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Mr.RC-Cam

Low Cost DiY 900Mhz Omni Dipole Antenna

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A few years ago I published the details to a DiY 2.4Ghz rubber duck antenna. It is described here: http://www.rc-cam.com/rc-cam4b.htm. The coaxial design is nice for those installations that need a flexible (nearly indestructible) antenna.

As an experiment, I rescaled it for the 900Mhz band. Because I needed a piece of coax that had an SMA connector on it, I just grabbed a junk WiFi antenna that I didn't need and cut the cable off of it. Below is a photo showing the dimensions that were found to offer decent performance (the Tx was set to CH1, 910MHz).

Although this antenna is not the most RF efficient, it had better performance than the cheap monopole antenna that came with my 900Mhz Tx. The nice thing is that you can use a slightly longer piece of coax (keep it reasonable) to create a nice remote antenna.

Stiffness of the antenna is up to you. Just add more heatshrink layers until it is solid enough for your needs. If necessary, add a coffee stirring stick or drinking straw to make it more rigid.

So, if you have a junk WiFi antenna that can be used as a donor for the coax, this antenna won't cost you a penny. smile.gif

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Edited by Mr.RC-Cam
clean up text

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I make all of my omni aerials that way. I made a great 459Mhz one for my TX and mounted in in a thin white plastic tube that pushes tight on to the N type plug at the base. It looks good and dose not need a ground plane like all the aerials I found in the shops.

I have also made them for 1.3Ghz and 2.4Ghz with great sucess. I often use a brass tube for the lower section which helps stiffen the aerial.

I read that this type of aerial should have the lower section shorter than the top due to the effect of it being so close to the input co-ax but it dose not seem critical to me. They also say it needs ferrite rings on the input co-ax but I have not needed them either ;)

Terry

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It looks good and dose not need a ground plane like all the aerials I found in the shops.

That is an important point. Simple 1/4 wave whip antennas perform best with a ground plane. That is something that is easy on the roof of a car, but not on a foam or wood model airplane. So, the beam pattern can be very odd. Dipoles like this duck variation don't suffer from that.

I often use a brass tube for the lower section which helps stiffen the aerial.

The really nice thing about a brass tube is that you can use it to increase the element's diameter. This increases bandwidth. But it's more work. I like easy. :)

I read that this type of aerial should have the lower section shorter than the top due to the effect of it being so close to the input co-ax but it dose not seem critical to me.

I've heard that too. The one I made (shown above) is shorter on the bottom (.1" less). But I didn't see any improvement over symmetrical lengths, so that is what I published. If someone wants to be fussy, go ahead and reduce the turtle-necked coax element to 3.1 inches long. Even if it is the wrong thing to do, the antenna won't care enough for you to ever know. :)

They also say it needs ferrite rings on the input co-ax but I have not needed them either

Ferrite rings / Toroids can be used as a choke balun that prevents the feed line from becoming part of the antenna (which can cause problems). But like you, I haven't found a need for them on these little duck antennas. I think it is mostly because the feedline section of the coax is quite short.

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I'm new to this forum but have been lurking all over the place researching antenna information.

Seems like each discussion forum has its specialties with great experts, and this is the one for transmitters and antennas. Thanks for sharing all the knowledge and experience!

I'm considering making a dipole antenna for a 500 mw, 900 MHz Range Video transmitter, to be mounted on an Align TRex 450 helicopter. I wanted to place the antenna way back at the tail rotor, out of the way of the main rotor blade. An RG172 cable leads to the antenna.

Attached is a crude diagram, not to scale. It shows the cable, a ferrite bead at the junction where the coax joins the dipole, and how the radiating/ground elements attach to the coax and shield.

Would this work? Would the helicopter's carbon fiber tail be a problem if it's adjacent to the ground half of the dipole? I thought an appropriate ferrite bead would act as a choke. Fair-Rite has a ferrite bead that's designed to work within the frequency range of 250-1000 MHz. Would I need more than one of these? Or would it be needed at all? Weight, of course, is a concern this far back on a TRex 450.

Needless to say, I don't know a whole lot about antennas and don't have any equipment to measure SWR, radiation energy, etc.

With a helicopter this size it's difficult to place an efficient antenna without it being at risk of getting hit by the main rotor or having to sit at some odd angle to prevent it from interfering with landing gear, etc.

Many thanks in advance.

Dean.post-5231-1231135587_thumb.png

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Would this work?

Yes

Would the helicopter's carbon fiber tail be a problem if it's adjacent to the ground half of the dipole?

It will have an effect but hard to say how bad. I always mount the aerial as low as possable.

Fair-Rite has a ferrite bead that's designed to work within the frequency range of 250-1000 MHz. Would I need more than one of these? Or would it be needed at all?

As was said earlier in this thread the bead has not been needed but may be worth fitting if there are problems.

Terry

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Thanks for the info, Terry.

I hope to be able to get a few of the right type of ferrite beads. Apparently they're designed for specific frequency ranges.

I'll give this a try and see how it goes.

Dean.

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I was using the "stock" Range Video antenna but read that it's not very efficient, and it gets worse when it's bent 90 degrees. I'm going to try a dipole that's in a better location and possibly more efficient.

