Gain of typical 72MHz Tx antenna

[AnthonyRC]

Does anyone happen to know the approximate gain of a typical 72MHz transmitter antenna (like the one on the futuba 9c series for example).

[stall warning]

Usually when you say an antenna has a 3dB gain, it's 3dB compared to
a dipole. A vertical dipole has a response pattern that looks like a
horizontal donut in free space. The antenna that got 3dB better gain probably
changed the response pattern to some thing other than a perfect donut.
It gave up some response in a certain direction to get gain in another.

What is a transmitter antenna? Could it be considered a dipole, grounded or
even single ended?

My guess it has no gain over a dipole unless there's something going on that
flattens the donut so you get more signal in the horizontal direction at the
expense of the vertical direction.

[jeffs555]

Like Stall said, gain is relative, not an absolute, so you need to know what it is relative too. For a portable antenna, gain is often specified as relative to a 1/4 wavelength monopole(ie whip). Since the typical 72mhz transmitter antenna is a 1/4 wavelength whip, it could probably be called unity gain or 0dB.

[albany tom]

Yes, it's all relative... With antennas, it's usually relative to whatever the company selling the antenna picks. Often, you'll see an antenna rated as a number of "dbi", which means relative to a theoretical isotropic radiator - something that radiates equally in all directions, and something that doesn't exist in reality as far as I know. A 1/4 wave dipole would have a 3 dbi rating, if sold by such a company. Some companies would just list this as "3 db", and some would list it as more, probably.

Anyway, to your question, an R/C antenna doesn't really have any gain. It's a 1/4 wave dipole, with you and the radio acting as the bottom half of the dipole. (Because you and the radio aren't great at being half of a dipole, an R/C antenna won't work as well as a real 1/4 wave dipole.) To add gain, you have to lengthen the antenna and load (match) it to the transmitter, or switch to a directional antenna such as a Yagi.

You can, however, REDUCE the gain of an antenna, a lot, if you want to. Any of the shortened, rubber duck style antennas will reduce the gain, probably by 6db or more. The shorter the antenna, the more gain reduction. Rubber duck type antennas are handy, but they all are crummy performing.

Also, it should go without saying, but don't point the antenna at the plane! The radiating pattern is from the side, not the end.

[Miami Mike]

It's more of a ground plane, but definitely not a dipole.

Rubber duck antennas are great for RC use because they're not as directional, and the way we usually hold our radios points the antenna null right toward our planes.

[pmackenzie]

IIRC, the gound plane acts like a mirror, so the other half of the dipole is "virtual"
Pat MacKenzie

[Miami Mike]

Whatever. I guess all antennas are really variations of other antennas.

A more important point is that they achieve gain by diverting energy from some directions and redirecting it in other directions, sort of like a reflector in a flashlight. And just as a lantern illuminates better than a flashlight if the flashlight is pointing in the wrong direction, a rubber duck antenna should theoretically get more signal to your plane than the stock antenna when you're holding your radio in the usual position, with the tip of the antenna pointing toward your plane.

A stock 1/4 wave antenna has its deepest null off the tip and exhibits gain from the side, so if you wanted to actually realize any of the potential gain that the antenna has, you'd have to tilt your radio so that the antenna is horizontal, or perhaps hold it up in front of your face with the antenna tilting back. If you don't habitually fly like that, the gain of your 1/4 wave antenna is only working against you by diverting energy away from the direction where it's needed.

I recommend Smiley Antennas (http://www.smileyantenna.com/product_info.php?cPath=43&products_id=86). Many of the guys in our sailplane club have them and they work great.

[albany tom]

It's really not a ground plane. A ground plane is a dipole where the bottom half is either the ground, or horizontal radials. There aren't any horizontal radials on an RC transmitter, and the physical ground is too far away, in terms of wavelength, to be a groundplane.

A groundplane would be WORSE than the imbalanced diplole that it is. The groundplane lowers the radiation angle and thus increases the null at the end of the antenna.

I don't know anybody that points the antenna at the plane. It's day one stuff, as far as I knew. To the side, below, whatever, but not at.

If a rubber duck antenna works for you, great! Some or all rubber ducks may not have a null quite as steep as a 1/4 wave - but in any other direction they're a lot worse.

[Miami Mike]

Some or all rubber ducks may not have a null quite as steep as a 1/4 wave - but in any other direction they're a lot worse.No, not at all. Here's an interesting report from an actual field test: http://www.bergent.net/antenna_field_test.html

[johnrobholmes]

is the installation of the smiley antenna pretty easy? Looks like a good one for using with RC cars and robots when they are close to you.

Would keeping my antenna in also accomplish the same effect? What about extending only one or two sections?