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H.J.Gelsthorpe

TELEMETRY ANTENNA

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Hello all

I am building a altimeter+ vario with data sent over a 434 mHz link. data rate is 1200 bps but transmitted at 2400 bps because of Manchester encoding.

Can I just expose the necessary 1/4 wavelength of coax on the transmitter without a ground plane without significantly reducing range?

would I be better off with a half wave dipole? and if so how would I match it to the 50 coax?

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Why not a ground plane?

fold a 1/4 wave of the shielding back over the coax exposing the inner conductor with insulation for a 1/4 wave element effectively giving you a 1/2 wave antenna. Tie with a string to a tree limb or tywrap the length to a fiber glass rod that is the length you want to get it into the air. This will give you a vertically polarized dipole.

To match it, trim both ends (shield element & driven element) to the lowest SWR possible. Make the feed line (coax) a multiple of a half wave in length.

But, you didn't say whether this was to be on the plane or on the ground. The instructions above are for on the ground.

In a plane, just expose a 1/4 wave, stick it out the bottom of the fuselage and use a good receiving antenna to make up for the "lossy" plane antenna. Taping it to the bottom of the fuse will effectively give a horizontally polarized antenna and an additional polarization loss unless the ground antenna is horizontal as well. Better to use vertically polarized systems for RC use.

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Thank you Ron for the reply,i was considering the antenna for the plane where it is difficult to get a neat installation with a ground plane.

Is the 1/2 wavelength multiples to ensure that voltage and current is arriving at the attenna in phase?.Would that make the worse case odd multiples

of 1/4 wavelength?

so all other things being equal what extra loss might be expected from a non ground plane transmitter antenna vers. a ground plane xmt attenna

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so all other things being equal what extra loss might be expected from a non ground plane transmitter antenna vers. a ground plane xmt attenna

Do you mean that you are interested in what occurs when a 1/4 wave vertical whip is operated without the required ground plane? Or, do you mean you are interested in how something like a Dipole would compare to a Ground Plane Antenna?

Please keep in mind that Ground Plane Antennas are just one type of vertical antenna. For your model aircraft telemetry installation I typically would suggest a simple dipole since they don't heavily rely on a neighboring ground system. However, at 434Mhz that would be a large antenna if built with a pair of 1/4 wave elements. But there are compact designs available too (helical loaded is common).

Antenna choice is all about compromises. Sometimes the best performance is not possible due to installation limitations. But it seems to me that you should be able to support a decent performing antenna if your model is not micro sized.

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Thank you for your reply Mr R.C.Cam,

I meant a 1/4 wave with and without a ground plane. As you correctly state a dipole would be to large in most models.

I will look into your helical anttenna suggestion,I may not stick with 433 mhz band this is a prototype and

i am keeping the rf transmitter as a seperate module/ pcb to allow chopping and changing.

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I meant a 1/4 wave with and without a ground plane.

There are a lot of small portable RF designs that use 1/4 wave elements without the ground element or suitable ground plane. Some rely on a human body to act as the counterpoise and some just ignore it altogether. It's not a great solution, but can be functional.

Even when a proper 1/4 wave vertical (monopole) design is used, the efficiency is often much less than a 1/2 wave design (one of the reasons I usually suggest dipoles). It is further handicapped by removing the ground element because the aperture is cut in half. If that was the end of the story then you would only see an additional -3dB loss. However, the feedpoint impedance is affected and so the matching errors cause significant efficiency losses. The radiation pattern can become unusual too.

But in practice, in absence of deliberate ground elements, there will be a ground system that is formed by the circuit board and adjacent wiring. These can be reasonable substitutes if they are sufficient in size. However, in cases where these are not optimized, the performance results will vary. With a 434MHz system, a sufficient counterpoise from the ground plane on the PCB would need to be inconveniently large for your application.

But I think you should look at the practical side of this. I have no idea what your design is, but I think I could make a good guess. I suspect you are using a commercial RF data receiver module that has -105 to -120dBm sensitivity at your chosen data rate. Your Tx is probably 10mW or more. Given the huge gain budget in that setup, you can implement a horrible Tx antenna and still experience longer range than most R/C and wireless video systems. So, I'd say you should just try it out and go from there.

As I have mentioned before, antenna choices are nearly always compromise selections. You have to think about what is most important in your application and then aim to satisfy those things. For many RF devices, longest range is NOT the top item in the list.

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Thanks again Mr R.C.Cam,for your indepth reply

I suspect you are using a commercial RF data receiver module that has -105 to -120dBm sensitivity at your chosen data rate. Your Tx is probably 10mW or more.

I am using the T7G transmit module and the R7G recvr.module which is in very good agreement with your asessment alink to the pdf

can be found here

http://www.okwelectronics.com/datasheets/radio_modules/DS307-7.pdf

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Once you try it out please report back the range test results. Assuming those RF modules work well, I expect your "crippled" 1/4 wave transmitter antenna will satisfy your R/C telemetry application.

Don't skimp on the Rx antenna -- use a proper design there (which should be easy to accommodate since it is on the ground station).

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