Here's how I had it set up. Unfortunately there's a lot of junk directly in the radiation pattern (tail boom, servo). It also works angled downward with the standard landing skids.

Edit: by the way, the transmitter's attached with something called 3M VHB tape. 3M developed it to fasten windows and other high-stress, critical materials. I believe it was used to mount the glass on a high-rise in Saudi. I saw a story about it on the Discovery Channel and found this stuff in Radio Shack. It requires an overnight set to attain full strength, but once it does, it's really stuck. Yet you can cut it free with a razor blade. It tolerates heat and moisture. A tiny strip was all that was needed to bond the transmitter to the carbon fiber rods.

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Edited by DCSensui

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Be sure & mount the dipole in a vertical position if you want omni directional radiation. In a horizontal plane it has directional radiation Foreward & Backward) & the polarization will be 90 degrees out of phase with a vertical Tx antenna.

Edited by W3FJW-Ron

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If you flip the aerial to point down it wont be so bad.

Terry

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On a heli the only good position for the video antenna is down. With all of the metal and carbon fiber any other position will almost certainly block the antenna at times. Ideally the antenna should also be straight up or down but don't be concerned about a slight angle. If it were that critical banking the aircraft would cause signal problems.

Double sided tape has its applications but I wouldn't recommend trusting two tiny strips to hold the Tx. I've used a lot of that tape and the adhesive is fine but the first thing to deterorate will be the foam. If the Tx comes loose the heli is very possible gone. I would at least put a velcro strap around it.

A 1/2 wave diople will provide slightly better performance over the wire antenna and I've used both with dual patches and a diversity on the ground station. I don't fly much beyond a 1/4 mile out with the heli and the wire antenna has proven to be adequate. The only time I've seen a signal issue is when its sitting on the ground with the antenna in tall grass.

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VHB isn't a garden-variety double-sided foam tape. This is an industrial product that's used in critical applications. So if it's applied to a properly cleaned surface, it'll stick extremely well. Among the applications include glass windows in high rises and certain aircraft components.

I did a quick Google search. Apparently I saw a story about VHB on Modern Marvels:

VHB is discussed at the 1:14 mark.

Another video that compares VHB to conventional double-sided tape:

A company like 3M wouldn't encourage its use in critical applications if it didn't work nearly perfectly: that would expose them to huge liabilities.

Edited by DCSensui

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I finally got the ferrite beads and assembled a very crude dipole as a test.

It's an RG-174 wire with a right-angle SMA connector. The ferrite bead sits right where the dipole starts. The center conductor is soldered to a short length of wire as is the shield.

Both wires were trimmed so the length of each element is 3.2", measuring from point where the elements start at the ferrite bead.

Everything was mounted to a piece of scrap plexiglas. Totally unflyable but it's just a "breadboard" version.

I don't have any test equipment except a digital thermometer that I checked by immersing it in ice water to check for accurate freezing point, then boiling water to check that, too. Both were within 0.5 degrees.

Ambient temp was 75 deg F (it's winter here in Hawaii). No wind in the enclosed garage.

With the stock Range Video antenna in a normal operating position (bent 90 degrees toward the ground), the temp got to 127 deg F in 10 minutes. I had the helicopter suspended one foot off the table on a wood block.

With the crude dipole the temp got to 118 deg F after 20 minutes.

Someone said that temperature might be a rough indicator of antenna efficiency or SWR? If there's a correlation, then perhaps this might provide some performance improvement over the stock antenna?

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Both wires were trimmed so the length of each element is 3.2", measuring from point where the elements start at the ferrite bead.

For 910Mhz operation that length seems a bit long. I get 3.085" for the estimated element length. How did you calculate it? I will say that 3.2" would be a practical length to start with if you are planning to accurately trim it down for best matching.

Someone said that temperature might be a rough indicator of antenna efficiency or SWR?

Temperature is not a good indication of antenna performance. Without a way to directly measure RF field strength, you need to do that with simple system range tests. You can dummy load the Rx end to help reduce the required field size. But regardless, plan on doing it in a wide open area far from other structures and reflective bodies. Besides buildings, that includes moving far away from innocent looking metal fences and parked cars, and other things that are often difficult to avoid.

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For 910Mhz operation that length seems a bit long. I get 3.085" for the estimated element length. How did you calculate it? I will say that 3.2" would be a practical length to start with if you are planning to accurately trim it down for best matching.

I got the length from an online calculator. Thanks for the corrected figure. Would both elements have to be trimmed? Or is the "ground" half of the element less critical?

Regarding the metal fences, etc., I noticed something quite interesting. Doesn't take much to foul up a signal. I noticed that the receiver antenna could find tiny dead spots that were about four inches in diameter, and I could actually blank out the signal if I carefully placed the antenna in just the right spot, even though the polarization was correct. And if I used the metal body of my iPod Touch like a mirror, I could interfere with the signal significantly.

Anyway, my 4-year-old granddaughter was asking about what I was making, and found it intriguing that you could get a live video signal from one point to another without any wires in-between.

Thanks for the guidance!

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Would both elements have to be trimmed? Or is the "ground" half of the element less critical?

