darryl123

My Feedback: (8)

hi
does it do any harm if you leave your antenna down with the radio on? if so how much time would be to much?

JoeAirPort

My Feedback: (41)

It harmed my plane when it went into a tree with the antenna down. Other than that I don't see how it could harm anything.

hilleyja

My Feedback: (3)

I don't know the technical description of it, but with the antenna down and the radio on for a significant time it could damage the RF circuitry due to overheating. It is OK to leave it down for short periods while performing range checks. When working on your model on the bench open up at least one length of the antenna.

mstroh3961

My Feedback: (4)

Also preform an extencive rainge check before flying!

Rodney

Yes, it can harm your output stage due to excessive heating. With the antenna down, the SWR (standing wave ratio) goes way up and the energy normally radiated from the antenna is now dissipated in the output stage and heats up the output transistor. When the heat builds up the failure rate skyrockets, doubles for every 10 degrees C. that the temperature rises. Even raising the antenna one section helps but DOES NOT ELIMINATE the heating, just decreases it a bit as the SWR is still higher than it will be with the antenna fully extended. Whenever possible, run with the antenna fully extended to minimize possible damage.

JoeAirPort

My Feedback: (41)

Wow good info. I did not know this. Especially important when I'm in the basement setting up a new plane.

Joe

ekrcflyer

Don't worry, Joe.

Just look at the definition of the SWR (standing wave ratio) here: http://www.its.bldrdoc.gov/fs-1037/dir-034/_5083.htm. As you can see, to create a "standing wave" you need two waves propagating in opposite direction.

The standing wave has nothing to do with the length of the antenna. The output cascade of the transmitter remains equally loaded with fully extended antenna and when it is collapsed. The length of the antenna only affecting the range of the transmittion.

And you were absolutely right by stating that only harm you may cause is having you plane flying into the tree.

But I do like when people making up stories or giving “scientific explanations� about things. This is usually how fairy tales are born.

mr.rc-cam

In this case, the standing wave issue is not a fairy tale; the two waves do exist. One is the RF source. The other is the reflected wave due the impedance mismatch that is caused by the retracted antenna.

If you do not believe this then measure the temperature of the RF PA transistor with the antenna extended for a few minutes. Then retract or remove the antenna and try again. The RF amp will be MUCH hotter. In a poor design, the transistor can fail (but nowadays that does not seem to happen much).

It is best to minimize the transistor's stress by pulling out a section or two of the antenna. Better yet, use the DSC feature if your R/C system has it.

ekrcflyer

OK, then.

Let's check out the definition of Impedance: [link]http://www.its.bldrdoc.gov/fs-1037/dir-018/_2678.htm[/link].
It is function of frequency which is remains constant no matter how long or short antenna is.

mr.rc-cam

Assuming that the frequency remains the same (such as in this R/C situation), antenna impedance changes with its physical length. In other words, the required length depends on the frequency and related source impedance loading. Any changes to the required length will impact antenna impedance. A fully extended R/C antenna versus a fully retracted antenna will have substantial impedance differences. Its radiated beam pattern changes too.

As a practical example, a classic 1/2 wave dipole will be nearly 50 ohms (37 ohms typical). Change its length to full wave and the impedance sky rockets to a few thousand ohms. Impedance charts for such experiments are listed in many RF design books.

These sort of RF rules are more complex than simple dictionary definitions. The www.ARRL.org site has a good selection of practical antenna design books that will offer excellent info on SWR/SWVR and impedance. And other good stuff.

darryl123

My Feedback: (8)

thanks for all the answers.
i had an old futaba that i left on and it burned up. i was wondering if they still will fry if left on?
with all of the features i can see having to bench setup a new model and taking a long time doing it and indoors with the antenna down might take 15 minutes or longer and just losing track of time. so it would be best to have the antenna at least up a few sections working indoors.
thanks
darryl

JPMacG

My Feedback: (2)

Rodney and RC-Cam are correct.

Bax

My Feedback: (11)

To put things in a very simple context, impedence is the resistance of the output system to the transmission of the radio energy. When you have an output device, like the output transistor, connected to a transmission network, like the antenna and its tuning elements, the transistor "expects" to see a certain amount of impedance to work properly. When the transmission network's impedence equals the impedence the output device "expects", you get the most efficient movement of radio energy into the atmosphere. Antenna length is a significant factor in the impedence of the output circuitry.

I've worked with older mobile communications transmitters where we adjusted the output impedence by adjusting the length of the transmitter antenna.

SWR (Standing Wave Ratio) measures the wave sent down the antenna versus the wave reflected back by the antenna. When it's 1:1, you also have good impedence matching, and most efficient power transmission.

With almost any RC transmitter, you should have the antenna partly-extended if you're turning on the transmitter for a short length of time. If it's on for a long time, then full extension would be recommended.

Please remember that this is a really, really, basic, "quick and dirty" explanation, and omits many of the particularly technical aspects of the subject.

mstroh3961

My Feedback: (4)

Thanks BAX for cutting through the BS.

smokingcrater

That's odd, since I just got in from adjusting my 2 meter antenna on my mobile. I was holding my SWR meter, and watching it change as I adjust the length of my antenna...

As others have mentioned, YES it does play a big part in it, and will destroy your transmitter eventually.

That is still done extensively on mobile ham gear. a common base loaded mag mount 2 meter 5/8 wave antenna is adjusted by sliding the antenna in or out of the base. Thanks BAX for the quick explanation, I've studied antennas a little as a hobby (and in part due to being a HAM that is active with high altitude balloon launches), it is something that can be REALLY, REALLY confusing at best!

Ryan
KC0LMO