. But, the proof is in the pudding, as they say. :)

Edited by Mr.RC-Cam

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OK, noted that. I had already thought of the signal overstrength with the preamp and was ready to have to retract TX antenna before being in the air. We'll see.

As a receiver I'll use my Hyperion DSP-8FS. It uses a TA31136F demodulator which has a RSSI pin. Cool. There is only one tunable coil in there.

"One carefully controlled experiment is often times worth more than a thousand expert opinions".

Definitely, and I've been surprised a few times.. :) This brings a question to my mind:

Perhaps have it go from the model's fuselage to a wingtip, then straight back to the tail.

That means the wire will run horizontally as it seems. BUT.. seconding the remarks about polarization, this doesn't seem the best way to go to me. At the beginning on my ugly foamy plane I had the antenna running along the fuselage, that is horizontal, with maybe a little 5cm bit trailing freely. I was getting very annoyed because I had really bad range, starting having glitches at maybe 300m, 500m was getting very bad and I couldn't think about going further. I was ready to change the receiver, and then I eventually changed something on the plane, and ended up leaving the antenna totally free, dropping vertically under the plane because I was too lazy to get some tape in the other room. But - laziness sometimes not being so negative :rolleyes: - the next flight I had went exceptionnally well, 1km without problems. Progressively pushed further, fixing the limit at 1.8km.

Nothing else changed, and never had any range issues again (I assume with standard gear - especially a micro-receiver on which is stated "only for parkflyers" - I shouldn't be expecting better :P )

I add that I have my TX antenna being vertical, wanting to fly around me at a certain horizontal distance. So definitely polarization has a huge influence.

Now to the point, should I try the same symmetrical arrangement, but with as many vertical segments as possible, or would it be advisable let's say to run each of the wires to the opposite wingtips, and then let them trail? Will uneven distance / alignment between wires matter?

I wait with eager anticipation!

Me too! preamps should arrive in 1 week or so. Will give it a try only with antenna change first, but I have to fly a bit with standard equipment before, as I don't have enough experience with this receiver. I know the R156F like my pocket with like 25 FPV flight hours now on this setup, but I only got the Hyperion one recently, plus I was using it in another model. I need to know how it behaves if I want to make reliable comparisons.

And to be able to fly I need to repair the plane as it did suffer a bit on my last flight over clouds... which was 100% predictable though <_<

Nevermind, was worth it :P

the other is attached to the plastic case of the Tx. I know this is to 'ground' the system, but I can't imagine the plastic case being much of a conductor.

TX cases are metallic-coated, at least Futaba "thumb-on" series ;)

That makes for a good contact with your hands. Though this is not right for guys like me who fly having the TX on the knees and not firmly holding the thing. I however certainly do rest my wristles on it, at least I think so :unsure:

Keep in mind that the tracking system you are speaking of would work opposite of what folks like cyber-flyer have done. So, to work via automation, the model would need to transmit its coordinates back to the ground station so that your Tx antenna would know where to point. I have not heard of any hobbyists that have done this trick and posted the tech details to it.

Isn't that precisely what he is doing? Or have I forgotten my lesson? (his flight log :) )

most R/C Rx's have RF sensitivity that is greater than 2uV.
Yup, at least that's what is usually stated ;) Edited by Kilrah

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At the beginning on my ugly foamy plane I had the antenna running along the fuselage ... I had really bad range, starting having glitches at maybe 300m, 500m ... ended up leaving the antenna totally free, dropping vertically under the plane ...

A cross-polarized RF signal will cost you up to -20dB, so it all adds up. But, glitches at 300 meters, with a full range Rx, does hint that you had more than a cross-polarization issue.

As you already know, interference can come from within the model (ESC, servos, carbon push-rods, video equip, gremlins, bad karma). Running the Rx antenna along the fuselage can put you closer to the internal RF noise sources, which may have reduced the range in your case (hard to say). Possibly your vertical antenna mounting may have worked better because it was further away from nearby gremlins. That, plus better polarization, may have been the winning combo.

Basically, it is all a compromise when it comes to R/C antennas. My experiences sort of direct me to offer the antenna advice I dish out here. But, since every installation and environment is different, the best course of action is to perform several experiments with different antenna installations. It would be nice if you could see the RSSI voltage on your video display for easy data comparisons. The Gecko board does this and I found it to be very helpful (the reported signal levels have been very believable -- not goofy at all). You could do the same with a DVM panel meter in the camera view or with a flexible OSD board.