Both lengths are important on a dipole.

Doesn't take much to foul up a signal.

You can blame multipathing for what you observed. That is why diversity Rx solutions are very desirable.

In a nutshell, with microwave RF, everything matters.

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the temp got to 127 deg F in 10 minutes.

Is it possible that your pigtail may be reverse polarity SMA? Most WiFi antennas are reverse polarity SMA and most video transmitters are regular polarity SMA. Perhaps you have an 'open' cable (no center conductor contact) creating a high SWR?

-dave

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Is it possible that your pigtail may be reverse polarity SMA? Most WiFi antennas are reverse polarity SMA and most video transmitters are regular polarity SMA. Perhaps you have an 'open' cable (no center conductor contact) creating a high SWR?

Thanks for the suggestions. This was the "stock" antenna that comes with the transmitter. And it was connected directly to the transmitter as seen in the photo, except that it was pointed downward instead of upward. I've read elsewhere that the stock anteanna can be pretty bad with a reported SWR of 3:1 or so. And pivoting the antenna 90 degrees can make it even worse.

The thought behind the solution that I'm cobbling together is to get the antenna way back on the tail, away from the R/C receiver and away from the rotor blades.

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For 910Mhz operation that length seems a bit long. I get 3.085" for the estimated element length. How did you calculate it? I will say that 3.2" would be a practical length to start with if you are planning to accurately trim it down for best matching.

Correction: I had it set at 3.12" Thanks again for your calculation.

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So here's the actual "flying" version of the antenna.

The elements consist of brass tubing with a diameter of 0.9". The length of the elements are 3.09", measured from where the conductor emerges from the ferrite bead.

The brass elements are supported on a plastic base with tiny Plexiglass blocks.

A quick range check -- almost line-of-sight -- got me 1/10 mile before the image started to break up a bit. I'm not sure how good the receiver antenna works since it's another crudely cobbled setup.

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Here are the basic parts. The mounting blocks are tiny Plexiglass strips that were drilled to fit the brass tubes. The brass elements were soldered to the conductors, then slipped into the Plexiglass blocks. That eliminated the concern of possibly damaging the blocks with any excess heat.

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Here's the antenna on the tail. The idea is to have the radiating element clear of any obstructions, away from the RC receiver, and away from the main rotor blades.

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This is the finished project with the antenna mounted on the tail of the TRex 450.

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The idea is to have the radiating element clear of any obstructions,

Seems very close to the carbon? tail fin.

Terry

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Seems very close to the carbon? tail fin.

Yeah, I'm wondering if the counterpoise being that close to the tail fin might have an adverse effect. If it's a problem I'll have to make a Plexiglass version of a tail fin.

At least the tail rotor blades are transparent to radio waves.

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A few years ago I published the details to a DiY 2.4Ghz rubber duck antenna. It is described here: http://www.rc-cam.com/rc-cam4b.htm. The coaxial design worked well for me.

As an experiment, I rescaled it for the 900Mhz band. Because I needed a piece of coax that had an SMA connector on it, I just grabbed a junk WiFi antenna that I didn't need and cut the cable off of it. Below is a photo showing the dimensions that were found to offer the best performance (the Tx was set to CH1, 910MHz).

It has higher gain than the antenna that came with my 900Mhz Tx. Plus, it is less susceptible to detuning from nearby structures, and is more omni. The nice thing is that you can use a longer piece of coax (keep it reasonable) and you can create a nice remote antenna.

Stiffness of the antenna is up to you. Just add more heatshrink layers until it is solid enough for your needs. If necessary, add a coffee stirring stick or drinking straw to make it more rigid.

So, if you have a junk WiFi antenna that can be used as a donor for the coax, this antenna won't cost you a penny. :)

If I may ask, what are the pros/cons of this antenna VS. a 1/2 wave whip?

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If I may ask, what are the pros/cons of this antenna VS. a 1/2 wave whip?

This is a 1/2 wave dipole antenna. Many 1/2 wave whips are simple monopoles, which require a counterpoise (surface such as car roof or human body for handheld apps) to work well.

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A few years ago I published the details to a DiY 2.4Ghz rubber duck antenna. It is described here: http://www.rc-cam.com/rc-cam4b.htm. The coaxial design worked well for me.

As an experiment, I rescaled it for the 900Mhz band. Because I needed a piece of coax that had an SMA connector on it, I just grabbed a junk WiFi antenna that I didn't need and cut the cable off of it. Below is a photo showing the dimensions that were found to offer the best performance (the Tx was set to CH1, 910MHz).

It has higher gain than the antenna that came with my 900Mhz Tx. Plus, it is less susceptible to detuning from nearby structures, and is more omni. The nice thing is that you can use a longer piece of coax (keep it reasonable) and you can create a nice remote antenna.

Stiffness of the antenna is up to you. Just add more heatshrink layers until it is solid enough for your needs. If necessary, add a coffee stirring stick or drinking straw to make it more rigid.

So, if you have a junk WiFi antenna that can be used as a donor for the coax, this antenna won't cost you a penny. :)

Dont understand why the coax dont affect the shield lengh (dipole)

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