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philthyy

TX antenna length?

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I read that Ahab had cut his TX antenna on his 1.2 system from 110mm to 65mm or something like that. Is there a known length that works best for airborne 1.2 systems?

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Hmmm, well

a 1/4 wave antenna for 1200 mhz is 2.34 inches or 59.436 mm.

a 5/8 wave antenna for 1200 mhz is 5.85 inches or 148.59 mm.

However, the antenna tuning circuitry has been matched to your specific antenna length in the transmitter or receiver. You can match your device to a different antenna length by tuning the antenna matching circuitry. Thats not optimum, but it gets done frequently.

In fact, just as a case in point, a standard 72 MHz RC radio system (US freq) uses a 39 inch antenna for a 1/4 wave length. You can swap out the RF module for a 50 MHz ham setup but you leave the 39 inch antenna on. It's not 1/4 wave length to 50 MHz. That would be a 56 inch antenna. But they tune the antenna matching circuitry in that 50 MHz RF module to make it work with your 72 MHz 1/4 wave antenna.

Lots of "what if's" come to mind. If Ahab took a 110mm antenna and cut it to 65mm, who knows whats going on. Those lengths don't match an optimum wavelength for 1.2 GHz to start with. There's more to the story than what we see here.

Unless you have equipment used to cut your antenna and look at power out while doing it, your best off not cutting the antenna to your 1.2 GHz wavelength as listed above. The tuning circuitry might not be tuned to the new length. Make sense?

I would leave it the way it is figuring someone cut and tuned it to optimum length during manufacture, (At least we hope so) I'm thinking Mr RC-CAM should add to this. He's the real brain in this area.

Dan

Edited by kd7ost

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It kind of depends depends on what sort of antenna you are making and the exact operation frequency. If it is a generic 1/2 wave dipole or 1/4 wave ground plane design, then use the first length in kd7ost's post (60mm) if transmitting on 1200Mhz. But, keep in mind that 1215 -1300 Mhz are the legal ham freqs in the USA, so you might not be exactly on 1200.

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Ok, thanks for all the info. I think I'll just leave it where its at, as I really don't have range issues just yet. I actually haven't had the courage to fly farther away than a quarter mile, or maybe its half mile, and the video is perfect the entire time. I don't know what kind of range my system has, but for now its definately enough. Every question I ask gives me answers that only further my interest in ham radio and wireless systems! I definately am going to take a class this fall to get my ham radio license and learn how to build my own radio and systems.

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I cut mine to 65 mm and it doubled my range. I have one of the cheap Hong Kong ones. I think mine operates between 1050 and 1150 mhz depending on battery voltage on the Tx.

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