BTW, you will earn more performance if you re-adjust the Rx's antenna matching coil with each change in installation. So, if you experiment with the antenna, prepare to tweak. However, I think the Hyperions are Berg designs that are not adjustable.

Isn't that precisely what he is doing? Or have I forgotten my lesson? (his flight log)

Oops, I appear to have my wires crossed. I recall reading about a tracking antenna on the old forum that used Rx signal strength. But, I think you are correct that Cyber-flyer's system went beyond that and actually followed the GPS data via a telemetry link. I'll have to revisit his site! :)

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I'll have to revisit his site!

I think, I can answer that :) . My tracking antenna is using GPS coordinates of the model to calculate antenna azimuth/elevation and this of course can be used to transmit RC controlling signal. There is going to be a conflict if both video downlink and RC signal are using 2.4 GHz band. So, probably it's a good idea to move video downlink to 1.2 Ghz or 5Ghz.

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Thanks for setting me straight on the tracking system.

One of the interesting things I have heard about the new 2.4Ghz Spektrum DSS (digital spread spectrum) R/C system is that it can be setup to avoid video system conflicts. As the story goes, you just turn on the 2.4Ghz video Tx before turning on the Spektrum R/C system. It will then avoid transmission on the frequencies that are in-use, or so they say. Some claim you need to perform the unit's freq "binding" process, other say it is not needed. Either way, it sounds like there is a work around.

Don't forget that the 2.4Ghz RF path losses are tremendous (-34dB) when compared to 6-meter operation (or any of the other VHF R/C freqs). Had they asked me, I would have wished for a dual RF channel UHF R/C system that used Spektrum's DSS protocol. :)

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Either way, it sounds like there is a work around.

You are probably right - it may be possible to make DSS system to avoid frequency range in use by downlink. I'd like to know if anybody tried it.

My guess is that the two signals will leak a bit into each others spectrum anyway. And because one will have a transmitter working right next to a receiver this may degrade receiver sensitivity a lot.

Don't forget that the 2.4Ghz RF path losses are tremendous (-34dB)
Do you mean loss in the air?

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...it may be possible to make DSS system to avoid frequency range in use by downlink. I'd like to know if anybody tried it.

A fellow reported on the rc-groups forum that this trick works and that he was able to avoid signal conflicts.

Do you mean loss in the air?

Yes

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Do you mean loss in the air? 

Yes

Hmm, Then how do you explain that I get the same range (~5miles) with 500mW 2.4 Ghz downlink (10-15 dBi RX antenna, 5 MHz bandwidth) and 200-300 mW 72 Mhz RC channel (10 Khz bandwidth, 3dBi dipole)?

The bandwidth differential itself should make video downlink range 10-30 times shorter compared to RC range. The bandwidth difference is partailly offset by 10 dB antenna difference but in no way I can account for additional 34 dB loss that you describe. It would imply that the 2.4 Ghz range shoule be half of 34dbi ~ 50 times smaller.

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It would imply that the 2.4 Ghz range shoule be half of 34dbi ~ 50 times smaller.

It's because of the input sensitivity of the Rx's. I expect that a good video Rx is about -90dBm or better. A typical R/C Rx is perhaps -70dBm.

FWIW, a lot of the 2.4Ghz data modems have Rx's with at least -105dBm sensitivity. That is how they get such nice range with a measly 100mW, even when burdened with the higher bandwidth obstacles. If we created a 72Mhz R/C Rx with that sensitivity we would be able to fly to the moon. :)

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And now imagine with the -160dBm of GPS receivers... If only we could get that for video and R/C... :rolleyes:

Theoretical range multiplication factor from -90dBm =~ 2048... aaah...

Edited by Kilrah

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The reason why radio modems have better sensitivity over video receivers is because of their narrower bandwidth. The same goes to GPS receivers. The data rates on GPS is so small that the sensitivity numbers appear very good. Per unit of Hz (or data bits per second) it's all up to semiconductor noise and it is not so different in GPS/Modem/Video.

What am I missing?

Edited by cyber-flyer

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I am not sure what to say. The free space path loss is what it is. Signal bandwidth is a burden too, as you say. The marriage of the two affects the range behavior we experience.

The reason why radio modems have better sensitivity over video receivers is because of their narrower bandwidth.

Sorry for the confusion. I was not really trying to show that they were better than video Rx's -- just trying to say that a low power 2.4Ghz data modem has great range due to it's Rx sensitivity spec, when compared to a R/C Rx. If their required bandwidth was as high as a video Rx's, they would get hit with a reduced sensititivity rating, just like the wireless video Tx.

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